Substrate-induced Effects on the Phase Transition in VO2 Thin Films

by Jian Li (Ph.D Candidate)

Thursday Nov 14, 2013, 2:10 PM → 2:50 PM Asia/Seoul

119호 (제1과학관)

Vanadium dioxide VO2 shows a metal-insulator transition (MIT) at a temperature of about 340 K. Even though the material has been intensively studied by many researchers since the first observation on MIT in VO2 by Morin (1959), the origin of the MIT is not completely understood. The MIT transition is accompanied by the structural transition from monoclinic to tetragonal as well as optical switching behavior and it is closely related to the oxygen stoichiometry of the material itself and lattice distortion. In this research, VO2 thin films were synthesized on various substrates and their phase transition properties were systematically investigated. First, VO2 thin films were deposited on c-plane sapphire substrate by pulsed laser deposition with various oxygen pressures, substrate temperatures, pulsed laser ablation frequencies, and laser powers. After those parameters analyzed, epitaxial VO2 films were synthesized on 2-side-polished TiO2 single crystal substrates of different orientations, which are (111), (110), (101), (100), and (001). The substrate-induced effects on the phase transition in VO2 thin films were studied. The metal-insulator transition temperature and the magnitudes of the resistance changes were proofed to be strongly related to the deposition parameters and the substrates. And then, in order to study the optical switching of VO2 and its thermal hysteresis behavior, VO2 films were fabricated on glass substrates. Upon heating and cooling these samples exhibited significant optical switching behaviors with a clockwise thermal hysteresis in the infrared range of >850 nm or with a counterclockwise thermal hysteresis in the near visible range of 650– 850 nm. Finally the photo-induced resistance in VO2 films on glass substrates with Nb doped TiO2 buffer layer was measured at 300 oC with illumination of UV light. It is found that neither VO2 films on glass substrates with pure TiO2 buffer layer nor Nb:TiO2 single layer shown photo-induced change in resistance like the case that VO2/Nb:TiO2 bilayers on glass. This photo-induced change in resistance is probably caused by photo-excitation and photo-carrier injection phenomenon at the interface VO2/Nb:TiO2.

Thesis Advisor: Prof. Joonghoe Dho