title: Nanoscopic Insights into InGaN/GaN Core-Shell Nanorods: Structure, Composition, and Luminescence.
abstract: Nitride-based three-dimensional core-shell nanorods (NRs) are promising candidates for the achievement of highly efficient optoelectronic devices. For a detailed understanding of the complex core-shell layer structure of InGaN/GaN NRs, a systematic determination and correlation of the structural, compositional, and optical properties on a nanometer-scale is essential. In particular, the combination of low-temperature cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM), and quantitative high-angle annular dark field imaging enables a comprehensive study of the nanoscopic attributes of the individual shell layers. The investigated InGaN/GaN core-shell NRs, which were grown by metal-organic vapor-phase epitaxy using selective-area growth exhibit an exceptionally low density of extended defects. Using highly spatially resolved CL mapping of single NRs performed in cross-section, we give a direct insight into the optical properties of the individual core-shell layers. Most interesting, we observe a red shift of the InGaN single quantum well from 410 to 471 nm along the nonpolar side wall. Quantitative STEM analysis of the active region reveals an increasing thickness of the single quantum well (SQW) from 6 to 13 nm, accompanied by a slight increase of the indium concentration along the nonpolar side wall from 11% to 13%. Both effects, the increased quantum-well thickness and the higher indium incorporation, are responsible for the observed energetic shift of the InGaN SQW luminescence. Furthermore, compositional mappings of the InGaN quantum well reveal the formation of locally indium rich regions with several nanometers in size, leading to potential fluctuations in the InGaN SQW energy landscape. This is directly evidenced by nanometer-scale resolved CL mappings that show strong localization effects of the excitonic SQW emission.
authors: Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg , Magdeburg, 39106 Germany.
authors: Marcus
authors: M
authors: Müller
authors: Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg , Magdeburg, 39106 Germany.
authors: Peter
authors: P
authors: Veit
authors: Institute of Solid State Physics, University of Bremen , Bremen, 28359 Germany.
authors: Florian F
authors: FF
authors: Krause
authors: OSRAM Opto Semiconductors GmbH , Regensburg, 93055 Germany.
authors: Tilman
authors: T
authors: Schimpke
authors: Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg , Magdeburg, 39106 Germany.
authors: Sebastian
authors: S
authors: Metzner
authors: Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg , Magdeburg, 39106 Germany.
authors: Frank
authors: F
authors: Bertram
authors: Institute of Solid State Physics, University of Bremen , Bremen, 28359 Germany.
authors: Thorsten
authors: T
authors: Mehrtens
authors: Institute of Solid State Physics, University of Bremen , Bremen, 28359 Germany.
authors: Knut
authors: K
authors: Müller-Caspary
authors: OSRAM Opto Semiconductors GmbH , Regensburg, 93055 Germany.
authors: Adrian
authors: A
authors: Avramescu
authors: OSRAM Opto Semiconductors GmbH , Regensburg, 93055 Germany.
authors: Martin
authors: M
authors: Strassburg
authors: Institute of Solid State Physics, University of Bremen , Bremen, 28359 Germany.
authors: Andreas
authors: A
authors: Rosenauer
authors: Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg , Magdeburg, 39106 Germany.
authors: Jürgen
authors: J
authors: Christen
keywords: HAADF STEM; InGaN; Scanning transmission electron microscopy; cathodoluminescence spectroscopy; composition determination; nanorods