Heterophase polydomain structure and metal-semiconductor phase transition in vanadium dioxide thin films deposited on (10(1)over-bar0) sapphire

Vanadium dioxide (VO2) thin films deposited on (10 (1) over bar 0) sapphire are composed of two mixed monoclinic phases, namely M1 and M2. The M1 phase is unstable because of the existence of a larger misfit strain in the ((1) over bar 02) VO2 film. The reduction of misfit strain in the film favours the formation of the M2 phase. The X-ray diffraction and pole figure results show that both M1 and M2 phases are well aligned with the substrate and both contain twinned structures. Therefore, the microstructure of the film can be regarded as being a transversely modulated heterophase polydomain. A higher electrical resistivity ratio of the semiconductor phase to the metallic phase (rho(s)/rho(m)) can be achieved only in single-phase VO2 thin films, either the M2 or Mt phase. Phase mixing degrades the ratio of rho(s)/rho(m). The film with a single M2 phase exhibits a lower transition temperature of 58 degrees C without any degradation of the rho(s)/rho(m) ratio.