Persistent M2 phase in strongly strained (011)-oriented grains in VO2 films grown on sapphire (001) in reactive sputtering

We report on the first observation of the persistent M2 phase in strongly strained (011)-oriented grains in VO2 films grown on Al2O3 (001) substrates by means of conventional rf reactive sputtering under adequate deposition conditions. Spatially resolved micro-Raman spectra clearly showed that (011)-oriented large crystalline grains with the cR-axis parallel to the substrate resulted in the appearance of the M2 phase over a wide temperature range of 30 °C. A close correlation of the appearance range of the M2 phase with the in-plane tensile stress of (011)-oriented grains was revealed by X-ray diffraction. We present a phase diagram for the M1, M2, and R phases in relation to the stress of (011)-oriented grains and temperature. It was shown that (011)-oriented micrometer-sized long grains play a crucial role in the emerging structural phase transition (SPT) via an M2 phase even in a film grown on Al2O3 (001), which is ordinarily reserved for the (020)-oriented VO2 growth. The results shown here will contribute to make clear the conditions for obtaining VO2 films with the appearance of the M2 phase in their SPT process.