  We report direct evidence of two mechanisms responsible for the excitation of
optically active Er3+ ions in GaN epilayers grown by metal-organic chemical
vapor deposition. These mechanisms, resonant excitation via the higher-lying
inner 4f shell transitions and band-to-band excitation of the semiconductor
host, lead to narrow emission lines from isolated and the defect-related Er
centers. However, these centers have different photoluminescence spectra, decay
dynamics, and excitation cross sections. The isolated Er optical center, which
can be excited by either mechanism, has the same decay dynamics, but possesses
a much higher cross-section under band-to-band excitation. In contrast, the
defect-related Er center can only be excited through band-to-band excitation
but has the largest cross-section. These results explain the difficulty in
achieving gain in Er doped GaN and indicate new approaches for realization of
optical amplification, and possibly lasing, at room temperature.
