Yellow photoluminescence in MOCVD-grown n-type GaN

The near-band gap and the "yellow" optical transitions in n-type GaN grown by MOCVD have been studied by photoluminescence experiments. The excitation density ranged from 5 × 10^-6 W/cm^-2 to 50 W/cm^-2. The UV PL intensity increases linearly in the entire range of excitation density. The yellow PL intensity exhibits a linear dependence at low excitation densities and a square-root dependence at high excitation densities. A theoretical model is developed describing the intensity of the two radiative transitions between continuum states and one defect level deep in the band gap as a function of the excitation density, free carrier and defect concentrations. The calculated dependences of the two luminescence channels follow power laws with exponents of 1/2 and 1 depending on excitation density. These dependences are in very good agreement with experimental results. The measured intensity of the yellow luminescence does not saturate at high excitation densities. This rules out the possibility that the yellow PL could arise from a sequential transition via two deep levels in the gap. It is shown that the intensity modulation that frequently appears in the PL spectra is caused by a micro-cavity which is formed by the semiconductor-substrate and semiconductor-air interfaces. Finally, the dependence of the yellow luminescence intensity on n-type doping concentration indicates that the deep center causing the yellow luminescence is an acceptor-like level.