Unusual M2-mediated metal-insulator transition in epitaxial VO2 thin films on GaN substrates

Abstract

We report on the epitaxial growth of vanadium dioxide thin films on (0001) GaN substrates using a radio frequency magnetron sputtering method and discuss their unusual M2-mediated metal-insulator transition (MIT) properties. We found that large lattice misfits between the film and the GaN substrate could favor the stabilization of the intermediate insulating phase, which is known to be observed only in either doped or uniaxially strained samples. We demonstrated that the MIT in films on GaN substrates could be mediated via a monoclinic phase during the transition from a monoclinic to a rutile R phase. This was confirmed by temperature-dependent Raman studies that exhibited both an evident upshift of a high-frequency phonon mode from to and a distinct peak splitting of a low-frequency phonon mode at for increasing temperatures. Moreover, a resistance change of four orders of magnitude was observed for thin films on GaN substrates, being indicative of the high quality of thin films. This study may offer great opportunities not only to improve the understanding of M2-mediated MIT behavior in thin films, but also to realize novel electronic and optoelectronic devices.