Sub-surface Imaging of Porous GaN Distributed Bragg Reflectors via Backscattered Electrons

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.