Properties of (In,Ga)N/GaN quantum wells grown by plasma-assisted molecular beam epitaxy

Abstract

We investigate the synthesis of (In,Ga)N/GaN multiple quantum wells by plasma-assisted molecular beam epitaxy (MBE). For metal-stable growth, unexpectedly strong In surface segregation is revealed. The In depth profiles obtained by secondary ion-mass spectrometry exhibit a top-hat In distribution and are thus indicative of a zeroth order segregation mechanism instead of a first order process as observed for other materials systems. As additionally evidenced by transmission electron microscopy, the segregation of In during metal-stable growth results in quantum wells with smooth interfaces but significantly larger width than intended. The resulting blueshift of the transition energy 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. For a thorough understanding of the spontaneous emission from these structures both electrostatic fields and compositional fluctuations have to be taken into account. Both the transition energies and radiative decay times in photoluminescence are shown to be in agreement with the quantum-confined Stark effect in these structures. Using cathodoluminescence spectroscopy, we investigate the dependence of transition energy and quantum efficiency on both temperature and excitation density. At low temperatures, recombination is governed by localized states, whereas for high temperatures extended states dominate. This finding is in agreement with the temperature dependence of the radiative decay time which is utilized as a probe of the dimensionality of the system. A quantitative analysis of these experiments via coupled rate equations shows that the localization depth in these MBE-grown (In,Ga)N/GaN quantum wells is around 20–30 meV. This shallow localization is found to significantly enhance the internal quantum efficiency up to a temperature of about 100 K.