Large photoconductive effects have been observed on single crystals of ZnO at 300°K and 78°K. At 300°K the decay of the photoconductivity is strongly dependent on the ambient conditions, being slow in vacuum and fast in wet oxygen. This shows that the conductivity is associated with the surface. The effects are essentially independent of the dark conductivity of the crystals and cannot be accounted for by the desorption of adsorbed oxygen. It is proposed that holes from hole electron pairs discharge lattice oxygen ions at the surface, producing a surface excess of zinc and an electron enrichment layer in which conduction occurs. The formation of this layer has been followed as a function of the total number of photons incident on the crystal. In the initial stages quantum efficiencies as high as 0.25 have been observed for this process.

• Received 15 May 1958

DOI:https://doi.org/10.1103/PhysRev.112.388