Structural properties of epitaxial TiO 2 films grown on sapphire (11$\overline 1$0) by MOCVD
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Titanium dioxide thin films were grown on sapphire (1120) substrates in a low-pressure metal-organic chemical vapor deposition system at temperatures ranging from 400 to 800 °C. Raman scattering, x-ray diffraction, transmission electron microscopy, and high resolution electron microscopy techniques were employed to characterize the structural properties of the deposited films. The resultant phases and structures of the deposited films depended on both the growth temperature and the substrate surface properties (surface imperfections, steps, etc.). At the growth temperature of 800 °C, single-crystal rutile films were obtained reproducibly with two possible epitaxial relationships. At lower temperatures (400 to 775 °C), the deposited films can be epitaxial or polycrystalline with highly oriented grains. The similarity between the atomic arrangements of the substrate and the film is discussed in detail to explain the observed epitaxial relationships and abruptness of the interfaces.
I. INTRODUCTION Oxide materials have a wealth of unique and interesting physical properties that can be used for various technological applications.1 Many oxide materials have been used extensively in the form of thin films because the applications involve microdevices that require the materials to be fabricated on micron or submicron scales.1'2 Although many applications employ the films in polycrystalline form, for others, single crystal films are either required or preferred.3"6 The great success of fabricating superlattice structures made of layered semiconductor materials has led to the creation of a new class of materials exhibiting potential for a variety of novel applications. One of the main restrictions of semiconductor superlattice structures is the limited number of materials that can be layered together, due to the stringent requirements of lattice matching. However, oxides form with all metallic elements of the periodic table; therefore, a much greater selection of compounds and crystallographic possibilities is available. The variety of available compounds multiplies rapidly when one includes multicomponent oxides. The range of physical properties is comparably varied. Thus, one can choose candidate substrate and epitaxial overlayer materials that meet the requirements of minimum interface strain while simultaneously selecting materials with varied or contrasting physical properties. The possibility for epitaxial fabrication of multilayer structures with individually tailored physical properties in each of the layers is almost unlimited. Of course, to achieve these potential technological benefits, the first step is to obtain the controlled growth of single
layer epitaxial films. However, very little work has been done on oxide epitaxial growth.7 It is the goal of our research to obtain an understanding of various fundamental aspects of epitaxial growth of oxide materials. At present, we have chosen TiO2, due to its wellcharacterized bulk crystal structure8"10 and interesting physical properties,11"13 as one of the startin
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