March 25, 2014
SSLS EFRC scientists Dan Koleske, Steve Lee, Mary Crawford, Karen Cross, Mike Coltrin, and Jeff Kempisty collaborated on a paper titled, “Connection between GaN and InGaN growth Mechanisms and Surface Morphology” for the Journal of Crystal Growth. They describe how smoothing and roughening mechanisms result in the growth morphologies that are observed during GaN and InGaN growth.
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 roughening becoming more evident at lower growth temperatures. Counteracting the surface roughening are two smoothing mechanisms that operate over different temperature ranges and length scales. The first an evaporation g 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.
March 15, 2013
SSLS EFRC scientists Dan Koleske, Steve Lee, Mary Crawford, Karen Cross, Mike Coltrin, and Jeff Kempisty collaborated on a paper titled, “Controlling indium incorporation in InGaN barriers with dilute hydrogen flows.” The purpose of the paper is to show that the extent of In incorporation into InGaN barriers could be controlled by changing the accompanying hydrogen carrier gas flow. They were successful in developing a chemistry-based model that suggests that hydrogen enhances indium surface desorption through the formation of more volatile indium-hydride species, thereby decreasing the surface indium concentration available for incorporation into the barriers.
Abstract: InxGa1−xN multiple quantum wells (MQWs) with InyGa1−yN barriers were grown by adding dilute hydrogen flows to the QW growth conditions in order to modify the barrier indium composition. With the H2 flow off, the indium concentration in the InxGa1−xN QWs were x=0.183, x=0.163, and x=0.096 for growth temperatures of 730, 750 and 770 °C respective. Using these same QW growth conditions, the H2 flow was increased up to 3 SLM resulting in a gradual decrease in the indium concentration in the InyGa1−yN barriers. Kinetic analysis suggests that hydrogen enhances indium surface desorption through the formation of more volatile indium-hydride species, thereby decreasing the surface indium concentration available for incorporation into the InyGa1−yN barriers. For the MQW structures grown at 750 °C, the photoluminescence (PL) wavelength blue-shifts from 477 to ~450 nm as the indium in the InyGa1−yN barriers increases as expected from the reduced influence of the piezoelectric fields. While a corresponding increase of spontaneous emission from the increasing overlap of electron states within the QW is also expected, the PL intensity of the QW instead decreases. This conflicting expectation of increased spontaneous emission and the observation of decreased PL intensity suggest either decreases in the QW-barrier height or increases in non-radiative defects offset any efficiency gains obtained when InGaN barriers to reduce polarization fields within the QWs.