Abstract
We study the optical properties of (In,Ga)N/GaN multiple quantum wells grown by plasma-assisted molecular-beam epitaxy (MBE). N-stable growth inevitably delivers rough interfaces, resulting in strongly red-shifted and broad emission bands. For metal-stable growth, secondary ion-mass spectrometry gives evidence of quantum wells with smooth interfaces but significantly larger width than intended, indicating a strong surface segregation of In. In addition, these measurements reveal that the redistribution of In leads to top-hat In profiles with abrupt leading and trailing edges. The strongly blue-shifted transition energy for these quantum wells may be the reason for the frequent conclusion that the theoretical polarization fields of Bernardini et al. [Phys. Rev. B 56, R10024 (1997)] are too large for (In,Ga)N. Being in possession of the (at least approximately) correct structural parameters, we find the theoretical fields to be in very satisfactory agreement with those deduced from experimental data. A further consequence of the In surface segregation is a pronounced reduction in the quantum efficiency due to a reduced electron-hole overlap. (In,Ga)N is additionally expected to be subject to In bulk segregation, leading to a localization of excitons at compositional fluctuations. Spot-excitation cathodoluminescence spectroscopy directly reveals the presence of localized states. For a quantitative understanding we investigate the temperature dependence of the radiative decay time which is measured by time-resolved photoluminescence spectroscopy and utilized as a probe of the dimensionality of the system. The analysis of these experiments within the frame of a coupled rate-equation model yields a localization depth in these MBE-grown (In,Ga)N/GaN quantum wells of around 20-30 meV. Even this comparatively shallow localization is found to significantly enhance the internal quantum efficiency up to a temperature of about 100 K.
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