Growth control of the oxidation state in vanadium oxide thin films

TL;DR

This study systematically explores growth conditions for vanadium oxide thin films to precisely control their oxidation states, enabling better understanding and optimization of their metal-insulator transition properties.

Contribution

It provides a detailed growth map for phase-pure vanadium oxides with specific oxidation states using pulsed laser deposition, and demonstrates how post-annealing enhances MIT performance.

Findings

Narrow oxygen partial pressure range needed for VO2 growth with pronounced MIT

Lower or higher P(O2) yields V2O3 or V2O5 phases, suppressing MIT

Post-annealing improves resistivity ratio by an order of magnitude

Abstract

Precise control of the chemical valence or oxidation state of vanadium in vanadium oxide thin films is highly desirable for not only fundamental research, but also technological applications that utilize the subtle change in the physical properties originating from the metal- insulator transition (MIT) near room temperature. However, due to the multivalent nature of vanadium and the lack of a good understanding on growth control of the oxidation state, stabilization of phase pure vanadium oxides with a single oxidation state is extremely challenging. Here, we systematically varied the growth conditions to clearly map out the growth window for preparing phase pure epitaxial vanadium oxides by pulsed laser deposition for providing a guideline to grow high quality thin films with well-defined oxidation states of V2(+3)O3, V(+4)O2, and V2(+5)O5. A well pronounced MIT was only observed in VO2 films grown in a very narrow range of oxygen partial pressure P(O2). The films grown either in lower (< 10 mTorr) or higher P(O2) (> 25 mTorr) result in V2O3 and V2O5 phases, respectively, thereby suppressing the MIT for both cases. We have also found that the resistivity ratio before and after the MIT of VO2 thin films can be further enhanced by one order of magnitude when the films are further oxidized by post-annealing at a well-controlled oxidizing ambient. This result indicates that stabilizing vanadium into a single valence state has to compromise with insufficient oxidation of an as grown thin film and, thereby, a subsequent oxidation is required for an improved MIT behavior.