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

• XAFS measurements for b-oriented VO2 films showed a first order structural phase transition from a monoclinic phase to a rutile phase at 70 °​C for a heating process.
• Temperature-dependent resistance measurements demonstrated a sharp MIT transition at 75 °C.
• XAFS first revealed a non-rigid change of V–O bond length of V–O octahedron in VO2.
• The discrepancy of the structural and electric properties showed that the first-order structural transition was insufficient to explain the MIT of VO2.
• XAFS revealed octahedron distortion in the M1 phase to the splitting of t2g from eg and to the lowering of the d|| (dxz, dyz) energy level.

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.