Growth of Bulk Hexagonal Boron Nitride from a Lithium Flux

The hexagonal polymorph of boron nitride (hBN) is a material of great interest for electronic and optoelectronic applications. There is a demand for large-area single crystals both as a bulk material and as a source of 2D monolayers for quantum and 2D devices. Recent work has produced millimeter-scale lateral dimensions but thicknesses only in the range of tens to lower hundreds of micrometers. The temperature (1400-1800 °C) and/or pressure conditions (2.5-2500 MPa) for crystal growth by existing methods provide significant limitations to upscaling and tend to produce very thin crystals. This study describes the growth of hBN crystals via two routes from a flux of lithium boron nitride in a novel parameter space: around 1200 °C with 0.1-1.5 MPa of nitrogen overpressure. Analysis of the hBN crystals provides evidence for optical transparency without coloration, phase purity, chemical purity, a narrow Raman peak width (8.2 cm^-1 for E _2g), and aspect ratios (thickness/width) of 0.1-0.25 for crystals with lateral widths in the c-plane of 0.5-1.5 mm, offering a path for the attainment of thicker crystals.