Raman spectroscopy and optical properties of GAZO thin films deposited at various substrate temperatures

We report the Raman study and optical properties of gallium and aluminium co-doped ZnO (GAZO) thin films deposited at various substrate temperatures by sputtering. The films exhibited Raman peaks at 99 cm^-1, 437 cm^-1 and 575 cm^-1 which were assigned to the ZnO E₂ (low), E₂ (high) and A ₁ (LO) modes, respectively. The wavelength dependence of the refractive index and the extinction coefficient were described by the Cauchy dispersion model, through theoretical modelling of the transmittance data. The refractive index showed an increasing trend while the extinction coefficient decreased with increasing substrate temperature in the visible region. This was attributed to the relative decrease in transmittance observed at higher substrate temperatures. The dielectric constant (ɛ) , dissipation factor (tan δ) and optical conductivity (σ) values were associated with the films’ high transparency. The Wemple and Didomenico single oscillator dispersion energies (E_o and E_d increased with substrate temperature which was attributed to the reduction in residual stress, improvement in microstructural order and crystallinity. The static refractive index (n₀) , optical band gap (E_g) , long wavelength dielectric constant (ɛ₀ , the M_{- 1} and M_{- 3} moments of the optical spectra and the non-linear refractive index (n₂ were determined. Optimum optical properties were obtained in films deposited on heated substrates (75 and 100 ^∘ C).