Thickness-modulated metal-insulator transition of VO2 film grown on sapphire substrate by MBE

VO2 films with precisely controlled thickness on the nanoscale ranging from 15 to 60 nm were grown on single crystal sapphire substrates by molecular beam epitaxy. X-ray diffraction and atomic force microscopy measurements indicated that high quality single phase VO2 films with condense and smooth surface and free of cracks could be achieved only when the film was thicker than 30 nm. The temperature-dependent resistance measurement indicated a drastic modification of metal-insulator transition (MIT) properties which was achieved through the variation of film thickness, especially the transition magnitude and curve abruptness. The corresponding mechanism was supposed to be associated with the tensile stress relaxation effect with increasing thickness caused by thermal mismatch within VO2 films, as demonstrated by Raman spectra. Our present finding provides an effective and convenient alternative to modulate the MIT properties of VO2 films.