### Session X40: Focus Session: Morphology and Evolution at Surfaces: Defects and Transport

 Friday, March 25, 2005 8:00AM - 8:12AM X40.00001: When Seeing is Not Believing: The case of O on Ag(111) Angelos Michaelides , Karsten Reuter , Matthias Scheffler A number of recent studies indicate that, under the oxygen rich conditions of oxidation catalysis, some transition metal catalysts are covered in thin oxide overlayers. Moreover, it has been suggested that such surface-oxide' layers are catalytically active, and that this role is not performed by the pure metal surfaces as was traditionally assumed. This contemporary picture can be traced back to Ag catalysis, where over 30 years ago it was suggested that the top layer of Ag(111) reconstructed to an epitaxial Ag$_2$O overlayer upon exposure to oxygen. Extensive experimental work, including scanning tunnelling microscopy studies in which the oxide was apparently imaged with atomic resolution [1], and density functional theory calculations [2,3] largely confirmed this interpretation. However, subsequent density functional theory results, presented here, augmented with thermodynamic calculations, indicate that previous conclusions are significantly incomplete and that the structure of this original surface-oxide must be reconsidered. [1] C. Carlisle \textit{et al.}, Phys. Rev. Lett. \textbf{84}, 3899 (2000). [2] A. Michaelides, M.-L. Bocquet, P. Sautet, A. Alavi, and D.A. King, Chem. Phys. Lett. \textbf{367}, 344 (2003). [3] W.X. Li, C. Stampfl, and M. Scheffler, Phys. Rev. Lett. \textbf{90}, 256102 (2003). Friday, March 25, 2005 8:12AM - 8:24AM X40.00002: STM study of C$_{70}$ and $C_{60}$/$C_{70}$ mixture on Ag(100) Woei Wu Pai , L. Y. Mandy Sin , C. H. Lin The adsorption structure of C$_{60}$ on Ag(100) is a very interesting one because it is a uniaxial incommensurate aperiodic (111) phase with extensive adsorbate-induced reconstruction [1]. Peculiar molecular ordering arises therein due to delicate balance of competitive interactions. In contrast, few adsorption studies were conducted for the next abundant fullerene, C$_{70}$, which has an ellipsoidal shape. We have recently studied C$_{70}$ and C$_{60}$/C$_{70}$ mixtures of controlled ratios on Ag(100). We compare structures and molecular ordering of these two systems to that of C$_{60}$/Ag(100), and seek possible structural phase transitions therein. Our results show that C$_{70}$/Ag forms a commensurate phase i.e., c(6$\times$4). The mixture adlayers evolve from c(6x4) to (111) with increasing C$_{60}$ ratios, as evidenced by low energy electron diffraction. In many cases, uni-directionally aligned bright lines were observed, e.g., at a C$_{60}$:C$_{70}$ ratio of 2:1. Such ordering cannot be explained by the model developed for C$_{60}$/Ag, and thus its origin remains controversial. Sequential deposition of C$_{60}$ and C$_{70}$ shows that C$_{60}$ intermix readily into the C$_{70 }$layer but not vice versa. This behavior suggests that kinetics of fullerene diffusion and intermixing may be crucial to determine the mixture structure. [1] W. W. Pai and C. L. Hsu, Phys. Rev. B \textbf{68}, 121403 (2003); \textit{ibid. }\textbf{68}, 245414 (2003) Friday, March 25, 2005 8:24AM - 8:36AM X40.00003: Imaging of fully depleted'' ultra-thin silicon-on-insulator by scanning tunneling microscopy Pengpeng Zhang , Emma Tevaarwerk , Mark Eriksson , Donald Savage , Max Lagally Silicon-on-insulator has emerged as an important substrate in MOSFET technology. Silicon template layers with normal doping level of 10$^{15 }$cm$^{-3}$ and as thin as 10nm will be depleted of free carriers by interface states at the Si-SiO$_{2}$ or Si-vacuum (when the silicon template is clean) interfaces [1]. As suggested by previous studies [2, 3], it should therefore be impossible to perform scanning tunneling microscopy (STM) measurements on very thin SOI surfaces. We show such considerations to be incorrect: we present high-quality STM images on fully depleted'' 10nm SOI(001) and 15nm strained Si-on-insulator [sSOI(001)], as well as Ge-covered SOI(001). We believe that surface $\pi$ and $\pi ^{\ast }$ bands are responsible for the ability to conduct even though there are no free bulk'' charges. Carriers can be easily thermally excited across the reduced surface bandgap (0.5-0.6eV). We discuss prior results in light of our findings, as well as the potential effect of surface states on conduction as the Si template layer becomes thinner and thinner. Research supported by DOE and AFOSR [1] S. Henaux, et al. J. Electrochem. Soc. 146, 2737 (1999) [2] K. C. Lin, et al. Appl. Phys. Lett. 72, 2313 (1998) [3] P. Sutter, et al. Appl. Phys. Lett. 85, 3148 (2004) Friday, March 25, 2005 8:36AM - 8:48AM X40.00004: Evidence of Copper Interstitials on Si(001) Arnaldo Laracuente , Lane Baker , James Sullivan , Lloyd Whitman We describe the surface structures following sub-monolayer Cu deposition on Si(001) and subsequent hydrogen termination as characterized by scanning tunneling microscopy. Cu adsorption at 870 K results in a characteristic (2$\times$8) island+vacancy structure, as previously reported. After H-termination, the dominant features of the island+vacancy structure remain, but the size and distribution of the structures are significantly altered. Based on the atomic-scale appearance of both the clean and H-terminated structures, we propose that Cu absorbs through interstitial locations ejecting Si surface dimers in the process. Density functional theory of the potential energy surface for Cu adsorption on Si(001) is consistent with this model, predicting that absorption takes place primarily between dimers and dimer rows. Most of the near-surface Cu is underneath the Si islands and within the vacancies, with the remaining residual in interstitial sites. After H-termination of the Cu/Si(001), all the surface atoms are H-terminated Si, and all Cu is actually subsurface. Friday, March 25, 2005 8:48AM - 9:00AM X40.00005: Scanning tunneling microscopy and spectroscopy of thin sodium-bromide overlayers on the NiAl(110) surface Gareguin Mikaelian , Xiuwen Tu , Wilson Ho Thin insulating films grown on metal surfaces are of a great technological interest due to their applications in microelectronics and corrosion protection. The ability to vary the thickness of the film proves to be a useful tool in the STM experiments with a double barrier tunnel junction geometry, where the coupling between single atoms and molecules adsorbed on the film and the metallic substrate can be controlled. Here we report atomically resolved scanning tunneling microscopy of mono-, bi-, and tri-layers of sodium-bromide adsorbed on the NiAl (110) surface. In addition to topography, differential conductance measurements were performed on these films and compared to those on the bare NiAl(110) surface. Friday, March 25, 2005 9:00AM - 9:12AM X40.00006: Length Scale Effects On The Electronic Transport Properties Of Cu/Nb Bilayers A.L. Lima , X. Zhang , A. Misra , C.H. Booth , E.D. Bauer , M.F. Hundley In this paper, bilayers Cu/Nb thin films of several repeat lengths and thicknesses have been studied using temperature dependent resistivity, transmission electronic microscopy (TEM) and X-ray absorption fine structure (XAFS) measurements. We investigated any possible structural change inside the Cu layer by using XAFS measurements which indicated that the local structure around Cu sites remains FCC-like even for the shortest bilayers period. Also, we obtained the grains size by using TEM measurements so that we could evaluate the scattering due to the grain boundaries. Our $\rho (T)$ measurements seemed to be related to the layers thickness of the thin film rather than the total thickness, and can be successfully compared to theoretical results using a model first proposed by Dimmich[1]. We found a clear correlation between fine structure length scales (grain size and layer thickness) and macroscopic properties (resistivity data). [1] R. Dimmich, J. Phys. F, Met. Phys. 15 (1985) 2477. Friday, March 25, 2005 9:12AM - 9:24AM X40.00007: Surface defect-promoted interaction of TiO$_2$(110) with molecular oxygen Ken Park , Minghu Pan , Vincent Meunier , William Shelton , Sergei Kalinin , Arthur Baddorf , E.W. Plummer We present recent results on the local interactions between surface defects on a TiO$_{2}$(110) and their reactivity with oxygen studied by a combination of Scanning Tunneling Microscopy and Spectroscopy. High-resolution STM images, interpreted with first-principles theory, show that the observed one-dimensional strands have partially reduced Ti atoms coordinated at oxygen octahedral sites forming edge-sharing octahedra. When strands are exposed to 5 x 10$^{-7}$ Torr O$_{2}$ at 300 K, oxygen is adsorbed and preferentially nucleated on and along the strands, indicating the presence of exposed Ti ions. The exposure to molecular oxygen also results in oxygen adsorption at Ti$^{4+}$ sites on (1x1)-terminated terraces. The adsorbed oxygen species result in dark `defective''features in STM, identical to those on the pristine TiO$_{2}$(110), and further diffuse on the surface. The implications of this behavior and specific interaction between defects and gas molecules for catalytic activity are discussed. Friday, March 25, 2005 9:24AM - 9:36AM X40.00008: Hydrogen and Carbon Effects on Al2O3 Surface Phases and Metal Deposition Xiao-Gang Wang , John Smith Effects of H and C impurities on $\alpha$-Al$_{2}$O$_{3}$ (0001) surface stability and metal wetting behavior are determined from first principles[1]. The \textit{ab initio} surface phase diagram for H and C on the alumina surface reveals six distinct surface phases. These different surface phases exhibit a variety of adhesion strengths with Cu and Co, and correspondingly different wetting behaviors. These results are consistent with the varied wetting characteristics observed experimentally. [1] Xiao-Gang Wang and John R. Smith, Phys. Rev. B70, Rapid communications, 081401 (2004). Friday, March 25, 2005 9:36AM - 9:48AM X40.00009: The Structure, Stability and Origin of beta-Phase Ta Aiqin Jiang , Trevor Tyson , Lisa Axe Tantalum thin films exhibit two crystalline phases, bcc ($\alpha$-phase, the bulk structure of tantalum) and metastable tetragonal $\beta$-phase, which differ in both mechanical and electrical properties. In order to understand the stability of the $\beta$ phase and the origin of the $\beta$-to-$\alpha$ phase transformation, molecular dynamics simulations have been performed on tantalum clusters. Molecular dynamics simulations show that the $\beta$ phase is stable with a very high melting point. No phase transformation was observed for pure $\beta$-Ta clusters from room temperature to the melting point. Simulations of Ta clusters with mixed $\alpha$ and $\beta$ phases revealed that inclusion of a small $\alpha$-Ta cluster inside a $\beta$-Ta cluster induces $\beta$ to bcc transformation at a temperature far below its melting point, depending on the cluster size and $\alpha$ to $\beta$ atom ratio. These results suggest that the observed $\beta$ to bcc transformation results from the presence of small $\alpha$-phase grains within the $\beta$-Ta films. The growth of $\beta$-Ta on substrate as a result of strain is being evaluated. Friday, March 25, 2005 9:48AM - 10:00AM X40.00010: The effect of inhomogenous diffusion on the formation of quantum dots Christian Ratsch , Raffaelle Vardavas , Xiaobin Niu , Russel Caflisch The simulation of the formation and self-organization of quantum dots is a major goal in modeling efforts of epitaxial growth. An anisotropic, spatially varying diffusion constant can lead to preferred island nucleation in certain regions. Such an anisotropic, inhomogeneous diffusion field can be achieved, for example, by burying defects (or other structures) underneath the surface: the inhomogeneous strain field leads to an inhomogeneous potential energy surface, and thus to an inhomogeneous diffusion field. The potential energy surface can change because the transition energy can be affected, as well as because the adsorption energy can be affected. The latter leads to a thermodynamic drift. Our results indicate that there is a competition between enhanced nucleation due to increased diffusion (kinetics), and enhanced nucleation due to a thermodynamic drift. Our results were obtained with an island dynamics model that employs the level-set technique. This approach is particularly well suited for this problem, as both, a spatially varying diffusion field, as well as microscopic events on vastly different time-scales can easily be implemented in our model, without extra computational cost. Friday, March 25, 2005 10:00AM - 10:12AM X40.00011: Atomic-Scale Analysis of SiH3 Diffusion on Surfaces of Plasma-Deposited Amorphous Si Thin Films Mayur Valipa , Tamas Bakos , Eray Aydil , Dimitrios Maroudas Under conditions of low SiH$_{4}$ dissociation during plasma-assisted deposition of hydrogenated amorphous silicon (a-Si:H) thin films, the dominant deposition precursor is the SiH$_{3}$ radical. Device-quality a-Si:H films grown under these conditions are remarkably smooth as the SiH$_{3}$ radical is very mobile on the films' surfaces and fills surface valleys during its diffusion. In this presentation, we discuss atomic-scale mechanisms of SiH$_{3}$ diffusion on a-Si:H surfaces based on molecular-dynamics simulations of SiH$_{3}$ radical impingement on a-Si:H film surfaces. The computed average activation barrier for radical diffusion on the a-Si:H surface is 0.16 eV; this low barrier is due to the weak adsorption of the radical onto the a-Si:H surface and its migration predominantly through overcoordination defects. The mechanisms and energetics of SiH$_{3}$ migration reported for the a-Si:H surface are consistent with our density functional theory calculations on crystalline Si surfaces. The diffusing SiH$_{3}$ radical also incorporates preferentially into surface valleys on the a-Si:H film when it transfers an H atom and forms a Si-Si backbond. X40.00012: Kinetic Monte Carlo Model Simulations of Nanoscale Oxidation Behavior Judith Yang , Xuetian Han , Richard McAfee Nucleation rate theory has been successfully used to describe metal heteroepitaxy and qualitatively explained the initial stage of oxidation behavior. To further quantitative understanding of these nano-scale processes and morphological evolution, the Thin Film Oxidation (TFOx) program based on Kinetic Monte Carlo (KMC) has been developed. The TFOx model can be used to simulate the general behavior of irreversible nucleation and growth of epitaxial islands. It explicitly considers many elementary steps, including deposition, adsorption, dissociation of gas molecules (such as O$_{2})$, surface diffusion, aggregation, desorption, and substrate-mediated indirect interactions between static adatoms. The diffusion behavior of the adatoms is determined by the orthogonal and the diagonal jump probabilities, which are considered independently. Potential gradients are used to model intermediate-range interactions, such as strain, between the adatoms. The large number of possible input parameters used in this program provides a rich environment for the simulation of surface dynamics, including the oxidation behavior of thin films. The current TFOx code simulates 2-dimensional transport, nucleation and growth on a square lattice. X40.00013: Ordered Oxygen Vacancy Structure on the Rutile TiO$_2$ (100) Surface Oliver Warschkow , Yingmin Wang , Arun Subramanian , Laurence D. Marks , Mark Asta , Donald E. Ellis We report the structure of a c(2x2) reconstruction of the Rutile TiO$_2$ (100) surface obtained by a combination of transmission electron diffraction (TED), direct methods analysis and density functional calculations. The surface layer is found to be Ti2O3 stoichiometric and characterized by an ordered array oxygen vacancies in the subsurface layer. Annealed under oxygen flow, the structure is metastable but relatively long-lived, suggesting that oxygen uptake from the gas-phase is kinetically hindered. A density functional theory construction of the surface phase diagram supports this interpretation. We compare and contrast our reconstruction with similarly reduced reconstructions reported in the literature for the TiO$_2$ (110) surface.