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MATERIALS RESEARCH SOCIETY
E-MRS Spring Meeting 2002 June 18 - 21, 2002 "Growth and Evolution of Ultrathin Films: Surface and Interface Geometric and Electronic Structure" Conference Venue: Palais de la Musique et des Congres, Strasbourg Ground Floor, Room ARP 1 Programme and Book of Abstracts Symposium Organizers: J.W.M. Frenken, Leiden University, The Netherlands Karsten Horn, Fritz-Haber-Institut, Berlin, Germany Guy Le Lay, CRMC2-CNRS, Marseille, France D. Phil Woodruff, University of Warwick, UK Papers will be published in Thin Solid Films SYMPOSIUM J
E-MRS 2002 SPRING MEETINGSYMPOSIUM JTuesday, June 18, 2002 Mardi 18 juin 2002 Session I Moderator: K. Horn08:45 (INVITED) X RAY DIFFRACTION STUDIES OF GAS MOLECULES ADSORBED ON METAL SURFACES FROM
UHV TO ATMOSPHERIC PRESSURES
C.J. Walker, C. Quiros, K.F. Peters P.Steadman, O. Robach, H. Isern and S. Ferrer, European Synchrotron Radiation Facility, BP
220, 38043 Grenoble cedex, France
The crystallographic structure of CO adsorbed on Ni(110) has been investigated with x ray diffraction at ambient pressures of
CO ranging from 2.3 bar to 10-10 bar. At room temperature the high pressure and vacuum structures are identical. Above room
temperature ( 130° C), the high pressure of CO induces a re-structuring of the Ni substrate which develops strained (111)
microfacets. CO on Ni(111)forms an incommensurate structure at elevated pressures whose lattice parameter depends of the
pressure of the ambient gas. Analysis of the compressibility of the adsorbed CO allows to get insight on the interaction energy of
the molecules.
HIGH-RESOLUTION Si 2p CORE-LEVEL STUDY OF THE Ca/Si(111) SURFACE Kazuyuki Sakamoto (a,b), Wakaba Takeyama(a), H.M. Zhang(b), Roger I.G. Uhrberg(b), (a)Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan, (b)Department of Physics and Measurement Technology,Linkoping University, 581 83 Linkoping, SwedenDesign of core-shell semiconductor nanoparticles is an important challenge because of the possibility to tune up theirphysicochemical properties. Due to recent advances in colloidal chemistry, synthesis of core-shell nanostructures has beensuccessfully performed in many research groups. However, colloidal chemistry routes are rather complicated and thus obtainednanoparticles are not stable enough to be suitable for many applications. We report a one-step preparation of core-shell PbS/Snanoparticles in inorganic matrix via decomposition of lead thiosulfate complex intercalated into hydrotalcite-type layered doublehydroxide (LDH). An LDH structure consists of positively charged hydroxide layers bonded with anions, which occupy theinterlayer space. During decomposition of anions in the interlayer space, the reaction zone is constrained by the rigid hydroxidelayers, giving rise to the conditions similar to those in 2D nanoreactors. Decomposition of [Pb(S O ) ]2- intercalated between the hydroxide layers is performed by UV-irradiation at 77 K and is not accompanied by destruction of the layers. Core-shell PbS/Snanostructures are formed in one step as a result of the simultaneous formation of PbS and elemental sulfur from [Pb(S O ) ]2-.
Nanoparticles are stabilized inside the matrix and show unusual optical properties, including the presence of excitonic peaks inabsorption spectrum and high quantum yield of photoluminescence. This work is supported by RFBR grant 00-03-32579.
AB INITIO CALCULATION OF Li ADATOM STRUCTURES ON THE Mo(112) (a,b) and R.M. Nieminen(a), (a)Laboratory of Physics, Helsinki University of Technology, P.O.Box 1100, 02015 HUT, Finland, (b)Institute of Experimental Physics, University of Wroclaw, Plac M. Borna 9, 50-204 Wroclaw, PolandThe formation and structure of the ordered phases of Li atoms adsorbed on the Mo(112) surface are investigated by performingfirst principles density functional calculations. Large inward relaxation for the first atomic interlayer distance of the cleanMo(112) surface is found. The vertical relaxations are relatively little influenced by the adsorbed Li atoms. Energetics of chain- structures of Li adatoms for coverages 0.125 < θ < 1 monolayer is determined in agreement with experiment. The binding energyof Li atoms is decreasing with increased coverage. The calculated work function changes with Li-adatoms coverage are in goodagreement with experiment.
STRUCTURAL STUDY OF THE Ge/Ag(110) C(4x2) RECONSTRUCTION BY SYNCHROTRON RADIATION XRD C. Leandri (a), H. Oughaddou(a), J.M. Gay(a), B. Aufray(a), G. Le Lay(a), O. Bunk(b), R.L. Johnson(b), (a)CRMC2-CNRS,Campus de Luminy, 13288 Marseille, France, (b)Institut für Experimentalphysik der Universität Hamburg, 22761 Hamburg,GermanyThe adsorption of metals on semiconductor surfaces (M/S) has been observed for many years whereas the case of semiconductordeposited (or segregated) on a metal has been much less studied. Nevertheless, it is very important to investigate this reversesemiconductor-on-metal interface since new properties can be expected. Indeed, an interface variation can induce amodification of the interface chemistry and hence lead to the formation of new compounds with novel properties. We chose tostudy a semiconductor on metal interface which presents a large miscibility gap in the bulk phase diagram and with a surfaceenergy of the adsorbate smaller than that of substrate: the Ge/Ag system. A previous study of Ge/Ag (001) has shown theparticular character of this system, with the remarkable stability of Ge tetramers on top of the Ag (001) surface observed atvarious coverages during growth and dissolution.In order to observe the differences induced by the substrate crystallographic, wehave started a comparative study on the silver (110) surface. The growth of germanium on silver (110) at room temperature isnearly layer by layer. The LEED reveals a sharp c(4x2) pattern at 0.6 ML during growth. We have studied this c(4x2)superstructure by Synchrotron Radiation Grazing Incidence X-ray Diffraction and an atomic model of 8 Ge atoms formingtetramers on top of the Ag (110) surface gives excellent fits with the measured data. Recent atomic scale observations by STMwill be also presented.
SURFACE REAL TIME CHANGES OF Pd8Ni92(110) DURING A SELECTIVE HYDROGENATION REACTION: A SYNCHROTRON IN SITU GRAZING INCIDENCE DIFFRACTION EXPERIMENT IN THE MBARS RANGEM.C. Saint-Lager (a), R. Baudoing-Savois(a), P. Dolle(a), J.C. Bertolini(b), Y. Jugnet(b), L. Picco(b), O. Robach(c),(a)Cristallographie-CNRS, BP 166X, 38042 Grenoble Cédex 09, France, (b)IRC-CNRS, 2 avenue Albert Einstein, 69626Villeurbanne Cedex, France, (c)Surface Diffraction beamline, ESRF, BP 220, 38043 Grenoble cedex, FranceThe Pd8Ni92(110) surface is a very active catalyst for the selective hydrogenation of butadiene, C4H6. There is a strong Pdsegregation, and a Nx1 reconstruction is observed by LEED and by X-ray diffraction. In the mbars range, the selective reactiontakes place in two steps; a) first, butadiene is rapidly (22 min in this experiment) turned into butene only (interesting productC4H8) b) butene is then transformed into butane (non interesting end product C4H10). Real time structural changes weremonitored with in situ synchrotron grazing incidence diffraction: the surface reconstruction signal as well as the truncation rod ofthe surface alloy was recorded during the reaction. In the same time, the reaction itself was followed by using a gas analyzer.
We clearly observe the effect of the two reaction steps on the diffracted intensity of the truncation rod. Inversely no significantchange has been observed on the superstructure features. The structural changes are mainly reversible upon pumping down atthe end of the reaction: the reaction resumes when the mixture is reintroduced as well as the structural changes. When thesurface is bombarded with Ar ions, but not annealed to yield the Pd segregated surface studied above, the reaction almost doesnot occur on a time scale of 60 min. This excludes any spurious significant contribution of the X-ray chamber alone to thereaction which can thus be attributed to the strained Pd atoms of the annealed Pd8Ni92(110) surface.
SYMPOSIUM J
ANGLE RESOLVED X-RAY PHOTOELECTRON DIFFRACTION ON CLEAN AND HYDROGEN TERMINATED 2x1-Si(100) SURFACESS. Dreiner(a), C. Westphal (a), M. Schürmann(a) and H. Zacharias(b), (a)Universität Dortmund, Fachbereich Physik, Otto-Hahn- Str. 4, 44221 Dortmund, Germany, (b)Westfälische Wilhelms-Universität Münster, Physikalisches Institut, D-48149 Münster,GermanyHigh-resolved X-ray photoelectron spectra were recorded of the 2x1 reconstructed Si(100) surface with and without hydrogenpresent at the surface in order to investigate the surface structure. The spectra were measured for the 2π half-space above thecrystal surface at BESSYII (U41-PGM) with a photon energy of hν=179.9 eV. The surface and bulk signals were separated by apeak-fitting procedure for all spectra. At the clean Si-surface with asymmetric dimers, the photoelectron spectrum contains asignal of an upper and a signal of a lower atom within the dimer. At the hydrogen terminated surface with symmetric dimers, asignal of both dimers was obtained. In this paper we present the first photoelectron diffraction patterns for these surfacecomponents obtained by high resolution spectroscopy. We compare the patterns with the results of a multiple scatteringcalculation [1] within an R-factor analysis. At the clean Si-surface asymmetric dimers in p(2x1)- and p(2x2)-reconstruction andat the hydrogen terminated surface symmetric dimers were included. Structure parameters of the first three layers at the surfacewere determined.
[1] R. Gunnella, F. Sohal, D. Sébilleau, C.R. Natoli, Comput. Phys. Commun. 123, 251 (2000) Session II Moderator: N.N.
11:00 (INVITED) STRUCTURE OF MAGNETIC ULTRATHIN FILMS STUDIED ON THE ATOMIC LEVEL
P. Varga, Institut für Angewandte Physik, TU Wien, 1040 Wien, Austria
Two systems which are prototype systems for the extrapolation of novel magnetic structures and thin film magnetism have been
studied by STM with atomic resolution. The first system is Co/Pt(111) [1] It will be shown that the first Co layer grows preferably
in the Pt fcc lattice site, with a high density of defects due to the lattice mismatch. The second layer is found to exhibit a moiré
structure, with Co in-plane lattice distance close to that of bulk Co. Up to a coverage of about 3.5 ML 2D growth is observed and
at higher coverage, Co grows in (3D) islands but still in fcc-like stacking.The 2D growth mode at low coverage is due to the
strained interface between Co and Pt resulting in large number of kinks and corners which facilitate interlayer diffusion [2]. The
second system was Fe on Cu(100) which have been studied extensively for the last three decade owing to its apparent ability to
stabilize the ferromagnetic fcc phase of Fe. In this talk it will be shown that ferromagnetic Fe layers are never fcc or fct but
always form nanosized domaines of strained bcc which we call nanomartensite[3]. Furthermore a direct view of the very initial
stage of the relaxed bcc nucleation on an atomic scale will be presented. The STM images reveal the nucleation centers as
nanometer sized bcc crystals. They form virtually without mass transfer under conservation of volume[4].
[1] E.Lundgren, B.Stanka, M.Shmid and P.Varga, PRB 62 (2000) 2843 [2] E.Lundgren, B.Stanka, G.Leonardelli, M.Schmid and
P.Varga,. PRL 82 (1999) 5068 [3] A.Biedermann, R.Tscheliessnig, M.Schmid and P.Varga, PRL 87(2001)086103
[4] A.Biedermann, M. Schmid and P.Varga, PRL 86(2001)464
TUNING THE MAGNETIC PROPERTIES OF Co-Au CODEPOSITED AT 30K , F. Scheurer and J.P. Bucher, Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504, Université Louis Pasteur, 23 rue du Loess, 67067 Strasbourg, FranceUltrathin films obtained by codeposition of CoxAu(x-1) on Au(111) at a substrate temperature of 30 K have been studied. Thelow temperature growth forces an intimate mixing of Co and Au clusters. Controlled segregation and growth by coalescence oflarger Co clusters is then achieved by annealing. Our goal is to test the interaction between particles and the evolution of theblocking temperature during the segregation process. Structural/chemical, as well as magnetic properties are studied in UHV byvariable temperature STM, LEED, AES and MOKE. For as-deposited films, no magnetic signal is observed. A signal onlyappears as a function of annealing temperature for Co concentrations in excess of 50%. For an equivalent thickness of 3.5 ML amagnetic signal appears in the polar configuration at about 150 K. The signal needs 120 min to stabilize in a rectangularhysteresis loop, with a coercive field of 60 Oe. For a 10 ML film, the annealing time is 330 min. The LEED diagram shows thatthere is a structural modification from an amorphous to a crystallized state above 150 K. This is consistent with the idea thatabove 150K the deposit adopts a well defined interface with the crystalline substrate. An out-of-plane magnetization is thenobserved. The coercive field can then be tuned over a wide range by annealing to higher temperatures. Coercive fields rangingfrom 20 Oe to 600 Oe are obtained for bare Co ultrathin films on Au(111) as a function of annealing temperature up to 500 K.
METAL-INSULATOR TRANSITION IN LaSrMnO THIN FILMS INDUCED BY EPITAXIAL STRAIN AND THE FORMATION OF LaSrMnO QUANTUM DOTSHanns-Ulrich Habermeier and G. Cristiani, Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, 70569 Stuttgart,GermanyPseudomorphically grown perovskite-type thin films can exhibit appreciable interfacial biaxial strain either compressive ortensile, depending on the lattice mismatch of substrate and film, respectively. Tensile stress causes the shift of the Curietemperature to lower values and an increase of the resistivity in LaCaMnO thin films in addition to modifications of themagnetisation curve especially in the range of the Rayleigh law and the approach to saturation.
In this contribution, the results of structural, transport and magnetic properties of films under tensile stress of LaCaMnO films arediscussed from the point of view of epitaxial stress. Additionally, recent results will be presented showing a film thicknessdependent transition from an insulating to a ferromagnetic metallic state in LaSrMnO thin films of different compositions. Thefilms have been prepared by PLD and analysed by XRD, TEM, transport and Raman scattering investigations. A generalisedpicture of the strain generation and relaxation emerges and will be discussed. Using the concepts of epitaxial strain LSMO filmsof thicknesses below 10 nm have been grown and the formation of LSMO quantum dots with dimensions 3 nm diameter and 4nmheight has been detected. It could be shown that these quantum dots are ferromagnetic with a Cuire temperatur well above roomtemperature.
(INVITED) DYNAMICS OF SURFACE MAGNETIZATION ON A NANOSECOND TIME SCALE
Giorgio Rossi, INFM-Dipartimento di Fisica dell'Universita di Modena e Reggio Emilia, via Campi 213/a, 41100 Modena, Italy
and TASC National Laboratory, Area Science Park - Basovizza, 34012 Trieste, Italy, Fausto Sirotti, LURE - CNRS, Université
de Paris-Sud, Campus d'Orsay, 91405 Orsay, France, Giancarlo Panaccione, TASC National Laboratory, Area Science Park -
Basovizza, I-34012 Trieste, Italy
A review is made of the study of the dynamics of surface magnetization with ns time resolution by means of spin-polarimetry of
the total photoemission yield excited by synchrotron radiation pulses. The surface and the bulk of ferromagnets have quite
different magnetization dynamics which can be interpreted as a weak surface-bulk coupling.
DOMAINS IN LaFeO3 THIN FILMS AND THEIR ROLE ON EXCHANGE BIAS (a), E. Fullerton(b), H. Siegwart(c), J. Fompeyrine(c) and J.-P. Locquet(c), (a)IPMC, EPFL, 1015 Lausanne, Switzerland, (b)IBM Research Division, Almaden Research Center, San Jose CA 95120, USA, (c)IBM Research Division, ZurichResearch Laboratory, 8803 Rueschlikon, SwitzerlandThe AF orthoferrite LaFeO3 is an interesting model system to explore this correlation, as the overlap of Fe-O-Fe orbitalsdetermines the AF as well as the structural ordering. Thin films of LaFeO3 have been grown by molecular beam epitaxy onSrTiO3 and MgO. Although the thin film is nearly lattice-matched with SrTiO3 (mismatch < 1 %), the thin film growsorthorhombic in the initial stage and leads to formation of 90° rotational twins, which are simultaneously AF domains.[1] We studythe growth of LaFeO3 thin films and explore the domain formation. In particular, we investigate how the parameters, such asgrowth condition, epitaxial relationship, film thickness and epitaxial strain control the domain morphology and discuss how the AF SYMPOSIUM J
domain configuration determines the exchange bias phenomenon.
[1] A. Scholl et al, Science vol 287, 1015, 2000.
Tuesday, June 18, 2002 Mardi 18 juin 2002 Session III Moderator: M. Giesen14:00 J-III.1 (INVITED) MESOSCOPIC SCENARIO OF STRAIN RELIEF AT METAL INTERFACES
V.S. Stepanyuk, Max-Planck-Institut für Mikrostrukturphysik, D-06120 Halle, Germany
Usually strain relaxations are predicted on the basis of the macroscopic lattice mismatch between the two materials. However, if
the deposited system is of the mesoscopic size of several 100 atoms, its intrinsic bond lengths are different from those in the bulk
material. In this case, the strain induced at the interface can locally distort the surface in an unpredictable way. Even in the case
of homoepitaxy, where there is no microscopic mismatch, the mesoscopic mismatch may lead to drastic strain relaxations.
Performing atomic scale calculations with ab initio based many-body potentials we demonstrate unexpected scenario of strain-
relief in mesoscopic islands in metal hetero- and homoepitaxy.We concentrate on the mesoscopic Co and Cu islands on Cu(001).
Our results lead to the conclusion that the size-dependent mesoscopic mismatch, rather than the macroscopic one, is the driving
force for strain-relieving effects on the mesoscale. The strain relief is predicted to have a profound effect on the shape of
islands. The islands locally distort the surface and induce strongly inhomogeneous displacement pattern in the substrate. The
effect of island interactrion and island coalescence on stress distribution on metal surfaces is studied. Strain relaxation in Co
islands leads to a reduced stress contribution in the partially filled layers. The stress in Co monolayers has been measured during
epitaxial growth on Cu(001). The Co-induced stress is found to oscillate with a period of one atomic layer. Our experiments and
atomic scale calculations ascribe the stress variation to the relaxation of epitaxial mesoscopic misfit strain in the islands.
J-III.2 ULTRA HIGH PRECISION OF THE TILT/TWIST MISORIENTATION ANGLES IN SILICON/SILICON DIRECT
WAFER BONDING
F. Fournel (a), H. Moriceau(a), B. Aspar(a), N. Magnea(b), J. Eymery(b), K. Rousseau(b), J.L. Rouvière(b), (a)CEA,
LETI/Département des Technologies Silicium and (b)Département de Recherche Fondamentale sur la Matière Condensée, 17
rue des Martyrs, 38054 Grenoble Cedex 9, France
New substrates for epitaxy with a nanometric sub-surface patterning are elaborated by direct bonding of ultra thin films of silicon
onto Si wafers. Due to the misorientation of the two crystals, the direct bonding produces nanometric networks of dislocations
localized at the bonding interface. An original direct wafer bonding process has been developed to accurately control both the
bonding interface twist and tilt angles between the two bonded monocrystalline crystals. This process is based on the bonding of
twin surfaces coming from a single wafer, using for instance the Smart Cut‚ process. The twist angle is achieved with an
accuracy of about ±0.005° without any alignment measurement based on diffraction techniques, but only by positioning
lithographic marks. Using such process, pure twist-bonded interfaces have been made between two (001) bonded silicon
surfaces. Pure square dislocation network is so obtained at the bonding interface. HREM observations show the excellent
regularity of such a network without the well known mixed dislocations which usually appear due to the tilt disorientation between
the two crystals. Grazing incidence X-Ray diffraction confirms the very good precision of the bonding alignment and the
regularity of the network over several mm2.
J-III.3 STRESS-DRIVEN RECONSTRUCTION OF AN OXYDE SURFACE: THE ANATASE TiO (001)-(1x4) SURFACE
Michele Lazzeri (a) and Annabella Selloni (b), (a)Laboratoire de Mineralogie et Cristallographie, UMR CNRS 7590, UniversiteParis VI, 4 Place Jussieu, F-75252 Paris, France, (b)Chemistry Dept. of the Princeton University, Princeton, NJ 08540, USAWe propose a new model for the recently discovered 1x4 reconstruction of TiO anatase (001) surface, on the basis of density functional theory calculations. Anatase is a TiO polymorph slightly less stable than rutile, that is widely used, especially for photocatalysis and charge separating devices. Besides being energetically much more favored than the unreconstructed 1x1surface and any other available model, our model provides an excellent description of of scanning tunneling microscopy images.
According to our analysis, the primary mechanism of the reconstruction of the anatase (001) surface is the relief of the largesurface stress present on the unreconstructed surface. The importance of this mechanism is well known in the context of metaland semiconductor surface reconstruction, but its relevance for oxide surfaces has not been established yet. We provide anexhaustive description of this surface reconstruction which is not explained by the electrostatic criteria which are usuallyemployed to rationalyze oxide surface structures.
M. Lazzeri and A. Selloni, Phys. Rev. Lett. 87 (2001) 266105.
J-III.4 SURFACE STRESS INDUCED BY ADSORBATES, MAGNETOSTRICTION, AND ION BOMBARDMENT
Klaus Dahmen and
, Forschungszentrum Juelich, ISG 3, 52425 Juelich, Germany Experimental methods for the determination of induced changes in the surface stress are known for some time [1]. The mostpopular and reliable one is to measure the bending of a thin sheet of material due to the change in the surface stress on one side.
The experimental data on the bending are converted into a change of surface stress using elasticity theory. However, until latelythe effect of the one sided clamping of the sample and the crystalline anisotropy was not taken into account[2, 3]. Based on finiteelement calculations, we have developed a general scheme which permits the transformation of bending into the isotropic oranisotropic surface stress for (100),(111), and (110) surface of cubic materials. As an example we discuss the adsorbate inducedsurface stress of Li on Mo(110). Comparison is made to the interface charge induced surface stresses in an electrochemicalenvironment. It is furthermore shown how the measurement of the bending of a nonmagnetic sheet covered by a thin magneticfilm due to a re-orientation of the magnetization can be converted into a particular magneto-elastic constant. Finally we discuss anovel set of experiments in which we have measured the stress induced by ion bombardment of a Cu(100) surface.
[1] H. Ibach, Surf. Sci. Rep. 29 (1997) 193.
[2] K. Dahmen, H. Ibach, Surf. Sci. 446 (2000) 161.
[3] K. Dahmen, H. Ibach, D. Sander, J. Magn. Magn. Mat. 231 (2001) 74.
J-III.5 SELF-ORGANISATION OF CHEMISORBED SYSTEMS: ELASTIC ORIGIN AND KINETIC FACTORS
G. Prévot and B. Croset, Groupe de Physique des Solides, UMR CNRS 75-88, Universités Paris 6 et 7 Tour 23, 2 place Jussieu,
75251 Paris cedex 05, France
Since the first theoretical predictions of Marchenko, self-organisation of two phases at the surface of a solid is assumed to result
from the competition between elastic relaxations in the bulk and boundary creation at the surface. For chemisorbed systems, this
has been experimentally shown only recently, for N/Cu(100). In this 2D system, N squares are organised in regular rows at the
Cu surface. By measuring bulk relaxations by GIXD, we have evidenced the elastic origin of self-organisation and determined a
surface stress difference between N-covered and bare Cu regions of 6.4 N/m [1]. Despite the universality of the hamiltonian
involved for self-organisation, only few systems have been up to now evidenced. This could be due to kinetic limitations. We
have analysed by SPA-LEED the kinetic of self-organisation in the case of O/Cu(110). After O adsorption at 130K, the system is
annealed at T>200K. Cu-O bands (with Cu-O added rows along (001)) then form, separated by bare Cu regions. The measured
period always increase during annealing. At 300K, this evolution takes place in the first hour of the annealing process. At lower
temperatures, the evolution is slower but the period of the system after annealing can reach higher values. In order to determine
the factors controlling the ordering, these results are compared to kinetic Monte-Carlo simulations taking into account the elastic
interactions between the different domains.
[1] B. Croset et al. PRL 88 (2002), in press.
SYMPOSIUM J
J-III.6 STRAIN ENERGY DENSITIES FOR HEXAGONAL CLOSE--PACKED ULTRATHIN OVERLAYERS
J-H.C. Schönfeldt and
H.W. Kunert , Department of Physics, University of Pretoria, 0002 Pretoria, South Africa The surface and near-surface monolayers of crystals are in different atomic environments and their interlayer and intralayerequilibrium spacings are accordingly different from those within the crystal interior. By this phenomenon, referred to as surfacemisfit, the surface layers are in a state of stress. Based on the set of stiffness constants transformation relations we have derivedexplicitly the Hookian relation and the strain energy densities for the stressed overlayers (10-10),(10-11), (11-20) and (11-22),.
Calculations have been performed by means of a computer programme. Our derived energy densities can be directly applied toseveral important planes of the hcp structure, on these, dislocation, kink, and stacking faults have been observed. For instance,they can be useful in the interpretation of the high energy stacking faults for (11-22) and (10-10) hcp planes. Moreover, in thesurface waves phenomenon, the transformed stiffness constants and strain energy density are related to elastic constants fromhigher-order surface waves of thin films. Our results can also be utilised in the calculation on piezoresistance tensors for sensorsbased on some hexagonal whiskers We have analysed some experimental data on Brillouin and enhanced surface Ramanspectroscopy of p-doped GaN, ZnO and 6H-SiC semiconductors it terms of strain energy density approach.
J-III.7 KINETIC ROUGHENING AND MOUND EVOLUTION DURING Ag/Ag(100) HOMOEPITAXY: COMPLEX
DYNAMICS IN A SIMPLE SYSTEM
K.J. Caspersen, A.R. Layson, C.R. Stoldt, V. Fournee, P.A. Thiel and J.W. Evans , Iowa State University, Ames, Iowa 50011, USA
Scanning tunneling microscopy studies combined with atomistic modeling for Ag/Ag(100) homoepitaxy reveal complex growth
behavior at 300K: initial smooth growth up to 25ML where 3D mounds develop from 2D islands; then an extended regime of
mound steepening for 1000ML producing unexpected rough growth; and finally an asymptotic regime with cooperative mound
ordering and coalescence dynamics quite distinct from that in much studied model systems with up-down symmetry. The
steepening regime is compressed upon lowering temperature, so that while initial growth is rougher, asymptotic growth is actually
smoother (contrasting traditional expectations).
C.R. Stoldt et al., Phys. Rev. Lett. 85, 800 (2000).
K.J. Caspersen et al., Phys. Rev. B 63, 085401 (2001); submitted.
Session IV Moderator: H. Pfnür16:30 (INVITED) EPITAXIAL GROWTH ON SURFACES - WHY IS STRUCTURE ANALYSIS IMPORTANT? K. Heinz, Lehrstuhl für Festkörperphysik, Universität Erlangen, 91058 Erlangen, GermanyThe heteroepitaxial growth of a thin film on a crystalline surface heavily depends on the electronic and geometric properties ofthe two materials at their common lateral lattice parameter. In case of a considerable misfit, the film structure will be modifiedalong the surface normal compared to the native structure. Even in the ideal case of zero or almost zero misfit, the stacking offilm layers can become different, at least up to a certain film thickness. In both cases, the film properties will be modified as well,so the knowledge of the film structure is essential for their quantitative understanding. Of particular importance is the interfaceforming in the very early stage of growth, so its structural analysis provides key information. In many cases, additional adatomson the surface and/or surface reconstructions influence the growth, acting in a way as surfactants.The various film andinterfacial structural features appearing within the above scenarios of growth will be illuminated through presentation of anumber of epitaxial systems for which structural information (by means of quantitative low energy electron diffraction(LEED)) isavailable. This covers film/substrate combinations involving metals, semiconductors as well as insulators (oxides) and includesexamples relevant for technological applications as, e.g., the formation of high quality, i.e. almost defect-free contacts orheterojunctions.
J-IV.2 ADDITIVE-ENHANCED COARSENING AND SMOOTHENING OF METAL FILMS: COMPLEX MASS-FLOW
DYNAMICS UNDERLYING NANOSTRUCTURE EVOLUTION
A.R. Layson, J.W. Evans and
P.A. Thiel , Iowa State University, Ames Iowa 50011, USA Exposure of Ag/Ag(100) thin films to molecular oxygen (O2) at 220-250K is shown to activate low-temperature coarsening ofsubmonolayer island distributions, and smoothening of multilayer films with "mounded" morphologies [1]. Dissociation of O2 atkink sites populates step edges with atomic oxygen (O), modifying the step-edge energetics, and facilitating Ostwald ripening offilm nanostructures. We propose that ripening occurs by "easy" detachment and terrace diffusion of an AgnO species. Clusterdiffusion does not play a significant role, contrasting the O-free system [2].
[1] A.R. Layson and P.A. Thiel, Surf. Sci. Lett. 472, L151 (2001).
[2] J.-M. Wen et al., Phys. Rev. Lett. 76, 652 (1996).
J-IV.3 SURFACTANT MEDIATED GROWTH OF Cu ON Co(0001) INVESTIGATED USING MEDIUM ENERGY ION
SCATTERING
T.C.Q. Noakes (a), P. Bailey(a), D. Dekadjevi(b), B.J. Hickey(b) and M.A. Howson(b), (a)CLRC Daresbury Laboratory,
Warrington, WA4 4AD, UK, (b)Department of Physics, University of Leeds, Leeds, L52 9JT, UK
The epitaxial growth of Cu on Co is of interest due to the importance of this material system in GMR films. The magnetic
properties of thin films are inextricable linked to their structure and hence considerable effort has gone into understanding and
improving the growth. Experiments were carried out using MEIS to compare the structural properties of Cu films deposited on
Co(0001) both with and without surfactant and before and after annealing. Films of 20Å thickness were deposited onto the clean
Co surface and onto a surface pre-dosed with 1 ML of Pb surfactant. MEIS data revealed a twinned fcc structure in all the
deposited films irrespective of the presence of surfactant or the use of post deposition annealing. In the case of the Pb deposited
surfaces the majority of surfactant was seen to float on the surface during growth but with a small quantity becoming distributed
throughout the film. Whilst twin formation was not suppressed, the MEIS data did provide limited evidence of improved
crystallinity for surfactant assisted film growth. Annealing the sample to 300°C appeared to have little or no effect on the
deposited films.
J-IV.4 MELTING IN ONE DIMENSION: THEORY OF THE BARIUM/Si(111)-3x2 HONEYCOMB-CHAIN CHANNEL
RECONSTRUCTION
S.C. Erwin
and C.S. Hellberg, Center for Computational Materials Science, Code 6390, Naval Reseach Laboratory, Washington DC 20375, USAAlkali metal adsorbates induce a 3x1 reconstruction of Si(111) widely believed to be a "honeycomb-chain channel" structure. Analkali coverage of 1/3 monolayer leads to a surface with no dangling bonds and thus a fundamental gap free of surface states.
Alkaline-earth adsorbates induce a closely related 3x2 reconstruction, which Lee et al. have shown results from an absorbatecoverage of 1/6 monolayer[1]. Experimentally, Ba/Si(111) indeed appears 3x2 in STM, but only 3x1 in LEED, with extremelyweak or absent half-order spots. We demonstrate that this apparent 3x1 periodicity can arise from melting of the Ba-adsorbateoverlayer. To show this, we carry out density-functional calculations to extract the Ba-Ba and Ba-substrate interactions; we findthat the Ba-adsorbate subsystem can be accurately described as a quasi-one-dimensional Coulomb lattice gas. Based on theseinteractions, we perform classical Monte Carlo simulations to explore the temperature dependence of adsorbate ordering. Despitethe one-dimensional structural character of the Ba adsorbate rows, the long-range character of the Coulomb interaction leads to aphase transition at finite temperature.
[1] G. Lee, S. Hong, H. Kim, D. Shin, J.-Y. Koo, H.-I. Lee, and D. W. Moon, Phys. Rev. Lett. 87, 056104 (2001).
J-IV.5 ULTRATHIN TRANSITION METAL FILMS ON W(211)
Jan Block and
Elsebeth Schroder , Dept. of Applied Physics, Chalmers University of Technology and Göteborg University, 41296 Gothenburg, Sweden SYMPOSIUM J
The transition metals Pd and Pt have, in bulk, approximately the same nearest neighbor distance as tungsten. Pd or Pt films cantherefore be grown on e.g. W(211) creating very little strain. We here study ultrathin Pd and Pt films on the atomically roughW(211) surface by first-principles density-functional theory (DFT) methods. The thicknesses of the films are a few atomiclayers: 0.5 to 3 physical monolayers (PML), corresponding to 1 to 6 geometrical monolayers. Experiments have shown [Pelhos etal., J. Phys. Chem. B, vol. 105 (2001) p. 3708] that in films thicker than 1 PML tungsten atoms of the substrate diffuse into the filmto create an alloy. The structure of the alloy is not known. Both to confirm the alloy forming from theory and to help characterizethe alloy we present studies of Pd and Pt films with included W atoms. The DFT methods allow us to point out the energeticallypreferred structures. We both studied pseudomorphically built alloy-films and alloy-films with the atomic positions locallyoptimized by minimizing the Hellmann-Feynman forces.
18:00-20:30 POSTER SESSION I
J/PI.01
MORPHOLOGY EVOLUTION IN A GROWING FILM Vladimir I. Trofimov , Institute of Radioengineering & Electronics of RAS, 101999 Moscow, Russia A quantitative description of morphology evolution in a thin film growing via 3D island mechanism is presented. The analysis isbased on the geometric-probabilistic growth models approach [1]. It is shown that this method allows to find out in a self-consistent manner the growth law evolution at all successive deposition stages. For the following morphological characteristics -the lateral growth front perimeter (GFP), the interisland boundaries length (IBL), the total boundaries length (TBL), the actualarea of a film surface (AFS) and the rms surface roughness - analytical formulas are derived at different nucleation mode(instant or continuous) and island form (described by a relevant shape coefficient). It is found that temporal evolution of each ofthem may be presented as a universal function of a substrate coverage only. Besides these mean parameters the important forsurface physics height-height autocorrelation function (ACF) and corresponding roughness spectrum are derived. Results ofcalculations with the derived formulas providing rather complete description of a surface morphology evolution in various growthregimes are presented, discussed and compared with available experimental data.
[1] V.I.Trofimov, V.A.Osadschenko. Growth and Morphology of Thin Films (in Russian), Moscow, Energoatomizdat, 1993,272pp.
THEORETICAL CONDITIONS FOR THE FIRST STAGES OF THE EFFECTIVE GROWTH OF HIGH QUALITY METAL FILMSDavid Fuks(a) , Simon Dorfman(b), Eugene A. Kotomin(c), Yury F. Zhukovskii and A.M. Stoneham(d), (a)Ben Gurion Universityof Negev, Beer-Sheva 84305, Israel, (b)Department of Physics, Technion, Israel Institute of Technology, Haifa 32000, Israel,(c)Institute for Solid State Physics, University of Latvia, Riga 1063, Latvia and Max-Planck-Institut für Festkörperforschung,Stuttgart 70569, Germany, (d)Center for Materials Science, Dept of Physics and Astronomy, University College London, LondonWC1E 6BT, UKIt is demonstrated how the growth mode of thin metallic films on insulating substrates could be theoretically predicted combiningthermodynamic approach and ab initio calculations for low-coveraged ordered metal/insulator interfaces. Theory predictsconditions for monolayer growth via spinodal decomposition, or for metal cluster nucleation-and-growth mode. This is illustratedby calculations for a long-debated Ag film deposition on MgO substrate. Our ab initio calculations predict high mobility ofadsorbed Ag atoms on MgO substrate even at low temperatures which stimulates greatly their aggregation.
RATE EQUATIONS MODEL FOR LAYER EPITAXIAL GROWTH KINETICS Vladimir I. Trofimov , Vladimir G. Mokerov, Institute of Radioengineering & Electronics of RAS, 1 101999 Moscow, Russia Recently, we have proposed a simple kinetic model for layer epitaxial growth [1], which combines the rate equations approachand a feeding zone that allows to account for the interlayer adatoms diffusion. With this model it has clearly been demonstratedhow with decreasing adatom diffusivity and/or increasing the repulsive Ehrlich-Schwoebel barrier height growth mode crossesover from atomically smooth layer-by-layer growth to a smooth multilayer growth and finally to a rough 3D growth, andcorresponding "phase diagram" of the growth mode in parametric space has been constructed. The purpose of this paper is toclarify the role of the growing island collisions in the epitaxial growth kinetics. To that end the comparative studies of the growthkinetics at two extreme cases of the island collision process behaviour, impingement and coalescence, are performed. By bothnumerical integration of the rate equations and analytically the coverage and island density kinetics in successive layers, thecritical island size and critical coverage for the next layer nucleation are obtained in these two cases and, based on this, theeffect of the collisions behaviour on the growth mode transitions and morphology evolution is analyzed. It is shown that thecharacter of the collision process affects quantitatively on the growth mode transitions and qualitatively on the island densitykinetics.
[1] V.I.Trofimov, V.G.Mokerov, Comput. Mater. Sci. 17, 510 (2000).
STRAINED EPITAXY: SELF-ORGANIZATION AND GLASSY PHENOMENA Vasyl Tokar (a,b), Hugues Dreysse(a), (a)IPCMS-GEMME, 23 rue du Loess, 67037 Strasbourg, France, (b)Institute ofMagnetism, National Academy of Sciences, 36-b Vernadsky str, 03142 Kiev-142, UkraineSince the number of atoms in nanostructures or glassy media is large, the description in the real space requires the use of manycontinuous atomic coordinates. The later task is known to be computationally very demanding. In order to be able to use theefficiency of the Monte Carlo simulations for strained epitaxial growth, we show that in the case of coherent deposition thesystem can be mapped on a lattice-gas model with a discrete energy spectrum leading to the appearance of cluster interactions. Arecursive approach leads to a very fast determination of the relaxation energy of very large systems. For strong lattice sizemismatch the largest term is a triplet repulsive linear term. An attractive chemical pair interaction induces frustrations and leadsto kinetic glassy phenomena, such as gelation and ageing. The size of the precusor platelets of quantum dots is also obtained anddiscussed.
ELEMENTARY TRANSFORMATION ACT MODEL OF CLUSTER STRUCTURE INTERACTING WITH IR LASER RADIATIONA.V. Stepanov , National Ozone Monitoring Research & Educational Center (NOMREC), Byelorussian State University, 7-906 Kurchatov Street, Minsk 220064, Republic of BelarusThe model of elementary transformation act of a cluster structure is proposed. It is based on two theses: 1. During thetransformation process, the potential energy of a cluster changes discretely or in quanta; the transformation process appears to bea series of quantum subsystems ocurring in sequence (these subsystems may also to be defined as identical quantum oscillators);2. In the intensive field of laser radiation, an energy exchange between IR radiation and atoms of cluster results in discretetranslation of these atoms, which absorb an oscillation energy by identical quanta up to cluster's structure complete transformationassociated with a semiconductor layer recrystallization. The numerical simulation for selfdiffusion processes of Si, Ge and GaAsis carried out. The possibility of prognostication and optimization of technological processes involving solid-phaserecrystallization of ion-implanted semiconductor layers is shown.
[1] A.V.Stepanov and V.L.Tavgin, Int.J.Quantum Chem. 59, 7 (1996).
[2] A.V.Stepanov, J.Mol.Struct.(Theochem). 538, 179 (2001).
CHARACTERIZATION OF THIN METAL FILMS WITH OPTICAL METHODS OF USUAL AND POLARITONIC REFLECTION ELLIPSOMETRY N.L. Dmitruk , O.V. Fursenko, O.S. Kondratenko, V.R. Romanyuk, Institute for Physics of Semiconductors NAS of Ukraine, prospect Nauki 45, 03028 Kyiv-28, UkraineNowadays the interest in thin semitransparent metal films increased strongly due to their applications for polaritonicoptoelectronics and optochemical sensor technique. However, the problem to determine their optical parameters and thethickness by optical methods is very difficult because for ultrathin films the corresponding problem of minimizing the least SYMPOSIUM J
squares functional is not correct due to existence of errors in the initial data. That is why special methods should be applied. Inthis paper the atomic force microscopy (AFM) technique was combined with several optical methods such asreflectance/transmittance spectroscopy, spectral and angular ellipsometry in the usual regime and at the surface plasmonresonance excitation. As surrounding media in ATR were used air, water and aqueous solution of alcohol. Au films wereevaporated in vacuum on glass or quartz substrates heated to different temperatures in the 293-425 K region. Influence of the Crsublayer and some organic overlayer on the gold film parameters have been investigated as well. A difference of opticalparameters (refractive index n and absorption index k) and thickness of Au films obtained by different methods was revealed andthe reasons of this difference is discussed in this paper. Optical parameters of thin gold films are discussed in connection with themicrostructure peculiarities investigated by AFM.
KINETIC MONTE CARLO SIMULATIONS OF HETERO-EPITAXIAL GROWTH M. Biehl , M. Ahr, W. Kinzel and F. Much, Sonderforschungsbereich 410 and Institut für Theoretische Physik und Astrophysik,Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, GermanyWe introduce an algorithm for off-lattice kinetic Monte Carlo simulations of hetero-epitaxial crystal growth. In hetero-epitaxy amismatch of the lattice constants in adsorbate and substrate can lead to a variety of phenomena within the first mono-layers ofgrowth, already. This includes the appearance of misfit dislocations or the formation of self-assembled islands. The onlyparameters of our model are the deposition flux, the temperature and a pairwise interaction potential of the particles. In order togain general insight in the relevant mechanisms we study, as a first example, a Lennard-Jones system in 1+1 dimensions. Thecritical layer thickness for the occurence of dislocations as well as their total number at later stages of the growth are determinedas functions of the misfit. The extension to 2+1 dimensions allows the more realistic investigation of the so-called 2D-3Dtransition in island growth or the effect of step-bunching in vicinal surfaces, for instance.
SINTERING OF 2D NANOCLUSTERS IN METAL(100) HOMOEPITAXIAL SYSTEMS: SUCCESSES AND FAILURES OF MULLINS-TYPE CONTINUUM MODELINGDa-Jiang Liu and J.W. Evans , Iowa State University, Ames Iowa 50011, USA Sintering and general shape relaxation of 2D nanoclusters in metal(100) homoepitaxial systems is driven by periphery diffusion(PD). A Mullins-type continuum theory for shape evolution [1] predicts that the relaxation time, T, for clusters with linear size oforder L scales like T L 4. For small L, deviations are expected: we find that behavior depends strongly on shape, and on thepresence of an extra barrier, d , for kink rounding in PD [2]. When d =0, we show that T L4 always applies for the relaxation of dumbbell-shaped clusters, but T L2 (cf. Ref.[3]) for the relaxation of rectangular clusters. However, distinct scaling is observedfor large dE. Experimental data for Ag/Ag(100) is consistent with the latter [4].
[1] Pai, Wendelken, Stoldt, Thiel, Evans, Liu, PRL 86 (2001) 3088. [2] Liu and Evans, PRB, submitted (2002).
[3] Combe and Larralde, PRB 62 (2000) 16074. [4] Stoldt, Cadilhe, Jenks, Wen, Evans, Thiel, PRL 81 (1998) 2950.
VALENCE BAND ELECTRONIC PROPERTIES OF CHLORINE COVERED SILVER SURFACES BY METASTABLE ATOM DEEXCITATION AND PHOTOEMISSIONL. Pasquali (a), S. Nannarone(a), M. Canepa(b), E.M. Staicu-Casagrande(c), V.A. Esaulov(c), (a)INFM, Unita di Modena andDipartimento di Ingegneria dei Materiali e dell'Ambiente, Via Vignolese 905, University of Modena, 41100 Modena, Italy,(b)INFM, Unita di Genova and Physics Dept., University of Genova, Via Dodecaneso 33, 16146 Genova, Italy, (c)Laboratoiredes Collisions Atomiques et Moléculaires, bât.351, Université de Paris-Sud, Orsay 91405, FranceAdsorption of chlorine on silver surfaces has been the subject of an intense study in the last decade. The interest in this systemwas stimulated by the important role played by Cl on Ag in the selective oxidation of ethylene. The selectivity increases with Clcoverage in the submonolayer range. A key role is certainly played by the evolution of the surface electronic states at differentadsorption stages.
We will present an experimental study of the evolution of the surface valence band of the three different high-symmetry low-Miller-index surfaces of silver during chlorine chemisorption. The electronic properties (both surface density of states andsurface charge density) were followed by He metastable atom deexcitation spectroscopy (MDS) and angular resolvedphotoemission. The behaviour of the three surfaces during chlorine exposure was seen to be similar, indicating a similar bondingcharacter for chlorine at the different (100), (110) and (111) Ag surfaces. Due to the high sensitivity to sp-type states, MDSallowed to follow the progressive depletion of the Ag(5s) surface charge due to bonding with Cl since the earliest exposurestages. The developing of Cl(3p) associated bands (both bonding and anti-bonding) was visible both in photoemission and MDS.
The increased lateral interactions between the adsorbed atoms at saturation coverages resulted in an increased separationbetween bonding and anti-bonding Cl-induced features.
INFLUENCE OF ANNEALING ON Co/Au MULTILAYERS: A STRUCTURAL AND MAGNETIC STUDY G. Gubbiotti(a), G. Carlotti(a), F. Albertini(b), F. Casoli(c) , E. Bontempi(c), L.E. Depero(c), H. Koo(d) and R.D. Gomez(d), (a)INFM, Unita di Perugia & Dipartimento di Fisica, Universita degli Studi di Perugia, Via A. Pascoli, 06123 Perugia, Italy,(b)Istituto Maspec del CNR, Parco Area delle Scienze 37/A, 43010 Fontanini, Parma, Italy, (c)INSTM and Laboratorio diStrutturistica Chimica, Dipartimento di Ingegneria Meccanica, Universita di Brescia, 25123 Brescia, Italy, (d)Department ofElectrical and Computer Engineering, University of Maryland, College Park Maryland 20742, USAThe effect of annealing treatments (at T=250 °C) on both the magnetic and structural properties of [Co(d )/Au(22 Å)]30 multilayers grown by RF sputtering was studied. The thicknesses of the Co layers (d ) were 6, 9 and 12 Å. The characterization carried out by XRR reflectivity allowed to model the bilayer thickness and the interface quality of the multilayers which stronglyinfluence the magnetic characteristics. All the as-prepared samples display perpendicular anisotropy. Upon annealing theperpendicular anisotropy for d =9 and 12 Å multilayers is enhanced, while a change of the easy-magnetization direction (from perpendicular to in-plane) is observed for d =6 Å. The results deduced from alternating gradient field magnetometry are in good agreement with the information obtained by a Brillouin-light-scattering (BLS) investigation of spin waves. After annealing wealways measured one peak in the BLS spectra which was identified as a surface mode. From the frequency dependence of thismode on the intensity of the applied magnetic field, the values of the saturation magnetization 4pMs, the g-factor and the uniaxialanisotropy were determined. A quantitative comparison of the values of these parameters in the as-prepared samples and afterannealing is presented. Finally, a direct visualization of the magnetic domains configurations and of their dependence on theapplication of parallel and perpendicular magnetic fields was obtained by magnetic force microscopy.
A FILM HETEROSTRUCTURE WITH SOFT FERROMAGNETICS TO IMPROVE LOW FIELD MAGNETORESISTANCEG.L. Yuan, J.-M. Liu, J.P. Li, X.Y. Yuan, X.B. Lu and , National Laboratory of Solid State Microstructures, Nanjing University, P.R. ChinaAt room temperature, a 5900-fold enhancement of the MR response at 10 Oe in La Ca MnO has been achieved by the use of heterostructures with soft ferromagnets (Mn,Zn)Fe O (H. Y. Hwang et all, Appl. Phys. Lett. 68, (1996) 3494). But its size is too large to use it in magnetic reading heads as it is made up of a single crystal of La Ca MnO (0.1 mm thick) and two long pieces of polycrystalline (Mn,Zn) Fe O (each piece 1.473 x 1.473 x 24.2mm3). We design similar heterostructures, which are composed of two pieces of NiFe alloy films and another piece of peroskite manganite film. This heterostructures is predicted toown the same enhancement on CMR as conventional heterostructures, and it can be dwindle to several micron. The same CMR isachieved in our rough heterostructures under half as much as applied magnetic field without heterostructures.
SHAPE OF SELF-ASSEMBLED Fe DOTS GROWN ON BCC METALS (110) SURFACES M. Eleoui , P.-O. Jubert, O. Fruchart, C. Meyer, Laboratoire Louis Néel (CNRS), BP166, 38042 Grenoble Cedex 9, FranceWe have grown Fe(110) self-assembled dots in the Stranski-Krastanov growth mode at elevated temperature (500-800K) onseveral bcc metal (110) surfaces. The dots fabricated at the highest temperatures are nearly fully relaxed elastically, and theirshape can be interpreted as a thermodynamical equilibrium shape, that minimizes the sum of surface plus interface energies of SYMPOSIUM J
the dots[1]. Using known surface energies values, the interface energy can be deduced from the dots' shape. For Fe/Mo andFe/W we find E 0.6 J/m2 and E 0.1 J/m2, respectively. In both cases E comprises both the purely interfacial energy (i.e. of chemical origin) and elastic energy (coming from interface dislocations). Experiments are under way in order to separate thesetwo contributions. These are based on the growth of an ultrathin pseudomorphic layer onto a buffer from another element, so asto control independently the in-plane lattice parameter and the chemical nature of the interface.
[1] P.-O. Jubert et al., Phys. Rev. B 64, 115419/1-10 (2001).
MICROSTRUCTURE AND TRANSPORT PROPERTIES OF Ni/NiO-MULTILAYERS S. Dreyer (a), Carsten Herweg(a), S. Sievers(a), J. Hoffmann(b), C. Borchers(a), H.C. Freyhardt(a,b), (a)Institut fuerMaterialphysik, Universitaet Goettingen, Windausweg 2, 37073 Goettingen, Germany, (b)Zentrum fuer FunktionswerkstoffeGmbH (ZFW), Windausweg 2, 37073 Goettingen, GermanyReactive ion-beam sputtering with a varying partial pressure of the ambient gas allows the deposition of a great variety ofpossible multilayer structures with different physical properties. Ni/NiO multilayers, a combination of a conducting ferromagneticand an insulating antiferromagnetic material, were prepared as a model system for an immiscible magnetic layered system. Themultilayers exhibit a characteristic microstructure determined by a pronounced columnar growth. The transport properties of thislayer combination are rather complex: Temperature dependent resistance measurements of the as-sputtered multilayers show alogarithmic increase of the resistance at low temperatures, indicating a Kondo-like behaviour in the Ni/NiO-system. Furthermoremagnetoresistive measurements were performed at T = 4.2 K which reveal that the exchange anisotropy due to the coupling ofinterface spins leads to an extremly anisotropic behaviour of the magnetoresistance. Upon the thermally induced disintegration ofthe multilayer, the increase of the resistance at low temperatures and the anisotropy in the behaviour of the magnetoresistancedisappear. Furthermore, the microstructure and magnetic properties of the metal/metal oxide multilayers are compared tomagnetron sputtered magnetic metal/metal multilayers.
STIFFNESS CONSTANTS FOR ANISOTROPIC HEXAGONAL CLOSE- PACKED ULTRATHIN OVERLAYERS H.W. Kunert and J.H.C. Schönfeldt, Department of Physics, University of Pretoria, 0002 Pretoria, South AfricaThe elastic properties of overlayers are of interest in epitaxy, interplay of surface misfit and steps on crystal surfaces. Theoverlayer is represented by thin slice regarded as an elastic continuum. In anisotropic crystals the stiffness constants C are usually specified in principle axes (x ,y ,z ) and must be transformed to the local (x,y,z) axes. We have computed a set of useful stiffness constant transformation relations for hexagonal close packed ultrathin overlayers. The transformed stiffness constantsare expressed in terms of bulk ones and matrix elements that transform the principle axes to the local overlayer system. Theresults can directly applied to several phenomena involving surface stresses i.e, i) surface misfit, ii) dislocations, iii) monatomicsteps, etc. For overlayers related to surface acoustic waves in GaN, molecular adsorption of CO on the Ru, structures of ZnOsurfaces, etc., the transformed stiffness constants are derived explicitly. The hexagonal GaN substrate has a small enough butfinite residual strain arising from the pileup of the lateral growth front on silicon oxide masks, and that the elastic stiffnessconstants C and C are more sensitive to the residual strain than the optical phonon frequencies. These stiffness constants can be modified using our approach. Other applications such as: sensors based on hexagonal whiskers, enhanced surface Ramanspectroscopy etc., are also discussed.
STRUCTURE AND ELECTRONIC PROPERTIES OF ULTRATHIN GOLDFILMS ON VICINAL Si(111) (a), C. Sürge(a), H. v. Löhneys(a,b), (a)Physikalisches Institut and Center for Functional Nanostructures, Universität Karlsruhe, 76128 Karlsruhe, Germany, (b)Forschungszentrum Karlsruhe, Institut für Festkörperphysik, 76021 Karlsruhe,GermanyVicinal surfaces can be used as templates for the synthesis of metallic nanostructures on the atomic level. For instance, atomicchains are obtained in ultra-high vacuum by evaporation of gold onto Si(111) substrates with an intentional miscut in the <-1-12>direction. We report on the preparation of such nanowires and their characterization by means of scanning tunneling microscopyand spectroscopy and in-situ electrical transport measurements. Substrates were cleaned by flash heating prior to depositionwhich gives rise to the Si(111) 7x7 reconstruction. After evaporation of gold corresponding to a coverage of 0.2 monolayers andsubsequent annealing, the surface is decorated by large domains of equidistant atomic chains parallel to the monatomic step edgesalong the <-110> direction promoted by the initial miscut.
Tunneling spectra taken on and off the chains show distinct differences ascribed to a lateral variation of the local electronicstructure at the surface. Preliminary resistance measurements between micrometer-spaced contacts evaporated onto the surfacewere performed in-situ for temperatures T = 1.8 - 300 K. In most cases, the resistance R, measured with the current parallel tothe wire axis, increases with decreasing temperature, i.e. a nonmetallic behavior is observed. This is discussed as being due todifferent reasons such as activated hopping between separated wire fragments or due to the intrinsic electronic structure of thereconstructed surface.
NEW MECHANISM FOR THE SURFACE DIFFUSION OF SMALL ADATOM CLUSTERS , Institute for Low Temperature Physics and Engineering, 47 Lenin Ave., 61103 Kharkov, Ukraine, A.I. Landau, Mivza Berosh, 11/24, 84799 Beer Sheva, IsraelAdatoms adsorbed on the furrowed surface of metals tend to form the linear complexes from 2-6 atoms which have a 1Ddynamics. At high temperatures the diffusion of these clusters is governed by an interaction with bulk phonons. At lowtemperatures the radically new mechanism for the diffusion is possible associated with a dynamic chaos in the system. Wepropose a simple model for the description of two-adatoms cluster dynamics in periodic field of substrate. In contrast to usualmodels in proposed one the infinite motion of the cluster is possible and we can compare the chaotic properties of the system withits diffusion at zero temperature. We study numerically different regimes of cluster dynamics in wide intervals of its initial energyE and calculate time dependences for coordinate of cluster center, its dispersion D(t) (averaged over the realizations and initialconditions) and diffusion index p=lnD(t)/ln(t). We found that there exist the windows in energy spectrum with the chaoticbehaviour which resulted in diffusion motion of cluster with p close to 1/2. Another interesting regime (similar to quantumAnderson localization) was observed when the long-time localization of adatom cluster takes place in the wide spatial regions.
For the long adatom clusters we predict another new mechanism of dynamics with shuttle-like motion of topological solitons alongthe cluster.
SURFACE STRUCTURE OF EPITAXIAL Fe O (001) FILMS - IN SITU STM AND CEMS STUDIES N. Spiridis(a), B. Handke(a,b), J. Barba(a), J. Hab(a), J. Korecki (a,b), (a)Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Krakow, Poland, (b)Faculty of Physics and Nuclear Techniques, University ofMining and Metallurgy, Al. Mickiewicza 30, 30-059 Krakow, PolandWe studied Fe O (001) epitaxial films in situ at ultra high vacuum conditions by combination of STM and the monolayer 57Fe probe Conversion Electron Mössbauer Spectroscopy (CEMS). Annealing of the epitaxial Fe O (001) films grown on MgO(001) considerably improves the surface quality (the lateral size of terraces) but it inevitably leads to the diffusion of magnesium intothe magnetite film. This phenomenon sets a temperature limit for the growth of Fe O (001) on MgO(001) by the reactive deposition of Fe. Such limitation could be avoided for ultrathin epitaxial magnetite films obtained for the first time by the post-preparation oxidation of epitaxial Fe(001) films on MgO(001). CEMS measurements at room temperature showed a deviation ofthe hyperfine pattern of the surface layer from bulk magnetite, suggesting a strong modification of the electronic properties forthe octahedral Fe ions. However, the surface Verwey transition is unambiguously evidenced. In situ STM results contributed tothe solution of the magnetite surface reconstruction, indicating that different atomic planes are imaged by the positive andnegative sample bias. It is concluded that the empty states STM images are dominated by the tunneling to the octahedral Fe ions,whereas the tunneling to the tetrahedral Fe ions prevails by imaging of the occupied states, in contradiction to the itinerant pictureof the octahedral irons.
SYMPOSIUM J
A QUASI-ELASTIC He-ATOM SCATTERING STUDY OF SELF-DIFFUSION ON Ag(110) , R. Tatarek and G. Bracco, Istituto Nazionale di Fisica della Materia and C.F.S.B.T.-CNR, Dipartimento di Fisica dell'Universita di Genova, Via Dodecaneso 33, 16146 Genova, ItalyThe mobility of silver atoms adsorbed on Ag(110) is investigated along the <1-10> and <001> azimuthal directions by means ofenergy resolved He atom scattering. Information on the diffusion process along both azimuths is gained investigating the evolutionof the quasi-elastic peak broadening with parallel momentum transfer and with temperature in the range between 340 and 800K.
The obtained results exhibit strong anisotropy. In fact, atoms are observed to diffuse along the <1-10> surface channels above600K while along the cross channel <001> direction diffusion is detected first above 750K. In particular, the main diffusionmechanism is pointed out, the diffusion coefficient and the effective diffusion barrier are measured. The results are comparedwith the corresponding estimations from molecular-dynamics calculations.
COMPARATIVE STUDY OF THE GROWTH INITIAL STAGES OF THE (Bi, Bi-Sb) FILMS PRODUCED BY THERMAL EVAPORATION AND SELF-ION ASSISTED DEPOSITIONA.V. Andreeva , A.I. Il'in, O.V. Kononenko, Institute of Microelectronics Technology RAS, 142432 Chernogolovka, Moscow Region, RussiaIn paper [1] submicron Bi films 50-60nm thick with conduction electron mean free path 1.5 microns were obtained by thermalevaporation (TE) in vacuum at 10-6 Pa. These properties were explained by strong influence of the structure of grain boundariesarising during evaporation and/or recrystallization on the electrical resistivity of Bi films. The present work deals with theinvestigation of initial stages of growth of (Bi, Bi-Sb) films produced by methods of different degree of nonequilibrium, such asTE and self-ion assisted deposition (SIAD) [2]. The comparative analysis of growth mechanisms and film island evolution(facetting and coalescence) was carried out depending on the type and temperature of substrate, evaporation rate, degree ofionization of particle current and film composition. The SIAD leads to films of higher density and reduced porosity, greaterstability than those deposited without ions. At all substrate temperatures the film grain orientation (111) R is more pronounced forSIAD than that for TE. As compared with the pure Bi films facetted island morphology of SIAD the Bi–Sb (20%) films is not soclear. This is explained by partial realization of the coalescence process by liquid fluidity mechanism and lower anisotropy of theSb lattice compared with that of Bi.
[1] Il'in A.I., Andreeva A.V., Tolkunov B.N.– Materials Sci. Forum, 1996,v.207-209, 625[2] Kononenko O.V., Matveev V.N., Kasumov A.Yu., Khodos I.I.– Vacuum, 1995, v.46, 685 GROWTH, STRUCTURE AND MORPHOLOGY OF CoFe /CoFe O MULTILAYERS , C. Meny, P. Panissod, N. Viart, G. Pourroy, C. Ulhaq-Bouillet, Institut de Physique et Chimie des Matériaux de Strasbourg (GEMM - GMI), 23 rue du Loess, B.P. 20 CR, 67037 Strasbourg Cedex, FranceThe metal/oxide CoFe /CoFe O system is interesting both from the fundamental and applied points of view. In the magneto- resistive devices, the possibility to pin the magnetic moment of one of the layers by exchange coupling with a ferrimagnet likeCoFe O has not been studied yet. From the fundamental point of view, the magnetic coupling processes between a magnetic metal and an oxide remain unclear. We prepared CoFe /CoFe O multilayers by rf and dc sputtering. A CoFe alloy layer was first deposited on a glass substrate and then partly oxidised by a rf plasma in an Ar-O atmosphere. This procedure was repeated to obtain the multilayer stack. Our films are polycristalline. The only phases detected on X-ray diffraction patterns are a bccCoFe alloy and a spinel CoFe O phase. HRTEM investigations showed that the ferrite and the iron-cobalt alloy layers are flat and have a constant thickness over several hundreds of nm. The CoFe alloy and ferrite grain in the multilayer have an average size of ca 100 nm. There is no increase in the ferrite thickness for oxidation times longer than 5 min. Lattice fringes are observedthrough several layers, indicating epitaxial growth at the metal/oxide interfaces, but also that the structural coherence ispreserved at the oxide/metal interfaces. This work shows that the elaboration technique we have used allows to prepare very thinand flat CoFe O layers.
SHORT AND LONG RANGE ORDERED SURFACE STRUCTURES OF SEGREGATED SPIECES ON Fe(100 J. Fujii(a) , G. Panaccione(b), and G. Rossi(b,c), (a)ELETTRA, 34012 Basovizza, TS, Italy, (b)TASC Laboratory, 34012Basovizza, TS, Italy, (c)INFM-Università di Modena e Reggio Emilia, Dipartimento di Fisica, Via Campi 213/A, 41100 Modena,ItalyFe(3%Si) single crystals exposing the (100) surface do segregate various impurities when annealed in the temperature range 600-950K. At every temperature a metastable surface is established where the segregated species form an adlayer structure withlocal coordination and with, or without, long range symmetry. The thermal promotion of a new segregating specie modifies thesurface by further superposing a new structure, or by atomic substitution. We have studied by STM the atomic structure ofsegregated C, P, S, and N from two different samples, with different impurity contents. The C segregated surface shows ac(3√.2x√.2) reconstruction above about 700K and a local zig-zag chain structure aligned with the <11> direction. The element ofthe chain is a dimer oriented along the <10> direction. The dimers are arrayed to align along the <11> direction. Arrays of morethan 10 „zig-zag" organized and <11> aligned dimers either break or change growth direction by 90 degrees. At highertemperatures about 850K, P segregation occurs. The P atoms bind first to the corners of the C zig-zag chains and form a twodimensional c(2x2) structure. At higher density of segregated P winding chains are formed.
FIRST STAGES OF Ni ADSORPTION ON Pt(111) M. Habar(a), L. Stauffer(a), P. Sonnet (a), L.T. Wille(b) and H. Dreyssé(c), (a)LPSE, FST, 4 rue des Frères Lumière, 68093 Mulhouse Cedex, France, (b)Department of physics, Florida Atlantic University, Boca Raton FL33431, USA, (c)IPCMS, 23 ruedu Loess, 67037 Strasbourg, FranceWe present a theoretical study of the initial stages of Ni adsorption (c<1) on Pt(111). The effective cluster interaction (ECI)which govern the ordering of Ni adatoms on the surface are computed in the framework of an extension of the DirectConfigurational Averaging method. We work in a tight binding model using the recursion method and the 'orbital peeling' schemeto compute directly the energies from the electronic structure. A variety of Ni cluster configurations formed on the surface(isolated impurities, pairs, trimers) are investigated, and a systematic study of the initial stages of Ni adsorption with respect to thenature of the occupied sites (fcc or hcp), and coverage (c=0.05,0.25,0.5,0.75) is performed. Monte Carlo simulations allow us toconsider the phases diagrams, and a competition between several overlayer structures is observed. Whatever the nature of theadsorption sites and temperature (T=11,6°K,116°K,232°K), the same superstructures, mostly c(2x6) and c(2x4) are observed invariable proportions. The c(2x6) superstructure appears since low temperatures and low concentrations. At increasingconcentration, the c(2x4) superstructure is also present. The two phases coexist at c=0.5, c(2x4) beginning to dominate. Atc=0.75, c(2x4) largely dominates, c(2x6) has desappeared. The c(2x2) superstructure, mainly observed in the fcc cases, appearsas a step of the c(4x4) phases formation.
MORPHOLOGY AND ELECTRONIC STRUCTURE OF Ag/Cu NANOSTRIPE ARRAYS GROWN ON VICINAL Cu(111) (a), F. Ostendorf(a), J. Lobo(b), E.G. Michel(b), J. Kuntze(c), A. Mugarza(c), J.E. Ortega(c), S. Speller(d), (a)Universität Osnabrück, Fachbereich Physik, 49069 Osnabrück, Germany, (b)Dpto. Fisica de la Materia Condensada, U.
Autonoma Madrid, Cantoblanco, 28049 Madrid, Spain, (c)Donostia International Physics Center and Dpto. Fisica Aplicada I, U.
Pais Vasco, Plaza de Onati 2, 20018 San Sebastian, Spain, (d)Research Institute for Materials, Katholieke Universiteit Nijmegen,Toernooiveld 1, 6525 ed Nijmegen, NetherlandsAdsorption of submonolayer amounts of Ag on vicinal Cu(111) induces periodic faceting in the sub-100nm regime . Below 1monolayer, the equilibrium structure is characterized by Ag-covered facets that alternate with clean Cu stripes. In the atomicscale, the driving force is the matching of Ag(111)-like packed rows with Cu(111) terraces underneath. This determines thepreference for the facet orientation and the evolution of different phases as a function of coverage. In the mesoscopic scale, theperiodicity of the striped nanostructure follows the predictions of continuum models with long-range elastic interactions betweenphases. ARPES measurements show that surface states at these Ag/Cu one-dimensional striped nanostructures are decoupled into SYMPOSIUM J
Ag- and Cu-like stripe states. A model potential scaled from the infinite system explains the results. In Cu-like stripe states weobserve a transition from dispersing bands for wide stripes to lateral quantum wells in narrow stripes. The local stripe scale isextended to the high energy photoemission final state as proved by the photon-energy dependent spectral function.
ON THE STRUCTURAL EVOLUTION OF ULTRATHIN Fe FILMS ON Ag(001) AT HIGH TEMPERATURE: A COMBINED PHOTOELECTRON DIFFRACTION AND X-RAY DIFFRACTION STUDYS. Terreni(a), G. Gonella(a), M. Canepa(a) , L. Mattera(a), L. Duo, F. Ciccacci(b), F. Bruno(c), D. Cvetko(c), L. Floreano(c), A.
Morgante(c) and A. Verdini(c), (a)INFM and Dipartimento di Fisica, Universita di Genova, Italy, (b)INFM and Politecnico diMilano, Italy, (c)INFM Laboratorio Tasc, Trieste, ItalyWe present a synchrotron-based experiment aimed to investigate the structural changes of an ultrathin (typically 1 nm) film of Fedeposited on a Ag(100) substrate at low temperature and annealed at relatively high temperature (700 K). Recent attention on theFe/Ag(001) growth system has been focused to annealing induced morphology changes [1-2] and the correlation with filmmagnetic properties [3]. In previous experiments we found Ag segregation ontop a 5 ML Fe film deposited at 140 K afterannealing at 500 K [1]. Results obtained for higher annealing temperatures (700 K) suggested film dissolution into the substrate[1]. In this communication we present combined photoelectron diffraction (PED and X-ray diffraction (GIXRD) experimentsperformed on this system at the beam-line ALOISA (ELETTRA). The PED measurements, performed in the forward scatteringregime, nicely confirmed the Ag layer segregation stage. They give, through comparison with multiple scattering calculations andGIXRD data, quantitative information on the structural evolution of the film as a function of temperature. Further, themeasurements definitely supports the hypothesis of film dissolution at 700 K. Work is in progress to clarify the modalities of filmdissolution.
[1] M. Canepa, P. Cantini, O. Ricciardi, S. Terreni, L. Mattera, Surf. Sci. 429 (1999) 34[2] D.E. Buergler et al, Phys. Rev. B 56 (1997) 4149 [3] D.M. Schaller et al, Phys. Rev. 59 (1999) 14516 CLUSTER CRITICAL SIZE EFFECT DURING GROWTH OF Co ON THE (111) HERRINGBONE RECONSTRUCTED SURFACE OF AuI. Chado , H. Bulou, C. Goyhenex and J.-P. Bucher, IPCMS-CNRS UMR7504, 23, rue du Loess, 67037 Strasbourg, France Due to its herringbone reconstruction [1], the Au(111) surface can be considered as an ideal substrate to grow nanostructuredarrays of metallic clusters. Evaporated Co starts to nucleate at the kinks of the reconstructed Au(111) surface, formingmonoatomic height islands[2]. These islands cease to expand laterally at an average cluster size of about 15-20 atoms. After thesecond layer is formed, they grow laterally with double Co(0001) step height[3]. In the present work, a detailed STM analysis ofCo deposit in the submonolayer range and for different temperatures is performed.It is shown that, at low temperature, bothmonolayer and bilayer clusters can be found on the same sample depending on the nature of the particular nucleation site while,at room temperature, only bilayer clusters are observed. This behavior is explained in the framework of a tight-bindingquenched-molecular dynamics study by calculating the relative stability of mono- and bi-layer clusters as a function of theunderlying Au(111) lattice, which, due to the reconstruction, shows a strong dispersion of the in-plane surface nearest-neighbordistances[4].
[1] J. V. Barth et al, Phys. Rev. B 42, 9307 (1990) [2] B. Voiglander, G. Meyer, and N. M. Amer, Phys. Rev. B 44, 10354 (1991).
[3] C. Tolkes, P. Zeppenfeld, M. A. Krzyzowski, R. David, and G. Comsa, Phys. Rev. B 55, 13932 (1997).
[4] H. Bulou and C. Goyhenex, Phys. Rev. B 65, 045407 (2002).
ANOMALOUS INTERACTION AND DRIVEN MOTION OF NEUTRAL ATOMS, MOLECULES OR CLUSTERS ABOVE (NEAR) MODULATED ELECTRIC OR MAGNETIC SURFACE
Vladimir I. Vysotskii , Nikolae V. Maksyuta, Kiev Shevchenko University, Radiophysical Faculty, 01033 Kiev, Ukraine
The paper discusses the new phenomenon of the physics of neutral atom collisions - non-diffraction velocity and frequency-
dependent collision and interaction between the beam of neutral atoms (molecules, clusters) - and the target surface with
periodical structure (e.g., a crystal lattice, sub-lattice or magnetic domain surface structure). The same phenomenon is observed
in the modulated surface moving in motionless cold gas of neutral atoms or molecules. The effect occurs at certain combinations
of relative (particle to surface) velocity, longitudinal period of the near-surface field (outside the crystal in the nearest vicinity of
its surface) and the frequency of one of the strong internal electron resonances in a moving neutral particle [1]. The problem was
solved precisely using the density matrix method.
At longitudinal relative motion of atoms in the near-surface periodic electric or magnetic field of a crystal lattice they are
influenced by alternate periodical excitation. In atoms the field induces electric moments (or magnetic moments in the case of
magnetic substructure) which oscillate with the same frequency and phase dependent upon the relation between the frequency of
excitation and resonant frequency of atom. Each of these moments in turn interacts with the field it's caused by. The sign of the
interaction energy depends on the dispersion of atom polarizability and is determined by the relation between the frequency of
excitation and resonant frequency of atom. If these frequencies equals, the energy of interaction equals zero. If the frequency of
excitation is smaller, the energy of atom-surface interaction is negative, which means the atoms are attracted to the surface. Such
case causes the existence of a potential pit near the surface and the increase of near-surface concentration of longitudinally
moving atoms. This leads to a sharp increase of friction between the neutral atoms and the surface. In opposite another case (if
the resonant frequency of atom is smaller) the energy of atom-surface interaction is positive and the atoms are repelled from the
surface. Here the near-surface layer will be depleted of atoms, which greatly reduces friction. For the case of multi-component
moving flow (with presence of different types of atoms with different resonant frequencies) it is possible to achieve strong
interaction (friction) for certain type of atoms (at large resonant frequency) and attractive regime of motion and reduce the
interaction for other types of atoms (at small resonant frequencies). This mechanism of controlled interaction between the neutral
atom and the crystal surface was investigated for two cases:
1. At atom motion along the usual (natural) crystal plane for realization of this phenomena the particles of a flow with kinetic
energy about 1 KeV for the H atom and about 30 KeV for the Na atom are required;
2. At motion of atoms of flow along the periodical 2-dimensional magnetic domain surface structure the optimal energy for B
atom equal 70 KeV.
[1]. V.I. Vysotskii, R.N. Kuzmin. Channeling of neutral particles in crystals // Soviet Physics - Uspekhi, v.35 (1992) 725-746
MICROSTRUCTURE AND MORPHOLOGICAL STUDY OF NANOMETER W AND W O THIN FILMS , C. Sant, C. Le Paven-Thivet, C. Buscema-Legrand, P. Garnier, Laboratoire d'Etude des Milieux Nanométriques (LMN), Université d'Evry Val d'Essonne, Rue du Père Jarlan, 91023 Evry Cédex, FranceMicrostructure properties of tungsten W thin films have been studied in order to improve mechanical properties. These films havebeen deposited by RF-sputtering on Si substrates. Samples have been grown using different growth parameters : total pressureranging from 0.2 to 1 Pa, target power ranging from 0.49 to 1.97 W/cm2, and negative bias applied to the substrate from 0 to -40V. A thickness of 50nm has been chosen because it is thick enough for X-ray diffraction (XRD) analysis and thin enough forX-ray reflectivity (XRR) study. High resolution scanning electronic microscope (SEM) and atomic force microscope (AFM)were used to analyse thin films microstructures surfaces. We note the presence of two phases with different morphology: (W O) with a nanograins structure presents in first step of the tungsten growth ; and, when the thickness increases, a pure tungsten phase(W Wolfram) with a columnar structure. We demonstrate that W O is due to a pollution target. We succeed to suppress this phase and to obtain a pure tungsten Wolfram layer.
INTERPLAY BETWEEN SURFACE STRUCTURE AND ELECTRONIC PROPERTIES IN EPITAXIAL Ag ULTRATHIN FILMS ON Cu(111).
A. Bendounan(a), H. Cercellier(a), B. Kierren(a), Y. Fagot-Revurat(a) , F. Bertran(a), D. Malterre(a) and V. Yurov(b), (a)Laboratoire de Physique des Matériaux, (UMR CNRS 7556), Université Henri Poincaré Nancy 1, B.P. 239, 54506Vandœuvre-Lès-Nancy, France, (b)Natural Sciences Centre of General Physics Institute, RAS, Moscow 117942, L2, Russia.
Shockley states which develop on (111) surfaces of noble metals have been studied by surface spectroscopies (ARPES, STS) to SYMPOSIUM J
investigate the relation between the surface atomic structure at nanometric scale and the electronic properties. Epitaxial growthof Ag on Cu(111) leads to a peculiar surface superstructure (9×9) which strongly affects the surface states. For submonolayerfilms grown at 300 K, STS reveals two surface states associated with Cu terraces and Ag islands, respectively. This behavior isconfirmed by ARPES measurements which show two parabolic dispersive bands separated by ∆E ≈ 210 meV. Deposition at 200 K leads to a reduced value of ∆E (120 meV). After annealing at 400 K, the former value is recovered. This result is explained by the metastability of the LT structure (Moiré) and illustrates the strong interplay between the overlayer structure and the energyof the surface state. The evolution with coverage of surface state parameters (energy position, linewidth and effective masses)will be presented and discussed.
ACTIVATION ENERGY DETERMINATION FROM RHEED LINESHAPE ANALYSIS IN METAL EPITAXY.
, F. Dulot and D. Malterre, Laboratoire de Physique des Matériaux, (UMR CNRS 7556), Université Henri Poincaré - Nancy 1, B.P. 239, 54506 Vandœuvre-lès-Nancy Cedex, FranceReflection High Energy Electron Diffraction (RHEED) is a suitable approach to monitor the growth processes. We combinedRHEED and STM measurements to follow the epitaxy of Fe on Fe(100). With increasing growth temperature, a continuousevolution from well resolved satellites to a broad diffraction line is observed. The satellites are interpreted in terms of coherentscattering associated with the island distribution on the surface (diffuse contribution). By determining, for each temperature, themomentum transfer (satellite separation), we obtained the surface diffusion barrier and critical nuclei energy. We also show thata coverage dependent intensity transfer between Bragg and diffuse contribution can explain, in a simple kinematic framework,the linewidth oscillations usually observed in layer by layer growth.
ELECTRON MICROSCOPY OF TRANSROTATIONAL THIN SELF-ORGANISED MICROSTRUCTURES FORMED IN AMORPHOUS FILMSV. Yu. Kolosov , Eng. Dept., Ural State Economic University, 8 March St. 62, Ekaterinburg, Russia As we reported earlier [1] thin (10-100 nm) amorphous films often undergo phase transition (sometimes spontaneously)associated with an unusual phenomenon: strong (up to 180 degrees per micrometer) regular dislocation independent rotation ofthe lattice round an axis (or axes) lying in the film plane of a growing crystal. Such transrotational [2] microstrucrures has beenrevealed and studied (in situ as well) using transmission electron microscopy (TEM) bend-contour technique [1]) for the films ofdifferent chemical bonding, prepared and crystallized by various methods: Se, Te, Sb Se , Sb S , Cu-Te, Cu-Se, Tl-Se, In-Se, Cd- Te, Ge-Te, Fe O , CoPd and some others.
In the present paper we describe geometry, morphology, texture, gradients of lattice orientations, dynamics of crystal growth forthe different kinds of self-organized microstructures that have different complex variations in the crystal lattice orientation andimperfection but form regular patterns. Video on in situ TEM is to be demonstrated. The dynamic changes of the diffractioncontrast for the large growth rates (revealed by the analysis of the video) fit the proposed mechanism of the phenomenon basedon the surface nucleation of small clusters which initially obtain strained lattice and later on relax by bending. Hypotheticalatomistic mathematical model corresponding to the conformal transformation of usual crystal lattice is also proposed.
[1] V. Yu. Kolosov, Proc. XIIICEM, 1, 574 (1990).
[2] V. Kolosov and A. Thölen, Acta Mat. 48, 1829 (2000).
MECHANICAL AND STRUCTURAL PROPERTIES OF TITANIUM CARBIDE THIN FILMS A. Mani (a), P. Aubert(a), F. Mercier(b), N. Barré(b), H. Khodja(c), P. Trocellier(c), P. Houdy(a), (a)Laboratoire d'EtudeMilieux Nanométriques (LMN), Université d'Evry, Bd F. Mitterand, 91025 Evry cedex, France, (b) Laboratoire Analyse etEnvironnement (UMR CNRS –CEA) Université d'Evry, Bd F. Mitterand, 91025 Evry cedex, France, (c)Laboratoire Pierre SüeCEA/DSM 91191 Gif sur Yvette, FranceTitanium carbide thin films of present attractive mechanical properties for engineering applications. The aim of this study is tocorrelate the mechanical properties with the structural properties. Thin films of TiC were deposited on <100> silicon substrate byRF sputtering from a TiC target. Various sputtering pressures and sputtering powers were carried out in order to observe theinfluence of these deposition parameters on chemical composition, structural properties and roughness. The sputtering pressureranges from 0.35 Pa to 1 Pa and the sputtering power from 100W to 350W. Rutherford Backscattering Spectroscopy (RBS), X-Ray Diffraction (XRD) and Atomic Force Microscopy (AFM) were used to analyse TiC Thin films. Mechanical properties wasstudied by nanoindentation. XRD experiments show that a higher sputtering pressure equal to 1 Pa must be used to obtain astoichiometric films with <111> texture and low roughness. Lower pressures induce the formation of two phases, titanium andtitanium carbide, while that RBS spectra show that the Ti/C ratio is constant for all these samples. Contrary X ray diffractionshows no effect of the sputtering power. Finally first results of hardness and Young modulus measurements will be presented.
THERMAL STABILITY OF METAL NANOCLUSTERS FORMED BY LOW PRESSURE PLASMA SPUTTERING , P. Brault, GREMI-ESPEO, Université d'Orléans, 14 rue d'Issoudun, BP6744, 45067 Orléans cedex 2, France, J.P.
Salvetat, Y. Breton, CRMD CNRS, 1B rue de la Ferollerie, 45071 Orléans cedex 2, FranceSince several years, we have been studying ultra-thin metal film deposition by a plasma sputtering method allowing low depositionrates. Previous works have shown that the metal growth modes depart from the usually observed ones in more conventionaldeposition techniques. They have been found to depend on the deposition conditions, which are tailored by the plasma conditions.
Indeed, the substrate surface is submitted to the plasma during deposition and especially to the flux of ions. For example, Pd hasbeen found to form either 3D nanometer clusters, or very thin continuous layers. These results are interesting because theyevidence that the thin film morphology can be controlled. However, for many applications such as heterogeneous catalysis, thesesupported aggregates have to be stable at high temperature. In the present work, we study the thermal stability of Pd nano-aggregates formed under various Ar-plasma sputtering conditions. The flux of Ar ion impinging onto the Si surface was varied aswell as their kinetic energy (negatively biased substrate). Since these objects were dedicated to the catalysis of carbon nanotubegrowth by acetylene decomposition at 600 °C, they were submitted to annealing at this temperature under N2 flux during (5 to 20)min. The morphology of the films was observed by non-contact AFM or TEM. We always observed a coalescence of Pd clustersto form nanoparticles of various sizes depending on the deposition conditions.
SELFASSEMBLY OF LOWDIMENSIONAL STRUCTURES BY SILVER DEPOSITION ON VICINAL SILICON SURFACESJ. Kuntze , A. Mugarza and E. Ortega, Donostia International Physics Center, 20018 San Sebastian, Spain We investigate the growth of Ag nanostructures on vicinal Si(111) surfaces miscut a few degrees towards the [11-2] direction byscanning tunneling microscopy and LEED. Whereas the clean Si vicinals of this step type exhibit spontaneous step bunching, Agdeposition at elevated temperatures promotes debunching of this surface, leading to a high-quality stepped surface with a welldefined terrace width. The terraces are characterized by a previously reported (3x1) Ag overlayer, stabilized in one domain dueto the step-step interaction. In addition, we find one atom wide one-dimensional structures at the step edges, which exhibit localcontrast in tunneling spectra. By further deposition on the debunched surface, chains of ordered Ag islands develop on the small(3x1) terraces.
A MOLECULAR-DYNAMICS STUDY OF Ag DEPOSITION ON SILICON(100) , C.N.R.-Istituto IMM Sez. Di Bologna, Via Gobetti 101, 40129 Bologna, Italy This study analyzes the effects of the surface morphology on the deposition of Ag onto Si by using a molecular dynamicssimulation method with classical interatomic potentials newly devised for this study. The silicon surface has either a singular (100)bulk termination or the dimerization pattern experimentally observed. In addition to a variable structure of the surface, aparametric study has been made on other physical quantities such as the interatomic potentials and the heat exchangemechanisms. The effects of all these parameters are discussed in terms of sticking, intermixing and channeling of the depositedAg atoms.
SYMPOSIUM J
Wednesday, June19, 2002 Mercredi 19 juin 2002 PLENARY SESSION
08:15-8:30
SESSION CHAIRS:
Giovanni MARLETTA, E-MRS President Mildred DRESSELHAUS, MIT, Cambridge, USA PS-1 J.M.
LEHN, ULP, Institut Le Bel, ISIS, Strasbourg, France (Nobel Prize 1987)
ARDEN, INFINEON Technologies - Corporate Logic, München, Germany
THE INTERNATIONAL TECHNOLOGY ROADMAP FOR SEMICONDUCTORS -PERSPECTIVES AND CHALLENGESON SILICON FOR THE NEXT 15 YEARSThe paper will give an overview on the structure and the contents of the International Technology Roadmap for Semiconductors(ITRS) 2001, with special emphasis on future requirements on semiconductor materials for manufacturing technologies on silicon.
The ITRS started in 1992 as a US-national roadmap and became a joint international effort in 1998 with all major five industrialglobal regions (US, Japan, Taiwan, Korea and Europe) participating in its definition. Its technical scope covers semiconductortechnology from design & test, device architecture, manufacturing processes, factory integration and environmental issues. Thetime horizon extends to the following 15 years. Representatives from IC manufactures, equipment & material suppliers, researchinstitutes and consortia totaling more than 800 experts are engaged in the definition of future requirements for the semiconductorindustry. The outlook in semiconductor manufacturing expects the continuous application of silicon technology for the next 15years where CMOS based devices will carry the development of the industry at least for one more decade. New devicearchitectures and concepts based on silicon wafer material are being developed to support the development of the IC industry foranother one or two decades. The talk will high-light the key issues of future semiconductor material requirements. These willcover material aspects for front end processing, interconnect, lithography, new memory concepts and new devices. Materials tobe discussed are, for example, high permittivity gate dielectrics, low k insulating layers for interconnect, ferroelectrica formemory applications as well as new resist materials for very deep UV imaging processes.
FROMHERZ, Max-Planck-Institute for Biochemistry, München, Germany
SEMICONDUCTORS WITH BRAINAssembling living matter and semiconductors may give rise to devices that combine analog biodynamics and digital electronicsfor information processing and medical prosthetics. We explore the fundaments of such future hybrid devices and study siliconchips with ion channel molecules, nerve cells and brain tissue. Two directions are followed: On a nanoscopic level we analyzeand optimize the interface of electronics and ionics. On a microscopic level we join neuronal networks with microelectroniccircuits. In the first area, solid state physics and DNA technology are used to improve the cell-chip interaction; fluorescencemethods elucidate the nature of the cell-chip contact. In the second area, small defined nets of snail neurons and tissue slices ofrat brain are electronically interfaced to semiconductor chips. The approach relies on an integration of concepts and techniquesin neurophysiology, molecular biology, electrochemistry, semiconductor physics and microelectronics.
D.B. GEOHEGAN, A. A. Puretzky, C. H. Schittenhelm, M. A. Guillorn, P. F. Britt, M. Lance, D. Austin,
H. Cui, G. Eres, M. L. Simpson, S. J. Pennycook, R. Buczco, L. Allard, J. Howe, A. Lupini, M. Chisholm and D. H. Lowndes,Oak-Ridge National Laboratory, USALASER SYNTHESIS AND PROCESSING OF SINGLE-WALL CARBON NANOTUBES FOR ADVANCED ELECTRONICSAND COMPOSITESSingle-wall carbon nanotubes (SWNTs) are just one example of a multitude of one-dimensional nanocrystals which areefficiently grown by catalytically-assisted laser-synthesis. SWNT show remarkable promise for the development of futuregenerations of multifunctional electronic and structural materials. Individual SWNT exhibit metallic or semiconducting behaviorsdepending upon their atomic structure, and have an axial Young's modulus 1 Tpa. Of the several methods to grow SWNT, thelaser vaporization (LV) approach naturally yields: the narrowest distribution of nanotube diameters, exclusively single-wallednanotubes, at the fastest growth rate, from a wide distribution of catalyst nanoparticle diameters. The LV approach is also themost well-suited to time-resolved diagnostic investigation. In this talk, time-resolved in situ spectroscopic investigations of SWNTgrowth are described which are yielding some of the first direct determinations of carbon nanotube growth mechanisms and ratesnecessary to evaluate strategies for controllable synthesis and large-scale production. Results will be presented to show thatSWNT grow over extended annealing times during the LV process by the conversion of condensed phase (cluster andnanoparticulate) feedstock. Methods to enhance the growth rate for faster SWNT production and the growth of SWNT by laser-annealing and other thermal treatments of condensed-phase precursors will be discussed. However, the synthesis conditions ofSWNT can dramatically affect strategies for their subsequent chemical purification and functionalization. Resonant Ramanscattering measurements are described for the monitoring and control of SWNT defect levels, a first step toward reproducibleSWNT chemistry for composites and electronics. Results and strategies for the formation of nanotube-based electronic devices,composite materials, and multifunctional composites will be discussed.
S. KALBITZER, Physics Department, University of Pretoria, South Africa
SEMICONDUCTORS FOR OPTICAL MEMORIESOptical nano-structures can be produced in semiconducting materials by irradiation with focused ion beams. Exposed target areasexhibit strong optical absorption in contrast with non-implanted regions. Geometrical limitations on pixel size are set by lateralstraggling of the ion beam. By proper choice of implantation conditions pixel diameters of less than 100 nm can be produced withan estimated limit of about 10 nm. For digital recording the resulting pixel densities reach ultra-high values of 1010 – 1012 pix/cm2.
Hetero-epitaxial silicon on sapphire, commercially available as thin films, constitutes an excellent recording material with anoptical working range up to photon energies of hn 3 eV. Diamond layers with a useful optical range of up to hn 6 eV have thepromise of better resolution due to a lower Abbé limit. The life times of these optical memories greatly exceed those of allpresently used storage techniques. The physical properties of this ionographic technique, i.e. of ultra-high pixel density andpractically unlimited life time of the recorded information, provide a perfect tool for permanent mass storage of archival data.
PETROFF, ALS-LBNL, University of California-Berkeley, USA
IMPACT OF SYNCHROTRON RADIATION IN MATERIALS SCIENCEThe brightness and the partial coherence of the beams produced by third generation S.R. sourceshave opened new possibilities, allowing the development of new techniques. This will be illustratedby describing very recent results in various areas:high pressure physics, magnetism, electronicproperties of solids and imaging.
SYMPOSIUM J
Wednesday, June 19, 2002 Mercredi 18 juin 2002 Session V Moderator: M. Henzler14:00 (INVITED) QUANTUM WELL EFFECTS ON FILM STABILITY
T.-C. Chiang(a) and T. Miller(a), D.-A. Luh(b) and J. J. Paggel(c), (a)Department of Physics, University of Illinois at Urbana-
Champagin, USA, (b)SSRL, (c)Freie University Berlin, D-14195 Berlin, Germany
Films of Ag on Fe(100) whiskers can be prepared with atomic layer uniformity. Angle-resolved photoemission from such films
shows very sharp quantum well states that can be resolved layer-by-layer, thus allowing atomic layer counting for an absolute
determination of the film thickness. The discreteness of the electronic states can cause significant layer-by-layer variations in
physical properties. We have examined the thermal stability of such films with thicknesses of N = 1-15 monolayers. In our
experiment, the temperature of each film was ramped up slowly, while spectra were taken repetitively to monitor any spectral
changes. The results show that films of initial thicknesses N = 1, 2, and 5 are structurally stable for temperatures over 800 K,
whereas films of other thicknesses are unstable and bifurcate into a film with N + 1 and N - 1 monolayer thicknesses at
temperatures around 400 K. The difference in stability is rather dramatic, and can be related to the total electronic energy of the
system that can be evaluated by summing over all occupied quantum well states. The calculated energy difference against
bifurcation is in good accord with the experiment, thus demonstrating the quantum electronic origin of the stability variation.
QUANTUM SIZE EFFECTS IN ULTRATHIN EPITAXIAL Al AND Mg FILMS ON Si(111) L. Aballe(a), C. Rogero(a), S. Gokha(b), P. Kratzer(a) and (a), (a)Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany, (b)Department of Physics, University of Poona, IndiaHighly perfect ultrathin epitaxial Al and Mg films are grown on Si(111), by evaporating at reduced temperature and annealing.
We have investigated the electronic structure of such films using photoemission. Discrete peaks in the s-p band are identified asquantum well resonances derived from bulk bands. An interpretation in terms of the phase accumulation model and the bandstructure accounts for the main features in the spectra. The thickness dependence spectral peaks is used for a parameter-freedetermination of bulk band structure, and an estimate of the decay length of the Al(111) and Mg(0001) surface states. Theanalysis of quantum well resonance dispersion within the surface Brillouin zone reveal striking features of the electronicstructure in the overlayers. Subtle intensity modulations and energy shifts of the Al quantum well features arise from the influenceof the substrate electronic structure on the reflection of the electron waves at the interface, giving rise to slight modifications inthe overlayer electronic structure. This inherently ground-state process gives access to the band edges of the substrate in theground state. Strong resonant enhancement of intensity, and line shape variations of quantum well resonances as a function ofphoton energy cannot be understood within a single-particle picture of photoemission; they are interpreted in terms of theinteraction of the electron gas with the electromagnetic field in the photoemission process.
GROWTH AND ELECTRONIC STRUCTURE OF MAGIC Pb FILMS ON Si(111) , A. Mans, J.H. Dil and H.H. Weitering, TU Delft, Physics Department, Lorentzweg 1, 2628 CJ Delft, The NetherlandsEpitaxial growth of metals on semiconductors usually follows the Stranski-Krastanov mechanism. It was found that atomically flatPb islands with a "magic" thickness can be grown on Si(111) between 100 K and 200 K [1]. This unusual growth morphology ismost likely stabilized by the Quantum Size Effect (QSE). Recently, it has been shown that the electronic energy of the quantumwell with a particular thickness has a significant contribution to the film stability [2]. The charge density associated with thestanding-wave quantum well states is expected to induce an oscillatory relaxation of the interlayer spacing in the film. Weinvestigated the lattice relaxations in this system with I(V) LEED and the electronic structure with angle resolved photoemissionexperiments and band structure calculations. We present our results of Pb films with a thickness varying from 3 to 23 MLdeposited at low temperature on Si(111). Dynamical LEED calculations reveal a significant oscillation of the relaxation in theoutermost layers.
[1] K. Budde et al , Physical Review B, 61, R10602, (2000).
[2] D.A. Luh et al, Science 292, 1131, (2001).
LATERAL ELECTRONIC CONFINEMENT IN Ag ISLANDS DEPOSITED ON Cu(111).
A. Bendounan, H. Cercellier, B. Kierren, Y. Fagot-Revurat, F. Bertran, V. Yurov and , Laboratoire de Physique des Matériaux, (UMR CNRS 7556), Université Henri Poincaré Nancy 1, B.P. 239, 54506 Vandœuvre-Lès-Nancy Cedex, FranceThe surface electronic state in ultra thin films of Ag deposited on Cu(111) has been studied by photoemission and scanningtunneling spectroscopy (STS). Different behaviors were observed according to the elaboration temperature and substratemorphology. For a stepped Cu(111) substrate, the mobility of Ag adatoms leads to the formation of large Ag monoatomic islandsalong the step edges. STS and photoemission spectroscopies reveal the presence of a surface state in the island exhibiting a free-electron like parabolic dispersion. The bottom of the surface band state is nearly coverage independent in the submonolayerrange. By contrast, deposition at low temperature (150 K) onto a Cu(111) substrate with large terraces gives small islands whoseaverage size depends on the Ag coverage. Photoemission spectroscopy reveals a shift of the energy of the normal emissionspectrum with respect to large islands. In the submonolayer range, this energy shift appears to be strongly coverage dependent.
STM exhibits an evolution of the island size with increasing Ag coverage. Then, the observed spectroscopic behavior illustratesthe lateral confinement of the surface electronic state in the nanoscale Ag islands growing on large Cu(111) terraces.
NANOSTRUCTURING OF MULTILAYERS BY A THERMALLY DRIVEN SELF ASSEMBLING PROCESS (a), S. Dreyer(a), P. Troche(a), S. Sievers(a), J. Hoffmann(b), H.C. Freyhardt(a,b), (a)Institut fuer Materialphysik, Universitaet Goettingen, Windausweg 2, 37073 Goettingen, Germany, (b)Zentrum fuer FunktionswerkstoffegGmbH (ZFW), Windausweg 2, 37073 Goettingen, GermanyMultilayers consisting of two immiscible metals (Nb/Cu, Fe/Ag, Fe/Au) were used to form thin films structured on the nanometerscale. The samples were prepared by dc-magnetron sputtering. For Ni/NiO multilayers (reactive) ion-beam sputtering wasemployed. The morphology of the interfaces and the particular microstructure allows to transform - by an appropriate heattreatment - the multilayers into a system of isolated (magnetic) particles. The particles with a size of 5-30nm are embedded in thenoble metal matrix. TEM investigations reveal the nanoparticles to be arranged along triple junctions of the initial growth columnsand thereby forming a pearl chain structure perpendicular to the substrate, laterally separated by the growth column width ( 30-70nm). In addition to the microstructural characterisation by means of X-ray diffraction and TEM, the conductivity wasmeasured during film growth and during subsequent heat treatment. At low temperatures (4.2K), the as prepared multilayersdepict a GMR with a magnitude of up to 6% and considerable changes in its characteristic appearance, as the layered structure istransformed into the nanostructured state. In order to tailor position and inter-particle distance, further investigations onprepatterned substrates, i.e. vicinal sapphire and well-defined ion-induced wavy substrates, are in progress. This work issupported by the Deutsche Forschungsgemeinschaft, DFG, project No. 452/4-2.
KINETICALLY CONTROLLED GROWTH MODES AND SPONTANEOUS PATTERNING IN MOLECULAR ULTRA-THIN FILMS AND SUPRAMOLECULAR ARCHITECTURESF. Biscarini(a) , F. Dinelli(a), M. Cavallini(a), M. Murgia(a), R. Zamboni(a), M. Brinkmann(b), D.A. Leigh(c), S. Leon(d) and F.
Zerbetto(d), (a)CNR-ISMN, Via P. Gobetti 101, 40129 Bologna, Italy, (b)CNRS-Inst. "Ch. Sadron", 67083 Strasbourg, France,(c)Dept. of Chemistry, University of Edimburgh, Edinburgh EH9 3JJ, UK, (d)Dip. Chim. "G. Ciamician", 40100 Bologna, ItalyConjugated molecules, such as sexithienyl, pentacene, and Alq , grown in high-vacuum, exhibit spontaneous self-organization in SYMPOSIUM J
their thin films. The large variety of morphologies can be classified in terms of the kinetic control during growth, usingmorphological analysis by means of fractal and Voronoi analysis. The spatial correlation of the interface fluctuations can beascribed to the nature of the conjugated molecules, and the processes of coarsening governed by the diffusional barriers. Thesecond part will focus on spinodal dewetting as a suitable mechanism for large area patterning of ultra thin films, with aperiodicity (less than 70 nm) controlled via film thickness and the size dispersion through the interface roughness. Spontaneousrupture (upon a thermal or mechanical perturbation) of thin films into spatially organised domains will be demonstrated in avariety of systems ranging from conjugated oligomers to bistable molecules (rotaxanes).
Moderator: Ch. Pettenkofer J-VI.1 (INVITED) THE ROLE OF SILICON MAGIC CLUSTERS IN DYNAMIC PROCESSES ON SILICON(111)
SURFACES
I.-S. Hwang, M.-S. Ho, and T.T. Tsong, Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan, R.O.C.
In a scanning tunneling microscope (STM) study of Si(111) surfaces from room temperature to 600¢J, we find a special type of
clusters which are not only stable with respect to surface diffusion, but are also the fundamental unit in mass transport
phenomena, step fluctuations in detachment and attachment of Si atoms at step edges, and epitaxial growth. At temperatures
above 400°C, the magic cluster usually hops within a half-cell of Si(111)-(7x7), but sometimes it makes long-distance hops. We
characterize its structure and derive path-specific hopping parameters using Arrhenius analysis. In the long hops, interestingly,
the magic cluster exhibits a strong bias for moving in the direction of the heating current. Effects of the directed motion in
electromigration and those in thermal migration are determined separately and quantitatively. We also observe fluctuations of
step edges through detachment and attachment of magic clusters. The filling of two- dimensional (2D) craters and the decay of
2D islands are also found to occur preferentially at the cathode side. These observations provide important clues for
understanding the atomic processes in epitaxial growth and in electromigration on Si(111) surfaces.
J-VI.2 DETERMINATION OF STEP AND KINK ENERGIES FROM ISLAND STUDIES ON Au(100) IN ELECTROLYTE
AND ON Pt(111)IN UHV
S. Dieluweit, J. Ikonomov, K. Starbova, H. Ibach and
, Forschungszentrum Juelich, ISG 3, 52425 Juelich, Germany Using recently developed theoretical methods [1-4] to analyze equilibrium shapes and shape fluctuations of monatomic highislands on surfaces we investigate the step and kink energies on Au(100) in electrolyte solution and on Pt(111) in vacuum. Wedemonstrate that our method is capable of detailed studies of the dependence of both energies on the electrode potential in caseof the sulfate covered Au(100) surface in sulfuric acid solution as well as of their temperature dependence in case of the cleanPt(111) surface in UHV. For Au(100), we find that the step and the kink energies increase for potentials higher than the potentialof zero charge (pzc) with a maximum around 100 mV above pzc, then they decrease again until they dramatically increase forelectrode potentials around 400 mV above pzc [5-8]. For Pt(111), we find values for the step and the kink energy in agreementwith recent ab initio calculations by Feibelman [9].
[1] M. Giesen, Prog. Surf. Sci. 68 (2001) 1. [2] M. Giesen et al., Surf. Sci. 471 (2001) 80.
[3] D. C. Schlöfler et al., Phys. Rev. Lett. 82 (1999) 3843. [4] C. Steimer et al., Phys. Rev. B 64 (2001) 085416.
[5] S. Baier et al., Surf. Sci. in press (2002) [6] S. Dieluweit, M. Giesen, J. Electroanal. Chem. in press (2002) [7] S. Dieluweit, M. Giesen, J. Phys. Condens. Matter in press (2002)[8] S. Dieluweit et al., Faraday Discuss. Chem. Soc. submitted (2002) [9] P. J. Feibelman, Surf. Sci. 463 (2000) L661.
J-VI.3 SECOND-MOMENT INTERATOMIC POTENTIAL FOR Pd BASED ON TOTAL-ENERGY CALCULATION AND
ITS APPLICATION TO MOLECULAR-DYNAMICS SIMULATION OF Pd/Pd(111) DIFFUSION
N.I. Papanicolaou
(a) and D.A. Papaconstantopoulos(b), (a)Department of Physics, Solid State Division, University of Ioannina, P.O. Box 1186, 45110 Ioannina, Greece, (b)Center for Computational Materials Science, Naval Research Laboratory,Washington DC 20375-5345, USAWe have obtained a many-body interatomic potential for Pd within the second-moment approximation of the tight-binding modelby fitting to the volume dependence of the total energy of the metal, computed by first-principles APW calculations. The abovepotential achieved good accuracy for the elastic constants, phonon spectrum, along with the temperature dependence of thelattice parameter of Pd. This was used, in tandem with molecular-dynamics simulation to study the self-diffusion of single adatomson Pd(111) surface. The diffusion coefficient of Pd adatoms was computed and presents Arrhenius behavior. The migrationenergy and pre-exponential factor for hopping mechanism were determined and compared with recent STM experimental data.
Furthermore, exchange type diffusion mechanisms were observed above 1000K, which present much higher migration energythan the hopping diffusion mechanism.
J-VI.4 THEORETICAL STUDY OF THE ROLE OF SURFACE DEFECTS ON THE DIMER DYNAMICS ON SI(001)
Chu Chun Fu
and Andres Saul, Centre de Recherche sur les Mecanismes de la Croissance Cristalline, CNRS, Campus de Luminy, Marseille, FranceIt is a well known fact that pairs of Si surface atoms on Si(001) form buckled dimer rows in order to reduce surface energy.
These dimer rows are maintained up to 1463K, close to the temperature at which the surface becomes rough. Despite theprevious extensive studies, the detailed dynamics of surface dimers near different surface defects are still not completely clear.
In this work we have performed long time scale tight-binding molecular dynamics simulations at finite temperatures to studydimer dynamics on Si(001) surfaces with defects: from a single dimer vacancy, monoatomic steps of type A and B, and thosesteps with kinks. The characteristic time between dimer flips changes for dimers near different defects, and the results arecompared with those of experimental STM images. The present study provides a more comprehensive understanding of Si(001)surface dimer dynamics.
J-VI.5 ATTRACTION TOWARDS STEPS AND STEP-EDGE DIFFUSION MODEL FOR THE UNSTABLE EPITAXIAL
GROWTH OF VICINAL SURFACES
A. Videcoq
and A. Pimpinelli, LASMEA, UMR 6602, Université Blaise Pascal/CNRS, Clermont-Ferrand, France Experimentally, growth on vicinal surfaces is often affected by instabilities such as step meandering and/or step bunching. Beingable to predict the evolution of the surface morphology during epitaxial growth is of crucial importance for technologicalapplications. Step-edge diffusion together with Ehrlich-Schwoebel (ES) barriers at kinks may produce step meandering,depending on the step orientation [1,2], and has been conjectured to produce step bunching [2]. But Ernst's experiments oncopper exhibit step meandering independently of the step orientation. We propose a novel scenario that just assumes an enhancedstep-edge diffusion along with a kink-adatom attraction, similar to the step-adatom attraction discussed by Amar and Family [3].
Kinetic Monte Carlo simulations of this simple, solid-on-solid model, exhibit step bunching as well as step meandering. Wecompare our results with the morphologies obtained in the case of the homoepitaxy of copper and give a quantitative comparisonbetween the structures given by this model and the ones given by other models such as our two-particle model [4] or the"inverse" ES effect, where bunching is present.
[1] O. Pierre-Louis, M. R. D'Orsogna and T. L. Einstein, Phys. Rev. Lett. 82, 3661 (1999)[2] P. Politi and J. Krug, Surface Sci. 446, 89 (2000)[3] J. G. Amar and F. Family, Phys. Rev. Lett. 77, 4584 (1996)[4] A. Videcoq, A. Pimpinelli and M. Vladimirova, Appl. Surface Sci. 177, 213 (2001) SYMPOSIUM J
Thursday, June 20, 2002 Jeudi 20 juin 2002 Session VII Moderator: S. Erwin08:45 J-VII.1 (INVITED) CONDUCTING CHAINS OF INDIVIDUAL ATOMS
J. Ruitenbeek, Kamerlingh Onnes Laboratory, Leiden University, 2333 Leiden, The Netherlands
When a metallic contact is gradually broken, in the final stages it consists of only a few atoms. Such atomic scale contacts can
now be routinely produced and their mechanical and electrical properties can be studied using a scanning tunneling microscope
or a mechanically controllable break-junction technique. The number of conductance modes through a single atom contact is
determined by the number of valence orbitals of the metal atom, as was recently demonstrated by analyzing the subgap structure
for superconducting atomic contacts [1-3]. All evidence shows that for a single atom contact for monovalent metals the current is
carried by a single mode, with a transmission probability close to one. Guided by this knowledge, in experiments on gold we
discovered that during the contact breaking process the atoms in the contact form stable chains of single atoms being up to 7
atoms long [4]. The formation of gold atomic chains was independently found in direct images by Transmission Electron
Microscopy [5]. Such chains constitute the ultimate one-dimensional metallic nanowires. Chain formation, however, is not
observed for the iso-electronic noble metals Ag and Cu. This has been a key observation in uncovering the microscopic
mechanism that allows Au to form such unusual 1D configurations. Au also differs from Ag and Cu in forming reconstructions at
its low-index surfaces. Using mechanically controllable break junctions we have demonstrated that all the 5d metals that show
similar surface reconstructions (Ir, Pt and Au) also form chains of atoms, while both properties are absent in the 4d neighbor
elements (Rh, Pd, Ag). This points at a common origin for these phenomena. We propose an explanation of chain formation in
terms of a competition between s and d bonding, modified by relativistic effects [6]
[1] E. Scheer, P. Joyez, D. Esteve, C. Urbina and M.H. Devoret, Phys. Rev. Lett. 78 (1997) 3535.
[2] J.C. Cuevas, A. Levy Yeyati and A. Martin-Rodero, Phys. Rev. Lett. 80 (1998) 1066.
[3] E. Scheer, N. Agrait, J.C. Cuevas, A. Levy Yeyati, B. Ludoph, A. Martin-Rodero, G. Rubio Bollinger, J.M. van Ruitenbeek
and C. Urbina, Nature 394 (1998) 154.
[4] A.I. Yanson, G. Rubio Bollinger, H.E. van den Brom, N. Agrait and J.M. van Ruitenbeek, Nature 395 (1998) 783-785; (cond-
mat/9811093).
[5] H. Ohnishi, Y. Kondo and K. Takayanagi, Nature 395, (1998) 780.
[6] R.H.M. Smit, C. Untiedt, A.I. Yanson and J.M. van Ruitenbeek, Phys. Rev. Lett. 87 (2001) 266102; (cond-mat/0106641).
J-VII.2 A SCANNING TUNNELING SPECTROSCOPY STUDY OF Sn/Si(111)
B. Ressel(a),
K.C. Prince(a) , C. Di Teodoro(b), L. Ottaviano(b), J. Slezak(c) and V. Chab(c), (a)Sincrotrone Trieste, Strada Statale 14, 34012 Basovizza, Trieste, Italy, (b)Dipartimento di Fisica, Universita degli Studi dell'Aquila, 67010 Coppito(L'Aquila), Italy, (c)Institute of Physics, Academy of Sciences of the Czech Republic, 182 21 Prague 8, Czech Republic The structure of the Sn-induced mosaic and α√3x√3 phases on Si(111) has been examined by scanning tunneling microscopy(STM) and atomically resolved STS spectroscopy. The mosaic phase has a coverage Sn , on the top layer, with 5% vacancies. Although the structure contains no long range chemical order, is not completely disordered (in the lattice gas sense) ona local scale. Short linear chains and partial or complete hexagons of like atoms are the most common motifs of the local order. InSTS, the derivative of the I-V curve shows a series of relatively sharp peaks, similarly to a previous observation on the closelyrelated Pb/Si(111) mosaic phase [1], but in contrast the Sn system also shows an average expected metallic behaviour. Someindication of these states can be found in the Sn root 3 alpha phase, while they were absent in the Pb correspondent phase. Theappearance of sharp peaks on a "disordered" surface is surprising, as a multiplicity of local bonding environments usually leads toa smearing out of the density of states. We interpret the states as bands derived from Sn 4p and Si 3p levels, polarised normal tothe surface. The low dimensionality of the system confines them into quasi-one dimensional chains. The splitting results frominteraction between atoms in a finite spatial range along a chain or in a ring, leading to a set of discrete bonding states.
[1] B. Ressel, J. Slezak, K.C. Prince, and V. Chab, submitted for publication.
J-VII.3 ADSORBATE INDUCED ONE-DIMENSIONAL LONG RANGE MODULATION OF AN EPITAXIAL
INSULATOR FILM
W. Ernst(a), M. Eichmann(a),
H. Pfnür(a) , K.-L. Jonas(b), V. von Oeynhausen(b) and K.H. Meiwes-Broer(b), (a)Institut für Festkörperphysik, Universität Hannover, Appelstr. 2, 30167 Hannover, Germany, (b)Fachbereich Physik, Universität Rostock,18055 Rostock, GermanyUsing low energy electron diffraction and scanning tunneling microscopy, we show that epitaxial NaCl films grown on Ge(100)at a thickness between two and at least 15 monolayers can be modulated uniaxially with a long range periodicity of six latticeconstants. The (6x1) periodicity was found to be induced by a preadsorbed Na layer at very low concentration (about 0.06 of amonolayer) on the Ge surface. As STM revealed, Na at these concentrations forms chain structures with a preferred distance ofsix lattice constants between the chains, destroying locally the (2x1) reconstruction of the bare Ge(100) surface. Along the chainsa preferred distance of three lattice constants between Na atoms was found, but no correlation of this periodicity betweendifferent chains. A quantitative LEED analysis of a 10 monolayer thick NaCl layer revealed that the modulation takes placemainly normal to the surface with an amplitude of 0.28 Å at this thickness. Details of the modulation that contains (at least) twoFourier components can be well described by a simple geometric model. This study shows that the interaction between asemiconductor and an insulating film can be enhanced sufficiently by an ordered array of atoms adsorbed at the interface so thatregular modulations of the insulator film are obtained. This opens a variety of possibilities for self-organized structures oninsulating films on the nanometer scale.
J-VII.4 ELECTRONIC STRUCTURE AT STEP SUPERLATTICES IN NOBLE METAL SURFACES
A. Mugarza
(a,b), S. Rousset(c), V. Repain(c), A. Mascaraque(d), F.J. Himpsel(e), F.J. Garcia de Abajo(a) and J.E. Ortega(a,b), (a)Donostia International Physics Center and Centro Mixto de Materiales CSIC-UPV, Spain, (b)Universidad del Pais Vasco,Departamento de Fisica Aplicada I, Spain, (c)Groupe de Physique des Solides, CNRS, Universites Paris 6 et 7, France,(d)Universidad Autonoma de Madrid, Departamento de Fisica de la Materia Condensada, Spain, (e) UW-Madison, Departmentof Physics, USALateral nanostructures are ideal systems for nanotechnological applications as well as for the study of basic phenomena in lowdimensional systems. Vicinal surfaces with a regular array of monoatomic steps can be considered as self-assembled lateralnanostructures, which in turn can be used to grow other self-assembled nanostructures such as nanowires or nanodots. The free-electron-like Shockley surface states at noble metal surfaces interacts strongly with the step superlattice leading to differenteffects, such as one dimensional confinement. In this work we show how the wavefunction of these electronic states can bereadily explored by the combination of Angle Resolved Ultraviolet Photoemission Spectroscopy (ARUPS) and synchrotron radiation. For vicinal Au(111) surfaces, a 3D Fourier map of these wavefunctions show that, in the case of big terraces (∼40 Å), the surface state is confined in the terraces, while a dispersing superlattice state is observed for smaller terraces (∼10 Å).
J-VII.5 (INVITED) THE GROWTH OF Pb ON VICINAL Si SURFACES : ULTRATHIN Pb FILMS ON Si(111) and Si(557)
M. Henzler(a), O. Pfennigstorf(a), E. Hoque(a), A. Petkova(a) and R. Zhachuk(b), (a)Institut für Festkörperphysik, Universität
Hannover, D-30167 Hannover, Germany, (b)Academy of Sciences, Novosibirsk, Russia
Metal films with dimensions of only a few atomic distances show many properties quite different from bulk both in structural and
electronic respect. Pb on Si forms continuous films after deposition at low temperatures. Depending on substrate and temperature
during deposition and annealing, disordered and epitaxial films have been grown and well characterized by SPA-LEED. The
SYMPOSIUM J
conduction mechanism in ultrathin films may differ from bulk materials due to differents effects: surface and interface scatteringand quantum size effects should be considered. Also weak localization and superconductivity may be important. For Si(111)substrates the different contributions are separated with the in situ measurement of conductance and magnetoconductance andtheir correlation with structural parameters is shown. The Hall effect changes its sign several times with film thickness.
Calculation of the band structure show the relevant variations of the Fermi surface and enable quantitative comparison. Thevicinal surface Si(557) has triple steps after appropriate annealing. Their orientation as [113] is derived from a special LEEDanalysis. The growth of Pb on this vicinal surface forms nanowires for submonolayers; for thick films the Pb(111) plains areinclined due to to vertical misfit accommodation. Electrical measurements are in progress.
Session VIII Moderator: P. A. Thiel11:00 J-VIII.1 (INVITED) CHIRALITY IN ACHIRAL MOLECULES: A SURFACE SCIENCE VIEW OF SELF-ORGANISATION
IN MOLECULAR MONOLAYERS
Neville V. Richardson and Qiao Chen, School of Chemistry, University of St Andrews, North Haugh, KY16 9ST, UK
Chirality is a subtle but vitally important property of molecules which influences there interaction with electromagnetic radiation
and crucially their biological activity. Species which are achiral as isolated species can, nevertheless, become chiral on
adsorption due to the restrictions imposed by the chemisorption bonding. This gives rise to novel, nanoscale, two-dimensional
chiral structures and self-organised patterns in a number of molecules on single crystal surfaces, including amino acids and
nucleic acids. LEED, STM and vibrational spectroscopies can be used to probe these phenomena and this talk will describe our
recent work in this area.
J-VIII.2 PENTACENE SELF-AGGREGATION AT NOBLE METAL SURFACES: INTERFACE ELECTRONIC STATES
Carlo Mariani(a), Maria Grazia Betti(a), Fabio Biscarini(b), Valdis Corradini(c) and Claudia Menozzi(c), (a)INFM,
Dipartimento di Fisica, Universita' di Roma "La Sapienza", Piazzale A. Moro 2, 00185 Roma, Italy, (b)ISM-CNR, 40129 Bologna,
Italy, (c)INFM, Dipartimento di Fisica, Universita' di Modena e Reggio Emilia, 41100 Modena, Italy
The interest in organic molecule self-assembling at surfaces is rapidly growing, in view of the development of novel nanoscale
organic devices for the "plastic" electronics era [1]. Order can be driven by using either molecules forming chemical bonds, or
single crystal templates with natural rows, like the Au(110)-(2x1) surface. Pentacene is a molecule with extremely interesting
electronic properties, recently prepared for advanced devices [2]. We present an experimental investigation of pentacene
ordered monolayer formation at the clean Au(110)-(2x1) surface, in ultra-high-vacuum (UHV) conditions. Surface order has
been determined by LEED, and interface electronic states by high resolution UPS. We bring to light interface electronic states at
the monolayer, while at higher coverage the typical molecular orbitals of pentacene appear, and correspond to a stacked array of
molecules, as observed by preliminary AFM data. Pentacene chemisorption, which interaction with the substrate is basically due
to pi-electrons, is discussed and compared with the 2-mercaptobenzoxazole chemisorption on noble metal surfaces [5], where
order is driven by a stronger interaction of the S atom acting as a "hook" at the surface.
[1] Sinova et al., Phys. Rev. Lett. 87, 226802 (2001); F.J. Meyer et al., Nature 412, 517 (2001)
[2] J.H. Schoen et al., Nature 406, 702, 2000p; Science 287, 1022 (2000)
[3] Di Castro et al. in press on Surf. Sci. (2002)
J-VIII.3 LEAD-PHTHALOCYANINES THIN FILMS ON METALS AND SEMICONDUCTORS
N. Papageorgiou
(a), H. Oughaddou(b), J.M. Layet(a) and G. Le Lay(b), (a)Université de Provence, PIIM UMR 6633 CNRS, 13397 Marseille, France, (b)CRMC2-CNRS, 13288 Marseille, FranceMetal-phthalocyanines (M-Pc's) are organometallic compounds with a large range of conductivities varying from 10-12 Sm-1(conductivities similar to that of diamond or nylon) up to 104 Sm-1 (conductivity similar to that of graphite or InAs). In additionthey present the major advantage of absorbing light in the visible (Soret band around 1-2 eV) and the ultra violet (3 - 4 eV)regions of the spectrum. M-Pc's form crystals in a chain structure through which electron or exciton transport is possible. Theabove properties make phthalocyanines very appealing in the domain of electrooptical devices. The key points for the fabricationof electrooptical devices based on organometalic compounds is the ordering of these materials on electrodes and the optimizationof the molecule-molecule and molecule-electrode interaction. Photoemission spectroscopy (PES) of thin films deposited on wellcaracterized substrates gives crucial informations about the molecule-surface contact by probing directly the surface andmolecular valence bands as well as the core levels shifts. Hence we have acquired high resolution synchrotron radiationphotoemission spectra of the valence bands and shallow core levels of lead phthalocyanine thin films deposited under UHVconditions on four different substrates: Pt(111) as a reactive metallic surface, Ag(100) as a less reactive one, clean, unpassivatedSi as a semiconductor of major practical interest and finally InSb as well as InAs as narrow gap semiconductors. In ourpresentation we will compare in detail several spectroscopic aspects of these interfaces which reflect different types ofinteraction between the molecule and the surface.
J-VIII.4 ADSORPTION OF ACRYLONITRILE ON Si(001)-2*1 AT ROOM TEMPERATURE STUDIED BY A
COMBINATION OF SYNCHROTRON RADIATION TECHNIQUES AND SCANNING TUNNELING MICROSCOPY
F. Bournel(a) , J.-J. Gallet(a), S. Kubsky(a), S. Rangan(a), G. Dufour(a), F. Rochet(a), M. Kneppe(b) and U. Köhler(b),
(a)Laboratoire de Chimie Physique, Matière et Rayonnement, 75231 Paris 05, France, (b)Institut fur Experimentalphysik IV,
Ruhr-Universität Bochum, 44780 Bochum, Germany
The adsorption modes of acrylonitrile (H C=CHCN) on Si(001)-2*1 at room temperature had been studied by synchrotron
radiation spectroscopies: Near Edge X-ray Absorption Fine Structure (NEXAFS) and photoemission (XPS) spectroscopies, incombination with Scanning Tunneling Microscopy (STM). The molecule adsorbs non-dissociatively, lying flat on the surface asshowed by XPS and N1s and C1s NEXAFS data. In fact, we observe two orthogonal π* transitions (one in the plane of thesubstrate and the other perpendicular to it) in the absorption spectra. The two outer atoms of the molecule link, after arehybridization of the electronic structure, to two surface silicon through their dangling bonds. This rearrangement is allowedbecause of the conjugation between the pi(CC) and π(CN) orbitals. Furthermore, STM images confirm this flat lying geometryand show that, at low coverages, the molecule bridges two adjacent dimer rows leading to a SiC=C=NSi adduct. This adsorptionmode induces a strong local dimer buckling.
J-VIII.5 PSEUODOMORPHIC STARFISH: NUCLEATION OF EXTRINSIC METAL ATOMS ON A QUASICRYSTALLINE
SUBSTRATE
T. Cai(a),
J. Ledieu(b) , R. McGrath(b), V. Fournée(c), T. Lograsso(c), A. Ross(c) and P. Thiel(c), (a)Department of Chemistry and Ames Laboratory, Iowa State University, Ames IA 50011, USA, (b)Surface Science Research Centre, The University ofLiverpool, Liverpool L69 3BX, UK, (c)Department of Chemistry, Ames Laboratory, Iowa State University, Ames IA 50011Direct observation of extrinsic Al atoms on the fivefold surface of icosahedral Al-Cu-Fe indicates that they form pseudomorphicislands resembling starfish. The starfish nucleate at specific types of sites on the laterally-bulk-terminated quasicrystallinesurface. The nucleation event consists of a diffusing Al atom dropping into an empty site at the centre of a substrate pentagon.
Growth consists of the addition of 5 Al atoms in (nearly) lattice sites. These 6-atom starfish do not grow laterally as coverageincreases, leading to islands of uniform size, and to early roughening.
J-VIII.6 GROWTH OF CADMIUM FILMS ON Cu(111) FROM SOLUTION
J. Hommrich, S. Hümann and
K. Wandelt , University of Bonn, Institute of Physical Chemistry, 53115 Bonn, Germany Cadmium monolayers deposited electrochemically on a Cu(111) electrode from a 10-4M CdCl / 10-2M HCl solution have been studied in-situ by means of cyclic voltametry (CV) and scanning tunneling microscopy (STM). Theatomically resolved STM images show a moiré-like superstructure which is analysed by means of an algebraic model. Asimulated hard-sphere model based on the resultant moiré periodicity shows remarkable agreement with the detailed tunneling SYMPOSIUM J
current densities of the measured STM images. The existence of translational and rotational domains is also demonstrated. Thisnew structure model is much more refined and precise than but consistent with previous (ex-situ) LEED observations of the samesystem [1] demonstrating both the analytical power of in-situ electrochemical STM and the stability of the deposited Cd overlayeragainst emersion. Previous ex-situ studies have shown that the Cd layer is stabilized by coadsorbed Cl anions [1].
[1] C. Stuhlmann, Z. Park, C. Bach and K. Wandelt, Electrochim. Acta 44 (1998) 993 Thursday, June 20, 2002 Jeudi 20 juin 2002 Session IX Moderator: J. J. Paggel14:00 J-IX.1 (INVITED) LOW DIMENSIONAL ELECTRONIC STRUCTURE ON In-TERMINATED InAs(001)
K. Hricovini(a,b), P. De Padova(c), C. Quaresima(c), P. Perfetti(c), R. Brochier(a), C. Richter(a), V. Ilakovac(a), O.
Heckmann(a), L. Lechevallier(a), (a)LMPS, Université de Cergy-Pontoise, Neuville/Oise, 95031 Cergy-Pontoise, France,
(b)LURE, Bat 209D, 91898 Orsay, France, (c)CNR-ISM; via Fosso del Cavaliere, 00133 Roma, Italy
The properties of low-dimensional structures depend on their size, and can be very different from those of bulk matter. One-
dimensional system of fermions shows a number of anomalous properties, which cannot be understood in the framework of the
Fermi-liquid theory of normal metal. Such a system can be described by a theory based on an exactly solvable Luttinger model.
An interesting feature of the Luttinger model is the complete separation of spin and charge degree of freedom [1]. One-
dimensional structures can be prepared on InAs(001) surface. In-terminated InAs(001) reconstructed surface gives a highly
ordered array of quantum stripes, In-chains, as determined by LEED and STM. In this contribution we report two new surface
states related emission on In-terminated InAs(001)4x2-c(8x2) clean surface as determined by ARPES measurements. The
observation of these features strongly depends on the surface quality, reconstruction and geometrical orientation of the In-stripes
with respect to the electric field of the synchrotron radiation Strong photoemission intensity oscillations from these emissions were
observed. ARPES measurements show a free-electron like behaviour of the peak energy–dispersion for the k vector parallel to
In-chains, but no evident dispersion perpendicular to the chains was found.
[1] M.G. Zacher et al. Phys.Rev. B 57 (1998) 6370, J. Voit Phys.Rev. B 47 (1993) 6740.
J-IX.2 PHOTOELECTRON SPECTROMICROSCOPY STUDIES OF DEPOSITION AND INTERCALATION OF Cs ON 4Hb-
TaS and 4Hb-TaSe SURFACES
and Sven Stoltz, Dept of Physics, Göteborg University and Chalmers University of Technology, 412 96 Göteborg, SwedenLayered transition metal dichalcogenides (TMDCs) are of great fundamental and practical interest because of unique propertiesassociated with their reduced dimensionality. The intercalation of TMDCs with alkali metals is of particular interest for advancedbattery applications. In the 4Hb polytypes of TaS and TaSe two distinctly different types of layers are stacked in an alternating fashion. The layer types differ in that the coordination of the Ta atoms is octahedral (OH) in one type, but trigonal prismatic (TP)in the other. Clean surfaces typically consist of small domains with either OH and TP termination, so that photoelectron spectrameasured from such surfaces will represent an average. We have used the photoelectron microscope at MAX-lab in Lund,Sweden, to study cleaved surfaces of 4Hb-TaS and 4Hb-TaSe before and after Cs deposition. In this instrument photoelectron spectra can then be measured from a micron-sized light spot on the surface. Alternatively, the distribution of a particular speciescan be mapped by scanning the light spot across the surface. By operating the instrument in the imaging mode, we were able toidentify the different domains, and to image the development and motion of intercalation fronts. We were also able to measurecore level spectra from OH and TP domains separately. Large differences in the intercalation behaviour of OH and TP domainswere observed, reflecting the different electronic properties of the layers.
J-IX.3 ELECTRONIC STRUCTURE OF LOW-DIMENSIONAL VAN DER WAALS-EPITAXY FILMS
A. Klein(a) , R. Fritsche(a), E. Wisotzki(a), W. Jaegermann(a), R. Rudolph(b), S. Tiefenbacher(b), C. Pettenkofer(b),
(a)Darmstadt University of Technology, Institute for Materials Science, 64287 Darmstadt, Germany, (b)Hahn-Meitner-Institut,
14109 Berlin, Germany
Monolayer films of transition metal-chalcogenides can be prepared by van der Waals-epitaxy on a variety of substrates. The
deposition on graphite leads to electronically decoupled films, with electronic structures described by those of non-supported
(monolayer) films. Growth of GaSe on Si(111) leads to a chemically and electronically passivated Si(111):GaSe surface with a
non-polar Si-Ga bond. The electronic structure of the surface can be described in terms of the GaSe electronic structure. Growth
and electronic structures of low-dimensional van der Waals-epitaxy films as determined using photoelectron spectroscopy, low-
energy electron diffraction and scanning probe techniques are presented.
J-IX.4 PERIODIC ONE-DIMENSIONAL DISLOCATIONS IN ISLANDS OF Ni-GRAPHITE INTERCALATION
COMPOUNDS
(a,b), S.T. Jemander(b), N. Lin(b), G.V. Hansson(b) and L.D. Madsen(b,c), (a)Department of Physics, Queens University, Kingston, Ont. K7L 3N6, Canada, (b)Department of Physics, Linköping University, 581 83 Linköping, Sweden,(c)Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USANickel, intercalated between layers of graphite, atomically orders in periodic 2D lattices. Commensurate with the graphite, theintercalated nickel atoms have nearest neighbour distances of 750pm, triple the distance of 249pm in nickel metal. Afterdeposition of 2.5 monolayers of Ni onto single crystal silicon carbide, 6H-SiC(0001), under ultrahigh vacuum conditions, scanningtunnelling microscopy analysis revealed nickel lattices that varied with anneal temperatures between 800C and 1000C. Thenickel-graphite intercalation compound (GIC) formed in small islands by a reaction between the thin nickel film and the siliconcarbide substrate, which yielded graphite intercalation islands, and nickel silicide compounds. At high anneal temperatures,periodic (3.7nm) one dimensional ordering was observed, similar to that observed for alkali metal GICs, but not previouslyidentified as dislocations. The nickel GIC, however, is imaged in clear atomic resolution; the nickel lattice is commensurate withthe graphite. Excellent agreement with the model lattice demonstrates that the one-dimensional features are line dislocations ofthe nickel lattice relative to the graphite.
J-IX.5 MBE GROWTH OF THIN Si FILMS ON CaF /Si(111) for RESONANT TUNNELLING STRUCTURES
, B.H. Mueller, K.R. Hofmann, Institute of Semiconductor Devices and Electronic Materials, University of Hannover, Appelstr. 11A, 30167 Hannover, GermanyCaF /Si heterostructures on Si substrates are highly promising for resonant tunneling diodes due to the large conduction band discontinuity at the heterointerface and the small lattice mismatch. To realize such devices, the growth of both CaF /Si and Si/CaF must be sufficiently well controllable to produce the required atomically abrupt interfaces. In this work we have investigated 10nm and 4nm thick Si films MBE-grown on CaF /Si(111) substrates at various temperatures and subsequent annealing steps. As expected the growth at elevated temperatures leads to non-wetting, and islanding of the Si. The sizes of anddistance between the islands increase with temperature. At 700°C, B-type Si islands are found, and caused by a Si/CaF interface reaction. Unexpectedly, we found a large discrepancy between the evaporated amount of Si and the amount measured on thesurface. For 500°C we estimated a loss of 70% and for 700°C of even 95%. To avoid both problems, Si was evaporated at roomtemperature and annealed. Below 500°C, the surface is smooth, but does not recrystallize completely. At about 700°C a 1x1diffraction pattern shows a recrystallized film, but deep holes appear. For the thinner Si films the crystallization temperatures arelower. Finally, Al/1.5nmCaF /2.5nmSi/1nmCaF /Si(111) double barriers resonant tunneling structures were fabricated, which exhibited negative differential resistance.
SYMPOSIUM J
J-IX.6 A NEW METHOD TO SUPPRESS THE In DIFFUSION OF GaInNAs/GaAs QUANTUM WELLS GROWN BY
MOLECULAR BEAM EPITAXY
C.S. Peng(a) , E.-M. Pavelescu(a), T. Jouhti(a), J. Konttinen(a), W. Li(a) and M. Pessa(a), I.M. Fodchuk(b) and Y. Kyslovsky(b),
(a)Optoelectronics Research Centre, Tampere University of Technology, 33101 Tampere, Finland, (b)Department of Solid State
Physics, Chernivtsi National University, Chernivtsi 58012, Ukraine
Diluted nitride lasers made of GaxIn1-xNyAs1-y exhibit excellent temperature characteristics suggesting that they may operate
as Peltier-free light sources in optical data transmitters. In addition, GaInNAs/GaAs has been used to fabricate photovoltaic cells,
resonant cavity enhanced photo-detectors, and hetero-junction bipolar transistors. However, the optical properties are rapidly
deteriorated with increasing nitrogen incorporation for longer wavelength emission, while the lasing threshold remains relatively
high. Although the photoluminescence can be improved significantly by thermal annealing, it can also induce a big blue shift,
which is due to the inter-diffusion of In and N (especially for In) between GaInNAs and GaAs(N) during the annealing. We
present a new method to suppress dramatically the In diffusion of GaInNAs/GaAs quantum wells during thermally annealing. By
adding very thin strain-mediating layer (SML) between GaInNAs and GaAs(N), both the diffusion of In and photoluminescence
(PL) blue shift are decreased significantly and the interfaces of the GaInNAs/GaAs(N) quantum well become much sharper
under the same annealing process. With such SMLs, the PL (after the same annealing) peak is shifted 33 nm to longer
wavelengths compared to the sample without the SMLs.
J-IX.7 C AND Si CO-DEPOSITION ON Si(100)-2x1: A METHOD TO OBTAIN EPITAXIAL 3C-SiC WITH GOOD
INTERFACE QUALITYC. Cepek (a), E. Magnano(a), and M. Sancrotti(a,b), (a)Laboratorio Nazionale TASC-INFM, Trieste, Italy, (b)Universita' Cattolica, Brescia, ItalySilicon carbide is one of the most promising materials for many electronic and opto-electronic applications. Recently, fullereneprecursors have been successfully used to synthesize good quality 3C-SiC films on Si substrates, with improved quality inmorphology with respect to more standard methods. The reduced growth temperature (= 850°C) of SiC growth improves thequality of the interface and the adhesion that otherwise would be poor because of the mismatch of lattices and different thermalproperties. Another advantage of this method is that C molecules do not contain and do not introduce any undesired species in the growing film. Here, we report a systematic spectroscopic (UPS and XPS), and structural-morphologic (LEED and SNOM)investigation of the early stages of growth of nano-sized SiC islands on Si(100)-(2x1) by co-deposition of C molecules and Si atoms. The co-evaporation promotes a scenario characterized by a more ordered reacted overlayer with respect to the solelyevaporation of C with unaltered conditions of growth. Moreover the interface quality between Si and SiC is substantially improved and characterized by a considerable reduction of voids and defects. We spanned over a wide range of parameters thatinfluence the growth process, including substrate deposition and post-deposition annealing temperature and relative effusionfluxes of C and Si.
Session X Moderator: K. Wandelt16:30 (INVITED) NANOSCALE ELECTRONIC AND MAGNETIC PROPERTIES OF ULTRATHIN FILMS PROBED BY
SPIN-POLARIZED SCANNING TUNNELING SPECTROSCOPY
Roland Wiesendanger, University of Hamburg, Institute of Applied Physics and Microstructure Advanced Research Center,
Jungiusstrasse 11, 20355 Hamburg, Germany
Ultrathin magnetic films have become of great interest in various fields such as ultrahigh density magnetic data storage, magnetic
sensor technology or magnetoelectronics. We have applied spin-polarized scanning tunneling microscopy (SP-STM) and
spectroscopy at variable temperatures and external magnetic fields to study the correlation of geometric, local electronic and
local magnetic structure of ultrathin films on the nanometer down to the atomic scale. By using ferromagnetically or
antiferromagnetically coated probe tips the spatial distribution of a particular spin component of spin-split electronic states could
be mapped, thereby allowing the direct real-space observation of magnetic domains and domain walls with subnanometer spatial
resolution. For antiferromagnetic samples, the different orientation of magnetic moments could even be made visible at the atomic
level. Magnetic field dependent SP-STM experiments allowed the investigation of magnetic hysteresis at the nanometer scale and
its direct correlation with microscopic processes of domain nucleation and domain wall motion. More recently, dynamic
phenomena, such as magnetization switching of nanoscale islands were studied by time-dependent SP-STM experiments.
Co NANOPARTICLES - MAGNETO-OPTICAL STUDIES B. Kalska(a) , Ch. Müller(a), J. Paggel(a) and P.Fumagalli(a), M. Hilgendorff(b) and M. Giersig(b), (a)Freie Universität Berlin,Institut für Experimentalphysik, Arnimallee 14, 14195 Berlin, Germany, (b)Hahn-Meitner-Institut Berlin, Glienicker Str. 100,14109 Berlin, GermanyNanomaterials are a new kind of materials and receive much interest because of their new fundamental properties andprospective applications in many new optical and electronic devices as well as in biology and medicine. For nanoparticles a largefraction of atoms reside on the surface of the particle compared to the total number of atoms per particle. This changes theirquantitative and qualitative properties with respect to the bulk values. It is a big challenge for physicists and chemists to prepareparticles with controlled and small size distribution. Numerous physical and chemical methods have been employed to producemetal nanoparticles. Our Co particles are chemically synthesised by thermal decomposition of dicobaltoctacarbonyl. The particlesshow narrow size distribution of the order of <5%. We will present magneto-optical spectra from Co samples. The samples formagneto-optical Kerr measurements were prepared on Al, Ag, Au, Si or Cu substrates by a magnetophoretic deposition methodfrom colloidal solution. A strong influence of the particle distribution on the magneto-optical Kerr rotation was observed. Themagnetic properties of the nanoparticle films are also influenced by the substrate material. We present data from Co-nanoparticles sandwich samples comprised of a stacked layer sequence of nanoparticles and (Cu or Ag)-spacer layers. Suchsamples show different magneto-optical properties compared to pure Co (particles) layers.
MAGNETIC ANISOTROPY AND STRUCTURAL INHOMOGENEITY IN SMALL CLUSTERS (a), F. Scheurer(a), P. Ohresser(b), N.B. Brookes(c) and C. Goyhenex(a), (a)IPCMS-CNRS UMR7504, 23 rue du Loess, 67037 Strasbourg Cedex, France, (b)LURE-CNRS UMR130, Bat. 209D, BP 34, 91898 Orsay Cedex, France, (c)ESRF, BP 220,38043 Grenoble Cedex, FranceThe effect of structural inhomogeneity on the magnetic anisotropy of nanostructures are addressed.
In a first part, a theoretical analysis of these effects in the framework of the semi-empirical Néel model is described[1]. It isshown that besides the usual reversal of the easy direction of magnetization related to the coordination reduction of the atomes atsurfaces and steps[2], a local variation of the nearest neighbour distances between the magnetic adsorbat atoms can result in areversal of the easy direction of magnetization too, for small adsorbed clusters. The second part will highlight these effects in thecase of iron nanostructures deposited on the (111) herringbone reconstructed surface of gold. It is shown that the model descibedin the first part of this presentation, combined with an atomistic description of the herringbone reconstruction obtained by meansof molecular dynamics[3] allows a good description of X-ray magnetic circular dichroism experimental results[4].
[1] L. Néel, J. Phys. Radium 15 (1954) 225.
[2] D. S. Chuang, C. A. Ballentine, and R. C. O'Handley, Phys. Rev. B 49 (1994) 15084[3] H. Bulou and C. Goyhenex, Phys. Rev. B 65 (2002) 045407[4] P. Ohresser, N. B. Brookes, S. Padovani, F. Scheurer, and H. Bulou, Phys. Rev. B 64 (2001) 104429 SYMPOSIUM J
4f-SPIN ORDER OF THE Gd SURFACE MONOXIDE (a), J.E. Prieto(a), G. Kaindl(a), S. Gorovikov(b), K. Starke(a), (a)Institut fuer Experimentalphysik, FB Physik, Freie Universitaet Berlin, 14195 Berlin, Germany, (b)MAXLAB, Lund University, P.O. Box 118, 22100 Lund, SwedenIt has been shown recently that oxygen adsorption on Gd(0001) leads to the formation of an epitaxial surface monoxide. Apronounced temperature dependent 5d valence-state exchange splitting [1] indicates the existence of long-range spin order,however it remains finite even above the bulk Curie temperature. Here we report on an MDPE study (Magnetic Dichroism inCore-Level Photoemission [2]) employing linearly and circularly polarized synchrotron radiation. A clear magnetic dichroismsignal was observed for the bulk component of the Gd 4f-photoemission line, indicating ferromagnetic spin order of the sub-surface atomic layers. Yet the chemically shifted surface-oxide 4f component reveals a complicated dichroic signal that widelychanges with temperature. Based on this finding, models of possible spin order of the surface monoxide phase are discussed. Thiswork has been supported by the Deutsche Forschungsgemeinschaft (Sfb 290, TP A6) and by the BMBF (DESY-HS, 05 KS1KEC/2).
[1] C. Schussler-Langeheine et al., Phys. Rev. B 60, 3449 (1999)[2] K. Starke "Magnetic Dichroism in Core-Level Photoemission", Springer, Berlin, Heidelberg (2000) IMPROVEMENTS OF THE LATERAL RESOLUTION OF THE MFM TECHNIQUE and U. Hartmann, Institute of Experimental Physics, P.O.Box 151150, 66041 Saarbrücken, Germany Microstructured thin film elements with dimensions of 1 µm or below play an increasingly important role in magnetic componentsfor information technology applications. Basic research on magnetic materials as well as industrial applications create anincreasing demand for high-resolution magnetic imaging methods. Here, we report on the preparation of high-resolution MFMtips using the e-beam deposition (EBD) method for MFM measurements on soft magnetic samples. E-beam lithography in a SEMmicroscope was used to define small particles of magnetic material at the very end of a commercial AFM tip to achievemaximum lateral resolution with low magnetic moment. Several approaches were tried out: (i) 30 nm CoCr thin film tips. Thetypical resolution is about 150 nm. These tips were found to cause strong disturbances of magnetic structures due to a large strayfield. (ii) EBD-tip. We show that these tips do not influence so much the magnetic structure of the sample; the resolution is similarto tips (i). (iii) Demagnetized tips for further reducing the stray field effect. Here, the achieved resolution is clearly below 100nm. (iv) Combination of demagnetized and EBD-tip. These tips show a reduced stray field, resulting in a reduced influence on themagnetic structure of the sample; estimated resolution 20 nm. The achieved resolution is clearly proven by the fact that moredetails of the magnetic structure can be observed. This work is subsidized by the EU-project 'MAGNETUDE'.
18:00-20:30 POSTER SESSION II
J/PII.01
HETEROEPITAXIAL GROWTH OF Si1-x Gex NANOSTRUCTURES ON (100) Si USING LPE (a), H. Wawra(a), S. Christiansen(b), M. Albrecht(a), H.P. Strunk(b), M. Schmidbauer(a), R. Köhler(c), (a)Institut für Kristallzüchtung, Max-Born-Str.2, 12489 Berlin, Germany, (b)Institut für Werkstoffwissenschaften, LehrstuhlMikrocharakterisierung, Universität Erlangen-Nürnberg, Cauerstr. 6, 91058 Erlangen, Germany, (c)ArbeitsgruppeRöntgenbeugung, Institut für Physik, Humboldt-Universität zu Berlin, Hausvogteiplatz 5-7, 10117 Berlin, GermanyIn the last years self-organising epitaxial growth mechanisms such as the Stranski-Krastanov growth mode have been utilised forthe fabrication of strongly strained coherent quantum dots. The key parameters of the system are shape, size distribution, strainand composition since they modify the electronic and optical properties. Precise control of the growth process is required in orderto produce highly regular monodisperse island arrays. However, many aspects of coherent islands formation and evolution arestill poorly understood. At LPE (Liquid Phase Epitaxy) the growth driving forces are very low so that growth can be considered totake place close to the thermodynamic equilibrium. This facilitates the interpretation of our results with heteroepitaxial Si1-xGexmodels based on energetic calculations. Consequently, the most relevant parameters controlling the LPE growth process are thestrain field within the islands and the surface-free energy, whereas the influence of kinetic processes can be neglected.
ANISOTROPICALLY ETCHED Si SURFACE AND THE ELECTRICAL PROPERTIES OF THE Si/CdTe and Si/HgCdTe HETEROSTRUCTUREST.Ya. Gorbach(a), P.S. Smertenko(a), S.V. Svechnikov(a), M. Kuzma(b) , G. Wisz(b) and P.Sagan(b), (a)Institute of Semiconductor Physics, National Ukrainian Academy of Sciences, Kiev 28, 252630 Ukraine, (b)Institute of Physics, RzeszówUniversity, 35-309 Rzeszów, PolandSilicon base (001) oriented substrates, chemically etched in special conditions were used as a substrate for laser epitaxy ofHgCdTe and vacuum etitaxy of CdTe. Four types of Si surfaces with different microrelief were obtained: flat, pyramid-like,plate-like and porous. Thin films of HgCdTe were deposited using YAG:Nd3+ laser. Structure properties of the layers wereinvestigated by electron diffraction. The mechanisms of film growth are discussed. The electrical properties of the structuresunder investigation were analysed on the basis of current-voltage characteristics. A model of the band structure of the HgCdTe/Siinterface and the carrier transport mechanism are proposed. The current-voltage characteristics were interpreted in the relationto the structure properties and the interface band structure.
SELECTIVE GROWTH OF Ge QUANTUM DOTS ON HYDROGEN-PASSIVATED Si(001) SURFACES (a), Tam T.T. Ngo(b), D. Bouchier(a), Tuyen T.T. Le(b), Khoi H. Phan(b), (a)Institut d'Electronique Fondamentale, UMR-CNRS 8622, Université Paris-Sud, Orsay, France, (b)Institue of Materials Science, NCNST Vietnam, Hoang Quoc Viet St.,Hanoi, VietnamDuring the last few years, the formation of self-assembled Ge/Si quantum dots (QD's) has received much interest since thistechnique could allow to produce quantum sized islands without any kind of mask or patterning. However, due to the randomprocess of island nucleation, the self-assembling technique lacks broad distribution both in size and in position.
We report, in this paper, on a new method to produce Ge quantum dots on Si(100) surfaces. Starting from the fact that theadsorption of hydride molecules (SiH4, GeH4) requires free adsorption sites on the surface, the basic idea of our approach is to limit the number of sites for molecular adsorption. By using in-situ Reflection High-Energy Electron Diffraction (RHEED),Thermal Desorption Spectroscopy (TDS), we show that etching of Si(100) surfaces in ammoniun fluoride (NH4F) solution initially produces a flat and dihydride-terminated Si(100) surface and that longer etching leads to the formation of microscopic(111) facets. Hydrogen atoms are found to desorb completely from dihydride sites at 400°C while those from monohydride-terminated (111) facets remain stable up to 650°C. Thus, for growth carried out in the temperature range of 400-650°C, theadsorption of hydride molecules occurs only on the sites that had been previously terminated by dihydride species, i.e. free ofhydrogen. Compared to self-assembled islands, we demonstrate, by using this new approach, that Ge islands with sizes beingreduced down to 200 Å can be achieved. First results on the optical properties of such small islands will be also reported.
INTERACTION OF IRON WITH NATIVE SILICON OXIDE STUDIED WITH PHOTOELECTRON SPECTROSCOPY T. de los Arcos , M.G. Garnier, J. Boudaden, and P. Oelhafen, Institut of Physics, Basel University, SwitzerlandTransition metals are widely used as catalysts for the growth of carbon nanotubes, and are deposited in many cases directly ontosilicon substrates. In order to clarify the metal-substrate interaction for such systems, iron was deposited on top of silicon waferscovered with its native oxide layer, and analyzed with in-situ photoelectron spectroscopy. Different iron deposition methods havebeen used, like thermal evaporation, pulsed arc, and sputtering. During the first deposition stages the reduction of the originalsilicon oxide layer by the iron was observed. This behavior has been investigated in terms of the energy of the incoming ions,substrate temperature, and background pressure.
STUDY OF SURFACE ROUGHENING OF TENSILY STRAINED Si Ge C FILMS GROWN BY UHV-CVD Cyril Calmes(a) , Daniel Bouchier(a), Catherine Clerc(b) and Yulin Zheng(c), (a)IEF, bât. 220, Université Paris Sud, 91405 Orsay,France, (b)CSNSM, bât. 108, Université Paris Sud, 91405 Orsay, France, (c)LMCP, Université Paris VI et Paris VII, F-75252 Paris05, France SYMPOSIUM J
A very attractive possibility offered by Si Ge C alloys is the achievement of N-MOSFET's with a thin buried channel built directly on the Si substrate. The required band offset implies a high tensile strain within the epilayer and, consequently, arelatively high amount of substitutional carbon atoms. Smooth interfaces are also needed to enhance the mobility of the carriersnear the interface A high growth rate and a low temperature are found to be favorable to obtain smooth surfaces. Whenthreshold conditions on these growth parameters are not fulfilled for a given SiH CH fraction in the gas phase, in situ RHEED measurements reveals that roughness develops at the film surface. This is accompanied by a dramatic decrease of thesubstitutional carbon content and simultaneously stacking faults develop within the epilayer. According to these observations, weproposed a model of surface roughening based on the formation of carboneous complexes on the film surface. This mechanism islimited by the diffusion length of carbon adatoms and by the growth rate. From that, a critical ratio between growth rate and Cdiffusion coefficient was assumed and its temperature dependence was determined. The activation energy of C adatoms diffusionwas found to be close to that known for hydrogen desorption. This suggests that diffusion of carbon is limited by the hydrogencoverage .
STRUCTURAL AND ELECTRONIC PROPERTIES OF CaF NANO-DIMENSIONAL ISLANDS ON Si(001) L. Pasquali (a), S. Suturin(b), V.P. Ulin(b), N. Sokolov(b), S. Nannarone(a), (a)INFM and Dipartimento di Ingegneria deiMateriali e dell'Ambiente, Via Vignolese 905, Universita di Modena e Reggio Emilia, 41100 Modena, Italy, (b)Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaInitial stages of epitaxial growth and formation of CaF nanostructures on Si(001) were studied. Different types of nanostructures were formed depending on the growth conditions. These include ultra-thin two-dimensional layers and quasi one-dimensionalstripes at T 700°C and nearly square-shaped dots at lower growth temperatures. Atomic force microscopy and reflection high-energy electron diffraction were used to measure the nanostructure shape and lattice orientation. The evolution of the surfaceelectronic structure under different growth conditions was studied by ultraviolet photoelectron spectroscopy at different photonenergies and metastable deexcitation spectroscopy. In particular, the formation of a high-temperature CaF wetting layer on Si(001) has been studied carefully using AFM. At very low coverage (below 0.1 ML) ultra thin 3 Å high lines nucleate along<110>-directions at silicon terraces with the lines on adjacent terraces running perpendicular to each other, reflecting thesymmetry of the Si(001) double-domain surface. At higher coverage, the density of the lines which develop along the terraceincreases with respect to those running across, causing an expansion of the terrace itself. In this way, the silicon surface transfersgradually from a two-domain (2x1 and 1x2) to an almost one-domain structure. Finally, the lines stick together to form auniformly covered surface.
THE ELASTIC CHARACTERISTICS OF AN INTERFACE Te INCLUSIONS AND MATRIX IN CdTe, HgTe AND CdHgTe CRYSTALS AND HETEROSTRUCTURES ON THEIR BASISI.V. Kurilo(a) , I.O. Rudyj(a), A.I. Vlasenko(b), I.S. Virt(c), (a)Lviv Polytechnic National University, 12 Stepan Bandera str.,79013 Lviv 13, Ukraine, (b)Institute of Semiconductor Physics, NAS Ukraine, 01028 Kyiv 28, Ukraine, (c)PedagogicalUniversity, 79720 Drohobych, UkraineOn the basis of the approach of van der Merve considering the contacting pair of Te inclusions/matrix as specific heterostructurewe have estimated the following of elastic properties and characteristics of misfit dislocations in CdTe, HgTe and CdHgTecrystals and hetero structures on their basis: the density energy of the inclusion - matrix interface; spacing between misfitdislocations; critical thickness for coherency loss in a matrix; elastic strain in the inclusion - matrix system; strains in systemcaused by the difference in the lattice parameters and thermal expansion coefficients; length of cracks in the matrix. The stressesvalues caused by difference in lattice parameters are close to values of the stress tensor components estimated on the basis of theMott-Nabarro model around inclusions. Such elastic stresses in the case of epitaxial heterostructures can be composed withstresses at the substrate/film interface which arise owing to misfit of lattice parameters and thermal expansion coefficients of thecontacting materials MODIFICATION OF PROPERTIES OF Cd Hg Te TARGETS BY PLD DEPOSITION OF NANOSTRUCTURAL FILMS I.S. Virt(a), M. Kuzma(b), I.O. Rudyj(c), M.S. Frugynskyj(c), I.V. Kurilo(c) and I.Ye. Lopatynskyj(c), (a)Pedagogical University, 24 Franko str., 293720 Drohobych, Ukraine, (b)Institute of Physics, Higher Pedagogical School, 35309, Rzeszow,Poland, (c)Lviv Polytechnic National University, 79646 Lviv, UkraineThe morphology and structure properties of surface layers of CdHgTe (CMT) targets (x=0.2) irradiated by laser pulses(YAG:Nd3+, 250 ms and XeCl, 20 ns) were studied. The modification of morphology, structure and some physical properties ofCMT targets was observed during the PLD process. The inhomogeneity of surface morphology of the targets was revealed bySEM. The change of microhardness, brittleness and surface energy was determined for irradiated targets. The modification ofsurface layers structure of targets was investigated by RHEED. The changes in electrical properties of targets due to thegeneration of dislocations and interstitial atoms were revealed. Concentration of the acceptor defects increases at irradiation withmillisecond pulses of energy over the threshold and concentration of donor defects decreases at irradiation with nanosecondpulses. The increasing of electrons concentration after irradiation by XeCl laser can be explained by Hg atoms leaving latticepoints and diffusing deep into the target bulk by means of the interstitial mechanism and formation of n+-type shunting canal. Themechanism of the defects generation is mainly determined by the pulses duration.
REAL TIME SPECTROSCOPIC ELLIPSOMETRIC ANALYSIS OF Ge FILM GROWTH ON Si(001) SUBSTRATES E. Palange(a) , L. Di Gaspare(b) and F. Evangelisti(b), (a)Unita INFM and Dipartimento di Energetica, Universita degli Studi diL'Aquila, 67040 Monteluco di Roio, Italy, (b)Unita INFM and Dipartimento di Fisica Universita degli Studi di Roma TRE, 00146Roma, ItalyEpitaxial growth of Ge films on Si is important for Si-based optoelectronic applications. In particular few nanometer thickstrained films can find potential applications for fast mesoscopic devices if surface roughening, ripening and structural defectsare absent. Owing to lattice mismatch between Si and Ge, the film growth in absence of surfactants follows the Stranski-Krastanov mechanism. Thus suitable materials require the control of the growth mechanism. In this paper we report on an in-situreal time spectroscopic ellipsometric studies of Ge first layer growth on Si deposited by low-pressure chemical vapour deposition.
The samples were grown at a temperature of 330 °C and the film thickness was varied in the 0.3 to 10 nm range, well above thecritical thickness for relaxation and island formation. The surface planarity was obtained by using the surfactant properties ofhydrogen present as reaction-by-product. We were able to measure the variations of the dielectric constant of the growing filmtogether with the change of the deposition rate. In this way it was possible to correlate the deposition rate with the film electronicproperties that develop towards the Ge bulk behaviour. In particular, we found that the growth rate reached its final value onlyfor thickness of the order of 10 nm, i.e. when the film dielectric constant is equal to that of bulk material. This finding proves thatchanges of the growth rate are related to the material strain status.
GROWTH MODE TRANSITIONS IN MOLECULAR-BEAM EPITAXY OF GaAs(001) Vladimir I. Trofimov and , Institute of Radioengineering & Electronics of RAS, 11 Mokhovaya Street, 101999 Moscow, RussiaEpitaxial growth mechanism of GaAs(001) as a function of MBE growth variables, deposition rate, growth temperature, substratemiscut and As/Ga flux ratio is studied with using in situ RHEED monitoring of a growing surface morphology. For analysis ofexperimental data a modified BCF model for MBE growth on a vicinal surface is developed, which takes nucleation and growthof islands on a terrace and the existence of repulsive Ehrlich-Schwoebel (ES) barrier at a descending step-edge into account. Onthe basis of model calculations using simple criteria for growth mode transitions, growth phase diagram in the space ofparameters, characterizing the ratio of the surface diffusion-to-deposition rates and the substrate misorientation, respectively, areconstructed. The inclusion of an ES barrier expands the region of island growth and shrinks the region of step-flow growth in thegrowth phase diagram. It is shown that the model predicted critical deposition parameters (substrate temperature at fixedsubstrate misorientation or critical misorientation at fixed substrate temperature) for the layer-by-layer transition; step-flow SYMPOSIUM J
growth can be fitted to the experimentally measured data at reasonable values of the parameters of elementary atomic eventssuch as durface diffusion barrier and frequency prefactor. This work was supported by the ‘Korea Signal' Corporation.
INTERFACE OF TANTALUM OXIDE FILMS ON SILICON BY UV ANNEALING AT LOW TEMPERATURE (a), J-Y Zhang(a,b), Z.M. Wang(b), J.X. Wu(b), B.J. O'Sullivan(c), P.K. Hurley(c), T.L. Leedham(d), M.A. Audier(e), J- P. Senateur(e) and Ian W Boyd(a), (a)Dept. Electronic and Electrical Eng., University College London, Torrington Place,London WC1E 7JE, UK, (b)Structure Research Laboratory, University of Science and Technology of China, Hefei 230026, P.R.
China, (c)National Microelectronics Research Centre, Lee Maltings, Prospect Row, Cork, Ireland, (d)INORG-TECH, 25 JamesCarter Road, Mildenhall, Suffolk, Suffolk IP28 7DE, UK, (e)Institut National Polytechnique de Grenoble, Ecole NationaleSuperieure de Physique de Grenoble - LMGP, B.P. 46, 38402 St. Martin d'Heres, FranceTantalum pentoxide (Ta O ) films appear to be one of the best alternative gate dielectrics for application in DRAMs. In previous work, we have grown 4-100 nm Ta O films by photo-CVD using a special precursor injection system, which exhibited leakage currents as low as 2.19x10-7 A/cm2 at 1 MV/cm. However, the properties of these films are known to deteriorate with decreasingfilm thickness and increasing interfacial SiO layer. UV annealing at low temperatures using an excimer UV source can improve the electrical properties of these films dramatically. In this paper, Ta O thin films with thicknesses of about 40 nm grown by photo-CVD have been annealed at low temperature using an excimer UV lamp. Film properties have been characterised usingellipsometry, FTIR, UV spectrophotometry, capacitance-voltage (C-V) and current-voltage (I-V) techniques. After UVannealing, improved leakage current densities as low as 1.5x10-8 A/cm2 at 1 MV/cm, and breakdown fields higher than 4.0MV/cm can be achieved. Investigation by XPS and TEM of the interfacial SiO layer formed after UV-annealing reveals that the thickness of the interfacial SiO layer slightly increases with annealing time and that the annealing converts the suboxide at the interface into stoichiometric SiO , leading to the improvement of electrical properties in the films. This technique of photo- induced deposition combined with UV annealing offers a very effective method for the fabrication of high quality films.
CRYSTAL QUALITY EVALUATION AND CARRIER PROFILING OF THIN AlGaAs (0.22<x<0.92) FILMS BY ELECTROCHEMICAL CV TECHNIQUEY. Fedorenko , T. Jouhti, J. Konttinen, M. Pessa, Optoelectronic Research Centre, Physics Department, TUT, 33101 Tampere, FinlandIn this paper we study details of the electrochemical capacitance - voltage method, or E-CV, for n-type Al Ga As layers with a special objective to estimate both the doping profile and the crystal quality by a single measurement. To the best of our knowledgethese exist no systematic E-CV study has been made for MBE grown Al Ga As over a wide composition range. We report here an E-CV measurement for range of x from 0.22 to 0.92. First, we studied the optimisation of E-CV method for Al Ga As. Three types of electrolytes were explored. We regard 0,2M NaOH: EDTA as the most appropriate enchant for this application. Thefrequency voltage to perform the C/V, and G/V measurements, the voltage of etching, and the intensity of light were optimised.
The details of the choice of these parameters will be presented in our paper. Secondly, we will discuss the results of etching.
Etching of Al-reach samples occurred at a very slow rate, yielding a rough surface, independent of the etchant. Therefore, to alarge extent the surface roughening is associated with an inherent property of Al Ga As at high x. We also compared the electrochemically-etched surfaces with chemically etched ones, and the mechanism of different pattern formations will bediscussed in the paper. The compensation effect was small, not exceeding 8-10 % for all the samples the growth temperaturesused. This observation provides evidence that the structural quality of all the samples was reasonably good.
THE Ge(113) SURFACE: A PLAYGROUND FOR STUDYING SURFACE STRAIN AND SELF ASSEMBLING NANOSTRUCTURESA. Hirnet , M. Albrecht, M. Gierer and W. Moritz, Universität München, Institut f. Kristallographie und Angewandte Mineralogie,Theresienstr. 41, 80333 München, GermanyThe (113) surfaces of Si and Ge are possible candidates for the growth of heteroepitaxy films and self-organized nanostructures.
These surfaces have the advantage that they do not exhibit rotational domains like the (100) surfaces. The adsorption of Sb and Bion Ge(113) has been studied by x-ray diffraction and SPA-LEED in order to understand their possible role as surfactants. Theresults show that Sb occupies 3 different sites on the Ge(113) surface. Two of six Sb atoms in the (2 x 2) unit cell break a Ge-Gebond and occupy an interstitial site. The structure can be explained by the release of tensile stress by the Sb which breaks the Ge-Ge bond. During growth of Si or Ge layers the removal of Sb from the interstitial site would require an activation energy whichmakes a role as surfactant very unlikely. Bi forms a series of superstructures (1 x n) with n=3,4,5,8 dependent on the coverage.
The x-ray analysis of the (1 x 4) structure shows that Bi causes a microfacetting to (111) and (114) facets. On the (111) facet Biforms a trimer like that found on the (111) surface, and on the (114) facet Bi forms dimers in substitution sites and occupies 3-foldcoordinated sites. Due to the microfacetting 26 Å wide trenches orientated along [1-10]are formed. The width of the trenchescan be controlled via the Bi coverage up to a width of 52 Å in the (1 x 8) structure. The well ordered array of trenches make thissurface a promising candidate for building 1-D quantum wires.
PERCOLATION NETWORK AND ABRUPT INTERFACES FOR Ag GROWN AT ROOM TEMPERATURE ON Si(111) , C. Ocal, M. Alonso and A. Ruiz, Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, 28049 Madrid, SpainObtaining two-dimensional or flat metal films on semiconductors with abrupt interfaces is a major objective in order to use thesefilms as buffer layers for metallic magnetic superlattices. In this sense, the non reactive Ag on Si(111) 7x7 system has a valuableinterest. However, a large mismatch exists between Ag(111) and Si(111) and, at room temperature (RT), a Stranski-Kastranovgrowth mode has been reported to occur. Our approach consists in the use of MBE protocols with deposition rates (0.1-1Å/s)much higher than those usually employed for surface science studies. The growth properties have been studied by means ofdifferent experimental techniques for structural, chemical and morphological characterization. Several deposition rates, substratetemperatures and metal coverages have been investigated. Whereas large and widely spaced three-dimensional crystallites areformed at 670 K, an absolutely different morphology is obtained for temperatures close to RT. As revealed by AFM, thetopography of the films grown at 325 K consists of a two level structure typical of a bi-dimensional system. Remarkably, the toplevel is extremely flat and homogenous. Depending on coverage this system evolves from island formation to a percolated layer.
The fraction of the covered surface in the percolated network increases with deposited material up to nearly 100%. Furthermore,these films exhibit an extremely abrupt Ag-Si interface, as seen by HRTEM.
SELF-ASSEMBLED Si STRUCTURES ON SINGULAR AND VICINAL Si(111) AS CANDIDATES FOR THE FABRICATION OF METALLIC NANOSTRUCTURESP.P. Martin , F.J. Palomares, M. Alonso and C. Ocal, Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, 28049 Madrid, SpainFabrication of ordered arrays of nanoscale structures is of enormous interest for the development of new devices based onquantum confinement effects. Our results confirm the capability to control the surface morphology and obtain quasi-regularpatterns on Si buffer layers grown, by Molecular Beam Epitaxy (MBE), on Si(111) wafers. It is known that instabilities duringhomoepitaxial growth give rise to different morphologies. We have studied thick Si layers (50-200 nm) grown at 1070 K onsingular and 4º-off Si(111) surfaces. The surface topography has been investigated by means of AFM and the film quality byAES, LEED and in-situ RHEED. For nominal (111) surfaces (<0.25º) an almost perfect honeycomb pattern, with lateralperiodicities of 550 nm and 200 nm, is developed. It consists of a network of steps of bilayer height plus out of phase boundariesdomains. The existence of these domains, which appear to be self-organized, is revealed by Si self-decoration during growth. Forvicinal surfaces, step bunching and meandering are followed by the formation of a sawtooth pattern. Lateral and vertical sizes ofthese triangular structures are 500 nm and 15 nm, respectively, preserving the total surface slope. Hence, we are able to obtainperiodic arrangements of Si structures with quite well defined shapes and uniform sizes. The viability of using these patterns astemplates to fabricate metallic and magnetic nanostructures by MBE is investigated.
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RAMAN STUDY OF NITROGEN-DOPED ZnSSe/GaAs EPILAYERS Y.S. Raptis (a), M. Strassburg(b), U.W. Pohl(b) and D. Bimberg(b), (a)Faculty of Applied Sciences, National Technical University, 15780 Athens, Greece, (b)Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin,GermanyZnSe-based II-VI compounds, in combination with p-type doping, is an active issue by now, one of its major questions being thepassivation of incorporated acceptors by hydrogen which is introduced during decomposition of the precursors.
In this work, N-doped ZnSSe layers, grown on (001)-oriented GaAs substrates using monomer (tBu)2Se or dimer Me Se precursors, are investigated. For the latter compound significantly reduced incorporation of hydrogen was previously reported[1]. The samples are investigated by X-ray and micro-Raman methods. Raman spectra, measured under parallel (yy) and crossed(xy) polarizations, show LO and TO modes for ZnSe and ZnS, in agreement with a two-mode behaviour, according to theliterature [2]. The phonon frequencies of these modes, in combination with the X-ray diffraction patterns, are analyzed in orderto evaluate both, elemental stochiometry and elastic strains of the epilayers, as well as their lateral inhomogeneities across thestructures. Shifts of the LO(GaAs) mode, from the epilayer-substrate interface, relative to its bulk value, are interpreted in termsof the interface state and possible influence on it of the N-dopants.
[1] P. Prete, N. Lovergine, A Tapfer, A.M. Mancini, and C. Zanotti-Fregonara, J. Cryst. Growth 209, 279 (2000).
[2] K. Hayashi, N. Sawaki and I. Akasaki, Jpn. J. Appl. Phys., 30 (1991) 501 INTERFACE PROPERTIES OF ZINCOXIDE FILMS: INTERFACIAL DEFECTS AND ELECTRONIC PROPERTIES C. Pettenkofer and U. Meier, Hahn-Meitner Institut, SE6, Glienickerstr. 100, 14109 Berlin, GermanyThin conducting oxide films play an important role in optoelectronic devices. Zincoxide is widely used as a window material inphotovoltaic applications. Nevertheless the morphology, doping characteristic and interface energetics in heterocontacts are notwell understood. For the system Si/ZnO a characterization of interfacial properties will be given in dependence of differentdeposition techniques (MOMBE, Sputterdep). Its implication on heterocontact formation and electrical properties will bediscussed on the basis of spectroscopic results.
SPIN EFFECTS ON THE OXIDATION PROCESS IN SiO2 , Antonio J. R. da Silva and A. Fazzio, Instituto de Fisica, Universidade de Sao Paulo, CP 66318, 05315-970, Sao Paulo, SP, BrazilInterstitial O plays an important role in the thermal oxidation of SiO as the promoter of the oxide growth after diffusing through the silica network. It is well known that the triplet state is the lowest energy configuration for an interstitial O . In this work we study the influence of the spin degree of freedom on the oxidation of SiO . We will present results, all based on total energy first principles calculations, for the interaction of an oxygen molecule with a Si-Si bond in the silica network, both for the triplet aswell as the singlet spin state. Our results show that in the triplet state the O bond is broken as the O molecule approaches the Si- Si bond. As a result, a perfect SiO structure is formed (Si-O-Si), together with an interstitial O atom. On the other hand, in the singlet spin state, the same reaction has a different final configuration. The O does not dissociate, being incorporated in the silica network as a peroxy defect (Si-O-O-Si). The total energy of this peroxy structure is lower than the final triplet configuration byapproximately 2 eV. However, as the probability for spin exchange is small along the reaction path, the reaction will most likelyoccur through the triplet channel. Therefore, the oxidation of SiO depends strongly on the spin degree of freedom, and the fact that the O-O bond is repulsive along the triplet pathway may help to explain why the Si/SiO interface has a very small density of INTERFACE PROPERTIES OF ULTRA-THIN HfO2 FILMS GROWN BY ATOMIC LAYER DEPOSITION ON SiO2/Si O. Renault(a) , A.-M. Papon(a), P. Holliger(a), D. Rouchon(a), J.-F. Damlencourt(a), A. Chabli(a), S. Marthon(a) and D. Blin(b),(a)CEA-DRT–LETI/DTS–CEA/GRE, 17 av. des Martyrs, 38054 Grenoble 9, France, (b)ASM France, F-34036 Montpellier 1,FranceThe interface properties of ultra-thin (1-8 nm) HfO films grown on 0.3 nm SiO /Si by Atomic Layer Deposition are studied by Angle-Resolved X-Ray Photoelectron Spectroscopy (ARXPS), High-Resolution Transmission Electron Microscopy (HRTEM)and depth profiling techniques using Auger Electron Spectroscopy (AES) and Secondary Ion Mass Spectrometry (SIMS) with theMCs + method [1]. For a 5 nm-thick as-deposited film, HRTEM shows a re-growth of SiO up to 0.55 nm upon HfO deposition.
On the ARXPS Si2p spectrum, the comparison of oxide and substrate intensities yields 0.61±0.14 nm, and a component at 102 eVcould possibly be assigned to Hf-O-Si bonds. Both AES and SIMS depth profiles show abnormal shape of the Hf signal, indicativeof a presence of Hf into the Si substrate, interpreted in terms of Hf diffusion on the basis of careful cross-check experiments.
Results concerning the evolution of the interface (thickness, chemistry) with post-deposition annealing temperature (425-800°C)and initial HfO thickness are also presented.
[1] P. Holliger, F. Laugier, J. C. Dupuy, Surf. Interface. Anal. 2002, to be published.
FIRST PRINCIPLES CALCULATION OF THE GEOMETRIC AND ELECTRONIC STRUCTURE OF (Al2O3)n AND (Al2O3)n(O2) CLUSTERS WITH n < 21.
and Luis C. Balbas, Departamento de Fisica Teorica, Atomica y Nuclear, Facultad de Ciencias, Universidad de Valladolid, 47011 Valladolid, SpainWe have studied the geometrical and electronic structure of the clusters (Al O ) clusters with n < 21, by means of the first principles self-consistent DFT-LCAO code SIESTA, using ab-initio norm-conserving atomic pseudo-potentials, double zeta pluspolarization basis sets of numerical pseudo-orbitals, and the local density approximation (LDA) for exchange and correlationeffects. The equilibrium cluster geometries have been determined by total energy minimization within the conjugate gradientmethod, starting with several initial geometries for each cluster size. We discuss the trends versus cluster size obtained for: i) thepreferred geometries (growth patterns), and ii) the binding energies per Al O molecule, and per each type of atom (Al or O).
Preliminary calculations of the total energy of (Al O ) (O ) oxide clusters, relaxing i) only the O-O distance, and ii) all the atoms in the complex, are presented and discussed for selected sizes (n = 6, 8 and 12).
GROWTH OF B C ON COBALT FROM THE DECOMPOSITION OF ORTHOCARBORANE ), J. Monson(a), T. Caruso(a), B. Xu(a), A. Sokolov(a), Zong-Yuan Liu(a), C.-S. Yang(a), B. Doudin(a) P.A.
Dowben(a), A. Harken(b), P. Welsch(b) and B.W. Robertson(b), (a)Department of Physics and Astronomy and the Center forMaterials Research and Analysis, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA, b)Departments ofMechanical Engineering and of Physics and Astronomy, and the Center for Materials Research and Analysis, University ofNebraska-Lincoln, Lincoln, Nebraska 68588, USA, (c)Ecole Polytechnique Federale de Lausanne, SwitzerlandWe investigated new type of insulating tunnel barrier material: the low Z inorganic boron carbide dominated by an icosahedralboron-carbon cage. This material can be grown by chemical vapor deposition without pinholes in the ultrathin film regime. Thescheme involve the adsorption and decomposition of 1,2 closo-dicarbadodecaborane (orthocarborane or C B Hy). Plasma enhanced chemical vapor deposited boron-carbide is an excellent dielectric with resistivities in the range of 1010 to 1013 Ohmcm,with a band gap of about 0.7 eV. Resistance curves of trilayer Co/ByC/Co "tunnel" junctions samples revealed strong non-linearcharacteristics at low bias and large magneto-resistance consistent with the polarization of cobalt. Photoemission studies of theadsorption and decomposition of orthocarboraneilluminate the boron carbide growth process from the molecular precursor.
While a variety of transport measurements have been used to characterize this relatively novel semiconductor. This is a relativelynovel semicondiuctor based on the largely boron icosahedral cage.
ORIENTED ZnO THIN FILMS SYNTHESIS BY SOL-GEL PROCESS FOR LASER APPLICATION L. Znaidi (a), G.J.A.A. Soler Illia(b), S. Benyahia(a), C. Sanchez(b), A.V. Kanaev(a), (a)Laboratoire d'Ingénierie des Matériaux et des Hautes Pressions - CNRS, U.P.R. 1311, Université Paris XIII, 99 Avenue J.B. Clément, 93430 Villetaneuse, France,(b)Laboratoire de Chimie de la Matière Condensée, Université Pierre et Marie Curie, 4 Place Jussieu, Tour 54 5e, 75252, ParisCedex 05, France SYMPOSIUM J
During the last decade a renewed interest has been devoted to ZnO-based materials owing to their applications in different fields:acoustic-wave and acousto-optic devices and short-wavelength semiconductor diode lasers. The sol-gel process offers a simple,low-cost and highly-controlled way for ZnO film elaboration. We have undertaken the systematic study of the key parameters,influencing the texture and preferential orientation of films elaborated by the sol-gel method. They are precursor, solvent andadditive natures and concentrations, coating and heat treatment conditions. The zinc acetate dihydrate is dissolved in ethanol andmonoethanolamine, which act at the same time as a base and a complexing agent. The aging of solutions at 60°C results in coloredsols. The solutions without aging and sols are deposited in different experiments on glass or quartz substrates by spin coating. Theas-deposited films are composed of amorphous zinc oxide-acetate submicronic particles. These particles are transformed intooriented ZnO upon thermal treatment. We show that the preferential orientation of films depends on the nature of the solution orsol, precursor concentration and thermal conditions. The sols with low Zn2+-concentrations result in films formed by rectangularplatelets oriented along [100] directions. For solution with high Zn2+-concentration the films are preferentially oriented along[002]. We discuss the preparation conditions and optical properties of these films.
EFFECT OF IMPLANTATION ON MICROWAVE-CVD DIAMOND LAYER H.K. Csorbai (a,b,c), G. Hars(a), E. Kalman(b,c), G. Peto(d), G. Kovach(d), P.M. Nagy(c) and P. Deak(a), (a)BudapestUniversity of Technology and Economics, Department of Atomic Physics, Budafoki ut 8, 1111 Budapest, Hungary, (b)Bay ZoltanInstitute for Materials Science and Technology Fehérvari ut 130, 1116 Budapest, Hungary, (c)Chemical Research Centre,Institute of Chemistry, Hungarian Academy of Sciences Pusztaszeri ut 59-67, 1025 Budapest, Hungary, (d)Research Inst. forTechnical Physics and Materials Science – MFA, P.O.Box. 49, 1525 Budapest, HungaryDiamond films exhibit unique mechanical, thermal, optical and electrical properties in combination with excellent chemicalstability. So this material is very perspective using a number of technologies. Usually one needs to combine the diamond withother layers, especially with other carbon layer (graphite or amorphous carbon). In this work we present our results in the field ofimplantation of diamond. Our results show that, using different conditions during implantation, different types of carbon werefound to be present. The measured diamond layers were deposited by Micro-Wave Chemical Vapour Deposition (CVD)technique. Reliable, clear and reproducible diamond layers can be deposited using this technique. SEM images were suitableideal to measure the orientation, grain size, and homogeneity of the layer. Due to the insulator nature of diamond, SEM imagescan be captured only after pretreatment to form a conducting layer (in this case with gold). AFM images can provide usefulinformation on surface roughness, secondary grains in a nano-scale and the homogeneity.
PREPARATION AND CHARACTERIZATION OF SILICA-TITANIA PLANAR OPTICAL WAVEGUIDE BY USING PHOTON-ASSISTED SOL-GEL METHODWei Chen (a), Jun-Ying Zhang(a,b), Qi Fang(a,b), Lei Shi(a), Jian-Xin Wu(a), Yan Di(a), Xin-yu Zhang(a) and Zu-Min Wang(a), (a)Structure Research Laboratory, University of Science and Technology of China, Hefei 230026, PR China,(b)Electronic and Electronical Engineering, University College London, Torrington Place, London WCIE 7JE, UKWe report the formation of planar waveguides of silica doped with titania on silicon (100) and silica substrates at lowtemperatures by a new photon-assisted sol-gel process. Polymeric sols were formed through hydrolysis and condensation oftetraethoxysilane mixed with tetrabutoxytitanate with the presence of SiCl as catalyst. These were then spin-coated on the substrates and irradiated by excimer lamps. The microstructure and optical properties of the composite films were studied by avariety of techniques, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infraredspectroscopy (FT-IR), Ellipsometry, and X-ray photoelectron spectroscopy (XPS). Slab waveguides from buffer layer, core layerand cladding layer were formed by photon-assisted sol-gel process at different substrate temperatures in the range of 100ºC to450ºC, in comparison to the thermal process. Preliminary experiments on the optical loss of the slab waveguides were reported. Itis found that low loss is achieved for lightly-doped waveguides by this new technique without any high-temperature annealing.
THE DIP COATING FORMATION OF NANOSTRUCTURED POLYMERIC LAYERS S. Tamulevicius , A.Guobiena, Institute of Physical Electronics, Kaunas University of Technology, Savanoric 271, 3009 Kaunas, LithuaniaPolystyrole (PS) and polymethylmethacrylate PMMA are optical polymers recently widely used to form thin correction layers inoptical devices. The present polymers of high technology have common solvents and can be formed of weak solutions of a low-viscosity. As a result of sorption processes of interaction between polymer macromolecules and substrate surface, thin polymericlayers with a structure determined by the substrate properties, solution concentration and temperature are formed. The presentwork is aimed to form PS and PMMA polymeric layers based on toluene solutions of 0.1-0.5 weight.% concentration andcharacterize these layers on various substrates (polyethylenterephtalate film PET, SiO glass, Si crystalline substrate) exposed to plasma processing. The dependence of the processes of formation and morphology of PS and PMMA films on solutionconcentration, type and conditions of substrate processing was studied by such methods as ellipsometry, laser light scattering,scanning electron microscopy (SEM), Fourier transformation infrared spectroscopy , and atomic force microscopy. It is shown,that PS and PMMA layers are formed as a result of macromolecules sorption from solutions, and interaction with the substratehave a tendency to nanostructurisation.
INVESTIGATION OF Ti DOPED HfO2 THIN FILMS DEPOSITED BY PHOTO-CVD (a), Jun-Ying Zhang(a) and Ian W Boyd(b), (a)Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, UK, (b)Structure Research Laboratory, University of Science and Technology of China,Hefei 230026, P.R. ChinaThe growth of Ti doped HfO thin films by photo-induced CVD using 222 nm excimer lamps at temperature of 400 °C is reported.
Vaporised titanium isopropoxide and Hafnium (IV) tetra-t-butoxide were used as the precursors, which were carried into thephotochemical reaction chamber by Ar carrier gas at a low pressures of several mbar. Films around 45 – 70 nm in thickness withrefractive indices from 1.850 - 2.424 have been grown, using different atomic ratios of Ti/Hf. The higher refractive index valuescompare favourably with the value of 2.58 recorded for the TiO bulk material. The microstructure and the optical and electrical properties of the deposited Ti doped HfO thin films were characterised by various techniques. XRD indicated that the films were amorphous when the atomic ratio of Ti/Hf was below 25%, while the crystalline TiO anatase phase was formed when the atomic ratio of Ti/Hf was more than 25%. The refractive index values of the films was found to sharply increase with increasing ratio ofTi/Hf over 25%. FT-IR, XPS and TEM were also used to characterise the Ti content and the interface and microstructure of thefilms deposited on Si-substrates. The effect of UV-annealing and Ti doping on the electrical properties of these films will bediscussed.
MOLECULAR LEVEL RECOGNITION ON MONOLAYER LANGMUIR-BLODGETT FILMS (a), M.O. Gallyamov(b), K.A. Zhavnerko(a), I.V. Yaminsky(b) and V.E. Agabekov(a), (a)Institute of Chemistry of New Materials, National Academy of Sciences of Belarus, Staroborisovsky Trakt 36, Minsk 220141, Belarus, (b)PhysicalDepartment of Moscow State University, Vorobievy Gory, Moscow 119899, RussiaThe goal of our presentation is to compare molecular packing in mono- and multilayer films on solid surfaces. Monolayers fromdifferent compounds, such as behenic acid, purified bee's wax, amphiphilic amides and ketones, have been prepared both onhydrophilic (mica) and on sufficiently hydrophobic (graphite) surfaces. Atomic Force Microscopy (AFM) has been used to imageLB monolayers with molecular resolution and to identify local defects and inhomogeneities in ordered surfaces. Reports fromother groups indicated that images with molecular resolution can be obtained mainly on layers of odd numbers starting from 3because of instability of a monolayer film on solid surface. We have prepared monolayers by specially developed horizontalprecipitation method (HP). Resulted monolayer films were stable enough to reach molecular resolution by AFM on monolayersamples. Detachment of molecules from the surface, their re-orientation and self-organisation has been observed during storage.
Repeatedly observed supramolecular formations on monolayer films of amide and ketone derivatives suggest that self-orgaizationis a very common phenomenon in monolayers. For instance, LB films of amphiphilic amides were found to reorganize into SYMPOSIUM J
superstructures with a hexagonal arrangement of hydrocarbon tails. It is conjectured that the driving force for the molecularreorganization is the surface tension gradient arising from the change in character of the hydrocarbon tail packing.
INVESTIGATION OF EVOLUTION IN INTERFACE BETWEEN ULTRATHIN TANTALUM PENTOXIDE FILMS AND SILICON SUBSTRATE DURING ULTRAVIOLET ANNEALINGWang Zumin (a), Qi Fang(b), Jun-Ying Zhang(a,b), J.X. Wu(a) and Chen Wei(a), (a)Structure Research Laboratory, University of Science and Technology of China, Hefei 230026, People's Republic of China, (b)Electronic and Electrical Engineering,University College London, Torrington Place,London WC1E 7JE, UKUltrathin tantalum pentoxide films with different thickness (4-15nm) were deposited directly on silicon substrate by photo-inducedchemical vapor deposition using a 222 nm excimer lamp. Some of the films were annealed in oxygen ambient under theirradiation of excimer ultraviolet sources. C-V and I-V measurements showed great improvement of electrical properties afterannealing. We investigated the evolution in tantalum pentoxide/silicon interfaces during annealing by X-ray photoelectronspectroscopy in combination with Ar+ etching and Fourier transform infrared spectrometry. It was found that there was siliconsuboxide in the interface for the as-deposited samples, which could be turned into silicon dioxide after UV annealing. Theremoving of tantalum suboxides in the film close to the interface was also observed. These evolutions are believed to be the originof improvement in electrical properties.
SURFACE SELF-DIFFUSION IN POROUS SILICON FILMS (a), E. Cing(a), B. Can Ome(a), C. Oru(a), Z.A. Allahverdie(b), (a)Department of Physics, Yildiz Technical University, 127 Davutpasa, 34210 Esenler-Istanbul, Turkey, (b)Department of Electric and Electronics, Karadeniz TechnicalUniversity, 61080 Trabzon, TurkeyThe crystalline structure of the porous silicon (PS) presents network of silicon in nanometer-sized regions, sarrounded by voidspace with very large surface to volume ratio (up to 103 m3/cm2). The pore surfaces are covered by silicon hydrides and siliconoxydes. Surface bonds, in particular Si-H bonds play an important role in regulation of electrical, optical, luminescence and otherproperties due to the large internal surfaces of PS. The aim of the present work is to study the self-diffusion of hydrogen andoxygen located in Si-H and Si-O bonds on surfaces of pores. The second aim is related with investigation of diffusion behaviourof hydrogen ions penetrating from ambient. The porous silicon films were prepared on n-type Si (111) substrates by anodicetching. Free-standing PS films were then detached from silicon substrates by electropolishing. The diffusion parametres of thebonded hydrogen and oxygen atoms in free-standing PS films were determined from analysis of intensity changes of Si-H(2100cm-1) and Si-O (1100cm-1) infrared absorption peaks as a function of temperature and duration of annealing in air. It isshown that in the range of 65-185 °C the temperature dependence of hydrogen and oxygen diffusion coefficients are describedby activation energy of 0.37 and 0.50eV respectively. Decrease of intensity of hydrogen-related absorption peak correlates withappropriate decrease of intensity of photoluminescence peak (at 570 nm) for PS films exposed to isothermal annealing in therange of 65-185 °C. It is assumed that Si-H centers are responsible for photoluminescence in PS. The humidity-voltaic effect, i.e.
the generation of the open-circuit voltage (V ) up to 440mV in metal(Cu,Ag,Au)-PS structures in humid atmosphere is discovered. It is shown that generation of the open-circuit voltage is mainly caused by hydrogen ions penetrating from ambient.
Data on V changes in depending on duration of annealing of metal-PS structures were used for estimation of diffusion coefficient of hydrogen ions in PS films. Diffusion of hydrogen ions in PS films at 60-200 °C proceeds with low activation energy(about of 0.13 eV) in compared with that for diffusion of hydrogen atoms located in Si-H bonds (0.37 eV).
Mechanisms of migration of hydrogen and oxygen along surfaces of pores were discussed.
MODEL CATALYST STUDIES: VANADIA PARTICLES ON A THIN-FILM ALUMINA SUPPORT , J.B. Giorgi, M. Bäumer and H.-J. Freund, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany A model system of alumina supported vanadia particles was prepared under ultra-high vacuum (UHV) conditions andcharacterized regarding its structural and electronic properties. As support oxide we used a thin well-ordered alumina film grownon NiAl(110) which allows the application of scanning tunneling microscopy (STM), infrared reflection-absorption spectroscopy(IRAS) and X-ray photoelectron spectroscopy (XPS) without charging effects. The vanadia deposits were prepared via metalevaporation in an oxygen ambient, leading to the growth of small, roundish oxide particles with vanadium in the +3 oxidationstate. The particles give rise to new features in the IR spectra thus providing insight into the inner structure of the particles.
Surface-localized vanadyl groups (V=O), interface-localized vibrations involving V, O and Al ions and lattice structures typicalfor bulk-V O could be identified. CO adsorption at 90 K displays a pronounced particle size dependance and results in weakly physisorbed species which interact with vanadyl groups present at the surface of the particles. Interestingly, CO can be oxidizedto CO by exposing the sample to slow electrons. For comparison, we also present results obtained for metallic vanadium particles prepared on the same alumina substrate. (This work was supported through SFB 546 of Deutsche Forschungsgemeinschaft) APPARATUS DEVELOPMENT FOR DIAMOND SYNTHESIS FROM ACETONE VAPOR WITH LOW ENERGY CONSUMPTIONFumitomo Onishi, Kaoru Gyoda, Yoshiki Takagi , Teikyo University of Science & Technology, Yamanashi, Japan In the fabrication of hydrogenated amorphous silicon (a-Si:H) based device, like the MIS (metal-insulator-semiconductor) or theheterojunction a-Si:H/Zn, the surface oxidation a-Si:H is highly crucial since it improves the device I-V characteristics bypassivation of surface defects. The main difficulty in oxidising a-Si:H is that the process must be done at low temperature sincefilm degradation starts at 350ºC. This means that thermal oxidation must be excluded, since at low temperatures the diffusion ofoxygen is insignificant. The anodic oxidation technique is a good alternative technique since it is performed at 20ºC, it has lowcosts and allows the growth of dense oxide films. In this work, spectroscopic ellipsometry was used to characterize the oxidesproduced using an ethylene glycol (0.04M KNO ) solution. The classical structure usually used is a simple Cr/a-Si:H structure where the Cr is the anodic back contact. From the data achieved we notice that to grow the oxide, an intermediate ZnO layer ofabout 10 nm thick between the Cr and the a-Si:H is required. This is essential to protect the Cr from direct solution contact trougha-Si:H pinholes which leads to the preferential Cr oxidation instead of the a-Si:H oxidation. We also observed that the oxidesgrown at constant voltage present less surface defects and higher density than the oxides grown in constant current mode, andthat the oxide thickness presents a linear relation to the applied voltage SURFACE ACTIVATION PROCESSES AND ION-SOLID INTERACTIONS DURING THE GROWTH OF ULTRA-THIN AMORPHOUS CARBON FILMSP. Patsalas (a), S. Logothetidis(a), S. Kennou(b) and A. Galdikas(c), (a)Aristotle University of Thessaloniki, Department ofPhysics, Solid State Physics Section, 54006 Thessaloniki, Greece, (b)Department of Chemical Engineering, University of Patrasand FORTH/ICEHT, Rion University Campus, 26500 Patras, Greece, (c)Physics Department, Kaunas University of Technology,73 Donelaicio st., 3006, Kaunas, LithuaniaAlthough many efforts have been devoted to study the growth of amorphous Carbon (a-C), the fundamental processes occurringat the film surface and the interactions with the energetic species are not quite clear, yet. The latter are very important for ultra-thin (<5 nm) a-C films, where the top 3-5 monolayers are a considerable portion of the film. In this work, a-C films have beengrown by processes using Ar+ or C+. The films' morphology and hybridization state were evaluated by Auger ElectronSpectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS) and X-Ray Reflectivity (XRR). The different sampling depth ofAES/XPS/XRR enabled the determination of the hybridization profiles in the films. It was found that the films grown using C+(Ar+) exhibit a sp2 (sp3) surface, while the deeper monolayers are sp3 (mixed sp3-sp2) bonded. These results were comparedwith Monte-Carlo simulations of the ion-solid interactions, which have shown that the surface activation dominate at lower(higher) energies when Ar+ (C+) ions are used. Therefore, we found two mechanisms of sp3 formation: on the surface and inthe bulk of a-C. Finally, we developed a phenomenological model to describe the initial stages of a-C growth and to estimate thebalance of the surface and bulk mechanisms. Based on the latter model, we conclude that at medium energy (>100 eV) thesurface effects are negligible (for both C+ and Ar+), while the surface processes dominate when low-energy ( 40 eV) Ar+ ionsare used.
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COMPARISON OF THE CORROSION BEHAVIOuR OF ARCHEAOLOGICAL AND MODERN BRONZES N. Souissi (a), L. Bousselmi(b), L. Robbiola(c), S. Khosrof(d), E. Triki(a), (a)Unité de Recherche "Corrosion et Protection desMétalliques", ENIT, 1002 Tunis, Tunisia (b)Laboratoire Eau & Environnement, INRST, 2050 Hammam-lif, Tunisia,(c)Laboratoire de Métallurgie Structurale, ENSCP, Paris Cedex 05, France, (d)Laboratoire de Conservation-Restauration, InstitutNational du Patrimoine, 1060 Tunis, TunisiaThis investigation is devoted to the comparison of the corrosion behaviour of two alpha-bronze alloys. The growth of the patinalayers in a chloride corrosive medium simulating the burying soil was characterized by electrochemical stationary techniques,impedance spectroscopy, SEM and EDAXS. The electrochemical behaviour exhibited by both alloys is characterized after onehour of immersion by a tin-selective dissolution. The pick formation at 100 mV/SCE is due to the oxidation of copper and tinrespectively into Cu O and/or CuCl and Sn(OH) and/or Sn(OH) Cl . In the potential range above 200 mV/SCE a blocking layer is produced. EIS diagrams and microscopic observations confirm that a stationary state is reached after 168 hours of immersionby the surfaces of both electrodes.
ADSORPTION PROCESSES IN LAYER-BY-LAYER FILMS OF POLY(O-METHOXYANILINE): THE ROLE OF Nara C. de Souza(a) , J.R. Silva(a), C.A. Rodrigues(a), L. da F. Costa(a), J.A. Giacometti(b), (a)O.N. Oliveira Junior, Instituto de Fisica de Sao Carlos, Universidade de Sao Paulo CP 369, 13560-970 Sao Carlos SP, Brazil, (b)Faculdade de Ciencias eTecnologia, Universidade Estadual Paulista CP 467, 19060-900 Presidente Prudente SP, BrazilThe adsorption of polyanilines in electrostatically assembled layer-by-layer (LBL) films has been investigated, especially forunderstanding the interplay between ionic interactions and H-bonding. The ability of polaynilines to H-bond makes it possible toobtain multilayers with controlled thickness that may vary from molecularly thin to very thick layers, including non-self-limitedadsorption. A key factor in the adsorption process is polymer aggregation, in solution as well as in the deposited films.
Aggregation affects film thickness and roughness. We investigate the effect from the solution concentration and polymermolecular weight on aggregation in LBL films of poly(o-methoxyaniline) (POMA) alternated with poly(vinyl sulfonic acid). Filmsare adsorbed on glass substrates and characterized with UV-vis. spectroscopy and AFM microscopy. The formation ofaggregates is favored in more concentrated solutions, leading to an increase in the diameter of the domains. The size of POMAaggregates in solution is estimated to be larger than in LBL films. This unexpected result may be explained by a swelling effect ofaggregates in the aqueous POMA solutions, consistent with other reports in the literature. Upon adsorption on a solid substrate toform the LBL film, a molecular reorganization probably takes place, resulting in smaller aggregates. It is also found that the sizedistribution of the POMA domains in the LBL films is determined by the concentration of the solution.
SURFACE ADATOM DIFFUSION IN LASER NANOTECHNOLOGIES L. Pyziak(a), M. Bester(a), M. Kuzma(a) and I. Virt(b), (a)Institute of Physics, University of Rzeszow, Rejtana 16A, 35-959 Rzeszow, Poland, (b) Section of Experimental Physics, Pedagogical University, Franco 34, 293-720 Drogobych, UkraineThe atomic beam energy in Pulsed Laser Deposition method was characterized by wide range of kinetic energy (from few to1000 eV). This energy has fundamental influence on the diffusion of adatoms and simultaneously on the growth of nanocrystals,clusters or thin films. In the paper the kinetic of incident particles energy influence on its surface diffusion and clusters formingby computer simulation method (Monte Carlo) were investigated. We propose the directional diffusion model connected withnon-perpendicular substrate placement to direction of incident particles. In the paper the influence of adatom diffusion tomorphology of Si on Si films were presented.
NON-DESTRUCTIVE PROBING OF INTERFACIAL OXIDATION AND NITRIDATION STATES AT RTA Si-OXIDES D. Schmeißer (a), G. Roters(b), Z. Nenyei(b), (a)Angewandte Physik-Sensorik, BTU Cottbus, Postfach 101344, 03013 Cottbus, Germany, (b)Mattson Thermal Products GmbH, Daimlerstr. 10, 89160 Dornstadt, GermanyThe quality of the SiO /Si interface is of crucial importance in the development and performance of sub 0.lµm technologies. The knowledge of the chemical composition of the interface is an important information in the preparation of ultra-thin oxides andhigh- k dielectrics in order to maintain a high-quality interface and channel mobility. Here we report on investigations of 2nmoxide layers formed by a RTA process under various gas mixtures containing N O and NH at different concentrations. We studied the SiO /Si(001) interface by a non-destructive depth profiling using Synchrotron Radiation Photoelectron Spectroscopy.
We determine the thickness of the oxide layer, the relative content and chemical state of nitrogen within the oxide, the thicknessof the interface layer, and the relative abundance of Si intermediate oxides within. In all systems we find the thickness of theintermediate layer to be around 0,17nm. The intermediate layer contains Si atoms in the oxidation states +1, +2, and +3 withsimilar relative abundance. Nitrogen is found predominantely close to the intermediate layer forming Si-N as well as Si-N-Obonds. Within the oxide there are only spurious nitrogen content with predominately Si-N-O bonds. Our method is able to detect anitrogen content within the intermediate layer as low as of about 5%.
GROWTH OF ULTRA-THIN PbS FILMS DEPOSITED BY SILAR TECHNIQUE (a), S. Tamulevicius(a,c), S. Lindroos(b), M. Leskelä(b), (a)Department of Physics, Kaunas University of Technology, Studenti 50, 3031 Kaunas, Lithuania, (b)Department of Chemistry, University of Helsinki, P.O. Box 55, 00014 Helsinki, Finland,(c)Institute of Physical Electronics, Kaunas University of Technology, Savanorii 271, 3009 Kaunas, LithuaniaLead sulfide, PbS, has a cubic crystal structure and a narrow direct band gap, small effective mass and high electron mobility.
One of the perspective trends is application of PbS ultra-thin film on crystalline silicon wafers. The successive ionic layeradsorption and reaction (SILAR) technique involves growth of thin films from solution, ionic layer by ionic layer at roomtemperature and normal pressure. The aim of this work is to characterize the PbS thin films (15-100 nm) grown on crystallinesilicon substrates using different Pb2+ precursor solutions (Pb(CH COO , Pb(CH OO) complexed with triethanolamine, Pb(NO ) ,) and substrate treatment. The x-ray diffraction (XRD), x-ray photoelectron spectrometry (XPS), scanning electron microcopy (SEM) and atomic force microscopy (AFM) investigations have been performed to explain formation and morphologyof the PbS thin films in relation to the residual stress of the film - substrate system. Intensity of the main reflections in XRD spectrathe PbS ultrathin films was dependent on the precursors used. The stress development, crystallinity and crystallite size,morphology, roughness and composition of the films were characterized as a function of the film thickness.
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Friday, June 21, 2002 Vendredi 21 juin 2002 Session XI Moderator: G. Le Lay08:45 J-XI.1 (INVITED) SURFACE AND INTERFACE ELECTRONIC PROPERTIES AND 2DEG CONCENTRATION AT
AlGaN/GaN HETEROSTRUCTURES
Angela Rizzi(a,b), Martin Kocan(a) and Hans Lüth(a), (a)Institut für Schichten und Grenzflächen (ISG1), Forschungszentrum
Jülich, 52425 Jülich, Germany, (b)INFM-Dip. di Fisica, Università di Modena e Reggio Emilia, 41100 Modena, Italy
In wurtzite GaN based heterostructures the presence of a spontaneous and piezoelectric polarization, the latter in the presence of
strain, gives rise to interface charges Q up into the range of several 1013 e cm-2. In combination with charge residing in surface
and interface states a detailed knowledge of charge balance at the interfaces is required to understand Schottky-barriers, bandoffsets, the formation of a 2-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures without modulation doping andfinally achieve reliable device simulation schemes in nitride heterostructures. In this work, first the in-situ determination by X-rayphotoemission spectroscopy of the surface potential at GaN(0001) surfaces is discussed in dependence on the Ga/N ratio in themolecular beam epitaxy (MBE) process. Different surface morphologies cause different surface Fermi level pinning positions.
However, after exposure to air the measured surface potential is the same, within the experimental error. The surface potentialhas also been measured on MBE Al Ga N layers and introduced as a surface boundary condition in the self consistent Schrödinger-Poisson calculation of the band scheme along an AlGaN/GaN heterostructure. It is shown that the Fermi levelposition has significant impact on the formation and concentration of the 2DEG at the AlGaN/GaN heterointerface. Finally, thestudy of the Schottky-barrier height, in particular its dependence on the polarity of the GaN, provides a further example of newaspects for the electronic properties at wurtzite nitride heterojunctions, due to the presence of polarization charges.
J-XI.2 TITANIUM SILICIDE FORMATION AND DIFFUSION BEHAVIOR ON SI(111)-7X7 SURFACE BY SCANNING
TUNNELING MICROSCOPY
H.F. Hsu(a),
(b), M.C. Lu(a) and L.J. Chen(a), (a)Department of Materials science and Engineering, National Tsing Hua University, Hsinchu 300, (b)Department of Physics, Tunghai University, Taichung 407, Taiwan R.O.C.
Scanning tunneling microscopy has been used to study the kinetics of formation and diffusion of very low coverage ( 0.04ML) Tion Si(111)-7x7 surface at elevated temperature. Preferential adsorption of Ti atoms on the Si center adatom sites and, to a lessextent, on the Si corner adatom sites was found while Ti deposition on Si(111)-7x7 surface at room temperature. With increasethe annealing temperature to 500°C for 30 sec, the Ti atoms coalesce and form three types clusters in the Si(111)-7x7 half unitcells. They are identified as titanium clusters (type A), transition-phase clusters (type B) and silicide clusters (type C). Furtherincreasing the annealing time to 5 minutes, the type A and type B clusters are almost transformed to C type clusters. Whileextending the anneal time, these clusters diffuse to the dimmer rows and the cluster size become larger. At 600°C, the STMimages reveal that the silicide islands formed, and the silicon atoms around the islands diffuse out to form the silicide islands.
These observations indicate that type C clusters are a meta-stable silicide phase on the center of the triangular subunit cells andmore stable to stay nearby the dimmer rows. The dynamical silicide structure transformation is leaded by the Si out-diffusion fromthe subsurface and the restructuring of the interface structures.
J-XI.3 Ge SUBMONOLAYER INCLUSIONS IN A SILICON MATRIX
G.E. Cirlin
(a), N.D. Zakharov(a), V.A. Egorov(a), P. Werner(a), U. Goesele(a), A.G. Makarov(b), D.V. Denisov(c), V.M.
Ustinov(c) and N.N. Ledentsov(c), (a)Max-Planck-Institute for Microstructure Physics, Halle, Germany, (b)Institute forAnalytical Instrumentation RAS, St.Petersburg, Russia, (c)A.F.Ioffe Physico-Technical Institute RAS, St.Petersburg, RussiaSilicon is a main semiconductor material using in microelectronics but it is still less applicable in optoelectronics due to indirectband-gap nature. To increase luminescence efficiency we proposed to insert Ge submonolayers (SML) in a Si matrix which mayresult in the formation of very small islands (smaller than the hole Bohr radii) and effective radiative recombination. All thesamples are MBE grown using RIBER SIVA MBE setup equipped with electron beam evaporators for Si and Ge fluxes. Toincrease luminescence efficiency, multilayer (20-100 sheets of SML Ge) structures are grown. Photoluminescence (PL) andTransmission Electron Microscopy (TEM) are used for samples characterisation. We have found a red shift of the PL band withincreasing of the Ge amount in a SML portions. Optimal growth temperatures are found to be in a range 620°C – 700°C. For thisgrowth conditions, pronounced Ge 3D islands are formed for Ge coverages less than 1 ML. However, for the 1 ML Ge amount ina SML portion PL intensity drastically decreases in comparison with SML regimes. High resolution TEM shows a very smooth(within 1-2 ML) of Ge atoms distribution for the later case. Sb doping of Si spacer layers significantly (up to 2 orders in anintegral intensity) increases PL intensity. Possible mechanisms responsible for the formation of Ge 3D islands in a SML regimeare discussed.
J-XI.4 INFLUENCE OF HYDROGEN-SURFACTANT COVERAGE ON Ge/Si(001) HETEROEPITAXY
M. Katayama
, T. Fujino, Y. Yamazaki, S. Inoue, T. Okuno, M. Shindo, R. Tsushima and K. Oura, Department of Electronic Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, JapanThe growth of Ge on Si(001) occurs by a typical Stranski-Krastanov mode. The successful usage of dynamically supplied atomichydrogen (H) as a surfactant has been reported for Ge/Si heteroepitaxy. With a high flux of H, the island formation is suppressedand layer-by-layer growth is achieved [1]. However, it is still unclear how the H atoms on the growth front influences localgrowth processes. In this study, the growth of Ge on Si(001) at 300°C in the presence of various thermal-equilibrium coverages ofH-surfactant has been monitored in real time using coaxial impact-collision ion scattering spectroscopy (CAICISS) and time-of-flight elastic recoil detection analysis (TOF-ERDA) feasible in gas phase atmosphere [2], a technique capable of surfacestructure and composition analysis [3,4]. It has been revealed that (1) submonolayer of H atoms (about 0.6 monolayer) mosteffectively act as surfactants, (2) beyond the submonolayer of critical H coverage, the roughening of Ge films occurs eventhough a monohydride phase is maintained on the growth front. Thermal stability of the flat Ge films grown with H-surfactant hasalso been investigated. On this basis, the mechanism of the H-surfactant effect will be discussed.
[1] A. Sakai and T. Tatsumi: Appl. Phys. Lett. 64, 52 (1994).
[2] M. Katayama et al.: Jpn. J. Appl. Phys. 40, L576 (2001).
[3] M. Katayama et al.: Phys. Rev. B 54, 8600 (1996).
[4] T. Fujino et al.: Appl. Phys. Lett. 79, 2716 (2001).
J-XI.5 ELECTROCHEMICAL DEPOSITION OF Au ON Si(111):H
S. Warren
(a), F. Maroun(b), T.-L. Lee(a), A. Reitzle(c), A. Kazimirov(d), J.C. Ziegler(c), O. Bunk(e), L.X. Cao(a), F.U.
Renner(a), D.M. Kolb(c), M.J. Bedzyk(f), P. Allongue(b), J. Zegenhagen(a), (a)ESRF, BP-220, 38043 Grenoble, France,(b)CNRS UPR 15, Physique des Liquides et Electrochimie, Université Pierre et Marie Curie, Tour 22, 4 Place Jussieu, 75005Paris, France, (c)Abteilung Elektrochemie, Universität Ulm, 89069 Ulm, Germany, (d)CHESS, Wilson Laboratory, CornellUniversity, Ithaca NY 14853, USA, (e)II. Institut für Experimentalphysik, Universität Hamburg, 22761 Hamburg, Germany,(f)Department of Materials Science and Engineering and Materials Research Center, Northwestern University, Evanston IL60208, USAElectrochemical deposition represents an appealing preparation method, with the possibility of selecting deposition sites andgrowth modes by changing the potential and electrolyte solution. Metal growth on semiconductors may occur as clusters, films, oreven nanostructures. A comparison is made between the structure and growth mechanism of two electrochemical preparationmethods for Au on Si(111) surfaces, using a variety of structure sensitive techniques.Electroless deposition of gold in monolayeramounts on hydrogen terminated Si(111) has been studied. AFM and Surface XRD studies indicate cluster formation, with theAu(111) face epitaxially aligned to the substrate. However, X-ray standing wave (XSW) experiments, at coverages below one SYMPOSIUM J
monolayer, suggest an ordered gold layer. Both clusters and isolated atoms are present at all coverages, which is explained by theformation of a 'wetting layer', in addition to cluster formation, which increases as the coverage increases. Deposition of Au onmiscut Si(111):H surfaces (using a different electrolyte) has also been studied. In this case, potential control can be used toselectively deposit gold clusters at the step edges, raising the possibility of nanowire formation. SXRD has been used to determinethe epitaxy and structure of the Au, and the effect of the steps on the Au island morphology. Initial XSW measurements suggestno ordered wetting layer is formed, in contrast to the electroless Au deposition studied previously.
J-XI.6 MICROSCOPIC STUDIES OF LIQUID GOLD-SILICON DROPLETS ON SILICON
Barbara Ressel(a) , Stefan Heun(a), Kevin C. Prince(a) and Yoshikazu Homma(b), (a)Sincrotrone Trieste, Basovizza Trieste
34012, Italy, (b)NTT Basic Research Laboratories, 3-1, Morinosato Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan
Gold is used to solder contacts to Si chips because it forms a low temperature eutectic that wets the substrate; this is a main
requirement for good soldered joints. In previous work we have used photoemission electron microscopy (PEEM) to determine
the stoichiometry of liquid Au-Si eutectic droplets. We found that the liquid has a surface composition that is enriched in Si with
respect to the bulk [1]. However, no clear information on the wetting angle of the droplets was obtained. Here we will present
investigations on the morphology of this system using ultra high vacuum scanning electron microscopy. As temperature is
increased, the wetting behaviour of liquid gold-silicon droplets on the (111) and (100) surfaces of silicon changes in different
ways. Although the shapes of the droplets are consistent with published models of etching, they are also consistent with an effect
due to anisotropic surface energy. We will illustrate the application of the droplets to obtain ordered surface nanostructures.
[1] B. Ressel, S. Heun, Th. Schmidt, and K. C. Prince: Defect and Diffusion Forum 183/185 (2000), p. 181-188.
Session XII Moderator: C. Mariani11:00 J-XII.1 (INVITED) NANOPARTICLES ON OXIDE SURFACES
Marcus Bäumer, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
Nanomaterials attract more and more attention in fundamental and applied research because of their outstanding physical and
chemical properties. One area where supported nanoparticles are technically used is heterogeneous catalysis. Due to the
complex structure of such systems, however, systematic studies regarding structure-performance relationships are difficult to
carry out. Therefore, model systems have been utilized instead which allow to study the interplay between structure and
adsorption/reaction properties at the atomic level. Starting with simple systems, it is important of course to increase the complexity
of the models step-by-step, i.e. to bridge the so-called materials gap. In this contribution, examples will be presented which show
how this can be achieved in the case of oxide-supported model catalysts. As supports, thin oxide films on metallic substrates are
used so that all surface science techniques relying on good thermal and electrical conductivity can be applied. Particles are
prepared by metal vapour deposition. By taking advantage of kinetically controlled nucleation and growth processes, a wide
spectrum of sizes and morphologies can be obtained. The systems which will be considered include Pd and bimetallic Pd/Co
particles on alumina and silica supports. After having discussed structural aspects, results regarding the adsorption and reaction of
simple molecules such as CO, hydrogen and ethene will be presented.
J-XII.2 THERMAL STABILITY OF ULTRATHIN METAL FILMS ON SrTiO (100)
and Thomas Wagner, Max-Planck-Institut für Metallforschung, Seestrasse 92, 70174 Stuttgart, Germany Interfacial reaction between metals and oxides is of fundamental interest for many technological applications. The interfaceformation depends strongly on thermodynamics and kinetics, and can thoroughly be studied by varying the deposition parameters.
To elucidate the effects of metal reactivity on the onset of interfacial reactions, different metal films (Mo, Cr, Al, Cu) weregrown on single crystalline SrTiO by molecular beam epitaxy (MBE). The goal of the work is to study the very initial stage of overlayer formation and thermal stability. The films were annealed in UHV and the chemical and structural modifications of themetal films were examined in-situ by different surface science techniques (e.g. XPS, STM, and RHEED). It could be shown thatthe transport parameters of the oxide and the reactivity of the metal overlayer are crucial for the reaction processes. Generaltrends will be given which allow the prediction of the interfacial reactions between metal films and oxides as a function ofprocessing and materials parameters.
J-XII.3 SELF-ASSEMBLED Fe3O4(111) NANOSTRUCTURES IN ULTRATHIN FeO(111) FILMS ON Ru(0001)
Guido Ketteler and Wolfgang Ranke, Fritz-Haber-Institute of the MPG, Faradayweg 4-6, 14195 Berlin, GermanyUltrathin films of FeO(111) can be grown on Ru(0001) by repeated cycles of evaporation of Fe and subsequent oxidation attemperatures around 870K. At equilibrium conditions, 1-2 ML FeO(111) wet the substrate before further growth proceeds by theformation of Fe O (111) islands (Stranski-Krastanov growth mode). However, if larger amounts of Fe are deposited in one turn on the substrate and oxidized afterwards, metastable FeO(111) films with a thickness up to 4 ML can be obtained. They havestrongly expanded lattice constants and form specific coincidence structures with the Ru(0001) substrate. In films with a thicknessof 4ML, self-assembled, periodically arranged Fe O (111) nanodomains with diameters of 2-3nm form in the FeO(111) film.
Further oxidation causes these domains to grow and finally coalesce into a closed Fe O (111) film. Self-organization and phase transition are discussed using thermodynamic and electrostatic arguments.
J-XII.4 NOVEL OXIDE NANOSTRUCTURES OF VANADIUM OXIDE ON Rh(111)
H. Schoiswohl,
, M.G. Ramsey and F.P. Netzer, Institut für Experimentalphysik, Karl-Franzens Universität Graz, 8010 Graz, AustriaUltrathin vanadium oxide films (0-5 ML) have been prepared by reactive evaporation of V metal in an oxygen pressure of 2x10-7mbar onto a clean Rh(111) surface held at 250°C. The growth and the structure of many different oxide phases are observed bySTM and LEED in the as-evaporated state and after annealing in UHV. Two different oxide phases coexist below 0.5 ML, whichexhibit (√13x√13)R13° and (√x√7)R19° surface structures. At around 1 ML a second oxide layer grows in the form of islands withan internal hexagonal tile structure. The latter represent the nucleation centres for the bulk-type V O corundum phase, which is fully developed above an oxide coverage of 3 ML. Annealing the submonolayer oxide films to 350°C results in the formation ofwell ordered islands with a (5x3√3)rect structure. Isolated star-like features with a lateral size of 15 Å are in addition observedon the bare Rh(111) terraces, which represent the building units of the (5x3√3)rect structure. For 1 MLE annealing to 350°Ccauses the appearance of a Moiré pattern in both STM and LEED, which replaces the hexagonally shaped islands. Interestingly,the observed growth behaviour and structure of the V-oxide films on Rh(111) is distinctly different from that of vanadium oxideson Pd(111) [1], and possible reasons are examined.
[1] S. Surnev et al., Surf. Sci. 495 (2001) 91.
*Supported by the Austrian Science Foundation J-XII.5 STUDY OF THE Al O /Ni Al(111) SURFACE BY X-RAY SURFACE DIFFRACTION AND LEED
(a,b),F. Wendler(c), X. Torrelles(d), W. Moritz(c) and G.R. Castro(a,b), (a)SpLine, Spanish CRG Beamline at the ESRF, BP 220, 38043 Grenoble, France, (b)Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, 28049 Madrid,Spain, (c)Institut für Kristallographie und Angew. Mineralogie, Ludwig-Maximilians Univ. München, Theresienstr. 41, 80333München, Germany, (d)Institut de Ciència de Materials de Barcelona (CSIC), Campus de la U.A.B., 08193 Bellaterra, SpainWell ordered ultra-thin films of Al O can be obtained from the oxidation of binary alloys that have aluminium as one of its constituents (NiAl, Ni Al, CoAl…). Despite of the different substrates, these films seem to have similar characteristics. Although many studies of these systems have been performed up to now, a complete structural model is still missing. We report here a studyof Al O /Ni Al(111) surface by LEED, and Surface X-ray Diffraction (SXRD). Ni Al is an ordered alloy with the bulk structure of Cu Au and a unit cell parameter of a=3.56 Å. The ideal Ni Al(111) face has a stoichiometric ratio of 3Ni:1Al. The CTRs reveal that the clean Ni Al(111) surface is flat (0.01±0.01 Å rippling of the outmost surface layer) and that chemical disorder exists in the 4-6 outmost layers. Segregation of Al is also possible. After oxidation at high temperature, a six-fold superstructure is SYMPOSIUM J
observed. A possible interpretation is a structure with a „parent" hexagonal unit mesh of r=3.01 Å, rotated 36.86° with respect tothe substrate and six-fold satellites around the main spots. The satellites reveal that the oxide film has a periodic distortion ormodulation. The thickness of the oxide film is 5.37±0.02 Å, as deduced from reflectivity measurements. A very complex atomicstructural model showing partial order is proposed. This model is supported by surface direct methods.
J-XII.6 INTERACTION OF GOLD PARTICLES AND CO WITH TiO2 ANATASE (101) BY DENSITY FUNCTIONAL
CALCULATIONS
A. Vittadini(a) and A. Selloni(b), (a)ISTM-CNR, via Marzolo 1, 35131 Padova PD, Italy, (b)Department of Chemistry, Princeton
University, Princeton NJ 08540, USA
Gold nanoclusters supported on titania are able to catalyze a number of reactions, such as CO oxidation. However, especially for
the anatase modification, it is not clear what is the role played by the substrate and by its defects, because both the surface
structure and the size of the supported clusters are often poorly defined. In the attempt to understand the basic features of the
gold-anatase system and its reactivity, we have performed density functional calculations within a slab geometry. We have
studied the interaction of Au, Au and Au with the stoichiometric and the partially reduced TiO anatase (101) surface. On the
stoichiometric surface, gold particles prefer anion sites and "standing" (non-wetting) geometries, in agreement with simple MOtheory arguments. On the defected surface, cation sites close to the vacancies are largely favored for Au atom adsorption. Forboth Au and Au , "lying" (wetting) adsorption geometries become more likely, and for Au there is a tendency towards mixed binding, where cation and anion sites are simultaneously involved. Clustering of Au atoms is favored on both the stoichiometricand the reduced surfaces. CO is found to interact strongly with gold particles adsorbed on the stoichiometric surface. As found inprevious gas-phase investigations, the CO-cluster bond is strengthened when the cluster carries a positive charge. Accordingly, aweak interaction is computed for a gold atom supported on the reduced surface.
Friday, June 21, 2002 Vendredi 21 juin 2002 Session XIII Moderator: N. N.
14:00 J-XIII.1 (INVITED) FRICTION AND WEAR ON THE NANOMETER SCALE
E. Meyer, R. Bennewitz, E. Gnecco, A. Socoliuc, V. Barwich, O. Pfeiffer, S. Schaer Institute of Physics, Klingelbergstr. 82, 4056
Basel, Switzerland
Friction experiments are performed under ultrahigh vacuum conditions. Velocity dependent atomic-scale stick slip experiments
show a logarithmic increase. The results are interpreted in terms of a Tomlinson model with thermal activation. Friction
experiments are done on different surfaces of metals, where regular stick-slip is found on close-packed (111)-surfaces. Atomic-
scale wear measurements are performed on ionic surfaces, where the structure of the grooves and the debris are investigated
with high resolution. Dissipation mechanism are investigated in the near-contact regime. Cantilever-type springs are oscillated in
the normal mode and the torsional mode. A quantitative analysis of the frequency shift curves is presented. Several models to
describe these dissipation measurements are discussed.
J-XIII.2 NON-CONTACT MEASUREMENT OF CONDUCTIVITY DURING GROWTH OF ULTRATHIN FILMS
G. Fahsold
, S. Diez, M. Lust, A. Priebe, M. Sinther and A. Pucci Kirchhoff-Institut für Physik, Universität Heidelberg, Germany Electron transport properties reflect both the size-dependent electronic structure of metal ultrathin films and their morphology.
We investigate these transport properties by means of IR-transmittance spectroscopy performed during growth of metal (Fe, Cu)films in UHV on insulators (MgO(001) and KBr(001)) and on semiconductors (Si(111)7x7). Careful analysis of the IR-spectraallows us to detect the beginning of island-island interaction as well as the percolation transition of a metal film and itsdependence on various growth parameters. Furthermore, we can quantify the charge transfer and the relaxation of electrons dueto various adsorbates at the film surface.
J-XIII.3 A NOVEL APPROACH FOR THE INVESTIGATION OF MESOSCOPIC CONTACT MECHANICS
R. Buzio, C. Boragno and
, INFM-Dipartimento di Fisica, Universita' degli Studi di Genova, Via Dodecaneso 33, 16146 Genova, ItalyWe present a novel experimental technique devoted to the investigation of contact mechanics on mesoscopic scale. It consists ofan Atomic Force Microscope (AFM) equipped with custom-designed probes with integrated flat micrometric tips. Samples arenormally compressed by the flat tips and load-displacement curves are acquired. The latter allow to investigate the mechanicalresponse under a multi-asperity regime not accessible by conventional AFM, in particular material strength can be directlyrelated to bulk properties and surface morphology. Preliminary results are reported for the contact mechanics of nanostructuredcarbon-based films films having a self-affine fractal morphology. Our measurements demonstrate a marked dependence of themesoscopic surface stiffness on fractal parameters.
J-XIII.4 INTERFACE COMPOSITION AND ELECTRONIC PROPERTIES OF CHROMIUM OXIDE JUNCTIONS
N. Pilet, C. Borca, A. Sokolov, E. Ovtchenkov, P.A. Dowben and
, University of Nebraska Lincoln, Physics and Astronomy, Lincoln NE 68588-0111, USAChromium oxide CrO is an important candidate material for 100% spin polarization. Our goal is to use chromium films as starting materials for making chromium oxide junctions, following the indications that electric properties of oxidized chromium junctionsare similar to CrO /Cr O composite materials. We show that properly oxidized chromium films show a mixture of CrO and Cr O at their surface, and investigate the oxidation conditions that allow Cr O to cover the surface (i.e. occupy the outermost surface layers), so that the Cr O surface layer can act as an insulating barrier. We also study trilayer structures made of chromium-oxide-superconductor and chromium-oxide-ferromagnet. Angle-resolved XPS is used to characterize the buriedinterface between a top Pb layer and the oxidized chromium. Oxidation and reduction reactions are identified at the Pb/Cr 0 J-XIII.5 NANOSCALE MONITORING OF ORGANIC THIN FILM FORMATION ON INSULATORS AND
SEMICONDUCTORS BY INFRARED SPECTROSCOPY
J.A. Mielczarski and E. Mielczarski, LEM, U.M.R. 7569 du C.N.R.S.-I.N.P.L., B.P.40, 54501 Vandoeuvre-lès-Nancy Cedex,
France
Infrared external reflection technique with very unique properties has been developed to characterize organic thin film
formation at solid substrates. This technique supported by computer simulation of surface composition and structure allows
obtaining almost all the information about the adsorbed layer including: (i) the nature of the adsorbed product(s), (ii) the adsorbed
quantity, (iii) the surface distribution (uniform layer or patches with determined thickness), (iv) molecular orientation of the
adsorbed species, (v) lateral interaction between adsorbed molecules, (vi) various kinetic phenomena of the surface processes
such as kinetics of adsorption/desorption, stability of the surface products and structures, mobility of adsorbed molecules on solid
surface. The variety, precision and reliability of information about interface phenomena delivered by this technique is
incomparable to other single technique. The experiments are fast and non-destructive. High sensitivity (starting from 20% of
monolayer), collected information in a multiphase system even in the region of a strong absorption of substrate makes this
technique a very valuable experimental tool. The complexity of recorded reflection spectra, their sensitivity to any variations of
the optical properties of all investigated phases in the system are in fact the major strength of the technique. The examples of
investigation of the formation of self-assembled monolayers will be discussed in this presentation.
SYMPOSIUM J
J-XIII.6 AN X-RAY AND He-SCATTERING STUDY OF THE FORMATION OF ULTRATHIN PENTACENE FILMS ON
OXIDE AND METAL SURFACES
B. Nickel , L. Casalis*, M.F. Danisman and G. Scoles, Princeton University, Department of Chemistry, Princeton NJ 08544, USA,
S. Iannotta, CeFSA Centro CNR-ITC per la Fisica delgi Stati Aggregati, 38050 Povo di Trento, Italy, (*)permanent address:
Sincrotrone Trieste, Trieste, Italy
Pentacene (C H ), a long, flat organic molecule, is the focus of much current research activities due to its versatile electronic
properties which allow for its use in electronic devices. The transport properties of pentacene single crystals are excellent anddepend strongly on the crystallographic direction since the molecules pack in a way which favors electronic transport onlyperpendicularly to their long axis. As a consequence, there is currently great interest in controlling the growth mode of thin films,i.e. to grow phases with few defects in which the axis of the molecule may also have a controlled direction with respect to thesurface normal. We have evaporated ultrathin films (1-10 monolayers) of Pentacene on silicon oxide and metal surfaces for abroad range of deposition conditions. The substrate temperature was varied between 200 and 500 K. The energy of the depositedmolecules was about 1eV (using a supersonic source) and about 50 meV (using a Knudsen cell). Using X-ray and Heliumscattering techniques, we observe a standing phase for deposition on SiO , but a lying down phase for the deposition on Ag(111).
The microscopic mechanisms, which control the various structures observed, will be discussed.
J-XIII.7 SUBSTRATE CONTROLLED CRYSTALLISATION OF ULTRA-THIN FILMS OF ORGANIC SEMICONDUCTORS
AND THEIR OPTICAL PROPERTIES
Q. Chen(a) , A.J. McDowall(a), T. Rada(a), S. Francis(a), N.V. Richardson(a), P.J. Unwin(b) and T.S. Jones(b), (a)University of
St Andrews, UK, (b)Imperial College of Science, Technology and Medicine, UK
There has been recent interest in the electronic and optoelectronic properties of polyaromatic hydrocarbons, such as tetracene,
pentacene, perylene and their derivatives, because of their potential use as organic lasers, OLEDs and OFETs. Device
performance is likely to be strongly influenced by the structural quality of crystals or thin films. We have carried out a detailed
investigation of the deposition, growth and evolution of ultra-thin films (0-18 monolayers) of these organic semiconductors on a
variety of substrates including Cu{110}, H/Si(111), InAs(111) and InSb(111), using STM, TPD, LEED and vibrational
spectroscopies. The substrate has a profound influence on growth and subsequent crystallinity of the film. For example, films
grown on a hydrogen terminated Si(111) surface show no evidence of layer-by-layer growth and, from the earliest stages, small
clusters of randomly oriented molecules are present which eventually cover the surface with a rough polycrystalline film
(perylene and tetracene) or large size bulk crystal (PTCDA). Deposition on a Cu{110} surface leads to large crystalline domains
upto one monolayer coverage. Multilayer growth of perylene takes place epitaxially layer-by-layer on this first monolayer into a
3D crystallined structure which is unrelated to the bulk crystal structure of perylene. With our newly developed in-situ
photoluminescence measurement, we are able to correlate the optical properties with the structure of well defined thin film.
Session XIV Moderator: K. C. Prince J-XIV.1 (INVITED) GROWTH DYNAMICS OF PENTACENE THIN FILMS
F.-J. Meyer zu Heringdorf(a), M.C. Reuter, R.M. Tromp, IBM T.J. Watson Research Center, Yorktown Heights NY 10598,
USA (a) present address: University of Essen, Germany
The growth of pentacene thin films is of major importance for the development of organic thin film transistor displays. In
particular, the pentacene transistor mobility depends significantly on the crystalline quality of the organic material. Here we
present an in-situ study of pentacene MBE growth on various Si surfaces. On clean Si, up to 50 µm large pentacene crystallitesare formed. The dendritic shape of these islands will be analyzed and simulated in the framework of diffusion limited aggregation(DLA). Growth of pentacene on Si oxide layers results in smaller crystal sizes, due to the large number of nucleation sitesprovided by the amorphous substrate. The growth of pentacene layers can be modified, and to some degree controlled, by pre-adsorption of simple organic monolayers.
J-XIV.2 AUTOCATALYTIC REACTION MODEL: A PHENOMENOLOGY FOR NUCLEATION-COALESCENCE-
GROWTH OF THIN FILMS
Maki Suemitsu
, RIEC, Tohoku University, Sendai, Japan In many thin film growth systems, the surface geometry develops via nucleation of 2D clusters, their growth, and coalescence.
When desorption of surface adsorbates is thermally activated, the growth dynamics becomes a delicate function of both thesubstrate temperature T and the impinging flux P of the precursors. The first objective of this paper is to show that the dryoxidation of Si(100)-2x1 provides one of the most ideal systems for studying such a T, P-dependent growth dynamics of thin films.
A series of real-time ultraviolet photoelectron-spectroscopy measurements showed that the oxide growth in the low-T-high-Pregime follows a Langmuir-type time evolution, characteristic of random adsorption, while the one in the high-T-low-P regimefollows a sigmoid-function-type time evolution, characteristic of nucleation-growth-coalescence of adsorbate clusters. Thesecond objective of this paper is to demonstrate that these two growth modes as well as the transition between them can beunifiedly and quantitatively described by a single autocatalytic-reaction model. Nonlinear as it is, this rate equation has ananalytical solution, which almost completely reproduces the time evolutions for both of the regimes. The two fitting parametersenables us to discuss such important kinetic parameters as the number of migrant species required in nucleation, the clusterdensity as a function of the coverage, and the activation energies for surface processes.
J-XIV.3 MICROSCOPIC MECHANISMS OF THERMAL AND DRIVEN DIFFUSION OF NON RIGID MOLECULES ON
SURFACES
C. Fusco
and A. Fasolino, Inst. Theoretical Physics, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The NetherlandsThe motion of molecules on solid surfaces is of interest for technological applications such as catalysis, lubrication and coating,but it is also a theoretical challenge at a more fundamental level. The concept of activation barriers is very convenient for theinterpretation of experiments and as input for Monte Carlo simulations but may become inadequate when mismatch with thesubstrate and molecular vibrations are considered. We study the simplest objects diffusing on a substrate at finite temperature T,namely an atom and a diatomic molecule (dimer), using the Langevin equation approach. In the driven case, we analyse the characteristic curves, comparing the motion for different values of the intramolecular spacing, both for T=0 and T≠0. Themobility of the dimer is higher than that of the monomer when the drift velocity is less than the natural stretching frequency. Therole of intramolecular excitations is crucial in this respect. In the undriven case, the diffusive dynamics is considered as afunction of temperature. Contrary to atomic diffusion, for the dimer it is not possible to define a single, temperature independent,activation barrier. This reveals a complex behaviour determined by the interplay between vibrations and a temperaturedependent intramolecular equilibrium length.
J-XIV.4 STRUCTURE AND MORPHOLOGY OF UNLTRATHIN NiO LAYERS ON Ag(001)
C. Giovanardi (a,b), A. di Bona(a), S. Altieri(a), P. Luches(a,b), M. Liberati(b), F. Rossi(b) and S. Valeri(a,b), (a)INFM, Unita di
Modena, via Campi 213/a, 41100 Modena, Italy, (b)Dipartimento di Fisica, Universita di Modena e Reggio Emilia, via Campi
213/a, 41100 Modena, Italy
The exciting catalytic and magnetic properties and the large variety of applications of NiO films supported a number of studies on
the epitaxial growth of ultrathin oxide films on metal substrates, oriented to the preparation of "perfect" layers. We report on a
structural/morphological study of ultrathin NiO layers in the thickness range from submonolayer to monolayer(s), prepared by
deposition of Ni in an O atmosphere onto Ag(001) substrate. The structure has been investigated by exploiting the scattering
SYMPOSIUM J
interference of either the incoming (primary-beam diffraction modulated electron emission, PDMEE) and outgoing (secondaryelectron imaging, SEI) energetic electrons. LEED spectroscopy has also been used to complement structural informations. TheNiO film morphology has been investigated by STM and AFM. It has been found that the low-deposition (= 1 monolayer) 2x1oxide phase deviates from the 1:1 NiO stoichiometry, being largely oxygen deficient. Stoichiometric and epitaxial NiO islandsform for a deposition larger than about 2 monolayers. The distance between the Ag surface and the first NiO plane is expandedwith respect of both Ag and NiO (001) interplanar distances. A complete (100% fractional coverage) is obtained for a depositionas large as about 8 monolayers.
J-XIV.5 THICKNESS DEPENDENCE OF PHOTOEMISSION AND X-RAY FLUORESCENCE SPECTRA IN EPITAXIAL NiO
LAYERS ON Ag(100)
S.A. Krasnikov
, A.B. Preobrajenski, T. Chassé and R. Szargan, Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstr. 2, 04103, Leipzig, GermanyInvestigations of NiO films with different thicknesses (0.8 ML - 12 ML) on Ag(100) substrate were carried out by low-energyelectron diffraction (LEED), X-ray photoemission (XPS), X-ray photoelectron diffraction (XPD), Angle-resolved ultravioletphotoemission (ARUPS) and X-ray emission spectroscopy (XES). All samples were prepared by thermal evaporation of metallicnickel onto cleaned Ag(100) substrates at 350 °C in an oxygen atmosphere with 8.0 x 10-6 mbar pressure. The photoemission (VGESCALAB 220i-XL) and XES (X-ray monochromator XES 300 using synchrotron radiation at the U41-PGM beam-line atBESSY II) in situ measurements on epitaxially grown NiO(100) films demonstrate the sensitivity of the electronic transitions to thegeometry of the first and the second coordination spheres. The relative intensities of particular structures in the Ni 2pphotoemission spectra are influenced by a competition between transitions of screening electrons coming from the surroundingligands and electrons coming from ligands around a neighboring transition metal ion. The results are consistent with theoreticalstudies of nonlocal screening effects explaining the shape of the main line in the Ni 2p spectra. Information about local partialdensities of states was obtained from a comparative analysis of NiL and OK X-ray fluorescence and VB photoelectron spectra.
J-XIV.6 ORIGINAL GROWTH MODE OF NiO ON A Cu(111) SINGLE CRYSTAL SUBSTRATE
S. Stanescu
(a), A. Barbier(b), J.P. Deville(a), J. Hommet(a), R. Lazzari(b) and Ch. Boeglin(a), (a)IPCMS - GSI, Strasbourg, France, (b)CEA/DRFMC, Grenoble, FranceThe study of the fundamental principles underlying spin-valve based systems experiences a tremendous expansion because oftheir properties and high application potential at an industrial level: MRAMs, GMR read heads, catalysis, etc. We report here anew MBE technique for the direct deposition of antiferromagnetic NiO. The growth and morphology of as deposited NiO layerson a Cu(111) single crystal substrate was checked in-situ through a complementary set of UHV surface techniques: AES, XPS,STM, GIXD, GISAXS (BM32-SUV beam-line at ESRF). The morphology studied by STM shows that in the early stages ofgrowth, nickel oxide induces at the Cu surface the formation of characteristic holes, which exhibit a strong shape and sizemonodispersity, the long-range morphology and correlation is confirmed by X-ray diffusion (GISAXS). This very unusual growthmode is due to the large mobility of Cu atoms at room temperature, while the deposited oxide is very homogenous, without thepresence of particular nucleation sites. The evolution of the oxide electronic structure, as seen by XPS, shows a transition from anon-stoichiometric regime (1 - 3 nm) to the characteristic electronic structure of bulk NiO above 3 nm. The deposited NiO has adramatic influence on the copper single crystal substrate with an oxide/metal interface, which behaves as a solid/oil interface.

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