Unusual M-2-mediated metal-insulator transition in epitaxial VO2 thin films on GaN substrates

Abstract

We report on the epitaxial growth of vanadium dioxide (VO2) 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 VO2 film and the GaN substrate could favor the stabilization of the intermediate insulating M2 phase, which is known to be observed only in either doped or uniaxially strained samples. We demonstrated that the MIT in VO2 films on GaN substrates could be mediated via a monoclinic M2 phase during the transition from a monoclinic M1 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 (ωV-O) from 618 cm-1 (M1) to 645 cm-1 (M2) and a distinct peak splitting of a low-frequency phonon mode (ωV-V) at 221 cm-1 (M2) for increasing temperatures. Moreover, a resistance change of four orders of magnitude was observed for VO2 thin films on GaN substrates, being indicative of the high quality of VO2 thin films. This study may offer great opportunities not only to improve the understanding of M2-mediated MIT behavior in VO2 thin films, but also to realize novel electronic and optoelectronic devices. Copyright © EPLA, 2015.