Iron Cyanide as a Surface State to Prevent ZnO Photolysis in Vacuum

When ZnO is intermittently irradiated with ultraviolet in vacuum, its dark conductance increases irreversibly, presumably due in part to oxygen photodesorption and in part to photodecomposition of the ZnO. From the theory of one-equivalent surface states and from earlier measurements of the relevant parameters associated with the surface states, it was concluded that an iron cyanide additive should provide characteristics suitable for surface stabilization. In particular, it should act as a recombination center for the photoproduced carriers. Single crystal studies of dark conductance after various time intervals of illumination in vacuum are reported, showing that an iron cyanide surface additive does indeed control the photodecomposition—less than a monolayer decreases the rate by several orders of magnitude. The action of iron cyanide as a recombination center is compared with theory, showing by semiquantitative analysis that carrier generation and recombination should be described by a modified Elovich equation rather than by the more usual formulations applicable to less dense recombination centers. The rate of ZnO decomposition is calculated using a simple model and it is shown that the rate should vary inversely with the square of the additive concentration, as observed for long exposure times.