Temperature dependence of the linewidths of free-exciton A and B transitions was investigated. Experimental linewidths were fitted to a theoretical model considering various interactions of excitons with phonons in addition to inhomogeneous broadening. It was shown that acoustic phonon scattering must also be considered to explain the emission linewidth broadening, in contrast to a recent report on luminescence linewidths in GaN. These exciton-acoustic-phonon interactions also explain the fast energy relaxation of free excitons to the bottom of the exciton band, which leads to generally observed short free-exciton lifetimes in GaN. The exciton-longitudinal-optical ͑LO͒ -phonon coupling constant was found to be extremely large. This was explained as being due to the Fröhlich interaction and the polar nature of GaN. The binding energy of both A and B excitons was found to be 26 meV. The relevance of exciton-phonon interactions and the binding energy of free excitons in achieving room-temperature exciton-based semiconductor lasers was discussed. Though exciton-LO-phonon interaction was very strong in GaN, it was still possible to observe room-temperature excitons since the exciton binding energy is very large.

GaN epitaxial layers on sapphire substrates were grown by the rotating disk metal organic chemical vapor deposition technique. Excitonic transitions from conduction band to spin-orbit split valence bands were observed. At 12 K we observed donor bound exciton and a very weak acceptor bound exciton. The temperature dependence of luminescence peak positions of free-excitons A and B were fitted to the Varshni’s equation to study the variation of the band gap with temperature. The linewidth of the free exciton (A) was studied as a function of temperature and was explained by theoretical model considering the scattering of excitons with acoustic phonons and longitudinal optical phonons. In the 12 K spectrum we also observed phonon-assisted excitonic transitions. The activation energy of the free exciton (A) was found to be 26 meV, while that of the donor bound exciton was 7 meV. The binding energy of the donor was estimated as 35 meV and that of the acceptor as 250 meV. The band gap of GaN was found to be 3.505 eV at 12 K and 3.437 at room temperature. All the parameters obtained in the present investigation are compared with those reported in the literature.

Magnesium doped GaN epitaxial layers were grown by metal-organic chemical vapor deposition on sapphire substrate. Energy levels of these acceptors were investigated by systematic photoluminescence measurements in the temperature range of 12–300 K. Magnesium concentration was varied from <1×1019 to higher than 5×1019 cm−3. Photoluminescence measurements were made on the as-grown and annealed samples. We have observed various transitions related to donor to acceptor and their phonon replicas, conduction band to acceptors, and free excitons. Their dependence on temperature, concentration of the magnesium impurity and annealing conditions was discussed. In our study, two important observations were made. First, very deep level luminescence was not observed even in the highly magnesium doped as-grown samples. Second, free exciton transitions including valence band splittings were observed for the first time in the Mg-doped materials, demonstrating the high quality of the samples.

A systematic study of time-resolved photoluminescence spectra from In0.15Ga0.85N/In0.015Ga0.985N quantum wells (QWs) with different Si-doping concentration in the barriers has been carried out. The 10 K recombination lifetimes in the QWs depend strongly on the Si-doping in the barriers, decreasing from ∼80 to ∼20 ns as the Si-doping level increases from 2×1018 to 1×1019 cm−3. The separation between the spontaneous and stimulated emission (SE) peaks and a shift of the emission peak to longer wavelengths with delay time after a pulsed excitation decrease with increasing Si doping. The increased recombination rate, the decrease of peak shift with delay time, and the reduced separation between the spontaneous and SE peaks with increasing Si doping are attributed to the screening of piezoelectric field by carriers originated from Si-doped barriers.

Effects of a thin AlAs layer (1 nm) with different position on InAs quantum dots (QDs) and wetting layer have been investigated by transmission electron microscopy (TEM), photoluminescence (PL), and photoreflectance (PR). The PL peak position of InAs QDs directly grown on the thin AlAs is blueshifted from that of InAs QDs grown on the GaAs layer by 171 meV mainly due to the high potential barrier and reduced dot size shown in the TEM image. As the additional GaAs layer (1 and 2 nm) is inserted on top of the AlAs layer, the PL peak position is systematically shifted toward longer wavelength with increase in the thickness. Temperature dependent PL of QD samples shows that a thin AlAs layer significantly influences the thermal activation energy. The wetting layer related peak in PR spectra is changed to lower energy with increase in the thickness of an additional GaAs layer, which is mainly caused by the reduction in the effects of the AlAs layer.