Behavior of the monoclinic order in VO2 thin films grown on sapphire near the metal-insulator transition

We report on the behavior of the monoclinic order near the metal-insulator phase transition (MIT) in vanadium dioxide (VO2) films grown on c-plane sapphire investigated by three-dimensional x-ray reciprocal space mapping (RSM). In the plane perpendicular to the film normal monoclinic b-axis [010] direction, pronounced diffuse scattering was observed in six specific directions whose origin was attributed to the rutile-like planar defects separating ordered monoclinic domains. The correlated region of the monoclinic domains was thin elliptical disk shaped with the disk normal along the monoclinic [001] direction. As the MIT was approached, the diffuse peaks first moved away from the (020) Bragg peak progressively and then disappeared rapidly, which suggests that the monoclinic-to-rutile structural phase transition accompanying the MIT was progressed in two stages. In the first pre-transitional stage, the rutile-like defect boundaries separating monoclinic domains are nucleated and grow progressively, which crosses over to the transitional stage where the rutile phase grows rapidly within the domains leading to the transition.