Twin structure and phase transition of VO2 film grown on r-plane Al2O3

Abstract

We report the twin structure and phase transition of the VO2 thin film grown on an r-Al2O3(0112) substrate. We found that the film is composed of nano-scale grains with their (211) or (200) crystallographic plane-normal close to the substrate-normal direction. The (211) grains exhibited a twin-domain structure separated by the (100) M twin plane, and their (211) plane-normal was tilted away from the substrate normal by 1.45 degrees to accommodate the twin formation. In situ 3D RSMs revealed a gradual reduction of the tilt angle from 1.45 degrees to 1.25 degrees during the monoclinic-to-rutile structural phase transition. In the rutile phase, the tilt angle remained to be finite, indicating that the (100) M twin plane is preserved even in the rutile phase. A comparison with electrical resistance measurements showed that the structural phase transition (SPT) occurs at a temperature lower than the metal-insulator transition (MIT), while the recovery of the tilt angle upon cooling proceeds gradually contrasting the steep resistance change. These results suggest that the twin structure may influence the kinetics of the structural phase transition in VO2 films grown on r-plane sapphire substrates.