Theoretical study on group III elements and F co-doped ZnO

Abstract

To realize the wide application of ZnO, one of the urgent and challenging tasks is the preparation of high-conductivity ZnO. Doping with group III elements (Al, Ga, and In) or F is adopted to achieve n-type ZnO. Their equal proportion co-doping (e.g. Al–F, Ga–F, and In–F) improves the conductivity with respect to mono-doping. Moreover, changing the doping concentration of Al and F further increases the conductivity. The physical mechanism behind the experimental phenomena has not been explored, thus far. Here, first-principles calculations were employed to explore the effect of cation-anion co-doping (Al–F, Ga–F, and In–F) and the variation of doping ratios on the geometric structure and the electron properties of ZnO. For equal proportion co-doping, the strong hybridization between the dopants in In–F induces a localized state in band gap, reducing the electrical conductivity, which is in sharp contrast with Al/Ga–F ones. Those mainly originate from the different arrangements of cation-anion dopants in the stable configurations (e.g. In and F form bond, whereas Al/Ga and F do not). Compared with Al–2F co-doping, 2Al–F can achieve the higher conductivity from the standpoint of lower defect formation energy and transition level. However, the 2In–F/2Ga–F becomes much worse with respect to 2Al–F. Our work not only rationalizes experimental observation but also provides useful information for improving the conductivity of similar binary compounds.