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T4.03 - Correlated XRD, Micro-PL and Atom Probe Tomography Analysis of Continuous and Discontinuous InGaN QWs 
December 2, 2014   2:30pm - 2:45pm

InxGa1-xN quantum wells (QWs) are widely employed as efficient blue emitters in solid state lighting, but improving the efficiency and achieving higher indium mole fractions to produce bright green emission remain important goals. While fundamental understanding of the factors controlling efficiency can contribute to advances in the technology, it is challenging to fully characterize the relationships between light emission and the spatial distribution of indium given the high density of dislocations threading through very thin QWs. Routine characterization techniques such X-ray diffraction (XRD) and PL mapping on the millimeter scale play important roles in the empirical optimization of growth conditions, but they fail to capture important nanoscale structure-property relationships. We have therefore conducted site-specific analysis of InGaN quantum wells through correlated micro-PL and atom probe tomography to elucidate the relationships between emission wavelength, efficiency, and defect distribution. Baseline studies on continuous InxGa1-xN QWs show good agreement between XRD and APT analysis of QW composition and width, though XRD fits are improved by incorporating composition profiles extracted from APT reconstructions, which have previously been shown to be reliable. [1] Hydrogen dosing after the QW growth is found to improve PL efficiency. Hydrogen preferentially etches indium rich regions, leading to the formation of discontinuous quantum wells with gaps. Standard XRD analysis underestimates the indium composition when continuous layers are assumed and transmission electron microscopy measurements average over depth. In contrast, APT reconstructions capture the three-dimensional distribution of indium with sub-nanometer resolution. In the etched sample, micro-PL mapping shows that red-shifted emitting regions are more efficient; we hypothesize that indium rich regions near trench-like defects are preferentially etched, leading to increased carrier localization in the remaining portions of the QWs. The highest indium content in the discontinuous QWs is 2% lower than continuous QWs, which is consistent with the observed overall blue-shift. More generally, these analyses indicate the important role that three decomposition mapping can play in the further development of InxGa1-xN QWs for energy efficient solid-state lighting.
The work at Sandia was supported by the Sandia’s Solid-State Lighting Science Energy Frontier Research Center, funded by the US Department of Energy, Office of Basic Energy Sciences. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
1. Riley, J., et al. Appl. Phys. Lett. 2014, 104, 152102.

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