Triple group-V donors in ZnO

Abstract

Triple donors have been explored in a few semiconductor materials; however, the conventional effective mass theory treatment fails at short length scales due to the high degree of localization implied by a 3+ nuclear charge. Using density functional theory, we consider the various charge states of group-V elements substituting for the Zn sublattice in ZnO under oxygen-rich conditions. For the case of Sb and Bi substitutional impurities, the (1+/0) charge state transition is shallow and has strong similarities to a (1+/0) charge transition of the more common shallow group III donors such as Ga and Al. We compare these calculations with extensive photoluminescence (PL) measurements that now exist for the Sb-related donor bound exciton in ZnO, which is known to contain substitutional Sb on Zn sites. We present new experimental data on the magneto-PL properties of the Sb-related donor bound exciton. These data confirm the strong similarity of the (+1/0) charge state transition of this center to the common group III shallow donors in ZnO. We propose that the very low binding energy (40.2 meV) of the neutral Sb donor is due to a combination of increased screening due to the two inner donor electrons, as well as the exclusion principle, resulting in a repulsive central cell potential close to the defect core.