Charge accumulation and barrier formation at grain boundaries in ZnO decorated with bismuth

Density functional plane-wave pseudopotential calculations have been performed on two high-angle grain boundaries in ZnO which have been decorated with various quantities of Bi. The results show that both grain boundaries, which have significantly different structures, can accommodate up to about 30% of substitutional Bi in qualitative agreement with experimental observations. The segregation of Bi to the boundaries results in local charge accumulation which is localized within Bi–Bi bonds or on Bi atoms. The charge accumulation in both boundaries results in fluctuations in potential across the interface and the formation of a barrier to electron transport. However, there is no evidence for a deep acceptor level usually associated with the Schottky barrier model. The present results suggest an alternative mechanism in which electrons are trapped in Bi–Bi bonds and depleted in an external field. However, defect states have not been ruled out and it is suggested that if they exist they are caused by more complex defects than those considered here.