Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H10: Semiconductor Surfaces and Interfaces |
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Sponsoring Units: DCMP Chair: Gary Kellogg, Sandia National Laboratories Room: D221 |
Tuesday, March 22, 2011 8:00AM - 8:12AM |
H10.00001: Reversible vertical manipulation of Ag atoms on Si(111)-(74$\times$7) at room temperature Fangfei Ming, Kedong Wang, Shuan Pan, Jiepeng Liu, Xieqiu Zhang, Jinlong Yang, Xudong Xiao We have demonstrated a technique to conduct reproducible and reversible vertical manipulation of Ag atoms on the Si(111)-(7$\times$7) surface at room temperature using a scanning tunneling microscope tip. The direction of the transfer of Ag atoms between the sample surface and the tip is simply controlled by the polarity of the bias voltage. Using the 7$\times$7 unit cell as a nanometer size template, complex Ag nano-clusters could be assembled or disassembled by adding or removing Ag atoms in an atom-by-atom manner. With controlled number of Ag atoms filled in a half unit cell, we can construct Ag clusters with up to 25 Ag atoms. The precise control of the number of Ag atoms in the Ag clusters can provide critical information for understanding their physical and chemical properties, and form a fundamental base for the relevant studies of the Ag/Si(111)-(7$\times$7) system and for fabricating nano-devices. [Preview Abstract] |
Tuesday, March 22, 2011 8:12AM - 8:24AM |
H10.00002: Probing surface electronic structure with conductance measurements on Si nanomembranes Weina Peng, James Endres, Shelley Scott, Donald Savage, Irena Knezevic, Mark Eriksson, Max Lagally The surface electronic structure of nanostructures has a strong, sometimes dominant, influence on their transport properties, because of their large surface to volume ratios. Different surface terminations result in different transport behavior, and therefore conductance measurements on nanostructures can be used to study surface and interface electronic spectra. In our experiments, the conductance of the thin (200nm or less in thickness) top Si layer in silicon-on-insulator is measured as the back gate voltage is varied, for both hydrogen terminations and clean reconstructed surfaces in UHV. Experimental results on samples of different thicknesses are compared systematically with simulations to understand the role of the Si/SiO$_{2}$ interface and the electronic structure of the front surface. We explain why the transport behavior of NMs with a clean Si(001) surface is distinct from that with hydrogen termination. Donor type surface states are present in the majority on the hydrogenated surface, and their concentration is on the order of 10$^{12}$ cm$^{-2}$. On the reconstructed Si surface, instead, pseudo-pinning of the Fermi level occurs because of the high density of states of the clean-surface band (2x1 reconstruction) and the presence of surface defect states. [Preview Abstract] |
Tuesday, March 22, 2011 8:24AM - 8:36AM |
H10.00003: Experimental evidence for trapped volumes in thin Ag films on Si(111)-7x7 S.T. Hayden, Yiyao Chen, M.W. Gramlich, R.S. Gari, G.M. King, P.F. Miceli Thin films of Ag on Si(111)-7x7 were prepared in UHV at room temperature by vapor depositing Ag at a glancing angle with respect to the surface normal. Comparing x-ray reflectivity and atomic force microscopy (AFM) measurements, it is found that both techniques give the same height distribution at the surface. However, for a given deposition angle, x-ray reflectivity measurements reveal that there is a significant portion of trapped volume that goes undetected by the AFM. Also, by increasing the deposition angle from normal to glancing incidence angles, both the roughness and the maximum height distribution profile increase. Experimental evidence for trapped volumes in Ag/Si(111)-7x7 will be discussed. Research funding is supported by NSF DMR-0706278. The Advanced Photon Source Sector 6 beam-line at Argonne National Laboratory is supported by the US-DOE through Ames Lab under Contract No. W-7405-Eng-82. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 8:48AM |
H10.00004: Growth of Iridium and Silver on Ge(111) Studied by STM Marshall van Zijll, Cory Mullet, Emilie Huffman, Shirley Chiang We have used scanning tunneling microscopy (STM) to characterize the growth of iridium and silver onto Ge(111) as a function of coverage and annealing temperature. Ir was deposited onto the Ge(111) c(2x8) surface at different coverages less than 1ML. The Ir forms islands with a ($\surd $3x$\surd $3)R30\r{ } phase and island size increasing with increasing annealing temperature. Stranski-Krastanov growth was observed at most coverages. Ag deposited onto the Ge(111) c(2x8) surface and annealed at 450K forms both a (4x4) phase and a (3x1) phase. The Ge(111) surface reorganizes to a (2x2) phase after deposition of both Ir and Ag. High resolution images have been obtained allowing direct observation of the different phases. [Preview Abstract] |
Tuesday, March 22, 2011 8:48AM - 9:00AM |
H10.00005: One-dimensional Mn atom chains templated on a Si(001) surface Sigrun A. K\"{o}ster, James H.G. Owen, Fran\c{c}ois Bianco, Alex M.P. Sena, David R. Bowler, Christoph Renner Single-atom chains on a wide gap substrate are a very attractive embodiment of a truly one-dimensional system to explore the remarkable physical properties emerging in such low dimensions. We present self-assembled single-atom Mn chains on a Si(001) surface with Bi nanolines, which serve to increase greatly the average length of the Mn chains. They grow perpendicular to the Si(001) dimer rows, at densities which can be adjusted by means of the growth parameter. High resolution scanning tunneling microscopy (STM) micrographs are in perfect agreement with density functional theory (DFT), providing detailed insight into the chain structure. We further discuss low temperature STM spectroscopy and spin dependent DFT modeling suggesting Mn-chains are indeed a suitable candidate to observe electronic and magnetic properties in one-dimension experimentally. [Preview Abstract] |
Tuesday, March 22, 2011 9:00AM - 9:12AM |
H10.00006: Low energy alkali ion-surface charge exchange for Si(111) as a function of doping Reuben D. Gann, Jory A. Yarmoff Alkali ion-surface charge exchange, which can be used to probe surface electronic states, is well understood within the context of the resonant charge transfer (RCT) model. Recent studies have extended the use of alkali ion scattering and the RCT model from metal surfaces to semiconductors and insulators. In the present work, we measure the effect of doping type and concentration on the neutralization probability of alkali ions scattered from semiconductors. Si(111) surfaces were prepared in UHV, and the neutralization probability of scattered Li$^+$ ions was measured for projectiles that were singly scattered from Si atomic sites. For the clean Si(111)-$7 \times 7$ surface, the neutralization is determined by the surface electronic states [1] and is independent of doping. Samples were then dosed with atomic hydrogen in order to passivate the surface states and unpin the Fermi level. This affects the neutralization probabilities and reveals differences between n and p-type materials. \\[4pt] [1] Y. Yang and J.A. Yarmoff, Phys. Rev. Lett. 89, 196102 (2002). [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H10.00007: Binding sites and diffusion barriers of a Ga adatom on the $\textrm{GaAs(001)-\emph{c}(}\textbf{4}\times\textbf{4}\textrm{)}$ surface from first-principles computations J. Roehl, A. Kolagatla, V.K.K. Ganguri, S. Khare, R.J. Phaneuf The Ga adatom adsorption and diffusion processes on the $\textrm{GaAs(001)-\emph{c}(}4\times4\textrm{)}$ surface were studied using \emph{ab initio} density-functional-theory computations in the local density approximation. Two distinct sets of minima and transition sites were discovered for a Ga adatom relaxing from heights of 3 and 0.5 {\AA} from the surface. These two sets show significant differences in the interaction of the Ga adatom with surface As dimers. An electronic signature of the differences in this interaction was identified. We computed the energetic barriers to diffusion for various adsorption sites. From these, we propose three pathways for diffusion of a Ga adatom on this surface which indicate anisotropic diffusion along different directions.\footnote{Supported by NSF DMR 0705464.} [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H10.00008: Surface Structure of Bi-terminated GaAs Grown with Molecular Beam Epitaxy Adam Duzik, Joanna Millunchick Control of III-V semiconductor surfaces is crucial for high-quality device production. A means of interface control involves the use of Bi as a surfactant, which both smooths the surface and alters the surface reconstruction. We examined the effects of Bi deposition via molecular beam epitaxy on the GaAs(001) surface structure using reflective high energy electron diffraction and scanning tunneling microscopy. After $\sim $0.2 ML of Bi deposition, scanning tunneling microscopy revealed a disruption in the initial c(4$\times $4) reconstruction. Coverages of $\sim $0.4 ML and $\sim $0.6 ML produced a (1$\times $3) and a (2x3) diffraction pattern, respectively, and an atomic surface structure consisting of a disordered row reconstruction, $\beta $2(2$\times $4) reconstruction rows, and surface clusters, with 1 ML deep pits at a coverage of $\sim $0.6 ML. Calculations show these changes in surface structure and morphology are likely not the result of As desorption, but due to the presence of Bi on the surface. These observations may help explain the origin of Bi clusters that give GaAsBi most of its unique properties. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H10.00009: Design of carbide thin film coatings from first principles Mikael R{\aa}sander, Biplab Sanyal, Ulf Jansson, Olle Eriksson Transition metal carbides have many interesting physical properties and have therefore been used in many technological applications, e.g. as thin film metal coatings. A commonly studied thin film coating material is nc-TiC/a-C, where nanocomposites (nc-) of TiC are dispersed in an amorphous (a-) C matrix. An interesting feature of these types of materials is the possibility to design the material to obtain new functionality, e.g. by tuning the C to Ti content. In this talk we will present results obtained by first principles density functional theory calculations of a different approach, where various metals have been alloyed into TiC. Depending on the alloying metals ability to form carbides this will yield different effects. One of these effects is the creation of a driving force for the release of C from the carbide. This C release has been shown to yield favorable lubricating properties of nc-(Ti,Al)C/a-C thin films. We will show that the C release can be tuned by a careful selection of the alloying metal in order to optimize the properties of these types of thin film carbide coatings. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H10.00010: THz optical Hall-effect and MIR-VUV ellipsometry characterization of 2DEG properties in HfO$_{2}$ passivated AlGaN/GaN HEMT structures S. Sch\"{o}che, A. Boosalis, C.M. Herzinger, J.A. Woollam, J. Shi, W.J. Schaff, L.F. Eastman, M. Schubert, T. Hofmann We present non-contact, optical measurements of free-charge carrier mobility, sheet density, and effective mass parameters of the 2DEG for different HfO$_{2}$ passivated AlGaN/GaN high electron mobility transistor structures at room temperature. Spectroscopic ellipsometry (SE) in the spectral range from THz and Mid-IR to the VUV and THz optical Hall-effect (generalized ellipsometry in magnetic fields) (OHE) are employed. Changes in the HfO$_{2}$ layer growth conditions are found to drastically influence the electron density of the channel. The sheet density and the carrier mobility obtained by the optical investigations are in excellent agreement with results from electrical Hall-effect measurements. The electron effective mass parameters determined here using the OHE corroborate previous SdH and cyclotron resonance studies. The surface sensitivity of VUV-SE in combination with OHE allows for correlation of surface passivation and changes in the 2DEG properties. [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H10.00011: Low Temperature Epitaxial Growth of Ge Quantum Dots on Si(100) Ali Er, Hani Elsayed-Ali The effect of laser-induced electronic excitations on the self-assembly of Ge quantum dots (QD) on Si(100)-(2x1) grown by pulsed laser deposition is studied. The experiment was conducted in ultrahigh vacuum. A chirped pulse amplified Ti:sapphire laser with $\sim $60 femtosecond pulse width, center wavelength $\sim $800 nm, and operating at 1 kHz repletion rate was split into two beams; one used to ablate a Ge target while the other to electronically excite the substrate. \textit{In situ} reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM), and \textit{ex situ} atomic force microscopy (AFM) were used to study the morphology of the grown QDs. For Ge coverage of 12 monolayer, it was observed that the excitation laser reduces the epitaxial growth temperature to 70 \r{ }C, at which no epitaxy is possible without excitation. By using nanosecond Nd:YAG laser for ablation and excitation, it was shown that applying the excitation laser to the substrate during the growth changes the QD morphology and island density and improves the size uniformity of the QDs at 390 \r{ }C. RHEED recovery curves show that the excitation laser increases the surface diffusion of the Ge atoms. A purely electronic mechanism of enhanced surface diffusion of the Ge adatoms is involved. [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H10.00012: Large scale atomic engineering of silicon (100) surfaces Kai Li, Pradeep Namboodiri, Sumanth Chikkamaranahalli, Joseph Fu, Richard Silver Control of atomic morphology at the micrometer scale has been a long term challenge to enable atomically precise manufacturing. In this presentation we describe our method to pattern micrometer scale, ordered features on a Si surface with subsequent etch and high temperature processing. Following high temperature UHV processing, high quality atomically-ordered surfaces are imaged using atomic-resolution STM. A significant attribute of these surfaces is that the micrometer scale features evolve, but persist, allowing external location of nanometer scale features as well as comprehensive control of atomic terrace sizes and step bunching. A multi step thermal process is used, resulting in surface with symmetric, reproducible step-terrace patterns and very wide atomically flat regions. A kinetic Monte Carlo (KMC) model is used to simulate the current induced electromigration process which is primarily responsible for the long range evolution of surfaces. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H10.00013: Theory of Scanning Tunneling Microscopy of Dangling Bonds on Silicon Surfaces Lucian Livadaru, Jason Pitters, Robert Wolkow Silicon surface dangling bonds (DBs) are electronic gap states with eigenenergy close to the middle of the bandgap of bulk silicon and can be explored as quantum dots. During exploratory fabrication and characterization of DB-structures on H-Si(100) surfaces, scanning tunneling microscopy (STM) imaging showed sharp halo-like features around single DBs that cannot be explained by the standard STM theory. Halo appearance varies with sample doping level and imaging conditions (sample bias and current). Here we investigate the nature of such features in the STM imaging of DBs. We propose a theory of image formation based on non-equilibrium charge transfer balance (via elastic and inelastic channels), from the STM tip to DB on one hand, and from DB to bulk Si on the other. For empty-state imaging mode, in the immediate proximity of a DB ($<$1nm) tip-induced band bending shifts the DB-level to a value between the Fermi levels of the STM tip and of the sample. Consequently, a steady-state of charge flow is established through the DB state, which dictates the time-average amount of charge on the DB. This in turn affects the total STM current in that proximity leading to the appearance of a halo. [Preview Abstract] |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H10.00014: The Electronic and Transport Properties of Si(111)-7$\times $7 and Related Reconstructions Manuel Smeu, Wei Ji, Robert Wolkow, Hong Guo The 7$\times $7 reconstruction of Si(111) has the interesting property of being metallic despite bulk Si being a semiconductor. This surface has a complex reconstruction that takes on a dimer-adatom stacking fault (DAS) structure composed of adatoms, rest atoms, and several other key features. It is believed that the conductivity occurs through the dangling bonds of the adatoms, and that it is entirely a surface effect. To elucidate the details of this mechanism, we have investigated a set of related Si(111) reconstructions of increasing complexity in order to resolve the effect of the different DAS features on the electronic and transport properties of the Si(111)-7$\times $7 surface. Density functional theory (DFT) calculations have been carried out on the $\surd $3$\times \surd $3-$R$30\r{ }, 2$\times $2, 5$\times $5, and 7$\times $7 reconstructions of Si(111). Additionally, our work is extended to electron transport simulations employing the non-equilibrium Green's function technique coupled with DFT (NEGF-DFT) to calculate the conductance for these systems. Finally, the effect of atomic steps and adsorbates on the conductive properties will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 10:48AM - 11:00AM |
H10.00015: Size dependent superconductivity of Pb islands grown on Si (111) Jiepeng Liu, Xuefeng Wu, Fangfei Ming, Xieqiu Zhang, Kedong Wang, Bing Wang, Xudong Xiao The superconductivity of nano-sized Pb islands grown on Si (111) with different size at 9 monolayer thickness was studied by low temperature scanning tunneling spectroscopy. By measuring the zero bias conductance as a function of temperature, for larger islands we observed a transition from pseudogap state at high temperature to superconductivity state at low temperature through two distinct slopes, where the superconductivity transition temperature ($T$c) of the island can be determined. For island size of $\sim $58 nm$^{2}$, a large drop in $T$c is found; when the size is further reduced to about 30 nm$^{2}$, no superconducting state was observed down to the measured temperature of 3.2 K. By properly subtracting the background and pseudogap effect, information on the temperature dependent superconductivity gap can be obtained. The ratio of $\frac{2\Delta _0 }{k_B T_c }$ decreased from 4.5 to 3.3 with the reduction of island size, showing that the electron-phonon coupling becomes weaker as the size decreases. [Preview Abstract] |
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