Tuning the properties of VO₂ thin films through growth temperature for infrared and terahertz modulation applications

Results are reported on tuning the electrical and optical properties of sputter-deposited vanadium dioxide (VO₂) thin films through control of substrate growth temperature (T_s). As T_s increases from 550 to 700°C, the morphology changes from granular to smooth film and finally to rough film. X-ray diffraction shows the presence of VO₂ along with additional weak features related to the presence of non-stoichiometric phases. Electrical measurements show the phase transition to change from abrupt to gradual as both the below- and above-transition resistivities vary with T_s. The transition and hysteresis dependences observed in electrical resistivity are similarly observed in infrared transmission. Terahertz transmission measurements show that high conductivity above the phase transition is more important in achieving high modulation depth than obtaining high resistivity below the transition. We attribute changes in the electrical and optical properties to the formation of V and O vacancies, which result in diverse valence states from the ideal V⁴⁺ of VO ₂. Low T_s produces material with V⁵⁺ states resulting in higher resistivity in both the insulating and metallic phases. Alternatively, high T_s introduces material with V³⁺ states leading to lower resistivity in the insulating phase but slightly higher resistivity in the metallic phase.