Thermopower analysis of metal-insulator transition temperature modulations in vanadium dioxide thin films with lattice distortion

Insulator-to-metal (MI) phase transition in vanadium dioxide (VO sub(2)) thin films with controlled lattice distortion was investigated by thermopower measurements. VO sub(2) epitaxial films with different crystallographic orientations, grown on (0001) alpha -Al sub(2)O sub(3), (1120) alpha -Al sub(2)O sub(3), and (001) Ti O sub(2) substrates, showed significant decrease of absolute value of Seebeck coefficient (S) from ~200 to 23 mu VK super(-1), along with a sharp drop in electrical resistivity ([rho]), due to the transition from an insulator to a metal. The MI transition temperatures observed both in [rho](T sub([rho])) and S(T sub(S))for the VO sub(2) films systematically decrease with lattice shrinkage in the pseudorutile structure along the c axis, accompanying a broadening of the MI transition temperature width. Moreover, the onset T sub(S), where the insulating phase starts to become metallic, is much lower than the onset T sub([rho]). This difference is attributed to the sensitivity of S for the detection of hidden metallic domains in the majority insulating phase, which cannot be detected in [rho] measurements. Consequently, S measurements provide a straightforward and excellent approach for a deeper understanding of the MI transition process in VO sub(2).