Photoluminescence analysis of Er-doped GaAs under host photoexcitation and direct intra-4 f -shell photoexcitation

Er-doped GaAs shows a sharp and simple intra-4f-shell luminescence spectrum of Er3+ ions under host photoexcitation, when the crystal is grown with deliberate oxygen codoping by metalorganic chemical-vapor deposition. Photoluminescence-excitation measurements by direct intra-4f-shell excitation, however, revealed that there are many kinds of Er3+ centers in the same crystal. To clarify the differences between these Er centers, photoluminescence was measured under both host photoexcitation and direct intra-4f-shell photoexcitation. It was found that there are three types of Er centers with distinctly different characteristics. The first type of Er center shows a sharp and simple photoluminescence spectrum with a high intensity under host photoexcitation. This center had been assigned as an Er atom coupled with two oxygen atoms (ErGa–2O center). The second type of Er center show sharp and simple spectra but only under direct intra-4f-shell photoexcitation and not under host photoexcitation. In a sample with Er concentration of 1.4×1018 cm−3, several such centers with distinctly different atomic configurations were found at substantial concentrations, probably exceeding that of the ErGa–2O center. As there is no intra-4f-shell luminescence for this type of center under host photoexcitation, no energy transfer path exists between the host and the 4f shells of this type of Er center. The third type of Er center shows complicated spectra even under intra-4f-shell photoexcitation with a specific photon energy. Analysis of photoluminescence spectra from this type of Er center revealed evidence of energy migration among Er3+ ions, suggesting that Er-rich aggregates are the origin of this type of PL spectra. Under host photoexcitation, this type of Er center shows luminescence but with a much lower intensity than the ErGa–2O center. The luminescence from Er centers similar to the third type probably is dominant in the PL spectrum of Er-doped GaAs grown without deliberate oxygen codoping, which does not contain an appreciable concentration of the ErGa–2O centers.