Manipulations from oxygen partial pressure on the higher energy electronic transition and dielectric function of VO2 films during a metal–insulator transition process

Abstract

Optical properties and metal–insulator transition (MIT) of vanadium dioxide (VO₂) films grown by pulsed laser deposition with different oxygen pressures (5 to 50 mTorr) have been investigated by temperature dependent transmittance spectra. Three interband critical points (E₁, E₂ and E₃) can be obtained via fitting transmittance spectra and the hysteresis behavior of the center transition energies E₁ and E₂ is presented. The VO₂ film grown at optimized oxygen pressure exhibits the well-defined resistivity drop (∼10³ Ω cm) across the MIT process. It is found that the metal–insulator transition temperature (T_MIT) increases with the oxygen pressure and the complex dielectric functions are drastically affected by oxygen pressure. It is believed that the oxygen pressure can lead to lattice defects, which introduce the donor level and the acceptor level in the forbidden gap produced by oxygen vacancies and vanadium vacancies, respectively. The donor level provides electrons for higher empty π* bands, which can make the energy barrier lower and decrease critical temperature. On the contrary, electrons jumping from the d_∥ band can be recombined by holes on the acceptor, impeding the MIT occurrence. It is claimed that the electronic orbital occupancy is closely related to oxygen pressure, which changes the energy barrier and manipulates the phase transition temperature. The present results are helpful to understand the fundamental mechanism of VO₂ films and practical applications for VO₂-based optoelectronic devices.