Mutual Passivation of Donors and Isovalent Nitrogen in GaAs

Using large supercell total energy and band structure calculations, we have studied the mutual passivation mechanism of isovalent N and shallow donors in GaAs. We find that all the donor impurities, Si$_{Ga}$, Ge$_{Ga}$, S$_{As}$, and Se$_{As}$, bind to N in GaAs:N, which has a large N-induced band gap reduction relative to GaAs. For group-IV impurity such as Si, the formation of the nearest-neighbor Si$_{Ga}$-N$_{As}$ defect complex creates a deep donor level below the conduction band minimum (CBM). The coupling between this defect level with the CBM pushes the CBM upwards, thus restoring the GaAs band gap; the lowering of the defect level relative to the isolated Si$_ {Ga}$ shallow donor level is responsible for the increased electrical resistivity. Therefore, Si and N mutually passivate each other's electrical and optical activities in GaAs. For group-VI shallow donors such as S, the Coulomb binding between S$_{As}$ and N$_{As}$ does not form a direct bond and a deep level inside the gap; thus, no mutual passivation exists in the GaAs:(S+N) system. We also explained the difference between the mutal passivation of Si and N and the mutal passivation of H and N in GaAs. Our study provides a deep understanding of the mutual passivation mechanism and explained some of the recent puzzling experimental observations.