Effects of copper doping of vanadium dioxide films on DC and terahertz conductivity

The transmission of terahertz (THz) waves in vanadium dioxide (VO₂) films decreases with the decrease in resistivity caused by the insulator-to-metal transition. Doping of VO₂ films with Cu leads to a narrowing of the hysteresis width and a decrease in the transition temperature. However, Cu doping affects both electrical properties and THz optical properties in such a way that for films with different doping ratios it is not possible to obtain similar resistivities and similar THz transmissions simultaneously. The investigations reported here reveal both the substitutional and interstitial roles of Cu doping, with the latter, in particular, leading to Cu separation. Further analysis shows that the conditions at grain boundaries mainly influence resistivity, while crystal quality significantly affects THz optical properties. Consequently, although the variations in resistivity and optical properties show similar trends, the ranges of variation are different. This can be attributed to the fact that, according to the Drude model, the relaxation time and static conductivity depend on the conditions at grain boundaries. The results are used to explore the possibility of balancing the electrical and THz optical performances of THz devices.