Carrier mobility and density contributions to photoconductivity transientsin polycrystalline ZnO films

Slow photoconductivity transients were comprehensively studied in ZnO films prepared by spray pyrolysis of the zinc-nitrate solution. Surface charge c ontrolled the film conductivity, and it was possible to reversibly change t he conductivity by many orders of magnitude using short-term annealing in h ydrogen and oxygen. Under illumination, the conductivity of as-grown films may increase by several orders of magnitude, depending on the dark conducti vity. Photoconductivity was due to the capture of nonequilibrium holes at s urface oxygen states to produce an equivalent number of excess electrons in the conduction band. Reverse process of the photoconductivity relaxation i s determined by an electron tunneling mechanism to the surface oxygen state s. (C) 2000 American Institute of Physics. [S0021-8979(00)05404-9].