When samples were co-doped with Er and O, via metalorganic chemical vapor deposition, it was found that the host-excited photoluminescence was dominated by a 4I13/2  4I15/2 intra 4f-shell luminescence (1.54) from a center that was identified as being an Er atom which replaced a Ga atom, together with two O atoms: Er-2O. A preferential alignment of the Er-2O center was revealed by the optical anisotropy which was exhibited by a host-excited photoluminescence that was polarized parallel to the <110> crystallographic directions. The preferential alignment of a specific symmetry axis of the Er-2O center was found to lie along the growth direction, [001]. This observation differed from previous work on rare-earth doped materials in which the rare-earth luminescence center itself exhibited anisotropic luminescent properties, but a random distribution of differently oriented centers resulted in an isotropic host-excited photoluminescence.

R.A.Hogg, K.Takahei, A.Taguchi, Y.Horikoshi: Applied Physics Letters, 1996, 68[23], 3317-9