Thermochromic VO2 thin films deposited by HiPIMS

Abstract

Thermochromic VO2 windows have the potential of managing heat transfer in an efficient way. However, several problems such as a high transition temperature ~68°C and high deposition temperatures (over 400°C) limit their applicability. We present a novel approach for the fabrication of thermochromic VO2 films in which High Power Impulse Magnetron Sputtering (HiPIMS) is used for deposition. Indeed, HiPIMS is known for its high ionization degree of sputtered species that allows one to control the evolution of the film microstructure by ion bombardment. The optical and other physical properties of the obtained HiPIMS VO2 coatings are first presented. Based on spectrophotometry, ellipsometry, AFM, SEM, TOF-SIMS, Raman spectroscopy and XRD results, we show that it is possible to deposit dense stoichiometric crystalline VO2 films at lower substrate temperatures (300°C) compared to other approaches. These films exhibit a high infrared modulation (ΔT2500nm of 61% between 30°C and 90°C), low surface roughness (Rrms under 10% of total thickness for films approximately 100nm thick), and lower transition temperatures than the bulk material (Tc down to 50°C for thicker films).