Ammonothermal Growth of Boron Nitride

Jacob Dooley
Lehigh University – Bethlehem, Pennsylvania, USA
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Boron nitride (BN) has numerous advantageous physical and electrical properties for mechanical and (opto-)electronic applications, that are useful towards high frequency applications, energy efficient power conversion, and 2D functional heterostructures. The challenge is growing large single crystals of BN required to build these devices. The most successful technique for bulk crystal growth is the high pressure, high temperature anvil syntheses, producing millimeter size crystals.

One largely unexplored synthesis path, successful in growing large single crystals of gallium nitride, is the ammonothermal technique. This is a solution growth technique where nitrides are dissolved in supercritical ammonia and recrystallized on seed crystal sites using a temperature controlled solubility gradient. Previously the temperature dependent solubility of BN in sodium-containing supercritical ammonia was demonstrated.1 This study examines the application of a temperature gradient across a baffled autoclave. Both rhombohedral and hexagonal BN crystals were grown on the autoclave walls and on seed crystals of cBN in the lower temperature, lower solubility zone of the autoclave.

The crystallography of the grown BN crystals was characterized via Raman spectroscopy and X-ray diffraction, providing evidence of sp2 bonded BN. Further investigation, using transmission electron microscopy, selected area electron diffraction, and electron energy loss spectroscopy, revealed that both the hBN and rBN phases grew in the ammonothermal system on the seed crystals of cBN. An epitaxial relationship was identified for the grown rBN on (111) cBN surfaces and will be discussed.

[1] Dooley, J., et al. J. Cryst. Growth 621, 127381 (2023).

The authors acknowledge the support from NSF DMR under Award No. 1832824, Northrop Grumman, and the Lehigh New Faculty Startup Funds.

Email: jrd419@lehigh.edu