Session Index

Gallium nitride (GaN) is central to modern optoelectronics and high-power electronics, but further progress depends on advances in crystal growth. This lecture reviews recent developments in bulk GaN crystallization, with emphasis on the basic ammonothermal method and its integration with halide vapor phase epitaxy (HVPE). Key challenges—including scalability, reactor design, and seed preparation—will be examined alongside issues in wafer processing and surface treatment. Particular focus will be given to hybrid growth strategies that combine the strengths of different techniques. Finally, future milestones toward large-diameter GaN substrates will be outlined, enabling next-generation lasers, transistors, and high-performance power devices.

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A novel seed architecture for ammonothermal GaN growth is presented, allowing simultaneous growth in the [000-1] direction on both sides of the seed through direct bonding of two (0001) crystal faces. Magnesium ion implantation into the (0001) faces of two GaN seed crystals, followed by precise alignment with the (0001) faces opposing and subsequent Ultra-High Pressure Annealing, resulted in the formation of a strong inter-seed bond, with clear evidence of interface recrystallization. This fabrication strategy removes the requirement for bulky seed holders in ammonothermal crystallization and has the potential to double GaN crystallization yield.

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In thick GaN crystal growth by oxide vapor phase epitaxy (OVPE), the previously addressed issue of polycrystal formation was resolved; however, a new challenge involving the formation of large pits has emerged. It was hypothesized that the large pits result from polycrystals generated within the reactor becoming airborne. By introducing a new setup that suppressed polycrystal generation, the occurrence of large pits was reduced. Furthermore, by clarifying the correlation between reactor wall polycrystals and large pits, the primary cause of pit formation was identified.

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The thermal conductivity of GaN substrates, grown by the Oxide Vapor Phase Epitaxy (OVPE) method, was investigated, specifically focusing on the impact of oxygen concentrations ranging from 10¹⁹ to 10²¹ cm⁻³. The results demonstrated a clear dependence of thermal conductivity on oxygen concentration. With increasing oxygen concentration, thermal conductivity decreased owing to enhanced phonon scattering, while electrical resistivity also decreased. The sample with the lowest oxygen concentration, around 10¹⁹ cm⁻³, exhibited relatively high thermal conductivity owing to the presence of c-plane growth sectors with lower oxygen incorporation.

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The thermodynamic stability of hexagonal boron nitride (hBN) in Ni–B solutions containing 10 at.% of boron under varying nitrogen pressure was investigated. A series of high-temperature experiments (1460 °C, 1500 bar Ar) with in-situ nitrogen release from GaN decomposition revealed that equilibrium stabilization of hBN occurs between 2.5 and 5 bar partial pressure of nitrogen. Higher pressures (e.g., 10 bar) lead to rapid polycrystalline growth of hBN, while lower pressures yield no crystalline hBN film. Experimental results align well with thermodynamic predictions. These findings enable controlled crystallization of hBN in selected melt regions, offering a pathway to optimize solution-based growth of high-quality hBN crystals.

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In this study, chlorine-based HVPE method with epitaxial lateral overgrowth (ELO) on N-polar GaN substrates were performed to reduce threading dislocation density (TDD). We compared two mask directions (m-axis and a-axis) and found that different crystal shapes and growth rates appeared when the stripe mask along to the m-axis leading to a larger reverse tapered structure. This method successfully demonstrated a promising way to grow high-quality, low-dislocation GaN crystals.

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We proposed a new method using meltback in an unsaturated Ga–Na melt to promote growth by {10-11}-facets. However, surface unevennesses caused by facet growth led to the formation of pits after polishing, complicating accurate evaluation of the overall threading dislocation density (TDD). Microcrystals released from the seed crystal during meltback induced polycrystalline formation, hindering surface planarization. In this study, meltback and growth were separated to eliminate microcrystals and suppress polycrystalline formation, thereby enabling surface planarization. The average TDD above pits was 8.5 × 10⁴ cm⁻², while other regions showed 7.0 × 10⁴ cm⁻². These results indicate that facet growth contributes to overall TDD reduction.

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In this study, the fabrication of AlN seed crystals and subsequent epitaxial growth were investigated using Fe-based fluxes. First, vapor-solid growth of AlN in Fe-Al flux was optimized to produce high-quality seed crystals, with XRC FWHMs of (0002) and (10-10) reflections reaching 18 and 32 arcsec, respectively. These crystals were further employed as seeds for solution growth in Fe-Cr fluxes. The diameter of the grown AlN increased from 114 μm to 515 μm at a growth rate of approximately 66 μm/h. This research proposes a novel method for synthesizing high-quality bulk AlN crystals exclusively using Fe-based alloys.

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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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