Preparation of Thin Films of Vanadium (Di-, Sesqui-, and Pent-) Oxide

Abstract

Polycrystalline thin films of V20~, VO2, and V205 have been prepared in the thickness range from 500 to 4000A. The films are obtained by a postdeposition heat-treatment of sputtered V or V-O films. The quality of the films has been studied by x-ray diffraction techniques and by measuring resistivity as a function of inverse temperature to check the magnitude of the metal to semiconductor transition. Electrically the films compare favorably with bulk crystals, and in addition the films are extremely stable during repeated cycling through the transition temperature.Since Morin (1) reported on the metal to semiconductor transition in certain oxides there has been considerable interest in the preparation and properties of single crystals of the oxides of vanadium. Vanadium dioxide, VO2, which gives rise to a conductivity change of up to 104 at 338~ has received the most attention (2-7) due to the very convenient transition temperature. However, for fundamental studies and some more restricted device applications vanadium sesquioxide, V208, with a conductivity change of greater than 106 at 168~ is also of great interest (8-10). A serious problem encountered with both V20~ and VO2 is a tendency for bulk crystals to fracture. This fracture, plus the need for material in configurations compatible with modern device technology, has stimulated our effort to deposit thin films of the above oxides.Previous work on thin films has been restricted to the deposition of VO2 films. The films were obtained either by vapor deposition techniques (11, 12) using VOCI~ or by reactive sputtering of vanadium in argon and oxygen mixtures (13). Since difficulties have been encountered in obtaining consistently reproducible results on these films simple postdeposition annealing of films was attempted. Considerable success was achieved in obtaining uniform large area films of V,~O5 and V203, as well as VO2, by choosing a suitable oxidizing or reducing atmosphere while treating sputtered V or V-O films. Apparatus and ProceduresExperimentally the vacuum system is identical to that used previously for the sputtering of ZnO (14) with the ZnO cathode replaced by V205. However, due to the complexity of the oxidation states of vanadium it was not possible to deposit crystalline films reproducibly and directly, as was accomplished with ZnO. Therefore, postdeposition treatments were devised which recrystallized the films such that their properties were comparable to bulk single crystals. Vanadium films sputtered in conventional bell jar systems were also studied. Substrate materials included single crystal sapphire and amorphous Pyrex glass slides with the best results obtained on the sapphire.Depending on the oxide desired the films were annealed in the presence of three different flowing gases in three separate open tube furnaces. The gases were water vapor, a mixture of 90% N2-10% 02, and H2 which had been bubbled through water. The optimum conditions for obtaining the V-O composition of interest are presented in Table I. No attempt has be...

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