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

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 (ω_V-O) from 618 cm^-1 (M₁) to 645 cm^-1 (M₂) and a distinct peak splitting of a low-frequency phonon mode (ω_V-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.