Eu was implanted into high quality cubic ͑zincblende͒ GaN ͑ZB-GaN͒ layers grown by molecular beam epitaxy. Detailed structural characterization before and after implantation was performed by x-ray diffraction ͑XRD͒ and Rutherford backscattering/channeling spectrometry. A low concentration ͑Ͻ10%͒ of wurtzite phase inclusions was observed by XRD analysis in as-grown samples with their ͑0001͒ planes aligned with the ͕111͖ planes of the cubic lattice. Implantation of Eu causes an expansion of the

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... ttice parameter in the implanted region similar to that observed for the c-lattice parameter of wurtzite GaN ͑W-GaN͒. For ZB-GaN:Eu, a large fraction of Eu ions is found on a high symmetry interstitial site aligned with the ͗110͘ direction, while a Ga substitutional site is observed for W-GaN:Eu. The implantation damage in ZB-GaN:Eu could partly be removed by thermal annealing, but an increase in the wurtzite phase fraction was observed at the same time. Cathodoluminescence, photoluminescence ͑PL͒, and PL excitation spectroscopy revealed several emission lines which can be attributed to distinct Eu-related optical centers in ZB-GaN and W-GaN inclusions.