Impact of Metal Solvent Composition on the Properties of hBN Crystals Grown from Molten Metal Solutions

Hexagonal boron nitride (hBN) is a wide energy bandgap (&gt;6.0 eV) semiconductor with a layered graphite-like structure and a high internal quantum yield for deep ultraviolet luminescence. These properties make it a good candidate deep UV light emitters and detectors, single photon emitters, and in nanophotonics by forming strong phonon polaritons. [1] While many researchers have focused on the heteroepitaxy deposition of hBN, there are many advantages to its bulk crystal growth via the solution growth method. Atmospheric pressure solution growth of hBN crystals using molten metals as solvents can produce material with crystal quality (Raman E_2g FWHM &lt; 8 cm^-1 [2]) that is as good or better than material produced by high temperature high pressure solution growth. Furthermore, it is a scalable process that is more capable at growing much larger crystals (up to 4 cm² [3]) than the high pressure process.<br/><br/>Here we report on the effects of the metal solvent composition on the crystal growth process and the quality of crystals produced. We analyzed how the process is affected using four different solvents: iron, iron-vanadium, nickel-chromium, and cobalt-chromium. These solvents represent a large range of nitrogen solubilities, with Fe and Fe-V having low solubility (0.04-0.1 mass% N) and Co-Cr and Ni-Cr having high solubility (0.15-0.3 mass% N). Crystal growth experiments were conducted with each of these solvents with varying compositions, the results of which were analyzed with optical microscopy, surface profilometry, and photoluminescence spectroscopy.<br/><br/>Based these results, the solvent appears to primarily affect three different characteristics: crystal thickness, the prevalence of certain defects, and the morphology of the bulk crystals. Crystals grown from Ni-Cr and Co-Cr solvents were hexagonal shaped with large steps and thicker (10-100 µm as-grown) than those grown from other solvents. In contrast, crystals grown from Fe and Fe-V were triangular-shaped with many wrinkles and thinner (5-30 µm as-grown). Comparisons between the photoluminescence spectra are also made, but the trends are less clear, though most spectra showed a strong peak at 5.75 eV, indicating high quality crystals can be grown with any of these solvents. To better understand these effects, the properties of the solvent such as nitrogen solubility are correlated with these observations and a model for their cause is proposed.<br/><br/>These results demonstrate that solvent choice is an important factor for optimizing atmospheric pressure solution growth of hBN and suggests a potential avenue to tailor crystals for specific applications. For example, in devices where a thicker crystal would improve efficiency, the crystals could be grown from a Ni-Cr solvent, while for devices that function better with thin crystals, they could be grown from an Fe solvent instead. Furthermore, these results provide insights into the mechanisms that guide hBN solution growth that can be used to optimize the process in general.<br/> <br/><b>References:</b><br/>[1] J.D. Caldwell, I. Aharonovich, G. Cassabois, J.H. Edgar, B. Gil, and D. Basov, Photonics applications of hexagonal boron nitride, Nature Rev. Mater. 4 552-567 (2019).<br/>[2] S. Liu, R. He, Z. Ye, X. Du, J. Y. Lin, H. X. Jiang, B. Liu and J. H. Edgar, "Large scale growth of high quality hexagonal boron nitride crystals at atmospheric pressure from an iron-chromium flux," <i>Crystal Growth & Design, </i>vol. 17, pp. 4932-4935, 2017.<br/>[3] J. Li, J. Wang, X. Zhang, C. Elias, G. Ye, D. Evans, G. Eda, J. M. Redwing, G. Cassabois, B. Gil, P. Valvin, R. He, B. Liu and J. H. Edgar, "Hexagonal boron nitride crystal growth from iron, a single component flux," <i>ACS Nano, </i>vol. 15, pp. 7032-7039, 2021.