Comprehensive study of the metal-insulator transition in pulsed laser deposited epitaxial VO2 thin films

In this paper, we present a comprehensive, correlative study of the structural, transport, optical and thermoelectric properties of high-quality VO2 thin films across its metal-insulator phase transition. Detailed x-ray diffraction study shows that it's textured polycrystalline along [010]M1, with in-plane lattice orienting along three equivalent crystallographic directions. Across the metal-insulator transition, the conductivity increases by more than 3 orders of magnitude with a value of 3.8 × 103 S/cm in the metallic phase. This increase is almost entirely accounted for by a change in electron density, while the electron mobility changes only slightly between the two phases, yet shows strong domain boundary scattering when the two phases coexist. Electron effective mass was determined to be ∼65m0 in the insulating phase. From the optical and infrared reflection spectra in the metallic phase, we obtained the plasma edge of VO2, from which the electron effective mass was determined to be ∼23m0. The bandgap of VO2 was determined from optical absorption to be 0.70 ± 0.05 eV at room temperature and rapidly shrinks before the phase transition occurs. In the temperature range where metallic and insulating phases coexist, the Seebeck coefficient was found to be significantly lower than that predicted by a linear combination of volumetric contributions from the insulating and metallic domains, indicating abnormal thermoelectric effect at the metal/insulator domain walls in such two-dimensional domain structure.