Effect of Ar pressure on phase transition characteristics and charge transport mechanism in VO₂ films grown by RF sputtering of V₂O₅

Abstract

In this study, we report the growth and characterization of VO2films deposited on YSZ (001) substrate employing RF magnetron sputtering of vanadium pentoxide (V2O5) target in pure Ar ambient. The VO2 film growth has been carried out at ~700 C for ~15 minutes at ~100 W RF power with a flow rate of ~20 sccm at Ar gas deposition pressure of ~3, ~6, ~20, and ~40 mTorr. X-ray diffractometry and Raman spectroscopy show that the nearly pure VO2 phase achieved at lower Ar pressure, e.g., ~3 and ~6 mTorr, transforms into a multiphase V-O system at ~20 and ~40 mTorr. This pattern is also supported by the electrical transport measurements through the occurrence of hysteretic IMT in films grown at ~3 and ~6 mTorr and the absence of this signature in these films deposited at ~20 and ~40 mtorr Ar pressure. The most pronounced hysteresis is seen in the Y6 film, and therefore, the optimum growth pressure in the present study is ~6 mTorr. The suppression of IMT in Y20 and Y40 is attributed to the appearance of other V-O phases, which smear out the phase transition. The activation energy at the beginning and end of the IMT is estimated from the Arrhenius fit to the ρ-T data on both sides of the transition is found to be ~0.081 eV (~0.223 eV) in metallic (insulating) phase at ~3 mTorr pressure which increases to ~0.102 eV (~0.311 eV) for ~6 mTorr film in the cooling cycle. The low temperature (120K≤T≤300K) transport conduction in all VO2 films is governed by Efros-Shklovskii’s variable range hopping (ES-VRH) mechanism with a systematic relation with phase transition characteristics. Thus, Argon gas pressure plays a critical role in growth and brings out the feasibility of VO2 films growth by RF sputtering of V2O5 target under Argon ambient.