The Growth Of VO₂Phase Transition Thin Film And Its Application Study

Vanadium dioxide (VO2), a typical strongly correlated material, shows a first-order metal-insulator transition (MIT) from high temperature tetragonal phase to low temperature monoclinic phase under the critical temperature of68℃. Across the MIT process, the resistance of VO2underdoes a sharp change, up to4-5orders of magnitude. Furthermore, the optical property of VO2also exhibits obvious switching behavior from high transmission to reflection in infrared region after the phase transition process. Because of the giant change in electrical and optical properties near room temperature, it has been attracted tremendous attention in many potential applications such as smart window, optical switches, memory devices, infrared laser radiation protective device and uncooled infrared detection.However, on one hand, the relatively high critical phase transition temperature of VO2has greatly hindered the practical applications based on VO2material. Accordingly, it is very important to modulate the MIT behavior of VO2and decrease the phase transition temperature (Tc) to room temperature. In fact, many previous reports have confirmed that the Tc value can be effectively regulated to room temperature by external heavy metal ions doping, by adding interfacial stress/strain on VO2film, or by electric-field effect in ionic liquid gated VO2three-terminal devices.On the other hand, in order to extend the VO2applications, we also need to integrate the VO2thin film with other material to form a hybrid device, which can gain all sorts of advantages from the composite materials. According to this idea, the main studies in this thesis and the related results achieved are listed as the following:(1) The rf-plasma assisted oxide-molecular beam epitaxy (OMBE) method has been adopted to prepare high quality VO2thin film samples. By this technique, we can precisely control the growth parameters including the deposition time, temperature and the final vanadium oxides with desired valence state by modifying the rate of vanadium and oxygen. In addition, this method has high repeatability for vanadium oxide film growth and up to2-inch film sample with atomic smooth surface can be obtained.(2) By using the OMBE method, high quality VO2epitaxial film has been prepared on p-GaN/sapphire substrates, forming the p-n junction of V02/p-GaN layers. The regular x-ray diffraction (XRD) and Raman spectroscopy were used to investigate the film growth behavior and phase components. High resolution synchrotron radiation φ-scan XRD was used to confirm the in-plane lattice matching relationship of VO2/p-GaN layers. Most importantly, the metal-insulating transition characteristics were investigated by modifying the carrier density in the depletion layer under different external bias voltages. The results showed that the phase transition of VO2film could be effectively controlled by modifying the carrier density within the depletion layer of p-n junction, which demonstrated that the phase transition behavior controlled by external bias voltage could have potential applications in a logical memory or in sensor devices.(3) We have investigated the preparation of VO2epitaxial film on HOPG or graphene substrates, which indicates that the high-quality flexible VO2/HOPG or VO2/graphene composite film can be prepared successfully. Based on this, we aim to prepare VO/graphene composite film and attempt to examine the potential applications in smart window area. In addition, we have prepared the V2O5/VO2composite structure by just annealing the prepared VO2thin film in air with suitable temperature. This V2O5/VO2composite shows some potential applications in uncooled infrared detection.