Session Index

The ammonothermal method enables bulk GaN growth under extreme conditions, limiting direct observation of internal processes. This study uses 3D Computational Fluid Dynamics (CFD) to investigate velocity and temperature distributions in large-scale autoclaves operated in a laboratory environment. A transient k-ω turbulence model was applied, with boundary conditions derived from real process parameters. Various internal installation geometries were examined for their effect on flow uniformity. Simulation results, supported by experimental observations, highlight CFD as a practical tool for design assessment and process optimization—providing insights into convective behavior, improving growth uniformity, and enhancing the structural quality of GaN crystals.

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This study presents the complete mechanical processing pathway for ammonothermal GaN (Am-GaN) substrates. Beginning with wire-saw separation from the seed, the crystal is aligned, machined, and sliced into wafers. Further steps include flat formation, lapping, backside roughening, edge grinding, and chemical-mechanical polishing (CMP), yielding atomically smooth (0001) surfaces (RMS < 0.1 nm). The work evaluates material losses during cutting and polishing, and applies both contact and non-contact metrology to assess surface quality. Subsurface damage detection and removal are discussed, along with process optimizations aimed at reducing waste and improving substrate preparation efficiency.

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Gallium nitride GaN crystals grown by HVPE and solution-based methods follow a sequence: step propagation, hillock formation, and hillock coalescence. The merging hillocks stage is critical for achieving uniform structural and electrical properties, as instabilities at hillock junctions can alter impurity incorporation and carrier concentration. We studied undoped ammonothermal and HVPE-GaN and Ge-doped HVPE-GaN grown on misoriented seeds. Photo-etching, X-ray topography, Raman spectroscopy, and electrical transport measurements revealed changes in carrier concentration in inter-hillock regions, with stronger effects at higher doping level. ToF-SIMS confirmed compositional differences. The results highlight importance of single-hillock crystal growth to improve GaN substrate uniformity.

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In this study, we investigated the dissolution behavior of GaN in supercritical ammonia under basic ammonothermal conditions with sodium as the mineralizer. Solubility measurements were carried out at temperatures from 300 to 550 °C, for Na:NH₃ molar ratios of 0.02–0.08, and ammonia pressures of 200–380 MPa. Based on experimental data, thermodynamic parameters (ΔG, ΔH, ΔS) were extracted using an extended van ’t Hoff model. The dissolution was found to be non-spontaneous (ΔG > 0) and strongly pressure-dependent. These results improve our understanding of GaN behavior in ammonothermal systems and provide a basis for optimizing growth conditions for high-quality GaN substrates.

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AlN and AlGaN are key materials for UV photonics and high-power electronics. High-quality bulk AlN crystals are essential and are best grown via physical vapor transport (PVT). To assess their structural perfection, non-destructive, high-resolution methods are required. This work analyzes PVT-grown AlN using Lang X-ray topography (L-XRT) for defect mapping and synchrotron monochromatic rocking curve imaging (RCI) for quantitative analysis. The crystals exhibited exceptional structural quality. RCI enabled detailed visualization of individual dislocations at sub-micrometer resolution, revealing lattice distortions in near-perfect regions, demonstrating the power of combined L-XRT and RCI characterization.

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Ammonothermal growth is a key method for producing high-quality GaN substrates, but regrowth steps often introduce crystal defects. This study investigates growth discontinuity-related defects using Lang laboratory X-ray topography and synchrotron rocking curve imaging. Dislocation or dislocation bundles chains form along a-plane facets during seed enlargement, visible as lines of contrast. RCI maps reveal lattice tilts up to ~13 arcsec and alternating strain fields of ±3 arcsec. Notably, regions adjacent to dislocation bundle chains show excellent crystal quality with ultra-low dislocation density. Both lateral and vertical growth discontinuities are considered to understand defect formation during crystal growth.

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