In situ electronic structural study of VO2 thin film across the metal—insulator transition

Abstract

The in situ valence band photoemission spectrum (PES) and X-ray absorption spectrum (XAS) at V LII — LIII edges of the VO2 thin film, which is prepared by pulsed laser deposition, are measured across the metal—insulator transition (MIT) temperature (TMIT = 67 °C). The spectra show evidence for changes in the electronic structure depending on temperature. Across the TMIT, pure V 3d characteristic d‖ and O 2p-V 3d hybridization characteristic πpd, σpd bands vary in binding energy position and density of state distributions. The XAS reveals a temperature-dependent reversible energy shift at the V LIII-edge. The PES and XAS results imply a synergetic energy position shift of occupied valence bands and unoccupied conduction band states across the phase transition. A joint inspection of the PES and XAS results shows that the MIT is not a one-step process, instead it is a process in which a semiconductor phase appears as an intermediate state. The final metallic phase from insulating state is reached through insulator—semiconductor, semiconductor—metal processes, and vice versa. The conventional MIT at around the TMIT = 67 °C is actually a semiconductor—insulator transformation point.