Heteroepitaxial Growth and Phase Transition Properties of Vanadium Dioxide Thin Films on Different Orientations of Sapph
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Heteroepitaxial Growth and Phase Transition Properties of Vanadium Dioxide Thin Films on Different Orientations of Sapphire Substrates Z.P.Wua, and H.Naramotob a Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R.Chima b Japan Atomic Energy Research Institute Watanuki 1233, Takasaki, Gunma 370-1292, Japan ABSTRACT Vanadium dioxide thin films have been deposited on sapphire substrates with different orientations by pulsed laser ablation. The samples are analyzed by x-ray diffraction and pole figures to determine epitaxial relationship. The crystal quality is evaluated by RBS/Channeling techniques. The results has shown that the growth of VO2 is of strong substrate dependence, the film on (0001) sapphire substrate exhibits better crystal quality than that on (1120) and (0112) sapphire substrates. The epitaxy can be divided into two processes, the first is surface symmetry determined oriented growth, and the second is in-plane oriented growth dominated by the minimization of in-plane lattice mismatch. More over, different substrates result in different defect microstructures, 120o twin is observed in VO2 film on (0001) substrate while 180o on (1120) and no twin on (0112), which also reflects the surface symmetries of the substrates.
INTRODUCTION Heterogeneous epitaxial growth is a important technique in thin film material science and semiconductor field, where the substrate plays an important role to the quality of the epitaxial films and their microstructures, because of the existence of lattice mismatch --- the differences from either lattice constant or angle of the lattice vectors between substrate and grown film. There are a lot of approaches to consider the coupling of the crystalline lattices. So far, it has been widely accepted that during the epitaxy, the lattice periods are changed before the completely matching at the interface (pseudomorphism), and that the natural lattice periods are preserved in the epitaxy (commensurateness) and the differences of the periods are compensated by the misfit dislocations1. Both cases are happened in epitaxial process, which results in the epitaxial growth of the thin film depending on the topography, symmetry of the substrates. In fact, substrate dependent growth was widely observed and well studied for some cubic crystal systems, such as diamond epitaxy on Si2, Pt on Cr3 and some perovskite-type oxide films, such as SrVO3 on SrTiO34, and BaTiO3 on SrTiO35 etc. Usually, the same crystallographic crystal is chosen as the substrate, i.e., cubic crystal grows on cubic substrate, and hexagonal grows on hexagonal. Because same crystallography can reduce the angle deviation to zero, so in order to reduce the lattice misfit, only the lattice constant are taken into consideration, which largely reduces the lattice misfit and improves the epitaxial quality of the films. But in some cases, it is hard to choose a same crystallographic crystal as the substrate, such as for the monoclinic crystal system. In this study, our interest is focused on the vanad
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