Application of Monte Carlo simulation of exciton (carrier) hopping for the analysis of the photoluminescence (PL) temperature behavior in In 0.2 Ga 0.8 N/GaN multiple quantum wells is reported. The PL linewidth and peak position measured in the 10-300 K range exhibited a W-shaped and S-shaped temperature behavior, respectively. The W-shaped linewidth dependence was fitted with the results of Monte Carlo simulation, which involved phonon-assisted exciton hopping through energy states confined in the band potential fluctuation minima. The simulation yielded the values of the standard deviation for potential fluctuations within In-rich regions (31 meV), dispersion of the average exciton energy in different regions (29 meV), and the temperature dependence of the band gap, which was found to be in a fair agreement with the photoreflectance data. Our results, which infer in-plane motion of localized excitons within the wells, are consistent with the model of large In-rich regions ("segmented quantum wells" or "quantum discs") with band potential fluctuations inside these regions. 1 Introduction InGaN/GaN multiple quantum wells (MQWs) are known to efficiently emit green to near UV light in spite of high density of dislocations due to lattice-mismatched substrates [1]. The unique optical properties of InGaN are believed to relate to compositional disorder. In particular, carrier and exciton localization in the band potential fluctuation minima should prevent them from reaching nonradiative recombination sites. However despite a commercial success of nitride-based emitters, the physical origin of efficient light generation in this partially disordered system is unveiled incompletely. In particular, carrier/exciton motion and the establishing of their distribution over the localized states in InGaN are not completely understood. Meanwhile, an anomalous temperature behavior of photoluminescence (PL) in group-III nitride structures designated as an S-shaped peak position dependence and Wshaped linewidth dependence [2][3][4] indicate that exciton/carrier motion occurs via phonon-assisted tunneling (hopping) through localized states [5,6], what results in incomplete thermalization of localized excitons at low temperature. Recently, an evolvement of the S-shaped and W-shaped temperature dependences of the PL peak position and linewidth, respectively, was observed with gradual incorporation of indium in to AlGaN alloy and accounted for by using Monte Carlo simulation of exciton hopping [7].Here we show that phonon-assisted hopping can account for nonmonotonous temperature dependences of the PL peak position and linewidth in InGaN MQWs. The PL linewidth temperature behavior is shown to be in quantitative consistence with the energy of the fundamental optical transition deduced from the photoreflectance (PR) measurements [8].

PACS 61.82.Fk, 72.40.+w, 78.55.Cr Effect of radiation defects on photoconductivity transients and photoluminescence (PL) spectra has been examined in semi-insulating GaN epitaxial layers. Manifestation of defects induced by x-ray irradiation with the dose of 600 Mrad and neutrons with the fluence of 5 × 10 14 cm -2 has been revealed through steady-state and pulsed PL as well as through contact photoconductivity (CPC) and microwave absorption (MWA) transients. Synchronous decrease in the PL intensity of yellow (Y), blue (B) and ultraviolet (UV) bands peaked at 2.19 eV, 2.85 eV and 3.42 eV, respectively, with increasing the density of radiationinduced defects was observed. The observed effect of radiation on the PL characteristics has been explained by interaction of the radiation defects with the native traps, responsible for Y-band PL or by structural modification of the "yellow" centers. The decrease in the PL intensity with concentration of radiation-induced defects is accompanied by an increase of the asymptotic decay lifetimes in time-stretched multi-trapping processes. The CPC and MWA decays, reflecting these processes, fit well a stretchedexponent approximation exp[-(t/τ) α ] with the time-stretching factor α that transforms from α = 0.7 for asgrown material to α = 0.3 for irradiated one.

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