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 t he growth temperature on the structural, optical, electrical, and rela xation properties has been studied. It was found that there was a crit ical temperature T-c = 180 degrees C below which the thermal decomposi tion to ZnO did not occur or was incomplete. Films grown above T-c sho wed strong preferred orientation of polycrystals along the c-axis, whi le 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 obse rved for as-prepared films as the substrate temperature was decreased near the critical temperature. Annealing brought all the samples to th e same band gag 3.30 eV measured at a half height of the maximum absor ption. 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 satu ration. Steady-state photoconductivity in the daylight can be as much as four orders in magnitude larger than the dark value. Annealing in n itrogen at 400 degrees C brought all samples to the same conductivity of 10(-3) (Omega cm)(-1) in daylight and 10(-4) (Omega cm)(-1) in the dark. The photoconductivity transients were complicated and changed fr om a power law to multiexponential time dependence after annealing. Th e data are discussed on the basis of model in which hole traps located at the grain boundaries play the major role. (C) 1998 American Instit ute of Physics.