Barrier properties in electrophotographic zinc oxide

A number of factors are known to influence greatly the photoelectrical and the electrophotographical properties of zinc oxide powder. Some of them, such as the degree of zinc excess and the state-species and amount-of chemisorbed oxygen, are in most cases uncontrollable, in the sense that they are almost determined at the time of manufacturing, unless some drastic treatment is imposed on samples before use. While some other factors such as the surface doping can be used to improve or control behaviors of samples at the time of use, these factors seem to be intimately related to the hard-to-control ones. After a brief review of studies of the oxygen states on zinc oxide surface, a discussion is developed concerning the photoconductive properties and the electrophotographic charge acceptance of zinc oxide bearing various degrees of zinc excess. The potential barrier at the surface of microcrystals, the height of which is estimated from dark conductivities, gives a good measure for the charge acceptance. Further, based upon the conventional theory of barrier development by oxygen chemisorption, the photo-to-dark conductivity ratio is argued in terms of the donor density, the barrier height, and other factors.