Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L52: Semiconductor Films and Interfaces |
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Sponsoring Units: DCMP Chair: Daniel Dougherty, North Carolina State University Room: Mile High Ballroom 1E |
Wednesday, March 4, 2020 8:00AM - 8:12AM |
L52.00001: A novel surface reconstruction of the TiO2 Anatase (001) surface from machine learning Maximilian Amsler, Ulrich Aschauer Surfaces of semiconductors often exhibit reconstructions that can significantly influence their physical and chemical properties. Especially for catalysts, studying the surface structures at the atomic scale is crucial to gain a better understanding of the catalytic activity and, ultimately, to design improved materials. Atomistic simulations can provide insight, but ab initio structure predictions are rather challenging due to the large supercells required to model 2D surfaces. |
Wednesday, March 4, 2020 8:12AM - 8:24AM |
L52.00002: Theoretical investigation of the GaN (0001) surface reconstruction Fatima AlQuaiti, Alexander Demkov Using density functional theory we study the surface reconstruction of GaN (0001). We compare energetics of the 2x2 and root 3 by root 3 R30 GaN (0001) surface structures and find them to be energetically equivalent. In addition, we systematically compare the results found when using the local density approximation and the generalized gradient approximation. We also consider Ga and La ad-atoms on Ga (0001), and determine the corresponding potential energy surfaces and diffusion rates and compare them with those previously computed for Eu, Ce, and Gd. In addition, similarly to the cases of Eu, Ce, and Gd, the La ad-atom exchange with a surface Ga atom is found energetically favorable and with a much smaller barrier of 0.15 eV. This may indicate a path to forming intermetallic LaGa2 through the surface exchange reaction. |
Wednesday, March 4, 2020 8:24AM - 8:36AM |
L52.00003: Two-dimensional overlayers of Au and Ag on the Ge(111) surface: insights from first-principles calculations Shree Ram Acharya, Shirley Chiang, Talat S. Rahman Experimental observations using low-energy electron microscopy (LEEM) of Au overlayers on the Ge(111) surface have revealed two-dimensional Ge(111)-Au(√3×√3)R30° structure as the only ordered structure being irrespective of Au coverage and sample temperature [1] while Ag overlayers form a number of structures on the same range of coverages and temperatures [2]. The charge density distribution analysis is performed using the density functional theory to see the variation in binding of Ag and Au on the Ge surface taking the thermodynamically stable √3 structures which shows that the Au atoms form strong covalent bond with Ge atoms (height = 0.56 Å) than bond between Ge atoms whereas the Ag atoms does not share charge with the Ge atoms indicating that they are trapped at height 0.73 Å inside the cage of the strong Ge-Ge bond. We will compare the electronic and vibrational structures of additional overlayer structures and strain on those systems to trace the reason behind such a difference in observed structures. |
Wednesday, March 4, 2020 8:36AM - 8:48AM |
L52.00004: 3D atomic mapping of rutile-TiO2(110) supported Vanadium oxide catalyst using X-ray Standing Wave excited XPS Anusheela Das, Yanna Chen, Tien-Lin Lee, KVLV Narayanachari, Michael J Bedzyk XSW excited XPS was used to study atomic-scale structure of 1/2 monolayer vanadium oxide catalyst supported on rutile TiO2(110) single crystal. The experiments were performed at I09 beamline at the Diamond Light Source. Collecting V 2p, O 1s and Ti 2p XPS signals as we scanned across a Bragg peak at different stages of a redox reaction allowed tracking of surface sites for chemically distinct V atoms, important for understanding their catalytic behaviour. V5+ and V4+ 2p XPS peaks were found to be separated by 1.4 eV and were found to have different XSW coherent fraction and position. Summation of these Fourier components for 5 symmetry inequivalent hkl Bragg peaks generates a chemical state sensitive 3D atomic map of vanadium. These results give us unique insights into this as well as other related oxide supported catalysts. |
Wednesday, March 4, 2020 8:48AM - 9:00AM |
L52.00005: Combinatorial Synthesis of a New II-IV-N2: MgSnN2 Ann Greenaway, Amanda L Loutris, Karen Heinselman, Celeste Melamed, Rekha Schnepf, Marshall Tellekamp, Rachel Woods-Robinson, Rachel Sherbondy, Dylan Bardgett, Stephan Lany, Steven Christensen, Adele Tamboli Nitrides with novel compositions are an emerging research topic in basic energy science, with applications in optoelectronics, catalysis, and magnetism. Ternary semiconductors are underexplored and have unique properties, such as transitions between cation ordered and disordered structures, which may be exploited to modify properties in a single composition. We demonstrate combinatorial co-sputtering of a new II-IV-N2, MgSnN2, in a phase-pure wurtzite structure across a range of cation compositions and from 200 to 400 °C. Below 200 °C, a metastable rocksalt phase is observed which is a potential wide bandgap, high dielectric constant material. Ellipsometry reveals an optical absorption onset at ~2 eV for the wurtzite phase, suggesting tuning via cation disorder from the predicted bandgap of 2.5 eV; disorder is confirmed by synchrotron x-ray diffraction. Finally, we demonstrate epitaxial growth via combinatorial co-sputtering on GaN. Based on this work, MgSnN2 is a target for fundamental exploration of novel nitride properties as well as a candidate for future device applications. |
Wednesday, March 4, 2020 9:00AM - 9:12AM |
L52.00006: Temperature-Dependent Spectroscopy of Rutile Tin Dioxide Hawazin Alghamdi, Benjamin Concepcion, Sugata Chowdhury, Prabhakar Misra The present spectroscopic investigation focuses on rutile tin dioxide (SnO2) in powder form. Raman spectroscopy with laser excitation at 780 nm has been used to characterize the different vibrational modes of SnO2. Thermal effects on the vibrational features in the Raman spectrum have been studied in the range 30 – 170 °C. We have demonstrated a red-shift in the Raman spectra as the temperature increases for both Α1g (634 cm-1) and B2g (775 cm-1), while Eg (475 cm-1) exhibited little change. FT-IR spectra have been obtained in order to study the IR-active vibrational modes for tin dioxide: Sn-O stretching vibration at 467.36 cm-1 and Sn-O-Sn asymmetric vibration at 569.37 cm-1. X-ray diffraction (XRD) spectra have been recorded to confirm the rutile structure of tin dioxide. Scanning Electron Microscope (SEM) images have provided information regarding the size of the spherical grain particles. Molecular Dynamic simulation (MD) has been utilized to study the various vibrational modes at different temperatures using the LAMMPS software. |
Wednesday, March 4, 2020 9:12AM - 9:24AM |
L52.00007: First-Principles Theory for Schottky Barrier Height Dmitry Skachkov, Xiaoguang Zhang, Hai-ping Cheng The physics of Schottky contact is very challenging and there are exist only simple empirical models for Schottky barrier. In current work we are developing the first-principles theory for the Schottky barrier height. Our method based on density functional theory and consists of calculation of evanescent states in the gap of the semiconductor based on complex band structure of the semiconductor surface, following by calculation of the induced charge in the bulk of semiconductor. Next we find iteratively an equilibrium between the induced charge and the electrostatic potential. The method is tested on the GaAs(111) – graphene system. |
Wednesday, March 4, 2020 9:24AM - 9:36AM |
L52.00008: Targeted High-Throughput Growth and Automated Phase Mapping of the Novel Semiconductor Zn2SbN3 Using the Analysis Package COMBIgor Allison Mis, Elisabetta Arca, Geoff Brennecka, Adele Tamboli High-throughput materials discovery has boomed in the past decade through the pairing of combinatorial growth methods and automated characterization routines. However, much of the data collected must still be processed manually, creating a bottleneck, notably for phase identification and phase space mapping. In this work, we will present semi-automated phase identification and targeted sputter growth of the novel semiconducting material Zn2SbN3, to date the only reported crystalline antimony nitride in which Sb functions as a cation with a positive oxidation state. This ternary nitride has a predicted effective electron mass of (0.15-0.19me) and direct band gap of 1.7 eV that may be tunable with cation disorder. X-ray diffraction patterns were run through an automated fitting routine, the fit parameters of which were fed into routines for the creation of phase maps, tracking thin-film texturing, and directed growth through the use of fit variables as proxies for properties like optical absorption. This work will present an expanded understanding of this unique and promising material as well as a widely-applicable add-on to the free, open-source data handling and analysis package COMBIgor. |
Wednesday, March 4, 2020 9:36AM - 9:48AM |
L52.00009: Transition-metal nitride semiconductors Baiwei Wang, Poomirat Nawarat, Kim Lewis, Panos Patsalas, Daniel Gall Semiconducting transition-metal nitride alloys emerge as a new category of semiconductors that are stable in hostile environments and are compatible with conventional semiconductor device processing. We report on the epitaxial growth and the electrical and optical properties of Ti1−xMgxN, Ti1−xMgxC0.2N0.8, and (Ti0.5Mg0.5)1−xAlxN on MgO(001) and (Ti1−xMgx)0.25Al0.75N on Al2O3(0001). X-ray diffraction reciprocal space mapping is used to quantify composition dependent lattice parameters and critical thicknesses for strain relaxation. Ti0.5Mg0.5N is a new semiconductor with a bandgap of 0.7-1.7 eV. Reducing the Mg-content in Ti1−xMgxN to x < 0.5 leads to a tunable plasmonic response that extends from infrared to visible (930 - 470 nm). The substitutional addition of Al and C on anion and cation sites, respectively, reduces the free carrier density: (Ti0.5Mg0.5)1−xAlxN layers show a negligible density of states at the Fermi level and a blue-shift of the absorption edge from 1.8 to 2.1 eV for x = 0 - 0.4. Ti1−xMgxC0.2N0.8 exhibits a metal to semiconductor to insulator transition as x increases from 0.3 to 0.4 to 0.5. Wurtzite-structure (Ti1−xMgx)0.25Al0.75N(0001) layers have a measured bandgap that increases from 5.1 to 5.2 to 5.9 eV for x = 0.0, 0.5 and 1.0. |
Wednesday, March 4, 2020 9:48AM - 10:00AM |
L52.00010: Unpinning the relationship between pinning factor and electronegativity by calculating Schottky barrier heights from first principles Nicole Hall, Ismaila Dabo We calculate the Schottky barriers that develop at the interface between a metal and semiconductor. These barriers govern the current-rectification ability of Schottky diodes and the electrical response of metal-semiconductor field effect transistors. A model that accounts for the alignment of the electronic levels across the metal-semiconductor interface and the long-range polarization within the space charge region enables us to predict the heights of the barriers as a function of the electronic offsets, including the effects of charge trapping and Fermi-level pinning. We validate our computational predictions against experimental measurements [1], providing a detailed understanding of trends in the pinning factor, i.e., the change in the Schottky barrier height as a function of the metal work function. The resulting model provides a comprehensive first-principles approach to simulate the characteristics of semiconductor-metal junctions under bias. |
Wednesday, March 4, 2020 10:00AM - 10:12AM |
L52.00011: Ultrafast Electron Diffraction of Monoclinic GaTe and the Dynamic Breaking of Friedel's Law Qingkai Qian, Xiaozhe Shen, Lanxin Jia, Renkai Li, Jie Yang, Duan Luo, Xijie Wang, Shengxi Huang GaTe is a low-symmetry layered materials with strong anisotropy, which has been widely studied in recent years. Various applications, such as photodetectors, nonlinear optics and thermoelectric devices, have been demonstrated based on GaTe. Here the electron-phonon coupling and the lattice dynamics of GaTe are studied by ultrafast electron diffraction (UED). Besides the Debye-Waller effect caused by the incoherent electron-phonon coupling, we observed a coherent breathing phonon mode in GaTe. Different from the LA breathing modes of high-symmetry 2D materials, the displacement direction of the LA breathing phonon modes in GaTe has deviated incommensurately from the stacking lattices. In-phase and out-of-phase oscillations of diffraction intensities are observed for each Friedel pairs, which in fact break the Friedel's law dynamically. This dynamic breaking of Friedel's law can be explained by the wobbling of the crystal lattice and the Ewald sphere, and the related study can be used as a reference for UED characterization of other low-symmetry thin films. |
Wednesday, March 4, 2020 10:12AM - 10:24AM |
L52.00012: Temperature dependent X-ray studies of negative thermal expansion (NTE) ScF3 thin films under strain Amani Jayakody, Zhiwei Zhang, Zhihai Zhu, Hope R Whitelock, Joseph I Budnick, Jason N Hancock, Barrett O Wells
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Wednesday, March 4, 2020 10:24AM - 10:36AM |
L52.00013: A revised three-dimensional electron gas model to describe surface plasmonic phenomena Jiantao Kong The long wavelength limit negative slope of the surface plasmon dispersion curve (for simple metals such as alkali) has been experimentally [1] and theoretically [2] investigated for a long time. Yet there is no universally accepted theory. On the other hand, the traditional 3D electron gas model such as Lindhard function [3] is successful in explaining many bulk properties, but not much attempt in literature has been made to account for the surface effects. We worked on the traditional 3D electron gas model, modifying the electron self-energy [4] with a correction due solely to surface, and produced the negative slope naturally from simple calculation. In other words, a revised 3D electron gas model can predict and describe surface plasmonic phenomena by itself. |
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L52.00014: Low energy excitons in GaN film under strong magnetic fields Yi wang, Liuyun Yang, Xinqiang Wang, Changli Yang, Chi Zhang We investigate magneto- far-infrared (FIR) transmission on a 100 μm thick GaN film, which is grown on a Si/HR-GaN substrate. Under zero magnetic (B) fields, we observe three absorption peaks, which are located at the energy of 30.6 meV (Peak I), 34.7 meV (II), and 40.0 meV (III), respectively. Under perpendicular B-fields, small energy shifts of the three absorption peaks (I, II, III) are all linear to B. Meanwhile, peak II splits into two distinct peaks, with a distance about 0.08 meV/T in between. And three distinct splits are observed around Peak III with a distance of about 0.11 meV/T between neighbor peaks. We propose that the patterns of the absorption can be attributed to the low energy excitons in GaN under strong magnetic fields. |
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