Identification of a N-related shallow acceptor and related EPR center in ZnO: N2 on Zn site

While the deep level of N$_{\rm O}$ makes it unsuitable for p-type doping of ZnO, a shallow level at about 165$\pm40$ meV above the VBM related to N is known to exist in ZnO (Zeuner et al. 2002). Here we show that a N$_2$ molecule on the Zn site behaves as a shallow acceptor. First-principles calculations show that when N$_2$ is placed on a Zn site, two electrons are removed from the $\sigma_g^+$ HOMO. The molecular levels line up with the ZnO band strucure in such a way that the $\sigma_g^+$ level forms a resonance near the VBM. In contrast, for N$_2$ on the O-site, two extra electrons occupy the $\pi_g$ LUMO of the N$_2$ molecule and form a donor level. The 0/- transition level of the acceptor is found at $\sim$0.2 eV above the VBM. When singly occupied the defect corresponds to a N$_2^+$ radical. We show that the $g$-factor, calculated within a simple tight-binding model, of this radical agrees better with an observed EPR center by Garces et al. (2003) than the N$_2^-$ radical. The $\sigma_g$ nature of the defect wavefunction for N$_2$ on Zn is consistent with a significant isotropic hyperfine interaction, while the $\pi_g$ character of N$_2$ on O is not. The lower value of $A_{iso}$ compared to the isolated molecule is consistent with the shallow nature of the defect.