Dependence of microstructure and thermochromism on substrate temperature for sputter-deposited VO2 epitaxial films

Abstract

A systematic study on the epitaxial growth of VO2 films deposited on sapphire by sputtering at various substrate temperatures (Ts) was carried out. The deposited films were characterized by x-ray diffraction, high energy electron diffraction, Rutherford backscattering spectrometry, atomic force microscopy, and spectrophotometry. Epitaxial VO2 was obtained from a Ts of 300 °C which is the lowest reported yet. Two epitaxial relationships, i.e., a dominant VO2(010)//sapphire (110), (001)//(001) and a secondary VO2(100)//sapphire (110), (010)//(001), were confirmed. The epitaxial films show strong dependence of the morphology and thermochromism on Ts. Films deposited at above 400 °C exhibit changes in resistivity of ΔR>104 upon switching, while those deposited at Ts=300 °C have reduced ΔR of 102. The film obtained at Ts=300 °C exhibits novel transition properties, i.e., a largely reduced τc (45 °C) even without any doping, and very little thermal hysteresis.