The effect of oxygen vacancies and strain on the optical bandgap of strained SrTiO$_{3}$ thin films

ORAL

Abstract

SrTiO$_{3}$ (STO) films were grown on single crystal SrTiO$_{3}$ p-Si (001) substrates using molecular beam epitaxy (MBE). The single-phase STO/Si films were of high crystalline quality as verified by x-ray diffraction (XRD) and atomic force microscopy (AFM) with an rms roughness of less than 0.5 nm. Oxygen vacancies were introduced by controlling the oxygen pressure (varied from 10$^{-8}$ to10$^{-7}$ torr) during growth. Both thickness variation and oxygen pressure alter the crystal structure and electronic properties of STO. The lattice mismatch of STO on Si causes a 1.7{\%} bi-axial, compressive strain. The oxygen vacancies cause a tensile strain because of the different Ti$^{3+}$ and Ti$^{4+}$ ionic radii. This agrees with our XRD measurements that show a decrease of the out of plane lattice constant as either the thickness or the oxygen pressure during growth increases. We used a Variable Angle Spectroscopic Ellipsometer M-2000 by Woolam and the VASE software to measure and model the optical properties of the films using Tauc-Lorentz oscillators for the STO layer and directly measured optical properties of Si and STO substrates. Our results show that the indirect bandgap of STO decreases as either the thickness increases or the oxygen vacancies decrease, in agreement with theoretical calculations. [1]

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Authors

  • Nathan Steinle

    Undergraduate Research Assisstant

  • Barry Koehne

    Undergraduate Research Assisstant

  • Ryan Cottier

    Texas State University, Postdoc

  • Darren Depoy

    University of Michigan, Univeristy of Michigan, Texas A\&M University, UC Riverside, University of Massachusetts, STScI, NOAO, University of Texas, Cyclotron Institute, Texas A\&M University, ENEA, Italy, INFN, Italy, University of Texas at Austin, TX, Cyclotron Institute, TAMU, TX, Cyclotron Institute TAMU, TX, Cyclotron Institute - Texas A\&M University, Texas A\&M Univ, Texas A\&M University, Baylor University, Department of Physics and Astronomy, Texas A\&M University, College Station, Texas, School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai, 200240, China, Department of Materials Science and Engineering, Texas A\&M University, College Station, Texas, Department of Mechanical Engineering, Texas A\&M University, College Station, Texas, Texas State University, University of Texas at Arlington, Lawrence Berkeley National Laboratory, DESY, Department of Physics and Astronomy, Texas A\&M University, Department of Physics, Syracuse University, Department of Physics and Astronomy, Vanderbilt University, Texas A\&M University, Princeton University and Baylor University, Texas A\&M Univ at Qatar, Kazan Federal University, Max Planck Institute for Nuclear Physics, Institue for Quantum Science and Engineering (IQSE) and Department of Physics \& Astronomy, Texas A\&M University, College Station, Texas 77843, USA, MIT, Sandia National Laboratories, Albuquerque, New Mexico, Postgrad, ORNL, Muons Inc., Texas Lutheran University, West Virginia University, Lawrence Livermore National Laboratory, University of Kentucky, US Naval Academy, University of Dallas, U.S. Naval Academy, Univ of Texas, Arlington, Department of Physics, Texas A\&M University, College Station, TX 77845, Science and Petroleum Departments, Texas A\&M University in Qatar, Doha, Qatar, Rice University, Houston, Texas, USA, Department of Physics, Texas State University, Florida A\&M University, Texas A\&M University in Qatar, Doha, Qatar, Physics and Astronomy Dept., TCU, Geology Dept., TCU, Colorado College, University of Texas at El Paso, University of Texas at Brownsville, Rochester Institute of Technology, Baylor University, Texas A\&M, Princeton University, Baylor University, Princeton University, Texas A\&M University, Materials Science and Engineering and Department of Physics and Astronomy, Texas A\&M University; WPI-AIMR, Tohoku University, Japan, WPI-AIMR, Tohoku University, Department of Chemistry, Texas A\&M University, Materials Science and Engineering and Department of Physics and Astronomy, Texas A\&M University, WPI-AIMR, Tohoku University, Japan, Department of Physics and Astronomy, Texas A\&M University, College Station, Texas 77843, USA, Department of Physics and Astronomy, Texas A\&M University and WPI-Advanced Institute for Materials Research, Tohoku University, Japan, University of Texas at Austin, Center for Nonlinear Dynamics and Department of Physics, The University of Texas at Austin, JILA, NIST, Department of Physics and Astronomy, Texas A\&M University and Institute for Quantum Studies and Engineering, College Station, TX 77843-4242, USA, Institute of Applied Physics of the Russian Academy of Sciences, 46 Ulyanov street, Nizhny Novgorod 603950, Russia, Department of Physics, University of Texas at Austin, Rice University, Texas A&M University

  • Nikoleta Theodoropoulou

    Texas State University, Research Advisor