Integration of Vanadium Dioxide Films on Aluminum Trioxide and Silicon (100) Substrates: Structure-Property Correlations and Applications

Many of the transition metal oxides have Semiconductor to Metal Transition (SMT), which provides the opportunity for the development of sensor and actuators, electrochromic, and optical devices. A critical issue in the application of those materials is the transition temperature, which is whether much higher or lower than room temperature, such as Ti2O3 (410 K) and V2O3 (150 K). However, vanadium dioxide (VO 2) exhibits a fast and first-order phase transformation at a critical temperature of ∼68°C. This transition can be modified to bring it down close to ∼25°C. However, the SMT characteristics are influenced by the nature of microstructure and density of defects as well as stoichiometry of VO2. Therefore, the epitaxial VO2 thin films must be developed by appropriate substrates and buffer layers.;This work is focused on developing integration methodology of epitaxial VO2 films with sapphire and silicon substrates using zinc oxide (ZnO) or chromium oxide (Cr2O3) template layers. Also, SMT characteristics related to the nature of microstructure (grain boundaries), defects, residual strain, and stoichiometry of the epitaxial VO2 films are discussed.;A comparative study of epitaxial VO2 films on c-sapphire and r-sapphire substrates, and the SMT characteristics of these films were performed. On c-sapphire, VO2 grows epitaxially in (002) orientation not (020) orientation. These (002) oriented VO2 films have 60° twin boundaries due to three equivalent in-plane orientations. The epitaxial VO2 films on r-sapphire consisted of two orientations, namely (200) and (2¯11). The coexistence of these two orientations of VO2 has been explained by the similar atomic arrangement (200) and (2¯11) planes. The SMT characteristics have been correlated to the nature of the microstructure and the twin boundary. Also, the shift in SMT temperature for VO2 films on r-sapphire has been correlated with the compressive strain along [001] direction of VO 2.;To investigate the influence of twin boundaries on the SMT characteristics, the different current flow directions with respect to twin boundaries were studied. Semi-insulating ZnO (normal to the boundary) and highly conducting metallic Ga:ZnO (along the boundary) were used. The thermal hysteresis was reduced from 4.5°C to 2.3°C as the current flow was changed from being perpendicular to being parallel to the direction of the twin boundaries, in agreement with our thermodynamic model.;The epitaxial VO2 films with Cr2O3 buffered on c-sapphire were used for multi-functional applications. Also, the SMT characteristics of VO2 were preserved. The shift in transition temperature to a higher value was due to the residual tensile strain along the monoclinic VO 2 [001] direction. The saturated magnetic moment and the coercivity were improved to be ∼30 emu/cm3 and 86 Oe for these VO 2films, compared to the VO2 films without Cr2O 3 buffer. This heterostructure has shown that VO2 films have a great potential for magnetic applications as well as for electrical and optical applications.;To integrate VO2 films with Si (100), Cr2O 3 was used for a template layer. Cr2O3 can be grown epitaxially on silicon substrates by controlling the deposition conditions. It is believed that SiO2(s) can be reduced to form volatile SiO (g) formed because of the large negative formation energy of Cr 2O3 (-1140 kJ/mol). This reduction prevents the formation of an amorphous SiO2 layer during the Cr2O3 growth. This silicon with epitaxial Cr2O3 template layer can be used to grow VO2 thin films. The SMT characteristics of the VO2 films were better than those VO2 films grown on SiO2/Si substrates. Thus, VO2 films with good SMT characteristics can be integrated with Si (100) by Cr2O3 template layer.