PACS : 61.66.Fn; 71.20.Nr; 78.20.Bh The photoluminescence (PL) and PL excitation (PLE) spectra of MOCVD grown double heterostructures GaN/InGaN/GaN with InN concentration in the solution range 0.4-2% were studied within the temperature interval 2-300 K. A fine structure of the PL band found at T ¼ 2 K is attributed to exciton localization at small In clusters of size s ¼ 2 and 3 in the cation sublattice. The temperature variation of the PL spectra shows that the PL band maximum position E max PL ðTÞ follows either ''normal" or ''anomalous" (''S-shaped") behavior in dependence on the concentration of In. We argue that the anomalous behavior is due to long-living excited (metastable) states of fractal-like complexes of s ¼ 2 clusters which occur in the region of the mobility edge due to a random spatial distribution of clusters and participate in the formation of the high energy part of the PL band. Their small relaxation rate at lowest temperatures and their high sensitivity to the temperature variation are considered as the reason of the ''anomalous" behavior of PL bands.Experimental Details The MOCVD grown GaN/InGaN/GaN double heterostructures (DHS) under investigation have well widths and the In contents ranging between 50-100 nm and 0.004 and 0.02, respectively (for more growth details see [1]). The PL spectra were obtained within the temperature interval 2-300 K using the 325 nm He-Cd laser excitation. PLE spectra were excited by a second monochromator with a Xe lamp.Exciton Localization by Clusters Two PL spectra presented in Figs. 1a, b for samples with In content 0.4 and 1.5% demonstrate a fine structure consisting of two components. Such a structure can appear as a result of exciton localization mainly by two centers characterized by considerably different attractive potentials. It can be attributed to exciton localization by clusters consisting of 2 and 3 In atoms occupying the nearest positions in the cation sublattice.Using the approach of Ref.[2] we have performed model calculations of the exciton density of states (DOS), spectral density of states (SDOS) and SDOS of radiative states, which has allowed us to describe the shape of PL spectrum for different concentrations of solid solutions. It was assumed in the calculations, that the distribution of In atoms over the cation sublattice is random, which allows to calculate the number of different clusters and to consider their spatial distribution as random.
We have studied photoluminescence (PL), PL excitation (PLE), and micro-PL spectra of single quantum wells (QWs) formed by CdSe insertions in ZnSe matrix with different nominal Cd thickness (1-3 monolayers (ML)). The PL spectra are considerably red-shifted with respect to the position expected for homogeneous Cd distribution over the QW and can be attributed to the luminescence of CdSe-rich islands. It has been found that PLE spectra of different points of the PL band show a characteristic divergence at excitation below some characteristic energy E ME . This energy is identified with the percolation threshold above which the exciton is able to move over the whole lateral plane of QW whereas below the E ME only a resonant excitation of island related states is possible.
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