Strain Effect on Metal–Insulator Transition and Electronic Structure of Epitaxial VO ₂ Thin Films: Comparative Structural, Transport, and Optical Studies

Extensive studies of structural, transport, and optical conductivity properties have been considered one of the most suitable approaches to investigate the phase transition‐induced electronic structure changes of strongly correlated electron systems. Here, we report the effect of strain on the metal–insulator transition (MIT) and electron correlation in VO 2 thin films. Stoichiometric VO 2 thin films (~10 nm thickness) with different strain states were epitaxially grown on TiO 2 (001) and Al 2 O 3 (0001) substrates using a pulsed laser deposition. Fully strained VO 2 /TiO 2 and relaxed VO 2 /Al 2 O 3 both have a critical carrier density of ~ 10 19 cm − 3 and exhibit the monoclinic‐rutile structural phase transition near the MIT temperature. The lowest lying optical transition of d || → π* in the insulating state transforms into a Drude peak in the metallic state. The effective plasma frequency of VO 2 /TiO 2 is found to be slightly larger than that of a VO 2 single crystal. These results suggest that the MIT in 10 nm‐thick VO 2 films should originate from electron–phonon and electron–electron interactions, and the electron correlation should be related to the strain‐induced orbital occupancy and bandwidth changes.