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 NO 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.