Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M53: Surfaces, Interfaces and Thin Films: Electronic Structures and Size Effects |
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Sponsoring Units: DCMP Room: Mile High Ballroom 2C |
Wednesday, March 5, 2014 11:15AM - 11:27AM |
M53.00001: Electronic structure of stacking faults in hexagonal graphite Klaus Koepernik, Manfred Taut, Manuel Richter We present results of self-consistent, full-potential electronic structure calculations for slabs of hexagonal graphite with stacking faults. There are two types of stacking faults, which differ qualitatively in their chemical bonding picture. We find, that both types induce localized interface bands near the symmetry line K-M in the Brillouin zone and a related peak in the local density of states (LDOS) very close to the Fermi energy, which should give rise to a dominating contribution of the interface bands to the local conductivity at the stacking faults. In contrast, a clean surface does not host any surface bands in the energy range of the $\pi$ and $\sigma$ bands, and the LDOS near the surface is even depleted. On the other hand, displacement of even one single surface layer induces a surface band near K-M. A special role play unsaturated monomer $p_{z}$-orbitals in the vicinity the stacking faults. The formation energy of both types of stacking faults and the surface energy are discussed. [Preview Abstract] |
Wednesday, March 5, 2014 11:27AM - 11:39AM |
M53.00002: Electromagnetic fluctuations near thin metallic films Luke Langsjoen, Amrit Poudel, Maxim Vavilov, Robert Joynt We compute the electromagnetic fluctuations due to evanescent-wave Johnson noise in the vicinity of a thin conducting film, such as a metallic gate or a 2-dimensional electron gas. This noise can decohere a nearby qubit and it is also responsible for Casimir forces. We have improved on previous calculations by including the nonlocal dielectric response of the film, which is an important correction at short distances. Remarkably, the fluctuations responsible for decoherence of charge qubits from a thin film are greatly enhanced over the case of a conducting half space. The decoherence times can be reduced by over an order of magnitude by decreasing the film thickness. This appears to be due to the leakage into the vacuum of modes that are well localized in the perpendicular direction. There is no corresponding effect for spin qubits (magnetic field fluctuations). We also show that a nonlocal dielectric function naturally removes the divergence in the Casimir force at vanishing separation between two metallic sheets or halfspaces. We include a treatment of both a Drude conductor and a superconducting material. [Preview Abstract] |
Wednesday, March 5, 2014 11:39AM - 11:51AM |
M53.00003: Temperature Dependence of Lateral Charge Transport in Silicon Nanomembranes Weiwei Hu, Shelley Scott, R.B. Jacobson, Pornsatit Sookchoo, Donald Savage, Mark Eriksson, Max Lagally Thin sheets of single-crystal silicon (nanomembranes), electrically isolated from a bulk substrate by a dielectric layer, are an exceptional tool for studying the electronic transport properties of surfaces in the absence of an extended bulk. Under UHV, we measure the conductivity, and a back gate allows us to look into the depletion region, where we can determine the minimum conductance. For hydrogen-terminated Si(001) NMs, for which the surface has no conductivity, the minimum conductance decreases with decreasing NM thickness (220-42nm), demonstrating the reduction in carriers for thinner NMs. For the clean Si(2$\times$1)surface, mobile charge exists in the $\pi^{\star}$ surface band [1]. For thicknesses below 200nm surface conduction dominates, rendering the thickness independence of the minimum. We determine a surface charge mobility of $\sim$50cm$^{2}$V$^{-1}$s$^{-1}$[2]. We have measured the temperature dependence of the conductance of a 42nm thick HF treated SiNM. The results show that the Fermi level is pinned 0.21 $\pm0.01$ eV below the conduction band minimum, in agreement with XPS results [3].\\[4pt] [1] P. P. Zhang, et al., Nature 439, 703-706 (2006);\\[0pt] [2] W. Peng, et al., Nature Commun. 4, 1339 (2013);\\[0pt] [3] R. Schlaf, et al., J. Vac. Sci. Technol. A 17, 164 (1999). [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M53.00004: Lateral extension of quantum well states: scanning tunneling spectroscopy study Sujit Manna, J\"urgen Kirschner Quantum well states(QWS) in thin metal films have been extensively studied mostly by laterally averaging techniqes such as photoemission or inverse photoemission.A complementary approach is opened by scanning tunneling spectroscopy(STS) and microscopy(STM), which extends the range of this extremely surface sensitive device into the interior of the sample, and make it possible to image features of a buried interface with lateral resolution on the atomic scale. We present low temperature STS results of ocuupied sp-QWS localized in Ag(111) films. For thin film with local varying thickness,we recall the fundamental question-how the transition of QWS takes place,and at what length scale? We demonstrate that the QWS of thin Ag(111) films are highly perturbed within the proximity of a step edge.Atomic resolved scanning tunneling microscopy/spectroscopy indicates that the energy of these states has a strong distance dependence within the proximity of the step edge with large energetic shift equaling up to $\sim$ 200meV. For an Ag layer of 30ML thick, we obtain a lateral extension of the QWS in the order of $\sim$ 10\AA. This spatial extension of QWS can be understood within the context of electron scattering within the proximity of the buried interface. [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M53.00005: Nanoscale Schottky Barrier Height Mapping at Metal/Semiconductor Interfaces Robert Balsano, Chris Durcan, Vincent LaBella Metal/semiconductor interfaces form a rectifying contact known as a Schottky diode characterized by a barrier height that is governed by the charge transfer and localized bonding at the interface. Conventional current voltage spectroscopy measures a spatially averaged barrier height. Ballistic electron emission microscopy (BEEM) is a scanning tunneling microscopy (STM) technique that can measure barrier heights with nanoscale resolution due to the nano-positioning of the STM tip. In this presentation, the Schottky barrier height is mapped with nanoscale resolution at several metal/silicon interfaces. These maps provide insight into the distribution and spatial homogeneity of the barrier height. In addition, they have the potential to identify and differentiate between different metal species at the interface as well as identify oxides and defects as well. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M53.00006: Band alignment and interface charge decomposition for abrupt and polar-compensated Si/ZnS interfaces David Foster, Guenter Schneider Using the DFT+$U$ method, we study abrupt and polar-compensated Si/ZnS interfaces in the (100), (110), and (111) directions, examining computational and physical aspects of asymmetric supercells. Distinct interfaces derived from the (100) and (111) directions have valence band offsets (VBO) near either -0.9 eV or -2.1 eV. This bimodal interface dipole distribution is surprisingly obeyed by single-substitution polar-compensated interfaces as well as abrupt polar interfaces. The VBO in the non-polar direction (110) is near the mean value of the distribution (-1.5 eV). Examining one stoichiometric (111) superlattice, we find that the interface free charge, estimated directly from partial occupation of nominally unoccupied surface states, is in good agreement with prediction from the calculated electric fields and dielectric properties. Specifically, the sum of the free charge determined by state occupation, the induced bound charge, and the compositional charge (interface theorem bound charge) is equal to the total interface charge. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M53.00007: Metal-insulator transition and nanoscale phase separation in a hole-doped surface reconstruction Daniel Mulugeta, Paul Snijders, Hanno Weitering Doping, the deliberate introduction of impurities to alter electronic or magnetic properties, has been a tremendously successful method to study and understand systems with multiple competing interactions, as reflected in both the widespread use of doped semiconductors and in the large number of emergent electronic phases in doping-dependent phase diagrams of e.g. complex oxides. In low dimensional systems, however, the perturbation to the crystal lattice by the dopant atoms can overwhelm a delicate balance of interactions in e.g. a ground state with coexisting phases. Here we introduce a modulation doping technique used to dope holes in a surface reconstruction of Sn on Si(111). Using variable and low temperature scanning tunneling microscopy and spectroscopy, we observe a doping-induced metal-insulator phase transition that is of a displacive nature, contrasting with the order-disorder nature of other surface phase transitions. Moreover, the transition leads to an intrinsic nanoscale phase coexistence at 5 K never before observed on semiconductor surfaces. Clearly, modulation doping allows us to study the delicate balance of interactions in the phase diagram of low-dimensional electronic surface systems that is otherwise experimentally inaccessible. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M53.00008: Measurement of work function difference between Pb/Si(111) and Pb/Ge/Si(111) by high-order Gundlach oscillation Hsu Sheng Huang, Wen Yuan Chan, Wei Bin Su, Germar Hoffmann, Chia Seng Chang Ge films can be grown between the Pb overlayer and Si(111) substrate by the surfactant-mediated epitaxy. We detect the high-order Gundlach oscillation revealed in scanning tunneling microscopy (STM) to measure the work function difference between Pb/Si(111) and Pb/Ge/Si(111). Owing to different dielectric responses of Si and Ge, the tunneling current on Pb/Si has to be larger than that on Pb/Ge/Si by a factor of 2-3 to establish the same electric field in STM gap on both regions. This condition leads us to obtain a work function difference of 200 meV from observing Gundlach oscillation. It is believed that the method developed in this work can be extended to measure the work function difference of bulk materials as well. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M53.00009: Effect of surface and interface states on the piezoresistivity of 2D electrons in III/V heterojunctions Miwa Hokii, Masato Ohmori, Hiroyuki Sakaki Uniaxial strains applied along the interface of n-AlGaAs/GaAs heterojunctions induce piezoelectric fields normal to the interface and change both the density $N_{\mathrm{s}}$ of 2D electrons and the resistance $R$ of the channel. We have measured this piezoresistance in a group of samples grown on (111)B, where the inversion symmetry is absent. Resistance changes$\Delta R/R$ of typically 1\% were observed for the external strain of 5 $\times$ 10$^{-4}$, indicating that the electron density $N_{\mathrm{s}}$ changed by about 10$^{9}$/cm$^{2}$. It should be noted that the change of $N_{\mathrm{s}}$ is affected not only by changes in the polarization charges at the surfaces and interfaces, resulting from the piezoelectric field, but also by changes in localized charges on the surface and interface states. Indeed, our measurements have shown that the magnitude of piezoresistance depends sensitively on whether the sample surface is kept bare or clad by SiO$_{2}$ and/or metal film. By analyzing these data, we have shown that the density of surface states can be quantitatively evaluated. This method is extended also to study n-AlGaAs/GaAs samples formed on (100) surface, in which external strains break the inversion symmetry. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M53.00010: Size effects in thin gold films: Discrimination between electron-surface and electron-grain boundary scattering by measuring the Hall effect at 4 K Raul C. Munoz, R. Henriquez, L. Moraga, G. Kremer, M. Flores, A. Espinosa We report the Hall effect measured in gold films evaporated onto mica substrates, the samples having an average grain diameter D that ranges between 12 and 174 nm, and a thickness t of approximately 50 nm and 100 nm. The Hall mobility was determined at low temperatures T (4K $\le $ T $\le $ 50K). By tuning the grain size during sample preparation, we discriminate whether the dominant collision mechanism controlling the resistivity of the samples at 4 K is electron-surface or electron-grain boundary scattering, based upon whether the Hall mobility depends linearly on film thickness t or on grain diameter D. \\[4pt] References:\\[0pt] R. Henriquez et al, Appl. Phys. Lett. \textbf{102} (2013) 051608. [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M53.00011: Electrolyte gating with ionic liquids -- structural and electronic characterization of the interface Trevor Petach, Apurva Mehta, David Goldhaber-Gordon Oxide dielectrics used for traditional gating suffer from breakdown, which limits the electric field that can be applied and thus the carrier density. Because of their wide electrochemical window, ionic liquids have recently been used to replace oxide dielectrics in hope of achieving higher carrier densities. We show that the specific capacitance of the interfacial layer for several common ionic liquids is 5 - 10 $\mu$F/cm$^2$, that the breakdown voltage is 3 - 6 volts, and that the characteristic time for the double layer to form is several milliseconds. We also show that the DC behavior of the ionic liquid interface at large potentials is not purely electrostatic. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M53.00012: Anomalous Hall effect in Pt thin films induced by ionic gating Sunao Shimizu, Kei S. Takahashi, Takafumi Hatano, Masashi Kawasaki, Yoshinori Tokura, Yoshihiro Iwasa Pt is an exchange-enhanced paramagnetic material, in which the Stoner criterion for ferromagnetism is nearly satisfied and thus external stimuli may induce unconventional magnetic characteristics. For example, nano-structure formation such as particles\footnote{Y. Yamamoto {\it et al.}, Physica B {\bf 329-333}, 1183 (2003).} or wires\footnote{X. Teng {\it et al.}, Angew. Chem. Int. Ed. {\bf 47}, 2055 (2008).} provides Pt with ferromagnetic-like properties even at room temperature. In this presentation, we report that a nonmagnetic perturbation in the form of a gate voltage applied through an ionic liquid induces a nonlinear Hall effect in Pt thin films,\footnote{S. Shimizu {\it et al}., Phys. Rev. Lett, in press.} which resembles the anomalous Hall effect induced by the contact to yttrium iron garnet.\footnote{S. Y. Huang {\it et al.}, Phys. Rev. Lett. {\bf 109}, 107204 (2012).} Analysis of detailed temperature and magnetic field experiments indicates that the evolution of the nonlinear Hall effect can be explained in terms of large local moments. The applied electric field triggers an electrochemical reaction at the solid/liquid interface and induces magnetic moments as large as $\sim$10 $\mu_{\rm B}$ that follow the Langevin function. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M53.00013: Novel Collective Excitation in Quantum Confined Pb(111) Films Ao Teng, Van Wesep Robert, Mustafa \"Ozer, Adolfo Eguiluz, Hanno Weitering Atomically smooth Pb(111) films were deposited on a Ge(111)-2x8 substrate and studied by high-resolution electron energy loss spectroscopy. A plasmonic feature with a positive momentum dispersion is observed at low energy, ranging in energy location from 0.3 eV in eight monolayer (ML) thin films to 1.7eV in 40 ML thick films. This excitation is no longer observable below 5 ML. Time dependent density functional theory (TDDFT) calculations of the dielectric function strongly suggest that in the bulk limit, the 1.7 eV feature can be visualized as being due to a modulation of the dielectric function by interband transitions in an otherwise simple-metal-like material. Ab initio calculations of the electronic structure within DFT have been performed to identify the specific nature of these interband transitions and their energy dependence as a function of the film thickness, from the ultrathin quantum size regime to the bulk limit. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M53.00014: Quantum confinement induced oscillatory electric field on stepped Pb(111) films and its influence on surface reactivity Xiaojie Liu, Cai-Zhuang Wang, Myron Hupalo, Hai-Qing Lin, Kai-Ming Ho, Michael Tringides Using first-principles calculations, we showed that such quantum size effects (QSE) can induce oscillatory electrostatic potential and thus alternating electric field on the surface of the wedge-shaped Pb(111) films. The alternating electric field has significant influence on surface reactivity, leading to selective even or odd layer adsorption preference depending on the charge state of the adatoms, consistent with the odd-layer preference of higher Mg coverage on wedge-shaped Pb(111) films, as observed in experiment. [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M53.00015: Ohmic contact formation assisted by metallic states at the interface of Cu2Te/CdTe Jin-Ho Choi, Ruilong Yang, Wenguang Zhu, Deliang Wang, Kai-Ming Ho, Zhenyu Zhang Ohmic contact to CdTe is a formidable task, but is indispensable for achieving high-efficiency in CdTe thin film solar cells. Using first-principles calculations within density functional theory, we study the structures and Schottky barrier heights (SBHs) of the widely used Cu$_{2}$Te/CdTe contact interfaces. We obtain two main structural configurations of the Cu$_{2}$Te layers on CdTe(111), with physically reasonable formation energies: epitaxial and bulk-like Cu$_{2}$Te layers. The epitaxial Cu$_{2}$Te layers possess Cu-Te bonds with the CdTe(111) surface, maintaining the hexagonal, unreconstructed CdTe(111) structure. In contrast, for the bulk-like Cu$_{2}$Te layered, both unreconstructed and reconstructed CdTe(111) surfaces are possible due to weak interactions between the Cu$_{2}$Te and CdTe interfaces. Detailed calculations of the SBHs at the Cu$_{2}$Te/CdTe interfaces show that the interfaces with an unreconstructed CdTe(111) surface have a low SBH of \textless 0.24 eV, consistent with our experimental observation of \textless 0.3 eV. These findings may serve as an important guidance in future efforts for improving contact quality of semiconductor devices. [Preview Abstract] |
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