Correlation between structure and semiconductor-to-metal transition characteristics of VO_2/TiO_2/sapphire thin film heterostructures

摘要

This study focuses on the role of strain and thin film epitaxy on the semiconductor-to-melal transition (SMT) characteristics of single crystalline VO_2 thin films. The VO_2/TiO_2 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 (20-θ and φ scans) and high-resolution transmission electron microscopy techniques to correlate SMT properties with microstructural characteristics. Mono-clinic (M_1) VO_2 thin films with (100), (001) and (201) out-of-plane orientations were grown on TiO_2(101)/r-sapphire, TiO_2(100)/c-sap-phire and TiO_2(001)/m-sapphire platforms, respectively. The in-plane alignments across the interfaces were established to be 010(100)_(VO2)010(101)_(TiO2), 100(001)_(VO2)001(100)_(TiO2) and 010(201 )_(VO2)010(001)_(TiO2) for r-sapphire, c-sapphire and m-sap-phire substrates, respectively. We were able to tune the SMT temperature of VO_2 epilayers from ~313 K to 354 K (bulk T_c ≈ 340 K). The SMT characteristics were interpreted based upon the residual strain in the VO_2 lattice, particularly along the c-axis of tetragonal VO_2. This research introduces the VO_2-based single crystalline heterostructures as a potential candidate for a wide range of applications where different transition temperatures are required.