(167j) Combined Experimental and Theoretical Study of Hexagonal Boron Nitride Crystal Growth

Song Liu, Bin Liu, and J. H. Edgar

Department of Chemical Engineering, Kansas State University, Manhattan, Kansas 66506

Hexagonal boron nitride (hBN) is a layered, 2D material structurally analogous to graphite. Traditionally, it has been employed as a refractory ceramic, but recently new device applications of hBN have been envisioned, which will utilize its unique optical, phononic, nuclear, and electronic properties, with examples including deep ultraviolet light emitters (i.e., light emitting diodes, and laser diodes), neutron detectors, nanophotonics, and quantum computers. For these applications, precise control of the thickness, area, crystal perfection, crystallographic orientation, and purity of the hBN crystals is essential.

Here we discuss the growth of high purity, low defect density hBN crystals at atmospheric pressure by cooling a molten Ni-Cr flux saturated with boron and nitrogen. Specifically, through optimizing the process conditions including the dwell temperature (for molten solution preparation), and the cooling rate, single crystals up to 4 mm² in area and up to 500 µm thick were grown, some of the largest ever produced. 

Reactive molecular dynamics (rMD) simulations, employing a ReaxFF force field established by our previous work on CVD hBN growth on Ni surfaces, have been designed and carried out to understand the molecular structures of dissolved N and B species in molten Ni-Cr solution, and the formation of hBN upon cooling. Furthermore, in order to reveal the role of Cr species in the Ni-Cr solution, the synthesis was simulated by density functional theory (DFT). In particular, the energetics and kinetics of elemental B and N on Ni-Cr alloy suggest that Cr increases the N solubility in Ni, which is growth rate limiting.