Seeded growth of bulk AlN crystals and grain evolution in polycrystalline AlN boules

Large AlN crystals were grown by powder sublimation in a nitrogen atmosphere at low supersaturation and growth rates of 0.1-0.3 mm/h. The starting deposition surface was a sintered TaC disc. An appropriate adjustment of the system pressure and source-seed temperature gradient during the early stages of growth allowed epitaxial re-growth on AlN seeds that had been exposed to air. Single-crystalline AlN grains of 1 cm in size were achieved through multiple sublimation growth runs conducted at P=500Torr and growth temperatures of 2050-2150 °C. Elemental analysis of impurities in the grown AlN boules confirmed low oxygen contamination levels of ∼1019/cm3. No discontinuities were introduced in the structural defect distribution in the individual single-crystalline grains by the multiple re-growth steps. Absence of preferred growth directions of grains suggest the epitaxial re-growth process is suitable for seeded single-crystal growth in any orientation.