The effects of doping a grain boundary in ZnO with various concentrations of Bi

We have made a systematic study of the Bi-decoration process in a Σ=13 [0 0 0 1] tilt grain boundary in ZnO by first-principles calculations. This grain boundary is taken as a model system for studying the microscopic properties of commercial Bi-doped ZnO-varistors. The calculations show that the decoration process is strongly site dependent and that there is a considerable segregation energy for the Bi-atoms at low concentration. Increasing the concentration lowers the segregation energy which sets an upper limit of approximately 32% for the Bi-concentration in this grain boundary. This implies that the Bi-atoms stay in the grain boundary region rather than diffusing into the ZnO grains during the manufacturing process, but the maximum Bi-concentration is limited which is consistent with the experimental observations. Bi-impurities in ZnO act as donors at low impurity concentration, but a localized Bi–Bi-bond is formed at higher Bi-concentration in the grain boundary. This Bi-state is located in the band gap of ZnO and it may be responsible for the varistor effect observed in Bi-decorated grain boundaries.