Terahertz transmittance and metal-insulator phase transition properties of M2 phase VO2 films induced by Cr doping

In this work, we investigate the terahertz transmittance and metal-insulator phase transition properties of M2 phase VO2 films induced by Cr doping. Firstly, Cr-doped VO2 films were successfully produced on silicon substrates using DC reactive magnetron sputtering and then fully characterized by XRD, XPS, Raman shift and SEM tests. These results demonstrate the formation of M2 phase and the effect of Cr doping on composition, crystal structure and surface morphology of VO2 films. Compared with undoped VO2, Cr doping significantly enhances the amplitude modulation in the frequency of phonon-absorption peaks by the suppression of infrared-active phonon modes. More concretely, the amplitude modulation is largely increased from 29.6% to 39.0% at 8.25 THz and from 24.1% to 37.1% at 9.33 THz, respectively. This enhancement strongly contributes to the development of VO2-based devices in high THz range. In addition, Cr doped VO2 films exhibit outstanding metal-insulator phase transition properties with very narrower hysteresis width (8.0 °C) as well as smaller transition sharpness (3.4 °C) than undoped film, which is ascribed to the increase of heterogeneous nucleation site density and the transformation of crystal structure from M1 phase to M2 phase, respectively. This work indicates that VO2 films with suitable Cr doping concentration have great potential for THz modulation applications.