We report on the optical properties of nitrogen acceptor-doped ZnO epilayers in the medium and high doping regimes using temperature and excitation power-dependent, as well as time-resolved photoluminescence experiments. The epilayers were doped with ammonia during homoepitaxial growth on ZnO single-crystal substrates with different surface polarities. Significant differences in the optical characteristics of the epilayers are observed between growth on nonpolar a-plane, polar c-plane Zn-face substrates and polar c-plane O-face substrates, which demonstrates different incorporation of the nitrogen acceptor depending on the substrate polarity. The incorporation of nitrogen into the ZnO films ranges between 10 19 and 10 21 cm −3 as determined by secondary ion mass spectrometry. Within this doping range the samples change from lightly compensated to highly doped compensated. We discuss the unique photoluminescence features of nitrogen-doped ZnO epilayers within the concept of shallow donor-acceptor-pair recombinations and at the highest doping level by the appearance of potential fluctuations.

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 À3 . 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.

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The back cover image shows the essence of this issue's Editor's Choice article by Stefan Lautenschlaeger et al. (). A complex model for shallow acceptors in ZnO, involving two group V acceptors (i.e., two nitrogen atoms) and one donor (in this case hydrogen), is presented and discussed. As you can see, three different nitrogen confi gurations are presented. One is the isolated NO which might, according to recent results and theoretical calculations, lead to deep acceptor states. The second configuration is the neutral N‐H complex. The third possible configuration consists of an acceptor‐donor‐acceptor complex and may lead to the observed shallow acceptor. The authors discuss the photoluminescence data obtained by ammonia‐doped samples with different amounts of nitrogen and hydrogen with respect to their acceptor model.

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