THz Transmittance and Electrical Properties Tuning across IMT in Vanadium Dioxide Films by Al Doping

Due to the insulator–metal transition (IMT) performance covering the full terahertz (THz) band, VO2 films were extensively investigated as an excellent candidate for modulating, switching, and memory devices. However, some remarkable absorption peaks owing to the infrared-active phonon modes suppressed the films’ modulation ability and restricted the films’ application in high THz frequency. Here we prepared Al-doped VO2 films on (111) directional silicon substrate, which rapidly counteracted the absorption peak and exhibited widely modulating properties. Al dopants introduced into the films brought a significant shift to high frequency in Raman spectra. The result was attributed to the effect of modifying VO2 crystal, leading the VO bond to be strained more intensively, contracting the distance of the VV dimers. All the Raman results indicated an oxidation effect by Al doping. However, the XPS results showed a valence reduction of the vanadium element, which was caused by the valence difference between V and Al atoms. In addition to the surface morphology characterization, the IMT properties of the shrinkage of hysteresis width and resistance variations in both electrical and THz optical aspects have been systemically analyzed. An additional difference is that the temperature of the optical transition behaves lower than the electrical transition observed, which resulted from the mechanism of transition propagation and boundary barriers.