Roles of a strain relaxation and an oxygen vacancy on nanoscale inhomogeneities in VO₂ thin film

We investigated structural and electronic inhomogeneities in a VO₂ thin film grown on a (001)-oriented TiO₂ substrate by exploiting nano-scale and macroscopic probing techniques. A compressive strain along the out-of-plane direction becomes additionally relaxed via microcracks which form a micron-sized rectangular pattern. A large inhomogeneity in the dielectric response is observed near the crack, and this signifies a strong coupling between electronic and lattice degrees of freedom. Interestingly, the strong inhomogeneity is observed also inside of the rectangular pattern, and it shows a gradient along one crystalline axis. We attribute such peculiar inhomogeneity observed in a relatively large length scale possibly to a combined effect of the strain relaxation and an oxygen vacancy distribution. As the nano-scale inhomogeneities in structural and electronic properties will eventually determine macroscopic responsivities, this work can be a good guide in designing VO₂ thin films with appropriate controls of the strain and the chemical composition to realize better functionalities.