The oriented growth of anatase in thin films of amorphous titania
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A. B. Harker Rockwell International Science Center, Thousand Oaks, California 91360 (Received 27 October 1986; accepted 9 February 1987) The growth of an oriented anatase grain structure from the post-deposition annealing of amorphous reactive ion-beam-deposited thin films of titanium oxide has been demonstrated. The transformation has been observed directly by electron microscopy and the crystallization of the films occurs only when their thicknesses are greater than about 50 nm. Substrate effects were not found to be significant to the crystallization, which could also be induced by annealing freestanding films. The transformation processes can be described in terms of existing theories of crystal growth where the amorphous film is described as a supercooled liquid.
I. INTRODUCTION The transformation and growth processes in titanium oxides are of direct interest both for optical film fabrication and for applications in high-energy laser optics where heating can induce their transformation. The alternative oxidation states and crystal structures are quite well known in this system,1'2 but when the oxides are prepared as thin films the density and thickness play a significant role in determining the transformation behavior. The purpose of the work described here was to identify the important characteristics of the transformations brought about by heating thin films of amorphous titanium oxide. II. EXPERIMENTAL The titanium oxide films for this study were reactively deposited from the argon ion-beam sputtering of titanium metal. The films were prepared in the presence of oxygen as a background gas at a pressure of 2 X 10~ 5 Torr using a Kaufman-type ion source equipped with a neutralizing filament to flood the target with low-energy electrons. The deposition was carried out on a rotating multisubstrate planetary at room temperature in an all metal gasket-sealed ultrahigh vacuum (UHV) chamber, turbo pumped to a base pressure of 10~ 9 Torr. During each run, films were deposited simultaneously on either freshly cleaved single crystal substrates of sodium chloride or polished silicon. The index of refraction of the films on the silicon substrates was routinely determined at 0.6//m using ellipsometry. The films deposited onto the NaCl samples were prepared for transmission electron microscopy by floating them off the substrate and then onto fine meshed titanium grids in deionized water. The sample thicknesses were determined independently by profilometry J. Mater. Res. 2 (2), Mar/Apr 1987
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and by ellipsometry, and the annealing of the films was carried out in a controlled atmosphere alumina tube furnace using either argon, oxygen or air. The in-situ annealing of several freestanding films was also carried out in the environmental chamber of the 1.5 MeV Kratos electron microscope at the Berkeley National Center for Electron Microscopy. In addition to the atmospheric and low-pressure annealing studies, several films were transformed by hot isostatic pressing at 400 °C under 140 MPa of argon
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