Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D12: Novel Instrumentation and Techniques in Surface Science |
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Sponsoring Units: DMP DCMP Chair: MIchael Dreyer, University of Maryland Room: 308 |
Monday, March 16, 2009 2:30PM - 2:42PM |
D12.00001: A Combined Scanning Tunneling Microscope-Quartz Crystal Microbalance Investigation of Heating and Liquid-Like Behavior at a Sliding Interface Benjamin Dawson, Jacqueline Krim The unique capabilities resulting from combining a scanning tunneling microscope and a quartz crystal microbalance have been used to characterize the heating and wear at the interface of a tungsten tip and an Indium substrate, with a change in the contact characteristics of the interface occurring for sufficient sliding speeds. The advantage of this system is the ability to probe subtle changes of a rubbing asperity contact, which will aid in developing a more complete understanding of the complex issue of heat generated via friction. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D12.00002: Dynamics and Spreading of pentanol and other alcohols for MEMS applications Brendan Miller, David Hook, Jacqueline Krim Microelectromechanical Systems (MEMS) have the potential to revolutionize widespread technologies, but tribological issues are currently preventing commercialization of some devices. Self-assembled monolayers (SAMs), while highly effective against release related stiction, are ineffective as MEMS lubricants [1]. Vapor phase lubrication has been proposed as a solution to the issue of tribological failure in MEMS with alcohol vapors attracting much interest. In an effort to understand the basic mechanisms of lubrication we have performed a quartz crystal microbalance (QCM) study of the uptake, sliding friction, and spreading rates of adsorbed alcohols on silicon and SAM treated substrates. [1] D. A. Hook, S. J. Timpe, M. T. Dugger, and J. Krim. \textit{Tribological degradation of fluorocarbon coated silicon microdevice surfaces in normal and sliding contact. }J. Applied Physics \textbf{104} (2008). [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D12.00003: Numerical Studies of Friction Between Metallic Surfaces and of its Dependence on Electric Currents Evangelos Meintanis, Michael Marder We will present molecular dynamics simulations that explore the frictional mechanisms between clean metallic surfaces. We employ the HOLA molecular dynamics code to run slider-on-block experiments. Both objects are allowed to evolve freely. We recover realistic coefficients of friction and verify the importance of cold-welding and plastic deformations in dry sliding friction. We also find that plastic deformations can significantly affect both objects, despite a difference in hardness. Metallic contacts have significant technological applications in the transmission of electric currents. To explore the effects of the latter to sliding, we had to integrate an electrodynamics solver into the molecular dynamics code. The disparate time scales involved posed a challenge, but we have developed an efficient scheme for such an integration. A limited electrodynamic solver has been implemented and we are currently exploring the effects of currents in the friction and wear of metallic contacts. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D12.00004: Combined conducting atomic force/scanning Kelvin probe microscope for investigating charge trapping on semiconductor surfaces James Moore, Sean Kenny, Monika Ruchala, Mikhail Reshchikov, Alison Baski A novel combination of conducting atomic force microscopy (CAFM) and scanning Kelvin probe microscopy (SKPM) was used to investigate charge trapping and transfer on semiconductor surfaces. CAFM is first used to locally inject charge at an oxide/semiconductor interface, and then SKPM is performed to monitor the evolution of the resulting surface potential. In a dark environment, the additionally charged interface states due to CAFM charge injection result in additional band bending that persists for hours in GaN, ZnO and Si. Specifically for GaN, a model based on a tunneling mechanism was used to describe the CAFM charge injection, where electrons travel from the tip through an oxide barrier and become trapped at the interface. The decrease in induced band bending with time shows a logarithmic behavior, similar to transients produced after exposure to light. This combination of techniques offers a relatively simple method for investigating induced band bending on semiconductor surfaces and could become a useful tool for understanding concentrations of charged surface states. Specifically, current collapse in GaN FETs and HFETs has been linked to concentrations of charged surface states at the contacts. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D12.00005: Hot Electron Transport Properties of Thin Copper Films Using Ballistic Electron Emission Microscopy J.J. Garramone, J.R. Abel, I.L. Sitnitsky, L. Zhao, I. Appelbaum, V.P. LaBella Copper is widely used material for electrical interconnects within integrated circuits and recently as a base layer for hot electron spin injection and readout into silicon. Integral to both their applications is the knowledge of the electron scattering length. To the best of our knowledge, little work exists that directly measures the scattering length of electrons in copper. In this study we used ballistic electron emission microscopy (BEEM) to measure the hot electron attenuation length of copper thin films deposited on Si(001). BEEM is a three terminal scanning tunneling microcopy (STM) based technique that can measure transport and Schottky heights of metal/semiconductor systems. We find a Schottky height of 0.67~eV and an attenuation length approaching 40~nm just above the Schottky height at 77~K. We also measure a decrease in the attenuation length with increasing tip bias to determine the relative roles of inelastic and elastic scattering. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D12.00006: A Comparative Study of Au-Au, Ru-Ru and RuO$_{2}$-Au RF MEMS Contacts in Controlled Vacuum Environments Matthew Walker, J. Krim, N. McGruer We have constructed an UHV system with in situ oxygen plasma cleaning capabilities in order to observe the impact of film contamination in reproducible conditions. We have performed measurements to allow comparison of soft, hard and combined soft-hard contacts. All switches are closed before applying a potential across the contacts to minimize e-field evaporation and material transfer between contacts. Prior to, and for short O2 plasma exposure times, the initial contact resistance measurements had larger variations. With longer O2 plasma exposure times initial and extrapolation resistances measurements converged. These results are consistent with prior reports, which showed that the oxide layer on a Ru surface thickens with exposure to O2 plasma [1]. Therefore, under stringent experimental conditions, we have demonstrated that operating RF MEMS contacts are comparable to those studied in the surface science literature. [1] Y. Iwasaki, A. Izumi, H. Tsurumaki, A. Namki, H. Oizumi, I. Nishiyama, \textit{Appl. Surf. Sci. }\textbf{\textit{253}}$, 2007$ [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D12.00007: Reduction of Barrier Height at Close Proximity between two Gold (111) surfaces. Yoshifumi Oshima, Yoshihiko Kurui Work function is one of important physical properties in order to understand electron emission or chemical reactions such as catalysis. Work function changes locally depending on a defect, step or adsorbate on the surface. A scanning tunneling microscope (STM) is one of powerful tools to investigate such a local work function, which is called a local barrier height. In this method, the barrier height is defined as $\phi =\left( {1/1.025\ast d\ln G/dz} \right)^2$, where G is conductance and z, the tip-surface distance. Experimentally, the barrier height has been reported to be constant independent of the distance. But, theoretically, it is suggested to be reduced at close proximity. In this study, we investigated the distance between two gold (111) surfaces in TEM observation simultaneously with measuring conductance value in a process of approaching each other. The distance changes controllably by a piezo actuator when it is above 1 nm, but the distance becomes narrower that the expected value obtained by a piezo actuator when it is below 1 nm. Obviously, structural relaxation is occurred when the distance between two gold surfaces is below 1 nm. Taking the structural relaxation into an account, we confirm that the barrier height is reduced at close proximity of two gold (111) surfaces. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D12.00008: Coherent X-ray Surface Diffraction: Speckle from the Surface Reconstruction Layer of Gold (001) Michael S. Pierce, Kee-Chul Chang, Daniel Hennessy, Alec Sandy, Michael Sprung, Hoydoo You We present preliminary results of the first coherent x-ray diffraction from an atomic monolayer surface reconstruction and demonstrate how this technique is capable of providing new information about surface dynamics. Speckled scattering patterns were successfully collected from ordered surface atoms on Au (001) crystals in high vacuum. We have collected data at the (001) anti-Bragg point as well as coherent scattering data directly from the in-plane hex reconstruction peak. These two points can provide complementary information about in-plane and out-of-plane surface dynamics. Below 1050 K, the system appears to remain in equilibrium maintaining a small constant fraction of non-reconstructed surface for a given temperature. However we observe the speckles continue to evolve within the collected scattering patterns indicating that the non-reconstructed portion of the surface rearranges slowly. Above about 1050K, we find that the rate of speckle decorrelation rapidly increases for very small changes in temperature. Signal to noise makes this a difficult experiment with existing light sources. However we expect this technique to become easier and more broadly applicable with future X-ray sources. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D12.00009: In-situ chemical and structural characterization via RHEED-total reflection analysis of x-rays (TRAXS). Sandeep Chandril, Cameron Keenan, Thomas Myers, David Lederman The use of x-ray fluorescence produced inside a molecular beam epitaxy chamber by the RHEED electron gun to simultaneously characterize the thin films for thickness, roughness and the chemical composition is described. This technique requires only slight modifications to the chamber and can be a powerful tool for beam flux calibration and in-situ analysis, especially where surfaces have to be protected under vacuum and for the stoichiometry control during growth. The angular dependence of the x-ray fluorescence signal from the thin film over the substrate is analyzed using Parratt's approach and simulating electrons' trajectories inside the film to account for grazing angle electron beam as a source for x-rays. We have found good agreement between the experiment and the theory for the thickness and roughness estimates. Experiments for chemical composition determination are currently underway. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D12.00010: High Energy XRD/XRF for High-Throughput Phase Mapping of Composition Spread Thin Films John Gregoire, Darren Dale, Alexander Kazimirov, Michele Tague, Hector Abruna, Francis DiSalvo, R. Bruce van Dover Analysis of thin film inorganic libraries is an increasingly popular technique for materials discovery and optimization. For ternary and higher-order libraries, the high-throughput determination of the crystalline phase fields is an active field of research due to its importance in understanding a given material system. We discuss our techniques for high-throughput data acquisition and analysis using a 60keV x-ray source at the Cornell High Energy Synchrotron Source. The techniques provide simultaneous mapping of the composition, crystalline phase, and fiber texture of a composition spread thin film. We also demonstrate the utility of this data in interpreting our measurements of the thin film's catalytic activity for the oxidation of methanol. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D12.00011: Low Energy Electron Microscope Imaging of Doped Si Structures Buried under Thermal Oxides Gary Kellogg, Meredith Anderson, Craig Nakakura We present recent progress towards low energy electron microscope (LEEM) imaging of doped-silicon, diode test structures buried under thermally grown oxides. The question addressed here is whether the observed contrast at incident electron energies just above the vacuum cutoff is due to differences in doping type or oxide thicknesses. To circumvent complications arising from charging of the when exposed to the imaging electron beam, we developed a method to measure ``pre-charging'' current voltage (IV) curves and applied it to three test samples with oxide thicknesses varying from 2.8 to 50 nm. The vacuum cutoff energies obtained from the IV curves depend on both doping type \textit{and} oxide thickness and are strongly influenced by external factors including surface contamination and UV exposure. The time dependence of the oxide charging increases significantly with oxide thickness providing further insights into the origins of LEEM contrast. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D12.00012: LEED structural analysis of strongly correlated systems: reaching the limit of the instrumentation? TeYu Chien, Biao Hu, Shuheng Pan, V. Braun Nascimento, E. Ward Plummer In strongly correlated systems, like transition-metal oxides and the Iron Pnictides, subtle changes in structural parameters can cause dramatic changes in the physical properties. Breaking the symmetry by creating a surface is a controlled way to explore the structure-functionality relationship. Low Energy Electron Diffraction (LEED) $I-V $has been one of the most used surface structural techniques, but there are inherent and instrumental limitations which will be discussed using data from surfaces of transition-metal compounds. Using CCD cameras and new data analysis three dimensional plots of diffraction vs parallel momentum can be created. Spot intensity, width, position, and profile, as well as the diffuse background can then be quantitatively extracted and evaluated. We will show how to couple the STM with LEED using data from the parent superconducting compound BaFe$_{2}$As$_{2}$. The inherent limitations of the existing system are tested using Cu(100). [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D12.00013: Electron emission from surfaces resulting from low energy positron bombardment S. Mukherjee, K. Shastry, A. H. Weiss Measurements of the energy distribution of electrons resulting from very low energy positron bombardment of a polycrystalline Au and Cu(100) surfaces provide evidence for a single step transition from an unbound scattering state to an image potential bound state. The primary positron energy threshold for secondary electron emission and cutoff in the secondary electron energy spectra are consistent with a process in which an incident positrons make a transition from a scattering state to a surface-image potential bound while transferring all of the energy difference to an outgoing secondary electron. Estimates of the probability of this process as a function of incident positron energy are also presented. Background free Auger spectra of the MVV transition in Cu and the OVV transition in Au were obtained by setting the incident positron beam energy below the secondary electron emission threshold. Auger electron emission resulted from the annihilation of surface state positrons with core electrons. The low energy tail associated with the low energy CVV Auger transitions in Cu and Au were found to have integrated intensity several times larger than Auger peak providing strong evidence for multi-electron Auger processes. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D12.00014: Hydrogen absorption by a Pd film detected by microgravimetry J.I. Avila, R. Trabol, U.G. Volkmann, A.L. Cabrera, C. Romero, P. Lievens A thin film of palladium (200 {\AA}) was e-beam evaporated onto a quartz crystal used in a SQM-160 Microbalance from Sigma Instruments. The crystal was then successively exposed to different hydrogen pressures, in situ, and the change in the resonant frequency was recorded as a function of time. Hydrogen absorption by the Pd film can be obtained by the shift of the resonant frequency upon hydrogenation at different pressures. A 14 Hz shift is obtained when the Pd film is saturated with hydrogen at 10 Torr of pressure. The different faces of the Pd-H system are observed with this technique. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D12.00015: Observation of In Plane Magnetization Reversal Using Polarization Dependent Magnetooptical Kerr Effect Hendrik Ohldag, Franz Hillebrecht We present an experimental setup for in plane two axis magnetometry by employing the polarization dependence of the magnetooptical kerreffect. (MOKE). The proposed approach allows for observing the complete in plane reversal process during a hysteresis loop. For this purpose a conventional setup to measure longitudinal MOKE with crossed polarizers is extended by a Faraday cell to compensate for the rotation of the polarization vector caused by the magnetized sample. This detection scheme enables us to observe hysteresis loops of single monolayer. Using a Jonesmatrix formalism we are able derive expressions for the kerr rotation using oblique incident polarization, allowing for extracting 2-dimensional vectorial information about the magnetization reversal process in the plane of the sample surface. The approach can be further extended to extract all three components of the magnetization by acquiring more than two loops. Since this setup does not require to change the sample geometry {\em in situ} it can be easily attached to an existing ultra high vacuum setup. [Preview Abstract] |
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