  Insulator-to-metal (MI) phase transition in vanadium dioxide (VO2) thin films
with controlled lattice distortion was investigated by thermopower
measurements. VO2 epitaxial films with different crystallographic orientations,
grown on (0001) alpha-Al2O3, (11-20) alpha-Al2O3, and (001) TiO2 substrates,
showed significant decrease of absolute value of Seebeck coefficient (S) from
~200 to 23 microV K-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 (Trho) and S (TS) for the VO2 films
systematically decrease with lattice shrinkage in the pseudo-rutile structure
along c-axis, accompanying a broadening of the MI transition temperature width.
Moreover, the onset TS, where the insulating phase starts to become metallic,
is much lower than onset Trho. 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 VO2.
