The stable geometry of two N atoms which were incorporated into the Se substitutional site was optimized by using  ab initio  molecular orbital theory and a Zn4N2H12 cluster model. It was found that the optimized geometry of the two N atoms was symmetrical with respect to the Se substitutional site, and that the N-N atomic distance was 0.1105nm. This was very close to the equilibrium bond length (0.1100nm) of the N2 molecule in the ground state. However, the bond population (0.74) of the two N atoms at the Se site was much smaller than that (2.62) of N2 molecules. The atomic charges of N, Se and of the two N atoms were -0.93, -0.12 and 0.01, respectively, according to a calculation which included electron correlation effects. This was attributed to corresponding electronegativities of N(3.04), Se(2.55) and N2 (approximately zero). According to a molecular orbital analysis, the pairing of N atoms at the Se site split the shallow acceptor level of atomic N into 2 levels, and the lower one was fully occupied by N-N bonding electrons. The pairing of N atoms thereby impeded the provision of a free hole carrier to the valence band of ZnSe.

T.Nakao, M.Suzuki, T.Uenoyama, Y.Funayose: Materials Science Forum, 1995, 196-201, 293-6