Millimeter-wave dielectric properties of epitaxial vanadium dioxide thin films

Abstract

The dielectric properties of grain-oriented thin-film vanadium dioxide (VO2) on single-crystal sapphire have been measured as a function of temperature across the phase transformation at 68 °C. The properties of the low- and high-temperature phases were determined by measuring the millimeter-wave transmittance and phase shift through the samples. The real part of the dielectric constant in the high-temperature phase increases with increasing thickness from less than 1000 for a 40-nm-thick film (the resolvable limit of this measurement technique) to over 90 000 for a 580-nm-thick film. The strong thickness dependence in epitaxial VO2 thin films can be attributed to a dielectric mixture phenomenon and can be described by effective-medium theory. The large real dielectric constant and its thickness dependence is likely due to bonding distortions at the grain boundaries. Analysis of the experimental data shows the VO2 thin films used in this study have no secondary grain-boundary phases, but do have interfacial stresses which affect the dielectric properties of the grain-boundary regions that lie near the VO2-sapphire interface. This study concludes that the millimeter-wave properties of VO2 thin films are strongly influenced by microstructure, crystalline orientation, grain-boundary stress, and stoichiometry.