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 which owned 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 a widely modulating properties. Al dopants introducing 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 strained more intensively and contracting the distance of V-V dimers. All the Raman results indicated an oxidation effect by Al doping. However, the XPS results showed a valence reduction of vanadium element, which was caused by the valence difference between V and Al atoms. Adding with surface morphology characterization, IMT properties of the shrinkage of hysteresis width and resistance variations in both electrical and THz optical aspects have been systemically analyzed. In addition, a difference that the temperature of optical transition behaves lower than electrical transition has been observed, which resulted from the mechanism of transition propagation and boundary barriers.