Correlation between structure and semiconductor-to-metal transition characteristics of VO2/TiO2/sapphire thin film heterostructures

This study focuses on the role of strain and thin film epitaxy on the semiconductor-to-metal transition (SMT) characteristics of single crystalline VO2 thin films. The VO2/TiO2 heterostructures of controlled orientations were epitaxially grown on m-cut, r-cut and c-cut sapphire substrates. Detailed structural investigations were performed using high-resolution X-ray diffraction (2θ–θ and φ scans) and high-resolution transmission electron microscopy techniques to correlate SMT properties with microstructural characteristics. Monoclinic (M1) VO2 thin films with (1 0 0), (0 0 1) and (2¯01) out-of-plane orientations were grown on TiO2(1 0 1)/r-sapphire, TiO2(1 0 0)/c-sapphire and TiO2(0 0 1)/m-sapphire platforms, respectively. The in-plane alignments across the interfaces were established to be [0 1 0](1 0 0)VO2||[0 1 0](1 0 1)TiO2, [1 0 0](0 0 1)VO2||[0 0 1](1 0 0)TiO2 and [010](2¯01)VO2‖[010](001)TiO2 for r-sapphire, c-sapphire and m-sapphire substrates, respectively. We were able to tune the SMT temperature of VO2 epilayers from ∼313 K to 354 K (bulk Tc ≈ 340 K). The SMT characteristics were interpreted based upon the residual strain in the VO2 lattice, particularly along the c-axis of tetragonal VO2. This research introduces the VO2-based single crystalline heterostructures as a potential candidate for a wide range of applications where different transition temperatures are required.