A model for acceptor doping in ZnO based on nitrogen pair formation

We reconsider acceptor doping of ZnO with Li and Cu published nearly 40 years ago by comparing it with the behaviour of nitrogen in ZnO. While Cu plays an exceptional role due to the d-shell configuration (acceptor level at 190 meV below conduction band) Li and N as single acceptors give rise to deep distorted acceptors with binding energies between 700 and 800 meV above valence band. With these binding energies no hole conductivity at room temperature can be expected, having in mind that typical background donor densities in ZnO are between 10(16) and 10(17) cm(-3). We propose a defect model to explain the role of lithium and nitrogen doping in ZnO which is based on two acceptor-one donor pairing as proposed by theory in the framework of co-doping. Hydrogen is a key candidate for the donor role. The luminescence properties in ZnO: N can be understood on the basis of the calculation of the hole densities without and with compensation assuming pair formation. We compare our experimental findings with published results on nitrogen doped ZnO explaining the limitations of p-type doping of ZnO with nitrogen. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim