We have investigated by extended x-ray absorption fine structure spectroscopy the local order around Er atoms introduced by ion implantation in substoichiometric silica films prepared by plasma enhanced chemical vapor deposition, where Si nanoclusters have been formed by different preimplantation annealing processes. The results show that Er atoms are surrounded by a first shell of O atoms and no Er-Si direct correlations are observed; moreover, while the variation of the preimplantation annealing temperature has no effect on the Er site, it is observed that the increase of the Er concentration determines an increase of both the Er first shell coordination number and the Er-O interatomic distance, becoming more similar to those of Er 2 O 3. In the presence of an extensive phase separation between Si and Si O 2 the local environment around Er plays a crucial role on the efficiency of the photoluminescence …