Light Emission from Quantum-Dot-Like Structures in Cubic GaN/InGaN/GaN Double Heterostructures and Quantum Wells
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Light Emission from Quantum-Dot-Like Structures in Cubic GaN/InGaN/GaN Double Heterostructures and Quantum Wells K.Lischka University of Paderborn, Department of Physics, 33095 Paderborn/Germany [email protected]
ABSTRACT Recent results of experimental investigations of cubic GaN/InGaN/GaN double heterostructures grown by molecular beam epitaxy are reviewed. Micro-Raman effect, photoluminescence and photoluminescence excitation spectroscopy as well as high-resolution x-ray diffraction measurements reveal clear evidence that the photoluminescence of these double heterostructures is related to In-rich quantum dot-like structures which are embedded within the InGaN layers. Annealing experiments corroborate the assumption that the formation of the In-rich clusters is terminated already during the growth process.
INTRODUCTION Enormous efforts with the epitaxy of semiconducting compounds of nitrogen and the group III-elements Ga, In and Al resulted in superb emitters of light in the short wavelength region of the visible spectrum. The group III-nitrides which crystallize in the hexagonal (wurtzite) structure emerged as the material basis for the production of highly efficient light emitting diodes and laser diodes. It is a common feature of these devices that they contain InGaN layers in their active zone. However, although InGaN-based structures are produced commercially in large quantities, details of the process of light generation are still subject to ongoing research. The reasons for that are experimental observations like (i) efficient photoluminescence (PL) despite an extremly high dislocation density in the layers, (ii) discrete states of varying energy inside the band gap in both quantum wells and epilayers, (iii) a large Stokes-shift between emission and absorption energies. These results have been unequivocally interpreted as being indicative for the localization of excitons. However, disagreement exists about the origin of the confining potentials. Generally speaking, two different models have been proposed. 1. Due to the low symmetry of the wurtzite lattice and the strain in pseudomorph quantum wells strong electric polarization fields can exist in h-InGaN layers. These fields can effectively separate electron and holes in quantum wells (Quantum Confined Stark Effect) which may lead to some of the effects observed in experiment 1. 2. The large difference in interatomic spacing between GaN and InN can give rise to a miscibility gap. Calculated phase diagrams of InN/GaN 2 3 4 reveal that at typical growth temperatures of 600 – 800 °C a strong tendency of InGaN layers exist to separate into regions of varying In-content. Therefore the mechanism of light generation in InGaN layers has been linked to the recombination of excitons localized at the potential minima originating from compositional fluctuations 5 6 7. The influence of these effects on the optical properties of InGaN quantum wells has been discussed extensively 8. In Ref. 8, the importance of submicrometer scale chemical inhomogeneities
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