Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session R28: Semiconductor Films, Defects, and Topological Materials |
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Sponsoring Units: DCMP Chair: Kenjiro Gomes, University of Notre Dame Room: 327 |
Thursday, March 17, 2016 8:00AM - 8:12AM |
R28.00001: Nanoscopic oxidation of p-type and un-doped Si (100) surfaces using un-externally biased atomic force microscope tips (AFM) in the presence of selected organic solvents Jeffrey McCausland, Sajeevi Withanage, Robert Mallik, Sergei Lyuksyutov A conductive un-biased AFM tip oscillating above p-type or un-doped Si (100) treated with toluene, butan-2-ol, and propan-2-ol creates nanostructures ranging in height from 1-100 nm. The tip was oscillated in ambient conditions (30-70{\%} Rel. Humidity) at frequencies in the 10$^{2}$ kHz range. It was repeatable with various concentrations of solvent in aqueous solution. It is suggested that mechanical oscillations of the AFM tip polarizes the solvent molecules deposited on the surface resulting in electron transfer from the tip to the surface followed by feature formation. This process effectively creates an electrochemical cell at the microscopic level and the miscibility of the solvents is the key to enabling the process. Species which ionize during the process may be consumed in irreversible reactions whereas the alcohols act as catalysts and are not consumed. The influence of boron defects in the Si substrates is also discussed. It appears that the observed oxidation is different from all other similar reported phenomena including local anodic oxidation, and chemo-mechanical lithographic techniques utilizing AFM. [Preview Abstract] |
Thursday, March 17, 2016 8:12AM - 8:24AM |
R28.00002: Interface investigation of solution processed high-$\kappa $ ZrO$_{\mathrm{2}}$/Si MOS structure by DLTS Arvind Kumar, Sandip Mondal, KSR Koteswara Rao The interfacial region is dominating due to the continuous downscaling and integration of high-$k$ oxides in CMOS applications. The accurate characterization of high-$k$ oxides/semiconductor interface has the significant importance towards its usage in memory and thin film devices. The interface traps at the high$-k$/semiconductor interface can be quantified by deep level transient spectroscopy (DLTS) with better accuracy in contrast to capacitance-voltage (CV) and conductance technique. We report the fabrication of high-$k$ ZrO$_{\mathrm{2}}$ films on p-Si substrate by a simple and inexpensive sol-gel spin-coating technique. Further, the ZrO$_{\mathrm{2}}$/Si interface is characterized through DLTS. The flat-band voltage (V$_{\mathrm{FB}})$ and the density of slow interface states (oxide trapped charges) extracted from CV characteristics are 0.37 V and 2x10$^{\mathrm{-11\thinspace }}$C/cm$^{\mathrm{2}}$, respectively. The activation energy, interface state density and capture cross-section quantified by DLTS are E$_{\mathrm{V}}+$0.42 eV, 3.4x10$^{\mathrm{11}}$ eV$^{\mathrm{-1\thinspace }}$cm$^{\mathrm{-2\thinspace }}$and 5.8x10$^{\mathrm{-18\thinspace }}$cm$^{\mathrm{2}}$, respectively. The high quality ZrO$_{\mathrm{2}}$ films own high dielectric constant 15 with low leakage current density might be an appropriate insulating layer in future electronic application. The low value of interface state density and capture cross-section are the indication of high quality interface and the defect present at the interface may not affect the device performance to a great extent. The DLTS study provides a broad understanding about the traps present at the interface of spin-coated ZrO2/Si. [Preview Abstract] |
Thursday, March 17, 2016 8:24AM - 8:36AM |
R28.00003: A New Approach for Surface Energy Calculations Applicable to High-throughput Design of New Interfaces Christian Ratsch, Jakub Kaminski In this talk we will present a new approach for the calculation of surface energies of periodic crystal. For non-polar materials slabs (which are terminated by two identical surfaces) the task of calculating the surface energy is trivial. But it is more problematic for polar systems where both terminating surfaces are different, as there is no single established method allowing for equal treatment of a wide range of surface morphologies and orientations. Our proposed new approach addresses this problem. It relies on carefully chosen capping atoms and the assumptions that their bond energy contributions can be used to approximate the total energy of the surface. The choice of the capping atoms is governed by a set of simple guidelines that are applicable for surfaces with different terminations. We present the results for different semiconductor materials and show that our approach leads to surfaces energies with errors as low as 2{\%}. We show that hydrogen is not always the best choice for a capping atom if accurate surface energies are the target of the calculations. Our approach is suitable for high-throughput screening of new material interfaces, as accurate calculations of surface energies can be performed in an unsupervised algorithm. [Preview Abstract] |
Thursday, March 17, 2016 8:36AM - 8:48AM |
R28.00004: Schottky Barrier Mapping to Nanoscale Dimensions Utilizing Ballistic Electron Emission Microscopy Vincent LaBella, Robert Balsano, Chris Durcan, Wes Nolting The Schottky barrier is the electrostatic barrier between a metal and a semiconductor that results in rectification and is found in many types of devices such as source drain contacts to sub 20-nm-node transistors. Nanoscale fluctuations in the Schottky barrier height can occur due to variations in bonding, compositional fluctuations in the materials, and the presence of defects. Measuring and mapping these electrostatic fluctuations to nanoscale dimensions can be achieved with ballistic electron emission microscopy (BEEM) an STM based technique. This presentation will demonstrate how the Schottky barrier height can be mapped to nanoscale dimensions using BEEM at 77K and under ultra-high vacuum. The STM tip is positioned on a regularly spaced grid and BEEM spectra are acquired from which the barrier height can be extracted. Maps and histograms can be generated by measuring and fitting thousands of these spectra. These maps provide detailed insight into the electrostatic fluctuations occurring at the buried interface with nanoscale resolution that cannot be accomplished with other bulk measurements. [Preview Abstract] |
Thursday, March 17, 2016 8:48AM - 9:00AM |
R28.00005: Hot Electron Scattering in Thin Metal Films Utilizing Ballistic Electron Emission Microscopy Christopher Durcan, Westly Nolting, Robert Balsano, Vincent LaBella Electron scattering in nm-thick metal films has fundamental and technological importance. Ballistic Electron Emission Microscopy (BEEM) an STM based technique can be utilized to measure the scattering rate and understand the scattering mechanisms. By injecting electrons from the STM tip in the energy range of 0.2 eV- 1.5 eV into the metal base of a metal semiconductor diode and measuring the amount of current collected in the semiconductor a Schottky barrier height can be measured. In addition, by measuring the decay in the collector or BEEM current vs. metal film thickness, an electron attenuation length can be measured. One question has always been; what are these BEEM attenuation lengths sensitive to? Intrinsic properties of the metal, or extrinsic effects such as the structure of the film? By measuring the attenuation length of W and Cr and comparing to prior measurements of Cu, Ag, Au a comparison between the BEEM attenuation length and resistivity can be achieved over an order of magnitude in resistivity. The results show an inverse relationship that one expects for mean free path and resistivity, indicating that BEEM measurements are sensitive to the intrinsic properties of the metal and not solely the structure of the films. [Preview Abstract] |
Thursday, March 17, 2016 9:00AM - 9:12AM |
R28.00006: \textbf{Surface phase stability and surfactant behavior of InAsSb alloy surfaces.} Evan M Anderson, Adam M Lundquist, Chris Pearson, Joanna M Millunchick InAsSb has the narrowest bandgap of any of the conventional III-V semiconductors: low enough for long wavelength infrared applications. Such devices are sensitive to point defects, which can be detrimental to performance. To control these defects, all aspects of synthesis must be considered, especially the atomic bonding at the surface. We use an \textit{ab initio} statistical mechanics approach that combines density functional theory with a cluster expansion formalism to determine the stable surface reconstructions of Sb (As) on InAs (InSb) substrates. The surface phase diagram of Sb on InAs is dominated by Sb-dimer termination $\alpha $2(2x4) and $\beta $2(2x4) and c(4x4). Smaller regions of mixed Sb-As dimers appear for high Sb chemical potentials and intermediate As chemical potential. We propose that InAsSb films could be grown on (2x4), which maintain bulk-like stoichiometry, to eliminate the formation of typically observed n-type defects. Scanning tunneling microscopy and reflection high energy electron diffraction confirm the calculated phase diagram. Based on these calculations, we propose a new mechanism for the surfactant behavior of Sb in these materials. [Preview Abstract] |
Thursday, March 17, 2016 9:12AM - 9:24AM |
R28.00007: Formation, properties, and function of vacancies in Si/Ge Clathrates: The importance of broken symmetries Amrita Bhattacharya, Christian Carbogno, Matthias Scheffler Inclusion compounds, such as clathrates, are cage-like crystal structures that can encapsulate guest atoms. Since this allows to tune their electronic and vibrational properties, they are regarded as interesting materials for thermoelectric applications. Progress in this field is, however, hindered by the fact that filling of group-IV clathrates often results in complex and unexpected structural changes, e.g., the spontaneous formation of vacancies in certain hosts: In Ge$_{46}$ clathrates filled with K or Ba, the most favourable phases K$_8$Ge$_{44}$/ Ba$_8$Ge$_{43}$ feature two/three vacancies. Conversely, the framework of the isoelectronic Si$_{46}$ clathrate remains intact~(K$_8$Si$_{46}$/Ba$_8$Si$_{46}$) upon filling with the exact same guests. Our first-principles calculations of the formation energies and of the thermodynamic phase stabilities confirm this experimental scenario and shed light on the underlying mechanisms. Due to the spatially more delocalized 4sp$^3$ orbitals in Ge compared to the more localized 3sp$^3$ orbitals in Si, fundamentally different symmetry breaking distortions become possible to stabilize the vacancies. Eventually, we discuss the implications of these findings for the thermoelectric properties of clathrates. [Preview Abstract] |
Thursday, March 17, 2016 9:24AM - 9:36AM |
R28.00008: Symmetry of Highly Strained ZnSnN$_{\mathrm{2\thinspace }}$Thin Films Nancy Senabulya, Yongsoo Yang, Christian Schleputz, Nathaniel Feldberg, Robert Makin, Christina Jones, Steven Durbin, Roy Clarke Zinc Tin Nitride (ZnSnN$_{\mathrm{2}})$ is a member of the ternary class of II-IV-V$_{\mathrm{2}}$ semiconducting materials that have gained significant research interest in the recent past as a cheaper, earth abundant and environmentally friendly alternative to Indium-based materials used in photovoltaic and solid state lighting applications. Surface x-ray diffraction measurements performed at Argonne National Laboratory on single crystal thin films of ZnSnN$_{\mathrm{2}}$ grown on (111)yttria stabilized zirconia(YSZ) substrates show a structural change from the wurtzite to the orthorhombic phase in films grown under low values of nitrogen flux and high substrate temperatures. This orthorhombic phase is characterized by in plane contraction and out of plane elongation of the unit cell lattice parameters, a phenomenon that theoretically results from the doubling of the wurtzite unit cell in the basal plane and ordering on the cation sub lattice [APL 103,042109(2013)].We are currently studying the crystal structure of ZnSnN$_{\mathrm{2}}$ thin films using 3-dimensional reciprocal space maps and pole figure measurements in order to characterize the high symmetry orthorhombic phase achieved using epitaxy. [Preview Abstract] |
Thursday, March 17, 2016 9:36AM - 9:48AM |
R28.00009: Surface charge transport in Silicon (111) nanomembranes Weiwei Hu, Shelley Scott, RB Jacobson, Donald Savage, Max Lagally Using thin sheets (“nanomembranes”) of atomically flat crystalline semiconductors [1], we are able to investigate surface electronic properties, using back-gated van der Pauw measurement in UHV. The thinness of the sheet diminishes the bulk contribution, and the back gate tunes the conductivity until the surface dominates, enabling experimental determination of surface conductance [2]. We have previously shown that Si(001) surface states interact with the body of the membrane altering the conductivity of the system. Here, we extended our prior measurements to Si(111) in order to probe the electronic transport properties of the Si(111) 7$\times$7 reconstruction. Sharp (7$\times$7) LEED images attest to the cleanliness of the Si(111) surface. Preliminary results reveal a highly conductive Si(111) 7$\times$7 surface with a sheet conductance $R_{\mathrm{s\thinspace }}$of order of $\mu $S/$\mathqed\Box$, for 110nm thick membrane, and $R_{\mathrm{s}}$ is a very slowly varying function of the back gate voltage. This is in strong contrast to Si(001) nanomembranes which have a minimum conductance several orders of magnitude lower, and hints to the metallic nature of the Si(111) surface. 1. Zhang, P. P.\textit{ et al.}, Nature 439, 703-706 (2006); 2. W. Peng, \textit{et al.}, Nature Commun. 4, 1339 (2013). [Preview Abstract] |
Thursday, March 17, 2016 9:48AM - 10:00AM |
R28.00010: Detection of a History Dependent Topological Hall Effect Due to Skyrmion Formation in FeGe Thin Films James Gallagher, Michael Page, Vidya Bhallamudi, Jack Brangham, Keng Yuan Meng, Bryan Esser, Hailong Wang, Dave McComb, Chris Hammel, Fengyuan Yang B20 phase crystal structures, such as FeGe and MnSi, have been of interest because they enable magnetic skyrmion phases, which can potentially lead to low energy cost spintronic device applications. We report the synthesis of pure phase FeGe epitaxial thin films grown on Si (111) substrates by ultra-high vacuum off-axis magnetron sputtering. The FeGe films were characterized by x-ray diffraction, scanning transmission electron microscopy (STEM) and Hall effect measurements. The topological Hall effect (THE) signals were extracted by subtracting out the anomalous Hall effect and ordinary Hall effect, demonstrating the existence of the skyrmion phase in FeGe films between 5 and 275 K. Topological hall effect was observed at zero field at all temperatures below the Curie temperature, showing the possibility of metastable skymion particles at zero field and high temperatures. We will also discuss the study of dynamics of the ferromagnetic phases using ferromagnetic resonance. [Preview Abstract] |
Thursday, March 17, 2016 10:00AM - 10:12AM |
R28.00011: Topological Imbert-Fedorov Shift in Weyl Semimetals Qing-Dong Jiang, Hua Jiang, Haiwen Liu, Qing-Feng Sun, Xin-Cheng Xie The Goos-Hänchen (GH) shift and the Imbert-Fedorov (IF) shift are optical phenomena which describe the longitudinal and transverse lateral shifts at the reflection interface, respectively. Here, we predict the GHIF shifts in Weyl semimetals (WSMs)—a promising material harboring low energy Weyl fermions, afermionic cousin of photons. Our results show that the GH shift in WSMs is valley independent,is analogous to that discovered in a 2D relativistic material—graphene. However, the IF shift hasbeen explored in nonoptical systems, and here we show that it is valley dependent. Furthermore, wethat the IF shift actually originates from the topological effect of the system. Experimentally, theIF shift can be utilized to characterize theWeyl semimetals, design valleytronic devices of high, and measure the Berry curvature. Morever, we investigate the transport properties of topological semimetal using the wave-packet dynamics, which give some interesting results. [Preview Abstract] |
Thursday, March 17, 2016 10:12AM - 10:24AM |
R28.00012: Hybridization-induced interface states in a topological insulator - ferromagnetic metal bilayer Yi-Ting Hsu, Priyamvada Jadaun, Craig Fennie, Eun-Ah Kim Recent experiments demonstrating large spin-torque in topological insulator(TI)/ferromagnetic metal(FM) bilayer, revealing their potential for spintroics applications raised much excitement. However, there is little understanding on the impact of the bilayer formation on the TI surface state and whether it is possible to represent such bilayer using a simple model. Moreover, due to the large charge-transfer from the FM layer, these Dirac surface states are unlikely to be anywhere near the fermi level to contribute to the observed spin-torque. In order to establish a theoretical starting point, we calculated the band structure of a TI-FM bilayer using density functional theory (DFT) and built a simple effective model that captures the essence of the DFT results. Through this double-pronged approach, we find new surface states we dubbed “reflection states” to form close to chemical potential due to level-repulsion between the original Dirac surface states and the energetically close-by FM states with the same momentum. Depending on the coupling strength, the ‘reflection’ states can carry a large weight of the original surface states and thus inherit not only the spatial localization but also the spin-winding of the original Dirac surface state. [Preview Abstract] |
Thursday, March 17, 2016 10:24AM - 10:36AM |
R28.00013: Local electronic structures and 2D topological phase transition of ultrathin Sb films SungHwan Kim, Kyung-Hwan Jin, Joonbum Park, Jun Sung Kim, Seung-Hoon Jhi, Han Woong Yeom We investigate local electronic structures of ultrathin Sb islands and their edges grown on Bi$_2$Te$_2$Se by scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations. The Sb islands of various thickness are grown with atomically well ordered edge structure over the 3 bilayers (BL). On the surfaces and edges of these islands, we clearly resolve edge-localized electronic states by STS measurements, which depend on the thickness. The DFT calculations identify that the strongly localized edge states of 4 and 5 BL films correspond to a quantum spin Hall (QSH) states while the edge states of 3 BL are trivial. Our experimental and theoretical results confirm the 2D topological phase transition of the ultrathin Sb films from trivial to QSH phase. [Preview Abstract] |
Thursday, March 17, 2016 10:36AM - 10:48AM |
R28.00014: The novel properties of epitaxial bismuth ultra-thin films on superconducting substrate NbSe$_2$ Dandan Guan, Haohua Sun, Meixiao Wang, Guanyong Wang, Dan Xu, Xiaojun Yang, Zhu-An Xu, Yaoyi Li, Canhua Liu, Dong Qian, Jin-Feng Jia Bismuth is theoretically predicted to be a quantum spin Hall (QSH) system. Such a QSH system, interesting itself with the genuine spin degenerated backscattering forbidden transport property, is also a potential platform to study the 2D topological superconductor with superconductivity induced by proximity effect. The epitaxial growth of ultra-thin Bi(111) film on superconductor NbSe$_{2}$ was investigated by scanning tunneling microscopy(STM) and scanning tunneling spectroscopy(STS). The orientation transition from Bi(110) phase to Bi(111) phase has been observed. One-dimensional topological edge states on the zig-zag edge of Bi(111) bilayers and proximity effect-induced superconductivity were also revealed by STS analysis. [Preview Abstract] |
Thursday, March 17, 2016 10:48AM - 11:00AM |
R28.00015: Vertical electrical field induced island growth in layered TiSe2. Husong Zheng, Salvador Valtierra, Nana Opoku, Chuanhui Chen, Liying Jiao, Kirk Bevan, Chenggang Tao For practical applications of atomically thin transition metal dichalcogenides, it is essential to characterize the structural stability under external stimuli such as electrical fields. Using vacancy monolayer islands on TiSe2 surfaces as a model system, we experimentally and theoretically investigated the shape evolution and growth rate by using scanning tunneling microscopy. The growth rate that depends on the tunneling current is experimentally determined. Our simulations of monolayer island evolution using a phase-field model are consistent with the experimental observations. The results could be potentially important for electronic device applications of ultrathin transition metal dichalcogenides and other 2D materials. [Preview Abstract] |
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