Effect of film composition on the orientation of (Ba,Sr)TiO 3 grains in (Ba,Sr) y TiO 2+y thin films
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Effect of film composition on the orientation of (Ba,Sr)TiO3 grains in (Ba,Sr)yTiO2+y thin films Debra L. Kaiser, Mark D. Vaudin, Lawrence D. Rotter, John E. Bonevich, and Igor Levin Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
John T. Armstrong Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Alexander L. Roytburd Department of Materials and Nuclear Engineering, University of Maryland, College Park, Maryland 20742
Darrell G. Schlom Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802-5005 (Received 8 February 1999; accepted 19 September 1999)
Thin films of composition (Ba,Sr)yTiO2+y, with 0.43 艋 y 艋 1.64, were deposited by metalorganic chemical vapor deposition on (100) MgO substrates at various growth conditions. X-ray diffraction and transmission electron microscopy studies showed that the films were composed of epitaxial Ba1−xSrxTiO3 (x ≈ 0.06) grains and an amorphous phase. The orientation of the tetragonal Ba1−xSrxTiO3 grains (pure a axis, pure c axis, or a mix of the two) was found to be strongly dependent upon film composition. This composition dependence is explained for the majority of the Ti-rich films by an analysis of average strains in the two-phase films, assuming a compressive strain of ≈1% in the amorphous phase.
I. INTRODUCTION
Thin films of barium titanate, BaTiO3, and barium strontium titanate, Ba1−xSrxTiO3, are of interest for a variety of electronic1,2 and photonic3,4 devices. For BaTiO3, many of the properties of the low-temperature tetragonal phase, which forms from the parent hightemperature cubic phase at the Curie temperature (Tc ≈ 130 °C5), are highly anisotropic. For example, the roomtemperature dielectric constants of bulk single crystal BaTiO3 along the c and a axis directions are 160 and 4100, respectively.6 Thus, the orientation of BaTiO3 is critical to its application in a ferroelectric device. For many devices, the BaTiO3 thin film must be electrically poled along the c axis, requiring that the c axis be normal to the substrate surface (c axis orientation) or in the plane of the substrate (a axis orientation). Recent studies have reported films containing epitaxial a-axisoriented BaTiO37–13 or c-axis-oriented BaTiO314–18 grown by a variety of techniques on single-crystal substrates of (100) MgO,8,11–15,17,18 (100) SrTiO3,7,14 (100) LaAlO3,9,10 (001) NdGaO3,10 (100) Pt/SrTiO3,7 and (001) BaO/Si.16 In each of these studies, only one set J. Mater. Res., Vol. 14, No. 12, Dec 1999
of deposition conditions was given for film growth, so the effect of film composition on BaTiO3 orientation was not reported. The present study was undertaken to investigate the effect of deposition conditions on the microstructure and orientation of barium titanium oxide thin films (containing a small amount of Sr as an impurity) that were grow
