Influence of post-deposition annealing in air and vacuum on the properties of thermally evaporated gallium oxide films

• Thermally grown Ga-oxide films are amorphous, oxygen deficient and absorbing.
• Air or vacuum annealing transforms them into UV transparent β-Ga2O3 films.
• Decomposition of Ga2O in vacuum annealing produces Ga2O3 phase.
• Vacuum annealed Ta-boat deposited Ga2O3 films consist of dendrites.
• Arrhenius plots of conductivity have both intrinsic and extrinsic regions.

Oxidative annealing in air or reductive annealing in vacuum around 773 K of thermally evaporated gallium oxide films produces monoclinic β-Ga2O3 films of distinctly different compositional, optical, morphological and electrical properties. The pristine films prepared by the evaporation of Ga2O3 powders are oxygen deficient, amorphous and absorbing in UV–visible region. The air annealed films are transparent (band gap ∼4.9 eV), display nanometric granular morphology and are characterized by <1.0 eV extrinsic and 1.2–1.6 eV intrinsic activation energies in the Arrhenius plots of electrical conductivity. The growth of Ga2O3 phase on vacuum annealing takes place through the decomposition of Ga2O, one of the constituents of the pristine films. The vacuum annealed films exhibit comparatively lower transparency (band gap <4.5 eV), comprise micron-sized dendrites or fibres and have <1.0 eV extrinsic and 1.7–2.0 eV intrinsic activation energies. The incorporation of these properties results from compositional changes in films induced by annealing in air or vacuum ambient.