Observation of multiple Er3+ sites in Er-implanted GaN by site-selective photoluminescence excitation spectroscopy

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the $\text{1540\hspace{0.17em}}\mathrm{nm}$ ${}^4{I}_{\text{13/2}}{\to }^4{I}_{\text{15/2}}$ emission of ${\mathrm{Er}}^{\mathrm{3+}}$ in Er-implanted films of GaN grown by metalorganic chemical vapor deposition. The PLE spectra exhibit several broad below-gap absorption bands, which excite three distinct site-selective ${\mathrm{Er}}^{\mathrm{3+}}$ PL spectra. The excitation of two of the site-selective Er PL bands involves optical absorption by defects or background impurities, rather than direct intra-$f$ shell absorption, with subsequent nonradiative transfer of the energy to nearby ${\mathrm{Er}}^{\mathrm{3+}}$ luminescence centers. The characteristics of the PLE spectrum of the third site-selective PL band suggest that an exciton bound at an Er-related trap is involved in the excitation mechanism.