We present a comprehensive study of the optical characteristics of Al x Ga 1Ϫx N epilayers (0рxр0.6) by means of photoluminescence ͑PL͒, PL excitation, and time-resolved PL spectroscopy. For Al x Ga 1Ϫx N with large Al content, we observed an anomalous PL temperature dependence: ͑i͒ an ''S-shaped'' PL peak energy shift ͑decrease-increase-decrease͒ and ͑ii͒ an ''inverted S-shaped'' spectral width broadening ͑increasedecrease-increase͒ with increasing temperature. We observed that the thermal decrease in integrated PL intensity was suppressed and the effective lifetime was enhanced in the temperature region showing the anomalous temperature-induced emission behavior, reflecting superior luminescence efficiency by suppressing nonradiative processes. All these features were enhanced as the Al mole fraction was increased. From these results, the anomalous temperature-induced emission shift is attributed to energy tail states due to alloy potential inhomogeneities in the Al x Ga 1Ϫx N epilayers with large Al content.

We report the results of an experimental study on efficient laser action in an optically pumped GaN/AlGaN separate-confinement heterostructure (SCH) in the temperature range of 10–300 K. The lasing threshold was measured to be as low as 15 kW/cm2 at 10 K and 105 kW/cm2 at room temperature. Strongly polarized (TE:TM⩾300:1) lasing peaks were observed over the wavelength range of 358–367 nm. We found high-finesse lasing modes that originate from self-formed microcavities in the AlGaN and GaN layers. Through analysis of the relative shift between spontaneous emission and lasing peaks, combined with the temperature dependence of the lasing threshold, we conclude that exciton–exciton scattering is the dominant gain mechanism leading to low-threshold ultraviolet lasing in the GaN/AlGaN SCH over the entire temperature range studied. Based on our results, we discuss possibilities for the development of ultraviolet laser diodes with a GaN active medium.

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The dynamic Stark effect of excitons in GaN was studied using femtosecond pump-probe spectroscopy with various polarization configurations and pump detunings at 10 K. In contrast to two-dimensional GaAs/AlGaAs quantum wells which have Bloch eigenstates similar to those of GaN and a large spin-orbit coupling, we observed that the Stark effect in GaN is strongly dependent on pump and probe relative linear polarizations. We found that this dependence results from the small spin-orbit splitting in GaN and a mixing of A and B valence bands induced by a linearly polarized pump. Using two different circular polarization configurations, we also observed splitting of degenerate excitons because of different optical Stark shifts. Our experimental results are explained by a simple theoretical model.

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