Er-doped semiconductors are promising materials for optoelectronic devices operating on the intra 4f-shell transitions of the rare earth impurity. GaN:Er is currently studied very intensively, because of its importance for modern optoelectronics. In spite of many efforts little is known, however, about Er energy structure, as generally Er creates complexes with another dopants, mainly with oxygen, which makes analysis difficult. To omit this Er should be introduced into substrates with low oxygen content and by ion implantation.
In this work we use site selective excitation of Er-centres to study their symmetry in cubic and hexagonal GaN. Photoluminescence excitation (PLE) spectroscopy at an energy range corresponding to the 4I15/2 - 4I9/2 absorption is applied to study splitting of energy levels of Er3+. The Stark split energy levels as determined from the PLE spectra serve then as a basis for model calculations. By means of point charge approximations we calculated the splitting of 4I9/2 free Er ion level in weak crystal field of the host material and we got good agreement with experimental results. Because the nature of this splitting directly depends on the symmetry of the crystal field, we can investigate the symmetry of Er centers and their position in host in such way. From our results one can make a conclusion that in cubic GaN sample there is a small distorsion in  direction of crystal growth and Er is in interstitial position. The symmetry of Er centers is C3v in hexagonal GaN samples.