Synthesis and temperature-dependent local structural and electrical properties of VO₂ films

We examine the local structural and electrical properties of VO₂ films showing metal-to-insulator transition (MIT) by using in-situ x-ray absorption fine structure (XAFS) measurements at the V K edge and resistance measurements in the temperature range 20-120 °C. VO₂ films are synthesized on Al₂O₃ (0001) substrates by DC magnetron sputtering deposition. X-ray diffraction measurements show that the films have b-oriented monoclinic-phase crystals at room temperature. XAFS measurements reveal a local structural transition in the films from the monoclinic (M₁) to the rutile (R) phase at ∼70 °C during their heating; further, temperature-dependent resistance (R-T) measurements showed a sharp MIT in the films at ∼75 °C. Extended XAFS (EXAFS) measurements reveal non-rigid changes of V-O and V-V bond lengths from the M₁ to the R phase via the M₂ phase. In-situ EXAFS and R-T measurements show that the synthesized VO₂ films act as Mott insulators and that their electrical property change is not proportional to their structural property change at their MIT temperature.