We report a comparative study of the emission and absorption spectra of a range of commercial InGaN light-emitting diodes and high-quality epilayers. A working definition of the form of the absorption edge for alloys is proposed, which allows a unique definition of the Stokes’ shift. A linear dependence of the Stokes’ shift on the emission peak energy is then demonstrated for InGaN using experimental spectra of both diode and epilayer samples, supplemented by data from the literature. In addition, the broadening of the absorption edge is shown to increase as the emission peak energy decreases. These results are discussed in terms of the localization of excitons at highly indium-rich quantum dots within a phase-segregated alloy.
Direct evidence for In-segregation in InGaN/GaN quantum-well structures is given via highly spatially resolved energy dispersive x-ray analysis performed in a dedicated scanning transmission electron microscope. The In fluctuations become increasingly pronounced in the vicinity of dislocations. The latter assist In diffusion and cause severe Ga/In intermixing.
A diffraction analysis in the transmission electron microscope was carried out on InxGa1−xN layers grown on (0001) sapphire by metalorganic vapor phase epitaxy on top of thick GaN buffer layers. It is found that the ternary InxGa1−xN layers can be chemically ordered. The In and Ga atoms occupy, respectively, the two simple hexagonal sublattice sites related by the glide mirrors and helicoidal axes of the P63 mc symmetry group of the wurtzite GaN. The symmetry of the ordered ternary is subsequently lowered by the disappearance of these operations, and it is shown to agree with the P3ml space group.
A comparative study of the optical linewidths of photo- and electroluminescence from high-quality InGaN epilayers and commercial single quantum well light emitting diode structures was undertaken. Optical linewidths in both cases are temperature insensitive and increase systematically with increasing indium concentration. We assess the contribution of three mechanisms to the luminescence linewidth: alloy fluctuations, well width fluctuations, and strain effects. It is found that the broadening of the emission line is an intrinsic property of InGaN alloys. The piezoelectric effect in wurtzite semiconductor is proposed as a novel line-broadening mechanism.
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