Optical and electrical properties of undoped ZnO films grown by spray pyrolysis of zinc nitrate solution

Undoped ZnO films were deposited by spray pyrolysis using aqueous zinc nitrate solution at different substrate temperatures. The effect of the growth temperature on the structural, optical, electrical, and relaxation properties has been studied. It was found that there was a critical temperature $T_c$ =180 °C below which the thermal decomposition to ZnO did not occur or was incomplete. Films grown above $T_c$ showed strong preferred orientation of polycrystals along the $c$ -axis, while the films grown at $T_c$ or below showed a powder-like, non-oriented polycrystalline structure when they were converted afterwards to zinc oxide by annealing. A slight increase of the optical band gap was observed for as-prepared films as the substrate temperature was decreased near the critical temperature. Annealing brought all the samples to the same band gap 3.30 eV measured at a half height of the maximum absorption. After illumination, the steady-state photoconductivity decayed very slowly with a time constant of about a week for as-grown samples. The steady-state photoconductivity in daylight was very close to saturation. Steady-state photoconductivity in the daylight can be as much as four orders in magnitude larger than the dark value. Annealing in nitrogen at 400 °C brought all samples to the same conductivity of ${10}^{\mathrm{-3\hspace{0.17em}}}\mathrm{\hspace{0.17em}(\Omega }$ cm)${}^{\mathrm{-1\hspace{0.17em}}}$ in daylight and 10${}^{\mathrm{-4}}$ (Ω cm)${}^{\mathrm{-1}}$ in the dark. The photoconductivity transients were complicated and changed from a power law to multiexponential time dependence after annealing. The data are discussed on the basis of model in which hole traps located at the grain boundaries play the major role.