On the microstructure, chemistry, and dielectric function of BaTiO 3 MOCVD thin films
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Thin films of BaTiO3 deposited on (100)LaAlO3 substrate by metal-organic chemical vapor deposition (MOCVD) are investigated using several electron-optical techniques. Combined high resolution transmission electron microscopy (HRTEM), electron energy loss spectrometry (EELS), and convergent beam electron diffraction (CBED) indicate a substantial influence of lattice strain on the structural and optical characteristics of BaTiO3 films. Spatially resolved EELS and CBED studies indicate that the substrate influence persists up to about 40 nm away from the interface. The changes in the dielectric function of the films, as inferred from spatially resolved EELS, appear to correlate well with internal lattice strain in the films as deduced from convergent beam electron diffraction (CBED).
I. INTRODUCTION There has been considerable interest in recent years in the development of ferroelectric perovskite thin films. The outstanding optical, dielectric, and ferroelectric properties of perovskites, coupled with the ability to deposit epitaxial single crystal thin films, make these materials extremely desirable for electro-optic and memory storage devices.1'2 The properties of thin films depend critically on the deposition conditions and on the nature and character of substrate-film interface. It is well established that substrate-film interactions can have a profound influence on the character and properties of deposited thin films.45 It is also known that a lattice misfit at the interface may result in formation of misfit dislocations (after a "critical" thickness of the film4), which generally deteriorate the properties. Furthermore, mismatch of the thermal expansion coefficients of the thin film and substrate can induce residual stresses in the films, which in turn may degrade the properties of the thin films. Other strain relaxation mechanisms include domain formation5'6 in perovskite thin films. A small amount of internal strain may have a large effect on the electronic and optical structure of thin films. Therefore, the influence of spatially varying strain on the electronic and optical properties of the films is of considerable interest. The effective width of these strained regions plays a significant role in numerous properties related to internal interfaces.7 Recently, the deposition of high quality epitaxial films of BaTiO3 on a (100)LaAlO3 substrate by metalorganic chemical vapor deposition (MOCVD) has been demonstrated.3 The bulk phase composition and epitaxial quality were controlled by adjusting the reactant 426
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J. Mater. Res., Vol. 9, No. 2, Feb 1994
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partial pressures and deposition temperatures. In-plane epitaxy was confirmed using x-ray diffraction and high resolution transmission electron microscopy (HRTEM). In this paper, we provide a detailed analysis of substrate-film interface and the thin films to further our understanding of the microscale character and interfacial disorder of the BaTiO3 MOCVD thin films. Moreover, we report the use of spatiall
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