Appearance of large crystalline domains in VO2 films grown on sapphire (001) and their phase transition characteristics

We report the first observation of large crystalline domains of several μm-size in VO2 films deposited on Al2O3 (001) substrates by rf-biased reactive sputtering technique. The large crystalline domains, dominated with random in-plane oriented growth of (011)M1-orientation, appear only under adequate substrate biasing, such as 10 W, while most biasing conditions result in conventional nanosized grains of highly oriented (010)M1-orientation. Two temperature-controlled analyses, x-ray diffraction and micro-Raman spectroscopy, have revealed that some parts of large crystalline domains undergo intermediate monoclinic (M2) phase during the thermally-induced structural phase transition from monoclinic (M1) to rutile-tetragonal (R) phase. As an effect of the appearance of large crystalline domains, the film showed in-plane tensile stress, resulting in high TIMT of 69 °C due to the elongation of the V-V distance in its low-temperature monoclinic phase.