It was noted that Er-doped material exhibited a simple sharp intra 4f-shell luminescence spectrum of Er3+ ions under host photo-excitation when the samples were grown, with deliberate O co-doping, by metalorganic chemical vapor deposition. Photoluminescence excitation measurements, using direct intra-4f shell excitation revealed that there were many types of Er3+ center in the same crystal. In order to clarify the differences between these Er centers, the photoluminescence was measured under both host photo-excitation and direct intra 4f-shell photo-excitation. It was found that there were 3 types of Er center with distinct characteristics. The first type of Er center exhibited a sharp simple photoluminescence spectrum, with a high intensity under host photo-excitation. This center had been attributed to an Er atom that was coupled to two O atoms (ErGa-2O center). The second type of Er center exhibited sharp simple spectra only under direct intra-4f shell photo-excitation, and not under host photo-excitation. In a sample with an Er concentration of 1.4 x 1018/cm3, several such centers with distinct atomic configurations were found in appreciable concentrations; which probably exceeded that of the ErGa-2O center. As there was no intra-4f shell luminescence for this type of center under host photo-excitation, no energy transfer path existed between the host and the 4f shells of this type of Er center. The third type of Er center exhibited complicated spectra, even under intra-4f shell photo-excitation with a specific photon energy. An analysis of the photoluminescence spectrum from this type of Er center revealed energy migration among Er3+ ions. This suggested that Er-rich aggregates were at the origin of this type of photoluminescence spectra. Under host photo-excitation, this type of Er center exhibited luminescence with a much lower intensity than that of the ErGa-2O center. It was concluded that the luminescence from Er centers that were similar to the third type probably predominated in the photoluminescence spectra of Er-doped material which had been grown without deliberate O co-doping. This did not contain an appreciable concentration of ErGa-2O centers.

K.Takahei, A.Taguchi: Journal of Applied Physics, 1995, 78[9], 5614-8