Sub-surface imaging of porous GaN distributed Bragg reflectors via backscattered electrons

Sarkar, Maruf and Adams, Francesca and Dar, Sidra A. and Penn, Jordan and Ji, Yihong and Gundimeda, Abhiram and Zhu, Tongtong and Liu, Chaowang and Hirshy, Hassan and Massabuau, Fabien C.-P. and O'Hanlon, Thomas and Kappers, Menno J. and Ghosh, Saptarsi and Kusch, Gunnar and Oliver, Rachel A. (2024) Sub-surface imaging of porous GaN distributed Bragg reflectors via backscattered electrons. Microscopy and Microanalysis, 30 (2). pp. 208-225. ISSN 1431-9276 (

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In this article, porous GaN distributed Bragg reflectors (DBRs) were fabricated by epitaxy of undoped/doped multilayers followed by electrochemical etching. We present backscattered electron scanning electron microscopy (BSE-SEM) for sub-surface plan-view imaging, enabling efficient, non-destructive pore morphology characterization. In mesoporous GaN DBRs, BSE-SEM images the same branching pores and Voronoi-like domains as scanning transmission electron microscopy. In microporous GaN DBRs, micrographs were dominated by first porous layer features (45 nm to 108 nm sub-surface) with diffuse second layer (153 nm to 216 nm sub-surface) contributions. The optimum primary electron landing energy (LE) for image contrast and spatial resolution in a Zeiss GeminiSEM 300 was approximately 20 keV. BSE-SEM detects porosity ca. 295 nm sub-surface in an overgrown porous GaN DBR, yielding low contrast that is still first porous layer dominated. Imaging through a ca. 190 nm GaN cap improves contrast. We derived image contrast, spatial resolution, and information depth expectations from semi-empirical expressions. These theoretical studies echo our experiments as image contrast and spatial resolution can improve with higher LE, plateauing towards 30 keV. BSE-SEM is predicted to be dominated by the uppermost porous layer's uppermost region, congruent with experimental analysis. Most pertinently, information depth increases with LE, as observed.