First-principles study of defect properties in ZnO and $SiO_2$

Abstract

Based on first-principles density functional calculations, we present a mechanism for the compensation of N acceptors in ZnO through real-space multigrid electronic structure calculations. We find that at low N doping levels using a normal $N_2$ source, O vacancies are the main compensating donors for N acceptors, while N acceptors are compensated via the formation of defect complexes with Zn antisites at high doping levels. When an active plasma $N_2$ gas is used to increase the N solubility, N acceptors are still greatly compensated by $N_2$ molecules at oxygen sites and N-acceptor-$N_2$ complexes, explaining the difficulty in achieving low-resistivity ρ-type ZnO. In ZnO codoped with N acceptors and Ga donors, the acceptor level of a N-Ga-N complex is similar to that for an isolated N acceptor, and hole concentrations are not enhanced due to the compensation by Ga donors. We suggest a method for fabricating ρ-type ZnO by using group I impurities such as Li and Na. The substitutional Li and Na are found to be shallower acceptors than the N acceptor, but they are severely compensated by Li and Na interstitials, respectively. However, a codoping of H with Li or Na greatly suppresses the formation of the compensating interstitials and increase the acceptor solubility by forming acceptor-H-interstitial complexes. We find that the H incorporated in Li- and Na-doped ZnO can be easily removed, thus, the low-resistive ρ-type ZnO is expected to be fabricated by controlling the doping level of H. We find that Co-doped ZnO energetically favors a spin-glass-like state due to antiferromagnetic interactions between transition metal atoms, while ferromagnetic ordering is stabilized via double exchange interactions by electron doping. We find a short range nature in both antiferromagnetic and ferromagnetic interactions, unlike the Ruderman-Kittel-Kasuya-Yosida interaction, and suggest that a very high doping level of Co ions is required to achieve ferromagnetism, together wit...