We report temperature-dependent time-integrated and time-resolved photoluminescence (PL) studies of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition. We observed anomalous emission behavior, specifically an S-shaped (decrease–increase–decrease) temperature dependence of the peak energy (Ep) for InGaN-related PL with increasing temperature: Ep redshifts in the temperature range of 10–70 K, blueshifts for 70–150 K, and redshifts again for 150–300 K with increasing temperature. In addition, when Ep redshifts, the spectral width is observed to narrow, while when Ep blueshifts, it broadens. From a study of the integrated PL intensity as a function of temperature, it is found that thermionic emission of photocarriers out of local potential minima into higher energy states within the wells is the dominant mechanism leading to the thermal quenching of the InGaN-related PL. We demonstrate that the temperature-induced S-shaped PL shift is caused by a change in the carrier dynamics with increasing temperature due to inhomogeneity and carrier localization in the InGaN/GaN MQWs.

Stable aqueous colloids of 2-3 nm In 2 S 3 nanocrystals have been prepared by using the classical method of nanoparticle stabilization by low molecular weight thiols. TEM crystal lattice spacing, X-ray diffraction, EDAX data, and electron diffraction indicate that the nanoparticles are predominantly in -In 2 S 3 form. They exhibit relatively strong excitonic emission at 360-380 nm with a quantum yield of 1.5%. The excitonic radiative lifetime is 350 ns, which indicates that a direct allowed electronic transition is responsible for this emission. The NMR lines of the stabilizer are strongly broadened and shifted as a result of deshielding induced by electron withdrawing by positively charged metal ions. This effect quickly wears off as the carbon chain becomes longer and the separation between the hydrogen atoms of the stabilizer and the semiconductor surface increases. The broadening is attributed to the reduced mobility of the stabilizer in the nanoparticle shell. For CdS nanoparticles of the same size, this effect was found to be substantially stronger than for In 2 S 3 . The lower density of metal centers in In 2 S 3 than in CdS, which serve as anchor points for the stabilizer, promotes greater mobility of the stabilizer moieties.

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.

Stimulated emission (SE) in optically pumped InGaN/GaN multiquantum well (MQW) structures grown by metalorganic chemical vapor deposition was experimentally studied in the temperature range of 175–575 K. The GaN barriers were intentionally doped with a different Si concentration ranging from 1×1017 to 3×1019 cm−3 and the effects of Si doping of GaN barriers on the optical properties of InGaN/GaN MQWs were investigated. The SE threshold was measured as a function of temperature and compared with bulk GaN. We observed that the SE threshold had a low value and a weak temperature dependence: for example, ∼25 kW/cm2 at 175 K, ∼55 kW/cm2 at 300 K, and ∼300 kW/cm2 at 575 K for one of the samples. Low SE thresholds are attributed to the high-quantum efficiency of the MQWs, possibly associated with the large localization of excitons. The characteristic temperature of 162 K was derived from the temperature dependence of the SE threshold. The integrated emission intensity versus pumping density was examined for different temperatures. This study shows that InGaN/GaN MQWs are suitable for development of laser diodes that can operate well above room temperature.

No abstract