Effect of annealing temperature on structural transformation of gallium based nanocrystalline oxide thin films and their optical properties

Nanocrystalline thin films consisting of pure phases of α-GaO(OH), α-Ga2O3 and β-Ga2O3 have been prepared. These were synthesized by sol–gel technique through varying annealing temperature from 300 °C to 1100 °C in air ambient. Single phase α-GaO(OH) was obtained at annealing temperature of 300 °C. With increase in annealing temperature a mixed phase of α-GaO(OH) and α-Ga2O3 was found. Pure α-Ga2O3 could be prepared at 500 °C. As the annealing temperature was further increased, α-Ga2O3 started to convert into β-Ga2O3. Only β-phase was observed at annealing temperature 700 °C and above. The average crystallite size of α-GaO(OH) and α-Ga2O3 phases was found to be ∼3 nm and ∼16 nm respectively, while that of β-Ga2O3 increased from ∼14 nm to ∼24 nm with increase in annealing temperature (700–1100 °C). The optical band gaps of α-GaO(OH) and α-Ga2O3, determined from transmittance measurements were 5.27 and 4.98 eV respectively and higher than that of β-phase. A nominal variation of band gap of β-phase (4.8–4.7 eV) was observed with the variation of annealing temperature (700–1100 °C). The structural defects of this phase, estimated from transmittance spectra using Urbach tail widths, were found to decrease with increase in crystallite size.