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GaN epilayers grown by metal organic chemical vapor deposition (MOCVD) were implanted with Tm and Eu ions with different energies and fluences and at different temperatures in order to optimize the implantation conditions. The recovery of the implantation damage was studied using both rapid thermal annealing and furnace annealing with nitrogen overpressure of 4 x 10(5) Pa. Rutherford backscattering spectrometry in the channeling mode (RBS/C) was used to monitor the evolution of damage introduction and recovery in the Ga-sublattice and transmission electron microscopy (TEM) was carried out for further structural analysis. The RBS/C spectra as well as TEM images show two different damage regions, one at the surface arising from an amorphous surface layer and another one deeper in the crystal coinciding with the end of range of the implanted ions. For implantation with 150 keV at room temperature, even for fluences as low as 3 x 10(14) at/cm(2), a thin amorphous surface layer, which becomes thicker with increasing implantation fluence, was observed by TEM. High temperature annealing of these highly damaged layers often results in loss of the amorphous layer and accumulation of the implanted species at the surface rather than a regrowth of the crystal. It was possible to prevent the formation of an amorphous layer by implanting at 500 degreesC. In those samples a large part of the lattice damage was removed during annealing at 1000 degreesC and the recovery of the lattice is similar for both applied annealing methods. (C) 2004 Elsevier Ltd. All rights reserved.