Ammonothermal Growth of Rhombohedral Boron Nitride

Boron nitride (BN) has numerous advantageous physical and electrical properties for mechanical and (opto‐)electronic applications, yet growing large single crystals is challenging. The ammonothermal method has demonstrated its suitability for large single‐crystal gallium nitride growth and is investigated for its ability to grow BN. The temperature‐dependent solubility of BN in sodium‐containing supercritical ammonia is previously demonstrated. Herein, the application of a temperature gradient across a baffled autoclave toward the continuous dissolution and crystallization of BN is examined. BN deposits on all wetted surfaces in the lower‐temperature zone, predominantly on the autoclave walls as polycrystalline sheets and to a lesser degree on suspended seeds of cubic BN as oriented, micrometer‐sized, repeated trapezoidal platelet units. Analysis of the grown BN crystals by Raman, X‐ray diffraction, and transmission electron microscopy suggests the growth of sp 2 ‐hybridized BN, predominantly the rhombohedral crystal structure on nitrided NiCr alloy surfaces and rhombohedral and hexagonal crystal structures on the cubic BN seed crystal surfaces. The interfacial relationships between the cBN and the sp 2 ‐hybridized BN phases are examined, where favorable attachment sites are discovered between the dissimilar lattices, further supporting the observed crystal growth.