Semiconductor-to-Metal Phase Transitions of Vanadium Dioxide (VO2) Thin Films

Among all the metal-oxide phase transition materials, vanadium dioxide (VO2) has attracted extensive research interest benefiting to its outstanding semiconductor-to-metal phase transition (SMT) properties with a transition temperature (Tc) close to room temperature. VO2 is known to exhibit an ultrafast (within 0.1 ⁰C) and reversible phase transition from a semiconductor phase to a metallic phase at ~68 ⁰C. Because of this fascinating characteristic, VO2 shows great potential in various devices including sensors, switches, thermo/electrochromics, thermal actuators, and memory devices. The semiconductor phase VO2 has a monoclinic crystal structure. By the first-order SMT process, VO2 transits to tetragonal crystal structure, which results in dramatic changes in its electrical and optical properties. Compared with single crystalline VO2, the properties of VO2 thin films can be largely affected by many factors including defects density, strain, and the existence of the multivalent vanadium ions(V2+, V3+, V4+, V5+). It’s quite challenging to synthesize high quality VO2 thin films with sharp transition width, narrow thermal hysteresis, and large electrical and optical property change. In order to address these issues, the work in this thesis is focused on optimizing the SMT properties of VO2 thin films and studying the defect effects in the SMT processes. Firstly, highly textured VO2 thin films have been achieved on amorphous glass substrates and compared with the ones grown on c-cut sapphire and Si (111) substrates, all by pulsed laser deposition. Excellent phase transition properties were observed for the films on glass substrates and were correlated with the large grain size and low defects density of the films. Based on the first work, VO2 thin films with controlled grain sizes were deposited on amorphous glass substrates by pulsed laser deposition. The VO2 films were found to exhibit a sharper SMT and larger transition amplitude with lower grain boundary (GB) density. The GBs were revealed to introduce disordered atomic structures and distorted crystal lattices in the films, which results in the drop of the film SMT properties. To enable VO2 thin films in practical devices, a tunable Tc of VO2 is necessary to satisfy the working environments of different devices. To achieve the tunable Tc, VO2 thin films with controlled thicknesses have been deposited on c-cut sapphire substrates with Al-doped ZnO (AZO) buffer layers by pulsed laser deposition. The Tc of the films was continuously tuned by the VO2 thickness and the VO2/AZO interface roughness, accompanied with no significant drop of other SMT properties. It shows that the Tc is correlated with the film strain, which increases with the decrease of film thickness or VO2/AZO interface roughness. Finally, the stability of the VO2 thin film phase transition was characterized. The VO2 film deposited on c-cut sapphire substrate has been founded to exhibit a Tc shifting and transition width broadening after tens of cycles of phase transition. In situ transmission electron microscopy (TEM) heating experiments revealed that the strain was accumulated around the domain boundaries during phase transitions, possibly because of the dimension changes of the crystals