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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