Influence of post-annealing on electrical, structural and optical properties of vanadium oxide thin films

Vanadium oxide thin films were grown onto quartz substrates using the pulsed DC reactive magnetron sputtering technique at room temperature and afterwards post annealed under vacuum conditions in the temperature range from 75 to 230 °C. The electrical resistance, temperature coefficient of resistance (TCR), optical energy gap and structural properties were investigated. The films are amorphous, nanoscale grained V2O5 phase and the mean grain size increases with increasing temperature. Additionally, the post-annealing at 230 °C induces formation of both V2O5 and V4O9 phases and pinholes on the film surface. The temperature dependent variation of the electrical resistance indicates two activation energy areas corresponding to two TCR values for the films post annealed up to 180 °C, but only one activation area was found after annealing at 230 °C. Analyses of the absorption coefficient versus photon energy revealed a direct forbidden transition. The mean grain size and TCR values increase with increasing post-annealing temperature, whereas the optical energy gap and electrical resistance do not follow this tendency. The evolution of the structure and its correlation to the optical energy gap, electrical resistance, activation energy and TCR were discussed by means of the results obtained in the present study.

► VOx (50 nm) thin films indicate amorphous and nanoscale grained V2O5 structure.
► Post-annealing under vacuum at 230 °C results in formation of V2O5 and V4O9 phases.
► Optical energy gap is significantly affected by the structural changes.
► Electrical resistance is directly correlated with optical energy gap and structure.
► Optical energy gap plays critical role on the activation energy and TCR value.