We present a two-step growth process for ZnO on GaN templates to achieve high-quality ZnO layers with XRD rocking curve FWHMs of the (0 0 0 2) reflection of similar to180" and narrow cathodoluminescence of 1.3 meV of the dominant 18 emission. These layers are applied for an investigation of potential nitrogen doping sources to achieve p-type doping of ZnO, namely NH3, UDMHy, and NO. Of these sources NO seems to induce only little changes in the optical and electrical properties of the material. A brownish colour of the samples is observed when using NH3 or UDMHy at higher dopant flows. With it we find an increase in the electron carrier concentration from 10(16) cm(-3) to above 10(18) cm(-3). For some samples grown with lower dopant flows, we observe a decrease in the carrier concentration upon annealing to values significantly lower than the background doping concentration when using UDMHy. The sample which shows the strongest decrease in the carrier concentration also shows a pronounced DAP emission which vanishes upon annealing, most likely due to activation of an acceptor state. (C) 2004 Elsevier B.V. All rights reserved.