Connection between GaN and InGaN growth mechanisms and surface morphology

Abstract

Power spectral density (PSD) analysis of atomic force microscopy (AFM) images is used to determine the roughening and smoothing mechanisms that contribute to InGaN and GaN morphology during metalorganic chemical vapor deposition epitaxy (MOCVD). The analysis finds that non-stochastic surface roughening occurs from deposition onto the pre-existing GaN surface with this roughening becoming more evident at lower growth temperature. Counteracting the surface roughening are two smoothing mechanisms that operate over different temperature ranges and length scales. The first is an evaporation and recondensation mechanism, which dominates at higher temperatures (>900°C) and longer length-scales ranging from one to tens of microns. The second is a surface diffusion mechanism, which dominates at lower temperatures (<800°C) and shorter length scales ranging from tens to hundreds of nanometers. The competition between roughening and smoothing leads to a characteristic length-scale for mound (or hillock) formation, which prevails for low-temperature growth of GaN and InGaN single heterolayers, as well as InGaN/GaN multiple quantum wells (MQWs). For InGaN/GaN MQW samples where the wells are similarly grown, the choice of the GaN-barrier growth conditions is shown to influence MQW-interface roughness. Moreover, increased green-wavelength photoluminescence intensity is observed when rougher (rather than smoother) interfaces are present. The results suggest that the operation of smoothing mechanisms during MOCVD growth can be tailored to influence InGaN/GaN surface morphology and possibly improve emission efficiency.