The effect of substrate temperature on structural and optical properties of D.C. sputtered ZnO thin films

Zinc oxide (ZnO) thin films were deposited on (100) silicon substrates via d.c. sputtering process with unbalanced magnetron configuration at various substrate temperatures (room temperature to 300°C in steps of 100°C) at 200 W power. Variable angle spectroscopic ellipsometry (VASE) characterization technique was used to study the optical properties of the samples in the UV-visible spectral range of 300-800 nm (around 1.5-4.2 eV). The optical properties of the films, including the index of refraction, the extinction coefficient, the band gap energy and the absorption coefficient were systematically investigated as functions of the substrate temperatures. Our mathematical modeling analysis shows that the refractive index obeys the second ordered Sellmeier's dispersion formulation, while Urbach's absorption energy and exponential tail are formulated with Cauchy-like dispersion model for the extinction coefficient. The band gap energy (extrapolated from Tauc plot) ranges from 3.19 to 3.33 eV with a maximum value at 300°C substrate temperature. The single oscillator energy (E_o), dispersion energy (E_d), wavelength (λ_o), average strength (S_o) moments of the optical spectra (M_-1 and M_-3) and the static index of refraction n() were estimated based on effective single oscillator model proposed by Wemple and DiDomenico. The films X-ray diffraction studies reveal a polycrystalline microstructure with preferred (002) orientation. Our results are physically interpreted and discussed.