A cascade model for the defect-driven etching of porous GaN distributed Bragg reflectors

TL;DR

This study addresses the mechanism of defect-driven porosification in GaN-based distributed Bragg reflectors by leveraging a lithography-free electrochemical etching process. Using serial-section FIB-SEM tomography, the authors reconstruct 3D pore networks across a 5-pair GaN-on-Si stack etched at 5, 8, and 10 V, and demonstrate that etching propagates as a cascade involving multiple dislocations rather than a single kebab pathway. Quantitative dislocation statistics reveal voltage-dependent increases in active dislocation density and a shift toward kebab-like networks at higher voltages, while many dislocations exhibit interrupted or activation-then-inactivation behavior. The proposed cascade model provides a mechanistic framework for understanding and optimizing porosification, with broad implications for designing high-reflectance porous GaN DBRs and for characterizing sub-surface porosity in complex multilayer materials.

Abstract

Fabrication of porous GaN distributed Bragg reflectors (DBRs) via the selective electrochemical etching (ECE) of conductive Si-doped layers, separated by non-intentionally doped (NID) layers, provides a straightforward methodology for producing highly reflective DBRs suitable for device overgrowth and integration, which has otherwise proven difficult in the III-nitride epitaxial system via conventional alloying. Such photonic materials can be fabricated by a lithography-free defect-driven etching process, where threading dislocations intrinsic to heteroepitaxy form nanoscale channels that facilitate etchant transport through NID layers. Here, we report the first three-dimensional characterisation of porous GaN-on-Si DBRs fabricated in this methodology with different ECE voltages, using serial-section tomography in a focused ion beam scanning electron microscope (FIB-SEM). These datasets reconstruct the pore morphology as etching proliferates through the alternating Si-doped/NID layer stack. Volumetric reconstruction enabled us to enhance the established `kebab' model for defect-driven etching by proposing a `cascade' model where etchant cascades through the material via vertical etching down nanopipes and horizontal etching across pores, forming complex networks directly related to the pathways taken. This accounts for premature nanopipe termination and discontinuities in nanopipe formation, where dislocations are observed to activate and deactivate individually. Statistical analysis of individual etching behaviour, across all dislocations for each tomograph, revealed a greater tendency to form continuous structures that follow conventional kebab behaviour at higher ECE voltages. We propose that higher ECE voltages alter the probability of dislocation etching relative to doped layer etching, thereby empowering morphological optimization through improved mechanistic understanding of ECE.

Figures (9)

• Figure 1: Simplified schematic of the as-grown epitaxial structure, prior to electrochemical porosification for the fabrication of 5-pair porous GaN-on-Si DBRs etched at 5 V, 8 V, and 10 V.
• Figure 2: Outline of the serial-section FIB-SEM tomography procedure, showing a) a simplified schematic representation of the sample preparation step, b) the five lines hosted by the platinum and carbon deposited in the target area that are used for 3D tracking and SEM autotune of focus and stigmatism, c) the coarse, side, and fine trenches, milled after tracking and protection are prepared, d) cross-sectional SEM of the 5-pair porous GaN-on-Si DBR etched at 5 V with three parallel lines in the centre as well as two converging lines on either side, hosted by the protective platinum pad and backfilled with carbon and e) 11 serial-sections from the same DBR, where the sections are related to each other with an average 5.1 nm ion slicing thickness (Z dimension) for voxel dimensions of (2.0$\,\times$ 2.0 $\times$ 5.1) nm owing to the use of 3D tracking and slice thickness interpolation.
• Figure 3: Serial-section FIB-SEM tomography example workflow with input serial-sections, output reconstructed plan-view images, and segmented tomograph 3D renders, shown for the 5-pair porous GaN-on-Si DBR etched at 10 V. a) Orthographic perspective 3D renders after tomograph segmentation with exaggerated inter-layer spacing, b) example serial-sections showing the porous region of interest, with two etched dislocation cores highlighted in red. Each of the scanning electron micrographs is captured with a 2.3 nm $\times$ 2.3 nm X and Y pixel size. The sections are related to each other with an 8.8 nm Z ion slicing thickness for tomographic voxel dimensions of (2.3$\,\times$ 2.3 $\times$ 8.8) nm, c) orthographic perspective 3D render of the porous layers with their real spacing, d) reconstructed plan-view of the first porous layer and e) sub-surface BSE-SEM micrograph of the first porous layer, depicting the target region of interest prior to correlative microscopy via serial-section tomography.
• Figure 4: Reconstructed plan-view images as a function of depth, derived from serial-section FIB-SEM tomography of the 5-pair porous GaN-on-Si DBR etched at 5 V. Images are extracted positioned at a) onset of layer 1, b) layer 1, c) onset of layer 2, d) layer 2, e) onset of layer 3, f) layer 3, g) onset of layer 4, h) layer 4, i) onset of layer 5 and j) layer 5. 'Dislocation 1' is highlighted in red throughout, alongside 'dislocation 2' in blue.
• Figure 5: Reconstructed plan-view images as a function of depth, derived from serial-section FIB-SEM tomography of the 5-pair porous GaN-on-Si DBR etched at 8 V. Images are extracted positioned at a) onset of layer 1, b) layer 1, c) onset of layer 2, d) layer 2, e) onset of layer 3, f) layer 3, g) onset of layer 4, h) layer 4, i) onset of layer 5 and j) layer 5. 'Dislocation 3' is highlighted in red throughout, alongside 'dislocation 4' in blue.