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Sub-surface Imaging of Porous GaN Distributed Bragg Reflectors via Backscattered Electrons
Maruf Sarkar1, Francesca Adams1, Sidra A Dar1
1Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK.
Summary
Backscattered electron scanning electron microscopy (BSE-SEM) offers a non-destructive method for characterizing porous gallium nitride (GaN) distributed Bragg reflectors (DBRs). This technique effectively visualizes sub-surface pore morphology, crucial for optimizing DBR performance.
Area of Science:
- Materials Science
- Nanotechnology
- Semiconductor Physics
Background:
- Porous gallium nitride (GaN) distributed Bragg reflectors (DBRs) are essential optical components.
- Characterizing the sub-surface morphology of these porous structures is critical for performance optimization.
- Existing characterization methods can be destructive or lack efficiency.
Purpose of the Study:
- To introduce and validate backscattered electron scanning electron microscopy (BSE-SEM) for non-destructive characterization of porous GaN DBRs.
- To investigate the relationship between electron landing energy (LE) and image quality (contrast, spatial resolution, information depth).
- To compare BSE-SEM imaging with other techniques like scanning transmission electron microscopy (STEM).
Main Methods:
- Fabrication of porous GaN DBRs via epitaxy and electrochemical etching.
- Utilizing a Zeiss GeminiSEM 300 for BSE-SEM imaging.
- Systematically varying the primary electron landing energy (LE) to assess its impact on imaging.
- Employing semi-empirical expressions to derive theoretical expectations for image parameters.
Main Results:
- BSE-SEM successfully imaged sub-surface pore morphology in both mesoporous and microporous GaN DBRs, consistent with STEM observations.
- Optimum contrast and spatial resolution were achieved around 20 keV LE, with improvements plateauing towards 30 keV.
- BSE-SEM detected porosity at depths of ~295 nm, even through a ~190 nm GaN cap, with improved contrast.
- Image analysis confirmed that BSE-SEM is primarily sensitive to the uppermost region of the topmost porous layer, and information depth increases with LE.
Conclusions:
- BSE-SEM is an efficient, non-destructive technique for characterizing the sub-surface morphology of porous GaN DBRs.
- Landing energy is a critical parameter for optimizing BSE-SEM imaging of these structures.
- The findings provide valuable insights for the design and fabrication of advanced GaN-based optical devices.
