Dynamics of anomalous optical transitions inAlxGa1−xNalloys

Abstract

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.

Excitonic localization in AlN-rich AlxGa1−xN/AlyGa1−yN multi-quantum-well grain boundaries

“…This trend is expected because the increase in E a should reflect an increase in trapping sites near the grain boundaries as AlN composition increases. 17,25 This is in line with the AFM results that show an increase in the density of grain boundaries with increasing AlN concentration. Although the thermal response of carrier lifetime generally gives a more accurate estimation of E a when the radiative lifetimes of the carriers are temperature- dependent, 26 the increase in E a derived from integrated PL intensity with AlN composition has been also reported in extant literature and was linked to an increase in trapped states.…”

“…16 On the other hand, the departure from Varshni's bandgap shrinkage model at temperatures higher than 150 K is most likely a result of thermally induced carrier population of the bandtail states. 17 In addition, sample D with lowest AlN composition shows an initial redshift followed by a final blueshift. This behavior has been reported in PbS quantum dot (QD) systems and in self-organised InGaN QDs, where it was attributed to thermally induced carrier trapping in shallow states.…”

“…Although the thermal response of carrier lifetime generally gives a more accurate estimation of E a when the radiative lifetimes of the carriers are temperature- dependent, 26 the increase in E a derived from integrated PL intensity with AlN composition has been also reported in extant literature and was linked to an increase in trapped states. 17,25 In order to confirm the effects of the grain boundary defects, we carried out RT CL hyperspectral imaging. Acquiring a CL spectrum at each point in a scan and peak fitting to the resultant data allows mapping of spectral parameters, such as peak wavelength, width, and emission intensity.…”

Dependence of carrier localization in InGaN∕GaN multiple-quantum wells on well thickness

“…14,15 However, these results point out that an increase of potential fluctuations can occur with well thickness as well. Previously, several reports suggested that the localized states may arise from In compositional fluctuation or QD formation, phase separation, monolayer thickness fluctuation, and surface segregation.…”

Time-resolved photoluminescence properties of AlGaN/AlN/GaN high electron mobility transistor structures grown on 4H-SiC substrate

“…It is known in AlGaN alloys grown by MOCVD that there are random fluctuations in the potential due to alloy fluctuations [16][17][18] and this gives rise to carrier localization. Thus, taking into account results of the TRPL measurements and the calculated band structure, we suggest that the AlGaN related emission likely occurred between electrons strongly confined in the triangular AlGaN QW and holes weakly localized on potential fluctuations.…”