Microstructural evolution of thin film vanadium oxide prepared by pulsed-direct current magnetron sputtering

Vanadium oxide (VOx) thin films have been deposited by pulsed-DC magnetron sputtering using a metallic vanadium target in a reactive argon and oxygen environment. While the process parameters (power, total pressure, oxygen-to-argon ratio) remained constant, the deposition time was varied to produce films between 75 +/- 6 and 2901 +/- 30 angstrom thick, which were then optically and electrically characterized. The complex dielectric function spectra (epsilon = epsilon(1) + i epsilon(2)) of the films from 0.75 to 5.15 eV were extracted by ex situ, multiple-angle spectroscopic ellipsometry (SE) measurements for the series of varied thickness VOx samples. Significant changes in epsilon and resistivity occur as a function of thickness, indicating the correlations exist between the electrical and the optical properties over this spectral range. In addition, in situ measurements via real time SE (RTSE) were made on the film grown to the largest thickness to track optical property and structural variations during growth. RTSE was also used to characterize changes in the film occurring after growth was completed, namely during post sputtering in the presence of argon and oxygen while the sample is shielded, and atmospheric exposure. RTSE indicates that the exposure of the film to the argon and oxygen environment, regardless of the shutter isolating the target, causes up to 200 angstrom of the top surface of the deposited film to become more electrically resistive as evidenced by variations in epsilon. Exposure of the VOx thin film to atmospheric conditions introduces a similar change in epsilon, but this change occurs throughout the bulk of the film. A combination of these observations with RTSE results indicates that thinner, less ordered VOx films are more susceptible to drastic changes due to atmospheric exposure and that microstructural variations in this material ultimately control its environmental stability. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4759255]