Metal-insulator transition in thin films of vanadium dioxide: The problem of dimensional effects

Vanadium dioxide has various potential applications in electronics due to the metal-insulator transition (MIT). it is known that oxide structures with nanometric dimensions exhibit properties different from bulk oxide materials because of the spatial confinement and the proximity of the substrate. However, in order to integrate VO2 into the thriving nano-scale electronics, it is necessary to explore the MIT in this material in thin film nano-structures. In this work, it is shown that there is a fundamental dimensional restriction for the transition to occur even for pure epitaxial VO2 nano-films and nano-wires. This is associated with the fact that any phase transition turns out to be impossible when the system size is decreased below a certain characteristic length d(c). This dimension is estimated to be d(c)similar to xi (where xi is the correlation length, similar to 2 nm for VO2), and, on the basis of the available experimental data, it is shown that the transition temperature falls as the characteristic size (film thickness or nano-wire radius) diminishes, though the predicted theoretical limit of 2 nm is not still experimentally achieved by now. Experimental results concerning the dependence of the threshold voltage on the film thickness at MIT-induced switching in VO2 based sandwich structures are presented. Finally, the comparison of the authors' experimental data with the literature data, as well as with the analogous features of superconducting phase transitions, is carried out. (C) 2009 Elsevier B.V. All rights reserved.