Using a molecular beam epitaxy system equipped with an inductively coupled radio frequency nitrogen plasma source, p-type GaN films were grown on sapphire substrates with no postgrowth treatment. Uniformity of the surface morphology and spatial homogeneity of the luminescence of the films were investigated using scanning electron microscopy and cathodoluminescence (CL) imaging, respectively. By examining the dependence of photoluminescence on the excitation laser power density at 6 and 300 K, three different emissions having different origins were identified. A blue emission at ∼3.25 eV is associated with shallow Mg impurities, while two different lower-energy emissions at ∼2.43 and ∼2.87 eV are associated with deep Mg complexes. The spatial distributions of the shallow and deep Mg impurities dominating the optical properties of the p-type GaN films were also examined along the growth direction by low- and room-temperature CL using an electron beam with a range of penetration depths

Site-selective photoluminescence (PL) spectra obtained at 6 K from the 1540 nm I413/2→I415/2 emissions characteristic of four distinct Er3+ centers in Er-implanted films of GaN are compared with the Er3+ PL excited by 325 nm above-gap pump light. Two of the site-selective 1540 nm Er3+ PL spectra pumped by below-gap, trap-mediated excitation bands dominate the Er3+ PL spectrum excited by above-gap light. A third broad band-excited spectrum and a fourth spectrum pumped by direct Er3+ 4f-band absorption are apparently not strongly excited by above-gap light. These results indicate that trap-mediated excitation dominates above-gap pumping of Er3+ emission in GaN:Er, and suggest an explanation for the reduced thermal quenching of Er3+ emission in GaN.

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the 1540 nm I13/24→I15/24 emission of Er3+ 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 Er3+ 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 Er3+ 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.

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