The solid-solid phase transition in vanadium dioxide thin films: synthesis, physics and application
THE SOLID-SOLID PHASE TRANSITION IN VANADIUM DIOXIDE THIN FILMS: SYNTHESIS, PHYSICS AND APPLICATION JOYEETA NAG DISSERTATION UNDER THE DIRECTION OF PROFESSOR RICHARD F. HAGLUND, JR. This dissertation is devoted to the impact of materials physics and materials synthesis on the characteristics of the solid-solid phase transition in vanadium dioxide thin films. In vanadium dioxide, the solid-solid phase transition comprises both an insulator-to-metal transition and a monoclinic-to-tetragonal structural transformation. The morphology, transition characteristics and energy cost of the phase transition in this strongly correlated material are shown to be strongly dependent on the details of the synthesis process. It was discovered that the conventional protocol for preparing epitaxial films on c-sapphire produces nanoparticulate samples, whereas room temperature deposition followed by annealing tends to produce remarkably smooth films, which are substantially better-suited for applications. For epitaxial VO2 films grown on c-sapphire, using both the above-mentioned protocols, it was found that the thermally-induced structural and metal-insulator transitions are not congruent due to the larger energy cost of the former. The energy of the phase transformation can be modified by doping with tungsten, and both the thermally induced transition and the ultrafast laser-induced switching between insulating and metallic states show evidence of this. The novel properties of the phase transition inVO2 thin films finds a compelling application in the modulation of a hybrid Si:VO2 ring resonator, a basic building block of photonic logic circuits.