Paper EM1-MoA2
Atom Probe Tomography Studies on Green Light Emitting InGaN/GaN Multi Quantum Wells Grown on GaN Substrates

Monday, October 31, 2011, 2:20 pm, Room 209

Carrier delocalization in InGaN/GaN multi-quantum wells (MQW) contained within green light-emitting diodes (LEDs) has been proposed as a contributor to LED efficiency droop. By contrast, interface roughness and fluctuations in composition within the MQWs may act to localize and confine carriers¹. In this study, InGaN/GaN MQWs were grown on both (0001)GaN layers and on In_xGa_1-xN buffer layer with graded In mole frations from 0 to 10%. Both heterostructures were grown on chemomechanically polished (0001)GaN substrates. Calculations using temperature-dependent photoluminescence spectra revealed a four-fold increase in the internal quantum efficiency (IQE) in the latter structure. A LEAP 4000X HR^TM pulsed UV laser (355 nm at 200 kHz) atom probe tomograph was used to investigate the elemental and spatial characteristics of the interface of the In_xGa_1-xN/GaN MQWs. To establish consistent atom probe operation parameters for reliable comparison among different samples, a systematic study was conducted to optimize the evaporation rate and laser energy. The concentration profile of In_xGa_1-xN/GaN MQW showed slightly varied In fraction among different QWs, ranging from x=0.21 to x=0.27, while the XRD results showed an average In fraction in all QWs of x=0.25. Furthermore, based on isoconcentration surface analysis and proximity histograms the upper surfaces of InGaN QWs appear to be more diffuse than the lower surfaces. These results indicate surface roughening of the InGaN layer. A detailed comparison of the two structures will be presented and the ability of 3-D atom probe tomography for such an analysis and the impact of the results on next generation LED technologies will be discussed.

1. J. Hader, J. V. Moloney and S. W. Koch, Appl. Phys. Lett. 96 (22), 221106 (2010).