Variant structure in metal-organic-chemical-vapor-deposition-derived SnO 2 thin films on sapphire (0001)
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Tin oxide (SnO2) thin films were deposited on sapphire (0001) substrate by metal-organic chemical vapor deposition (MOCVD) at temperatures of 600 and 700 °C. The microstructure of the deposited films was characterized by x-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). At the growth conditions studied, films were single-phase rutile and epitaxial, but showed variant structures. Three distinct in-plane epitaxial relationships were observed between the films and the substrate. A crystallographic model is proposed to explain the film morphology. This model can successfully predict the ratio of the width to the length of an averaged grain size based upon the lattice mismatch of the film-substrate interface.
I. INTRODUCTION Over the past decades, deposition techniques for the epitaxial growth of semiconducting thin films have been advanced to such a level that it is now possible to synthesize new classes of artificially structured materials exhibiting new or improved physical properties.' Quantum well structures2 and superlattice structures with periodicity of a few atomic layers3 are some examples. However, one of the main restrictions of semiconductor superlattice structures is the limited number of elemental materials that can be layered together, due to the stringent requirements in lattice matching. On the other hand, oxide materials can be formed 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, it is possible to 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. Many oxide materials have been fabricated in thin-layer form with micron and submicron features for extensive applications in microdevices, including, but not limited to, nonvolatile memories, pyroelectric imagers, and microactuators.4"6 Tin oxide (SnO2) with the rutile structure has been known as an ionic metal oxide exhibiting unique elec1516
J. Mater. Res., Vol. 10, No. 6, Jun 1995
http://journals.cambridge.org
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tronic and optical properties.7-8 One of these properties, which has been utilized for commercial gas sensors, is that a reversible change in chemisorption is often accompanied by a reversible change in the near-boundary conductance. It has been found that high surface area, porous thin films may lead to higher gas sensitivity and faster responses to gases.9 Therefore, thin-film techniques, by providing the highest ratio of surface to volume, seem the most suitable for the development of gas sensors with high sensitivity. Film textures and the degree of crystallinity may be directly controlled by varying substrate materials and their orientations, as well as by adopting different
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