Ab-initio studies on Li doping, Li-pairs, and complexes between Li and intrinsic defects in ZnO

First-principles density functional calculations have been performed on Li-doped ZnO using all-electron projector augmented plane wave method. Li was considered at six different interstitial sites (Li-i), including anti-bonding and bond-center sites and also in substitutional sites such as at Zn-site (Li-zn) and at oxygen site (Li-o) in the ZnO matrix. Stability of Li-Zn over Li-i is shown to depend on synthetic condition, viz., LiZn is found to be more stable than Li-i under O-rich conditions. Hybrid density functional calculations performed on Li-Zn indicate that it is a deep acceptor with (0/-) transition taking place at 0.74 eV above valence band maximum. The local vibrational frequencies for Li-dopants are calculated and compared with reported values. In addition, we considered the formation of Li-pair complexes and their role on electronic properties of ZnO. Present study suggests that at extreme oxygen-rich synthesis condition, a pair of acceptor type Li-Zn-complex is found to be stable over the compensating Li-i + Li-Zn pair. The stability of complexes formed between Li impurities and various intrinsic defects is also investigated and their role on electronic properties of ZnO has been analyzed. We have shown that a complex between Li-Zn and oxygen vacancy has less formation energy and donor-type character and could compensate the holes generated by Li-doping in ZnO. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729774]