Impact of Interfaces on Photoluminescence Efficiency of High-Indium-Content (In,Ga)N Quantum Wells

Abstract

$(\mathrm{In},\mathrm{Ga})\mathrm{N}$-based light-emitting diodes (LEDs) are known to suffer from low electron-hole wave-function overlap due to a high piezoelectric field. Staggered $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum wells (QWs) are proposed to increase the wave-function overlap and improve the efficiency of LEDs, especially for long-wavelength emitters. In this work, we show evidence that the growth of staggered QWs by plasma-assisted molecular beam epitaxy has another beneficial effect, as it allows a reduction in the formation of defects, responsible for nonradiative Shockley-Read-Hall recombination, at the bottom interface of the QW. Staggered QWs comprised an $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ layer of an intermediate $\mathrm{In}$ content between the barrier and the QW. We show that insertion of such a layer results in a significant increase of the luminescence intensity, even if the calculated wave-function overlap drops. We study the dependence of the thickness of such an intermediate-$\mathrm{In}$-content layer on photoluminescence intensity behavior. Staggered QWs exhibit increased cathodoluminescence homogeneity that is a fingerprint of a lower density of defects, in contrast to standard QWs for which a high density of dark spots is observed in QW-emission mapping. Transmission electron microscopy of standard QWs reveals the formation of basal-plane stacking faults and voids that can result from vacancy aggregation. A stepwise increase of the $\mathrm{In}$ content in staggered QWs prevents the formation of point defects and results in an increased luminescence efficiency. The $\mathrm{In}$-composition difference between the barrier and the well is, therefore, a key parameter to control the formation of point defects in the high-$\mathrm{In}$-content QWs, influencing the luminescence efficiency. Characteristics of the cyan laser diode (LD) utilizing staggered QWs are presented.