We present a nanometer-scale correlation of the structural, optical, and electronic properties of InGaN/GaN core-shell microrod LEDs: The microrods were fabricated by MOVPE on a GaN/sapphire template covered with an SiO2-mask. Through the mask openings, Si-doped n-GaN cores were grown with high SiH4 flow rate at the base. Subsequently, the SiH4 flow rate was reduced towards the microrod tip to maintain a high surface quality. The Si-doped GaN core was finally encased by an InGaN single quantum well (SQW) followed by an intrinsic GaN layer and a thick Mg-doped p-GaN shell.
Highly spatially resolved cathodoluminescence (CL) directly performed in a scanning transmission electron microscope (STEM) was applied to analyze the free-carrier concentration within the Si-doped GaN core and the luminescence properties of the individual functional layers. The CL was supported by Raman spectroscopy directly carried out at the same microrod on the thin TEM-lamella.
The cross-sectional CL of a single microrod resolves the emission of the single layers. CL and Raman measurements reveal a high free-carrier concentration of 7x1019 cm 3 in the bottom part and a decreasing doping level towards the tip of the microrod. Moreover, structural investigations exhibit that initial Si-doping of the core has a strong influence on the formation of extended defects in the overgrown shells. However, we observe the most intense emission coming from the InGaN QW on the non-polar side walls, which shows a strong red shift along the facet in growth direction due to an increased QW thickness accompanied by an increased indium concentration right at the intersection of generated defects and InGaN QW, a red shifted emission appears, which indicates indium clustering.
Frank Bertram, Marcus Müller, Peter Veit, Christian Nenstiel, Gordon Callsen, Matin Mohajerani, Jana Hartmann, Hao Zhou, Hergo-Heinrich Wehmann, Axel Hoffmann, Andreas Waag, and Jürgen H. Christen, "Direct comparison of structural and optical properties of a nitride-based core-shell microrod LED by means of highly spatially-resolved cathodoluminescence and μ-Raman(Conference Presentation)," Proc. SPIE 10104, Gallium Nitride Materials and Devices XII, 101041V (Presented at SPIE OPTO: February 02, 2017; Published: 19 April 2017); https://doi.org/10.1117/12.2251748.5391659260001.
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