Photoluminescent, undoped ZnO films have been fabricated using spray pyrolysis of zinc nitrate solution. The luminescent films had a polycrystalline hexagonal wurtzite type structure with no preferred orientation. Photoluminescence intensity was critically dependent on substrate temperature during spray pyrolysis and on post-annealing temperature. Green, photoluminescent films possessed a porous structure while orange films possessed a close packed granular morphology. Green luminescence appears to be due to oxygen vacancies in a layer just below the crystallite surface.

The relative intensities of the green and blue luminescence of a ZnO film was shown to depend on the excitation regime. Time-resolved and steady-state luminescence were studied along with photoconductivity transients. Under continuous excitation the film emitted green light, while under pulsed excitation the luminescence was either blue or green, depending on the intensity of the excitation pulse. The intensity of the blue component depended linearly on the pulse intensity while the green intensity followed a sublinear power law dependence with the exponent α=1/3. The transient luminescence exhibited fast (below nanosecond) and slow (microsecond) decay components at room temperature. The fast component was ascribed to interband exciton recombination, and the slow component was attributed to an electron-hole recombination involving a donor-acceptor complex, which most likely consisted of oxygen and zinc vacancies. In this model, the complex can emit light only when it is activated, i.e., oxygen vacancy is in its singly ionized state and the acceptor (zinc vacancy) captures a hole. The density of the activated complex depends on the Fermi level position, bend bending, and thickness of the depletion layer.

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 Tc=180 °C below which the thermal decomposition to ZnO did not occur or was incomplete. Films grown above Tc showed strong preferred orientation of polycrystals along the c-axis, while the films grown at Tc 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 10−3  (Ω cm)−1 in daylight and 10−4  (Ω cm)−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.

Slow photoconductivity transients were comprehensively studied in ZnO films prepared by spray pyrolysis of the zinc-nitrate solution. Surface charge controlled the film conductivity, and it was possible to reversibly change the conductivity by many orders of magnitude using short-term annealing in hydrogen and oxygen. Under illumination, the conductivity of as-grown films may increase by several orders of magnitude, depending on the dark conductivity. Photoconductivity was due to the capture of nonequilibrium holes at surface oxygen states to produce an equivalent number of excess electrons in the conduction band. Reverse process of the photoconductivity relaxation is determined by an electron tunneling mechanism to the surface oxygen states.

Response of steady-state photoconductivity to changes in oxygen partial pressure (10 Ϫ3 to 1 atm) has been quantitatively studied in thin-film polycrystalline TiO 2 :Nb and ZnO at 80-120ЊC. The magnitude of photoconductivity varied as a square root of illumination intensity regardless of oxygen pressure. Both materials showed fast response to oxygen, although in different pressure ranges. Zinc oxide was more sensitive to lower oxygen pressures while titanium dioxide worked better at pressures close to 1 atm.