Strain and thickness dependence of the metal-insulator transition in VO2 epitaxial films
Résumé
One of the emerging fields in nano-electronics is the adaptive electronics based on resistive memories integrating functional oxides. In this context, the most promising memories and devices make use of the Mott Metal-Insulator Transition (MIT) in which electric charge injection into a transition metal compound induces a transition from a strongly correlated insulator into a weakly electron correlated metal (Mott or Mott-Hubbard transition). The transition is accompanied by large resistance changes of the material and of the associated devices, on very short timescales.
Among the possible materials exhibiting a MIT, vanadium dioxide (VO2) is of particular interest since it exhibits a huge resistivity change between the two stable phases (five orders of magnitude). In this work, pure and W-doped VO2 epitaxial films with thickness ranging between 5 and 200 nm have been grown on Al2O3 and TiO2 substrates of various orientations using both pulsed laser deposition and electron beam evaporation. The films have been characterized using high-resolution X-ray diffraction, ellipsometry and 4-probe electrical resistivity measurements. We show that there is clear dependence between the magnitude of the MIT and the thickness of the deposited films. This behavior can be explained by the fact that upon decreasing film thickness the level of strain in the films increases, which results in a lowering of the electrical gap in the insulating state by ~0.1 eV and hence a degraded resistivity and resistivity ratios reduced to two orders of magnitude for the thinnest films.