A computational study of SrTiO 3 thin film deposition: Morphology and growth modes
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The growth of SrTiO3 (STO) thin films is examined using classical molecular dynamics simulations. First, a beam of alternating SrO and TiO2 molecules is deposited on the (001) surface of STO with incident kinetic energies of 0.1, 0.5, or 1.0 eV/atom. Second, deposition of alternating SrO and TiO2 monolayers, where both have incident energies of 1.0 eV/atom, is examined. The resulting thin film morphologies predicted by the simulations are compared to available experimental data. The simulations indicate the way in which the incident energy, surface termination, and beam composition influence the morphology of the thin films. On the whole, some layer-by-layer growth is predicted to occur on both SrO- and TiO2-terminated STO for both types of deposition processes, with the alternating monolayer approach yielding thin films with structures that are much closer to that of bulk STO.
I. INTRODUCTION
Strontium titanate (SrTiO3 or STO) possesses several properties that make it useful in electronic device applications. For example, STO’s high dielectric constant and low dielectric loss make it an attractive material in ferroelectric thin film capacitors,1 microwave applications,2 tunable resonant circuits,3 and dynamic random access memory applications.4 STO has a paraelectric cubic perovskite crystal structure5,6 above its transition temperature of approximately 105 K; below the transition it displays a tetragonal antiferrodistortive phase.5 Thin films of SrO and TiO2, STO’s component oxides, have been of interest due to their catalytic applications (TiO2)7 and the use of atomic-layer deposition methods to create atomically smooth STO surfaces for use in herterostructures.8 In situ reflection high-energy electron diffraction (RHEED) measurements during lasermolecular beam epitaxy (laser-MBE) of TiO2 films on STO (001) substrates performed by Ong and coworkers indicated various growth modes of TiO2 films.9 In addition, growth dynamics studies of epitaxial SrO films grown on STO (001) substrates carried out by Takahashi et al. indicated that after deposition of the first SrO layer, subsequent layer-by-layer growth of SrO occurred with the atoms occupying positions consistent with bulk SrO.10 a)
Current address: University of California–Berkeley, Berkeley, California 94720. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0229 1994
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J. Mater. Res., Vol. 24, No. 6, Jun 2009 Downloaded: 13 Mar 2015
In addition to examining the deposition of the components of STO, numerous experimental studies have been conducted to better understand STO thin film growth under varying conditions. For example, Khodan and coworkers11 investigated the pulsed laser deposition (PLD) of epitaxial STO films. They observed the formation of smooth multilayer films at temperatures above 873 K. Additionally, they concluded that, although annealing in an oxygen environment at 1373 K improves the functional properties of the STO films significantly, only films deposited at pressu
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