Thermochromic properties of vanadium oxide thin films prepared by reactive magnetron sputtering at different oxygen concentrations

Abstract

Vanadium dioxide (VO2) is a thermochromic (TC) material, exhibits a reversible transmittance change in the near-infrared (NIR) region upon heating above a critical temperature τc, due to an insulator-to-metal phase transition. Numerous studies on the V–O system can be found in the literature, including the synthesis of VO2 films. The phase diagram of the V–O system involves complicated suboxide phases that prevent pure VO2 formation and obscures quantitative analysis. Here, VO2 thin films were systematically prepared with various oxygen flow ratios (Γ) using reactive magnetron sputtering. Thin films prepared with Γ = 4.1% show dominant VO2 phase and the highest TC performance. The Γ-range producing dominant VO2 phase is found to be narrow: 3.7% < Г<4.2%. At lower and higher Γ, TC characteristics are shown to be accompanied, as expected, by electrical conductivity changes, but also by microstructure transformations. The latter producing textured films that gradually develop upon cycling around τc. In particular, a change of texture, yielding oriented VO2 crystallites, is observed by in-situ XRD around τc. Our results shed light on the VO2 formation and associated structural, chemical and electrical properties under various oxidizing conditions during magnetron sputtering, which calls for careful preparation and strategies to stabilize the VO2 phase.