Terahertz Transmittance of Cobalt-Doped VO2 Thin Film: Investigated by Terahertz Spectroscopy and Effective Medium Theory

Vanadium dioxide (VO
₂
), due to its characteristic semiconductor-metal transition (SMT) at 341 K, is a promising material for optical modulators in the terahertz (THz) regime. In this study, doping Co ions into epitaxial VO
₂
films have been demonstrated to acquire SMT with a reduced critical temperature, a large modulation depth, and a narrow transition region in the THz regime, which would benefit the applications of VO
₂
film in THz switches and memory devices. Utilizing Co ions, a 77% THz modulation ratio within a 3 °C transition region appeared when the doping content was 4.0 at. %. Moreover, more doping content induced two different phases in VO
₂
films, identified by X-ray diffraction (XRD), which would decrease the THz modulation ratio. Theoretical analysis based on the Drude-Smith model and the Bruggeman effective medium theory predict that the phase containing more Co ions might be transparent to THz waves and void of switching ability in the temperature range from 30 °C to 80 °C. This phase separation may be responsible for the complex correlation between the Co doping concentration and the SMT properties.