Role of Si and Ge as impurities in ZnO

The electronic and structural properties of Si in ZnO are studied using density functional calculations with both the generalized gradient approximation and a hybrid functional. Our calculations show substitutional Si on the Zn site (SiZn) to be significantly lower in energy than the Si interstitial (Siᵢ) and Si on an O site (SiO). SiZn is predicted to be a shallow donor in ZnO with a formation energy low enough to lead to significant incorporation of Si in the ZnO crystal. Interestingly, we find that SiZn has a lower formation energy (and therefore higher solubility) under Zn-rich conditions than under O-rich conditions. We also study the properties of Ge in ZnO for comparison, finding behavior similar to the Si impurity, but with an even lower formation energy. Our results suggest Si and Ge as suitable n-type dopants in ZnO.