Papers

AVS1997 Session SS3-ThA: Metal and Semiconductor Surface Structures

Thursday, October 23, 1997 2:00 PM in Room A1/2-A

Thursday Afternoon

Start	Invited?	Item
2:00 PM		SS3-ThA-1 Thermal Dependence of fcc(001) Surface Structure: Failure of the Quasiharmonic Model A.P. Baddorf, V. Jahns (Oak Ridge National Laboratory) Anomalously large thermal expansions observed for Ag(111), Cu(110), Pb(110) and other surfaces have prompted development of a series of theoretical models. The most successful to date, the quasiharmonic model, combines first principles calculations for T=0 K with Helmholtz free energy minimization of selected harmonic oscillators representing the surface phonon modes.¹ In this model, softening of (harmonic) in-plane vibrations as the out-of-plane lattice spacing increases contributes more to thermal expansion than does anharmonicity. Using x-ray diffraction, we have measured the surface structure and RMS vibrational amplitudes of Cu(001) and Ag(001) between 300 K and 1000 K (Cu) or 600 K (Ag). Both surfaces exhibit large in-plane vibrational amplitudes, but do not conform to the quasiharmonic model. In particular, the outermost layer of Cu(001) relaxes inward, instead of expanding, up to 800 K. Only at the highest temperatures is expansion observed. The contracting force can be explained by a dynamic application of Smoluchowski smoothing, which is not accounted for in earlier models. The magnitude of the dynamic contribution of smoothing can be obtained from a "universal" relationship between static corrugation and inward relaxation observed empirically on metal surfaces.² ¹S. Narasimhan and M. Scheffler, accepted in Z. Phys. Chem, 1997. ²ORNL is managed by Lockheed Martin Energy Research Corp. under U.S. Department of Energy contract DE-AC05-96OR22464.
2:20 PM		SS3-ThA-2 Nature of Adatom Bonds to Transition-Metal Surfaces: O/Pt P.J. Feibelman (Sandia National Laboratories) First-principles total-energy calculations provide an accurate account of the structural energetics of O on Pt(111) and on surfaces vicinal to it. The calculated geometry of the saturation p(2x2)-O/Pt(111) structure is in excellent agreement with the recent LEED analysis of Materer, et al., the theoretical O-induced stress is within 20% of Grossmann, et al.'s crystal-bending measurement, computed stretch vibration frequencies for p(2x2)-O/Pt(111) and for O in edge-bridge sites at A-type steps agree with loss-spectroscopy measurements to a few percent, and finally, Local Density Approximation (LDA) calculations account for the remarkably different appearances, in STM images, of O at A- and B- type steps on Pt(111). This extensive record of predictive success invites us to explore the LDA results for O/Pt in greater depth, as a way of learning how electronegative adatoms interact with a late transition metal. As an example, I show how charge density plots and calculated bond-lengths explain a surprising result of the LDA calculations, that O prefers fcc to hcp 3-fold hollows by several tenths of an eV, for instance, 0.5eV at saturation on Pt(111). I show that this preference is a consequence of d-electron depletion between first and second neighbor Pt atoms in the fcc geometry that does not occur at an hcp site. The source of the effect is the electrostatic requirement that each nearest-neighbor Pt atom's d-hole lie along the O-Pt bond. Sandia National Laboratories is operated by the Sandia Corporation, a Lockheed-Martin Company, for the United States Department of Energy. This work was supported by the U. S. DOE under Contract DE-AC04-94AL85000.
2:40 PM		SS3-ThA-3 A Low Temperature Scanning Tunneling Microscopy Study of Benzene and Carbon Monoxide Coadsorbed on Pd(111) C.A. Pearson, G.W. Anderson, S. Chiang (University of California, Davis) We have imaged the surface structure of benzene and carbon monoxide (CO) coadsorbed on a Pd(111) surface in ultra high vacuum using low temperature scanning tunneling microscopy (STM). For saturated coverage we identify distinct ordered overlayers as a function of CO coverage using both STM and low energy electron diffraction (LEED). STM imaging is performed at both 90 K and room temperature. At high CO coverage we observe the previously studied hexagonal (3x3) ordered structure¹. By reducing the CO coverage the formation of a hexagonal (2√3x2√3)R30° structure is observed. Upon further reduction of CO coverage a three domain rectangular ordered structure is observed. Internal structure of the benzene molecules is evident in the STM images. Domain boundaries are clearly visible on single structure overlayers. Overlayers with two or more structures coexisting are also observed by STM. Observation and identification of ordered overlayers by STM is a necessary step to advance the understanding of chemical reactions at the molecular level. ¹A. Barbieri, M.A. Van Hove and G.A. Somorjai, Surf. Sci. 306 (1994) 261.
3:00 PM		SS3-ThA-4 Cr-induced Self-organization of Nanostructures on a Pt(111) Surface L.P. Zhang, J. van Ek, U. Diebold (Tulane University) Cr-induced self-organization of nanostructures on a Pt(111) surface have been investigated using STM, ISS, and XPS. The electronic structure of the Cr/Pt system has been explored also within the local density approximation of density functional theory. The chromium films are prepared by vapor-phase deposition of Cr on a Pt(111) surface. There is a large lattice mismatch between a quasi-hexagonal Cr bcc(110) surface (2.49 Å and 2.88 Å) and a hexagonal Pt fcc(111) surface (2.77 Å). Within the first two monolayers (ML), Cr grows pseudomorphic to the substrate Pt(111) surface at room temperature. Though the film is under a non-isotropic strain, compressed 4% and extended 10% along the two directions of the hexagonal unit cell, respectively, generally no reconstruction is observed for the Cr/Pt(111) system. After annealing various Cr overlayer coverages to different temperatures, metastable surface alloys characteristic for a miscible metallic system have been observed. However, a highly ordered hexagonal pattern appears for a limited coverage range from 1.5 to 3.0 ML and an annealing temperature of 800 K. This strain-related reconstruction shows remarkable features. It appears only along the step edges of the Pt(111) substrate, which is directly correlated to a high atomic density and strong strain at the reconstructed areas. The reconstruction is composed of a highly-symmetric hexagonal network of one-atom-wide dislocation lines. Relative to the second layer, the first layer of Pt(111) forms a (sq39 x sq39)R16.1° network, with unit cell dimensions of 17.3 Å. The contrast in STM images is sensitive to the STM bias voltage, reflecting a strong local variation of the electronic structure due to the presence of a self-organized array of Cr clusters, in which the Pt reconstruction forms a template for the two-dimensional Cr clusters containing only ten atoms.
3:20 PM		SS3-ThA-5 Nonstoichiometry on TiO₂(110) and Cu- TiO₂ Interfaces M. Wagner, O. Kienzle (Max-Planck-Institut für Metallforschung, Germany); D.A. Bonnell (University of Pennsylvania); M. Rühle (Max-Planck-Institut für Metallforschung, Germany) TiO₂ Rutile continues to be a model of semiconducting transition metal oxide surfaces. A number of reconstructions have been observed on the (110) surface that accommodate oxygen deficiency. Structures have been described in terms of missing rows of bridging oxygen. We present atomically resolved STM images and LEED/Auger analyses that describe a new (3x2) reconstruction. We discuss a model for the observed reconstruction where the (3x2) symmetry is achieved by removing 2/3 of the bridging oxygen in the bridging oxygen rows such that a shift in position of missing oxygen by one lattice space vector a¹ along the [001] direction occurs in every third row. This is in contrast to previous structures which remove entire rows. The question whether Ti or O is imaged will be discussed in terms of spatially resolved tunneling spectroscopy. The surface structure dictates, not only surface properties, but also the behavior of interfaces formed with these surfaces. We combine STM, STS and transmission electron microscopy (TEM) to examine properties at nm sized Cu/TiO₂ interfaces. Nanometers sized Cu/TiO₂(110) interfaces were fabricated by vapor deposition at room temperature in UHV. Our TEM studies show that the initial growth mode for this system is Volmer-Weber growth in contrast to previously reported studies. Cu is found to form faceted island (40 nm in width) on the surface.
3:40 PM		SS3-ThA-6 Ab Initio Calculation of the Stoichiometry and Structure of the (0001) Surfaces of GaN and AlN J. Fritsch, O.F. Sankey, K.E. Schmidt, J.B. Page (Arizona State University) We have investigated the stoichiometry and the atomic and electronic structure of the anion and cation terminated (0001) surfaces of wurtzite-phase GaN and AlN using (\it ab initio) tight-binding calculations based on the local-density approximation and the pseudopotential method. All stable surface configurations differ in atomic composition and periodicity from the ideal bulk-like termination. We compare the total energy computed for various $p$(2$\times2$) geometries of GaN and AlN(0001). Vacancy structures are found to be the most stable configurations for the anion and cation terminated surfaces. For metal rich growth conditions our calculations favor the adsorption of metal atoms on the cation terminated surface while the adsorption of hydrogen stabilizes the surfaces in the presence of hydrogen. Surface chemical reactions relevant for the growth of thin nitride films like the adsorption of hydrogen and nitrogen from decomposed ammonia are discussed. The validity of the autocompensation principle is verified by a close inspection of all relevant surface electronic states. Work supported by the Alexander-von Humboldt foundation through a Feodor-Lynen-Forschungsstipendium, by the Office of Naval Research (N00014-95-1-0122), and by the National Science Foundation (MRSEC, grant number DMR-9632635).
4:00 PM		SS3-ThA-7 The Structure of Sb-Terminated GaAs(001) Surfaces. L.J. Whitman, B.R. Bennett, E.M. Kneedler, B.T. Jonker, B.V. Shanabrook (Naval Research Laboratory) A variety of novel electronic and optoelectronic devices are being developed employing AlSb, GaSb, or InSb thin films grown on GaAs(001) substrates via molecular beam epitaxy (MBE). Usually the GaAs surface is prepared under As flux, with the transition to antimonide film growth preceded by a switch to Sb (in the absence of Al, Ga, or In flux). Therefore, Sb-stabilized GaAs(001) surfaces are the actual substrates for the heteroepitaxy. We have studied these surfaces in situ following MBE with RHEED, XPS, and STM. Exposing an As-terminated GaAs(001)-(2x4) surface to Sb at 530°C induces a (2x8) reconstruction. Heating the (2x8) surface to 460°C in vacuum converts it to a (2x4) reconstruction incorporating about half the initial surface Sb. Both surfaces can be produced with wide (>100 nm) terraces having a high degree of both long-range and atomic-scale order. Atomic-resolution images of the Sb-(2x4) reconstruction clearly show a "ß(2x4)"-like structure consisting of rows composed of three dimers. The bias-dependent contrast within the images suggests that 2/3 of the dimers are Sb, with the other 1/3 As. These results are in clear contradiction to previously proposed models of pure Sb-termination.^1,2 STM images of the (2x8) reconstruction (the first, to our knowledge) reveal a more complex multilayer structure that also appears to include a mixture of Sb and As dimers on the surface. Possible models of this more Sb-rich reconstruction will be discussed. Supported by the Office of Naval Research. ¹Esser et al., Phys. Rev. Lett. 77, 4402 (1996). ²Moriarty et al., Phys. Rev. B53, R16148 (1996).
4:20 PM		SS3-ThA-8 Dynamics of Phasons Formed on Si(100) Surface Dimer Rows Studied by Scanning Tunneling Microscopy H. Shigekawa, K. Hata, S. Ozawa, K. Miyake, M. Ishida, H. Oigawa (University of Tsukuba, Japan) Movable phase defects of a new type was observed to exist on Si(100) surface, dynamics of which was studied in detail from 6K to room temperature (RT) by using scanning tunneling microscopy (STM). Interaction between dimer rows was found to depend on temperature, and the characteristic properties of which strongly affected the dynamics of the solitary phase defects; phasons formed on dimer rows. Migration of the phasons occurred remaining the c(4x2) arrangement down to 30K, however, the migration resulted in an apparent phase transition between c(4x2) and p(2x2) arrangements at 6K. Anti-feromagnetic correlation between dimers was confirmed to be stronger along a dimer row compared to that perpendicular to the dimer rows, which is in good agreement with the results previously obtained for this surface by using photoemission spectroscopy and LEED measurements, suggesting that the anti-feromagntetic correlation exists even at RT. Taking account the observed results into consideration, the symmetric dimers appearing in STM images at RT is supposed to be caused by the rapid migration of the phasons introduced onto the surface as predicted by Northrup. H. Shigekawa et al., Jpn. J. Appl. Phys. Lett., 35, L1081(1996). H. Shigekawa et al., Jpn. J. Appl. Phys. Lett., 36, L294(1997).
4:40 PM		SS3-ThA-9 New Structural Model for the Si(111)4x1-In Reconstruction A.A. Saranin, A.V. Zotov, V.G. Lifshits (Institute of Automation and Control Processes, Russian Federation); M. Katayama, K. Oura (Osaka University, Japan) We have proposed new structural model of the Si(111)4x1-In surface reconstruction based on our Auger-electron spectroscopy, low-energy electron diffraction and scanning tunnelling microscopy data. The proposed 4x1 substrate reconstruction contains faulted and unfaulted Si rows (sixfold rings) connected through π-bonded chains (sevenfold rings) and fivefold rings. So we call it π-bonded-chain-stacking-fault (π-SF) model. π-SF 4x1 structure presents a remarkable example of a new Si(111) surface reconstruction which adopts simultaneous the features of the metastable Si(111)2x1 cleaved surface (π-bonded chain) and the stable Si(111)7x7 surface (stacking fault). Indium overlayer in the π-SF model incorporates 0.75 ML of In adatoms residing above sixfold and fivefold rings of the reconstructed Si layer. As a result, three parallel rows of In atoms are formed. Indium atoms of the edge rows saturate the dangling bonds of sixfold rings leaving no unsaturated Si dangling bonds on the surface. Indium atoms of the central row (located above fivefold rings) form metallic bonds with In atoms of the edge rows. The π-SF model of Si(111)4x1-In surface appears to account successfully for most if not all of the available experimental data obtained by variety of surface-sensitive techniques.