Microscopic Description of Radiative Recombinations in InGaN/GaN Quantum Systems
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Microscopic Description of Radiative Recombinations in InGaN/GaN Quantum Systems Aurelien Morel, Pierre Lefebvre, Thierry Taliercio, Bernard Gil, Nicolas Grandjean1, Benjamin Damilano1, and Jean Massies1. Groupe d’Etude des Semiconducteurs, CNRS, Université Montpellier II, CC 074, 34095 Montpellier Cedex 5, France. 1 Centre de Recherche sur l’Hétéro-Epitaxie et ses Applications, CNRS, rue Bernard Grégory, 06560 Valbonne, France.
ABSTRACT Recombination dynamics in a variety of InGaN/GaN quantum systems has been studied by time resolved photoluminescence (PL). We have discovered that the time-decay of PL exhibits a scaling law: the nonexponential shape of this decay is preserved for quantum wells and quantum boxes of various sizes while their decay time varies over several orders of magnitude. To explain these results, we propose an original model for electron-hole pair recombination in these systems, combining the effects of internal electric fields and of carrier localization on a nanometer-scale. These two intricate effects imply a separate localization of electrons and holes. Such a microscopic description accounts very well for both the decays shape and the scaling law.
1. INTRODUCTION Optical recombination processes in InGaN/GaN quantum systems are one of the central issues of light-emitting devices based on group-III nitrides. Previous studies have shown that radiative recombination in these systems is influenced by huge internal electric fields and by local potential fluctuations [1]. Envelope-function calculations of transition energies and oscillator strengths, including internal electric fields [2], account very well for experimental results, yielding an electric field value of ~2.5 MV/cm along the growth axis. On the other hand, the local potential fluctuations, together with large carrier effective masses in group-III nitrides, induce the localization of carrier wave-functions on nanometric scales [3], which prevents their efficient capture by nonradiative centers, related to the high density of threading dislocations. This ensures purely radiative recombination at low temperature. The two intricate effects of electric field and localization imply a complex recombination dynamics. By performing time resolved photoluminescence (PL), we have determined how these two effects influence respectively the recombination dynamics. We recall, in a first part, the experimental results; then we present our nanoscopic model of recombination mechanisms.
2. EXPERIMENTAL RESULTS We have performed time-resolved PL experiments at varying temperature on series of InxGa1-xN/GaN quantum well (QW) and quantum box (QB) samples of similar compositions (0.15 < x < 0.20). The samples are grown by molecular beam epitaxy on sapphire substrates. The QB size is typically of 10 nm in diameter and 2-5 nm in height. As the lateral dimension of QBs
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is larger than the exciton Bohr radius (for GaN, we can estimate ~3 nm, from the exciton binding energy and the effective masses [4]), the effects of lateral confinement ar
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