  Highly oriented VO2 thin films were grown on sapphire substrates by the
sol-gel method that includes a low pressure annealing in an oxygen atmosphere.
This reduction process effectively promotes the formation of the VO2 phase over
a relatively wide range of pressures below 100 mTorr and temperatures above
400oC. X-ray diffraction analysis showed that as-deposited films crystallize
directly to the VO2 phase without passing through intermediate phases. VO2
films have been found to be with [100]- and [010]-preferred orientations on
Al2O3(1012) and Al2O3(1010) substrates, respectively. Both films undergo a
metal-insulator transition with an abrupt change in resistance, with different
transition behaviors observed for the differently oriented films. For the
[010]-oriented VO2 films a larger change in resistance of 1.2x10^4 and a lower
transition temperature are found compared to the values obtained for the
[100]-oriented films.
