Undoped (as-grown) ZnO films grown by pulsed laser deposition on Al2O3 (0001) substrates were doped with nitrogen by means of an ion implantation process. Post-implantation annealing behavior in the temperature range between 500 and 700 degrees C has been studied by photoluminescence and Hall effect measurements. The implanted films show no peak other than the excitonic recombination emission in the asimplanted state, however, after rapid thermal annealing at 700 degrees C they reveal a nitrogen acceptor related emission at 3.273 eV The as-implanted ZnO films show more electron concentrations than the as-grown, unimplanted ZnO film. In contrast, after annealing, the electron concentration in the implanted films is significantly reduced, indicating that the incorporated nitrogen becomes activated after the thermal annealing, then produces holes and eventually compensates for certain amount of electrons. The results imply that a proper nitrogen implantation and subsequent annealing may be a way to produce p-type ZnO films. (C) 2007 Elsevier B.V. All rights reserved.