Explaining relative spectral red shifts in InGaN/GaN micropillars

Abstract

There is common agreement that dimensional downscaling of III-nitride light-emitting diodes leads to spectral blue shifts due to strain relaxation of the quantum wells (QWs). Near-field photoluminescence (nf-PL) mapping of micropillars with InGaN/GaN QWs of different indium compositions using scanning near-field optical spectroscopy reveals that the nf-PL spectrum blue-shifts at the edge of a micropillar with respect to the center for QWs with a high indium composition, whereas a relative red shift is observed for QWs with a low indium composition. This observation suggests that the strain relaxation mechanism in micropillars is dependent on the indium composition, evident from changes in lattice parameters determined from calibrated diffraction patterns obtained by transmission electron microscopy. As indicated by molecular dynamics simulations, the strain of a micropillar is influenced by competing strain relaxation mechanisms between the lattice mismatch strain from the QWs, and residual strain from other layers and their interactions with the edge of the micropillar. First-principle calculations of GaN/InGaN/GaN heterostructures confirmed the effect of strain relaxation on the potential profiles, and, thus, on the spectral shifts from the micropillars. The findings of this work provide insight into strain-induced band profile engineering in optoelectronic devices built on lattice-mismatched systems.