Structural changes across the metal-insulator transition in thin epitaxial VO2 films

Vanadium dioxide (VO2) got much interest in the recent years not only from the fundamental point of view as a correlated electron system but as well as due to its intriguing electrical and optical properties, like the metal-insulator transition (MIT) close to room temperature. This makes VO2 favourable for optoelectronic, switching or even memory devices. The main challenge for device applications is the epitaxial growth of VO2 on suitable substrates. Sapphire seems to be one of the promising substrate candidates for the growth of high quality epitaxial VO2 phases.
Referring to literature, the MIT is directly connected with a change in the crystal structure, namely the transition from the low temperature monoclinic phase (P21/c) to the high temperature tetragonal (rutile) phase (P42/mnm). However, this symmetry change at the transition temperature should be strongly influenced by the epitaxy itself. Comparing our structural investigations and electrical measurements the results indicate that the MIT as observed by the resistance measurement in epitaxial VO2 thin films seems to be not necessary accompanied by a complete monoclinic to rutile phase transformation. A slight lattice distortion causing a possible change in the atomic positions without breaking the existing the epitaxial relationship appears to be sufficient.