arXiv subjects

Ben Thornley

Publications and source records attributed to Ben Thornley.

3 recordsLinked to original sources

arXiv2026

Improving feature resolution and pore back effect in focused ion beam tomography of porous GaN thin films

Porous gallium nitride (GaN) is a mesoporous crystalline material, typically in the form of a thin film on an unlike substrate, prepared by electrochemically etching conductive GaN. The use of porous GaN in electronics and optoelectronics is rapidly expanding, but is held back significantly by a lack of structural control and understanding of the principles of pore formation from underlying electrochemical mechanisms, where high-quality characterisation of pore morphology is essential to understanding these principles. Focused ion beam (FIB) tomography is an invaluable tool for such characterisation, but is hindered greatly by the pore back effect, where unwanted contrast appears in an image due to electrons scattering out from the back wall of a pore. No major attempts to formally quantify or assess the extent of the effect for different tomography experiments has been demonstrated. In this work, we demonstrate an advanced methodology for performing FIB tomography on porous GaN thin films, where the experiment is rotated to image pores perpendicular to the surface of the sample, and introduce new voxel intensity-based formalisms for assessing the impact of the pore back effect based on voxel intensities for individual features and the whole dataset. The new approach, which requires more complex preliminary setup, is found to significantly mitigate the pore back effect in porous GaN thin films with a range of pore morphologies without increasing the total experimental duration. The pore back effect can thus be quantified and mitigated in FIB tomography of porous GaN and similar mesoporous thin films.

cond-mat.mtrl-sci↗

arXiv2026

A stochastic simulation of the dislocation-mediated etching of porous GaN distributed Bragg reflectors

Distributed Bragg reflectors (DBRs) can be fabricated by electrochemically etching nitride epitaxial structures consisting of alternating layers of highly n-type doped and non-intentionally doped (NID) GaN. Threading dislocations (TDs) can be electrochemically etched into transport pipelines that can carry the etchant through the NID layers to access the doped material. Experimentally this has been shown to involve a mechanism where the etching pathway may follow one TD into a doped layer and then propagate sideways through the doped layer to continue via a different TD. Across multiple layers this process creates complex pore structures that have been described as 'cascades'. Here, we build a stochastic simulation for the DBR etching process that can reproduce some key features of the observed microstructures including the cascade morphology. By comparing the simulation output to samples etched at a range of voltages, we show that we can reproduce variations in experimental chronoamperometry data with applied bias by varying the probability of etching the doped layers within the simulation. The outputs of the resulting simulations replicate the experimentally observed cascade morphology. At higher voltages, experimental data reveal a lower proportion of cascade features, a trend that is also replicated by the simulations for relevant probability values. Outputs of the simulations also correlate well with experimental chronoamperometry data for samples where - unlike in a DBR - the thicknesses of the doped layers vary through the epitaxial multilayer, suggesting that the probabilistic simulation can be applied to a range of structures to help understand the dislocation-mediated electrochemical etching process.

cond-mat.mtrl-sci↗

arXiv2025

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

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.

cond-mat.mtrl-sci↗