The information on the variations of indium composition, aggregation size, and quantum-well width is crucially important for understanding the optical properties and, hence, fabricating efficient light-emitting devices. Our results showed that spinodal decomposition could occur in InGaN/GaN multiple quantum wells with indium content in the range of 15%-25% ͑grown with metal-organic chemical-vapor deposition͒. A lower nominal indium content led to a better confinement of indium-rich clusters within InGaN quantum wells. The InGaN/GaN interfaces became more diffusive, and indium-rich aggregates extended into GaN barriers with increasing indium content. It was also observed that indium-rich precipitates with diameter ranging from 5 to 12 nm preferred aggregating near V-shaped defects.

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Two-component decay of time-resolved photoluminescence (TRPL) intensity in three InGaN/GaN multiple quantum well samples were observed. The first-decay component was attributed to exciton relaxation of free-carrier and localized states; the second-decay one was dominated by the relaxation of localized excitons. The second-decay lifetime was related to the extent of carrier localization or indium aggregation and phase separation. The lifetime of free-carrier states was connected with the defect density. Based on the temperature-dependent data of PL and stimulated emission (SE), the localization energies of the three samples were calibrated to show the consistent trend with the second-decay lifetime and previous material analyses.

We report the study results of an InGaN/GaN multiple quantum well structure with a nominal indium content of 25%. The high-resolution transmission electron microscopy and x-ray diffraction show clear indium aggregation and phase separation. Stimulated emission data always show two major peaks in spectrum. The long-͑short-͒ wavelength peak is assigned to the recombination of localized state carriers ͑free carriers͒. At low temperatures or optical pump levels, the localized-state recombination dominates the stimulated emission; however, at high temperatures or pump levels, the free-carrier recombination becomes dominant. The peak position corresponding to localized states changes little in spectrum as temperature or pump level varies. This result is attributed to carrier overflow, strain relaxation, and carrier shielding in increasing temperature or carrier supply.

Efficient activation of Mg acceptors for obtaining a high hole concentration is a challenging topic. In this paper, we report the results of Mg acceptor activation in GaN with irradiation of the second-harmonic photons (532 nm in wavelength) of a Q-switched Nd:YAG laser. This laser was used to irradiate two Mg-doped GaN samples of different doping concentrations. With doping concentration of 1.2 Â 10 18 cm --3 , a hole concentration of 2.66 Â 10 17 cm --3 was obtained after laserinduced activation. The average temperature of samples during laser irradiation was around 30 o C. Hence, it was speculated that the irradiation process was very unlikely to be thermal annealing. PL measurements revealed that laser-induced activation could ionize not only the shallow but also the deep donors. IntroductionThe second harmonic photons (532 nm) of a Q-switched Nd : YAG laser were used to irradiate two Mg-doped GaN samples. The MOCVD grown samples consisted of Mg-doped GaN of 1.6 mm for sample A and 0.615 mm for sample B after the growth of 0.2 mm (sample A) and 1.6 mm (sample B) nucleation layers on sapphire substrates. The growth temperature was 1020 o C. The concentrations of Mg doping were 6.3 Â 10 17 cm --3 (sample A) and 1.2 Â 10 18 cm --3 (sample B). To obtain uniform laser fluence, the laser output of 1 mm in beam radius was first expanded into a radius size of 2 cm through a defocusing lens. The central portion of the enlarged laser beam, which was believed to be quite uniform in fluence, was used for laser irradiation onto samples of 5 Â 5 mm 2 in dimension. After laser irradiation, metal contacts were prepared, followed by thermal annealing at 750 o C for 20 s in ambient nitrogen. It was confirmed that this short-period thermal annealing process could not activate the GaN samples.