First-principles study of the formation mechanisms of nitrogen molecule in annealed ZnO

Recently, N₂ molecule was reported to induce localized states in the band gap and trap two holes in ZnO. In this Letter, the detailed mechanism for the formation of N₂ molecule in high temperature annealing process in ZnO was investigated based on density-functional theory. By analyzing the interactions between N-related defects, we found that the nitrogen molecule would form by the binding of two interstitial nitrogen atoms. Interstitial oxygen facilitated the formation of N₂ by kicking out N_O to interstitial site. The formation of nitrogen molecule in ZnO would cause low doping efficiency and degeneration of the p-type in annealing process. Our results could explain the recently reported formation of N₂ molecule in high temperature annealing process in N-doped ZnO. Appropriate annealing conditions were suggested in order to get p-type ZnO.