Growth of hBN Using Metallic Boron: Isotopically Enriched h 10 BN and h 11 BN
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Growth of hBN Using Metallic Boron: Isotopically Enriched h10BN and h11BN
T.B. Hoffman1, Y. Zhang1, J.H. Edgar1, and D.K. Gaskill2 1 Department of Chemical Engineering, Kansas State University, 1005 Durland Hall, Manhattan, KS 66506, U.S.A. 2 Electronics Science and Technology Division, United States Naval Research Laboratory, 4555 Overlook Ave., SW Washington, DC 20375, U.S.A. ABSTRACT Hexagonal boron nitride (hBN) crystals enriched in 10B and 11B isotopes were synthesized using a high temperature (1500° C) Ni-Cr-B reactive-precipitation growth under a N2 atmosphere. Two growth mechanisms were observed: conventional defect-facilitated bulk growth which produced crystals with a platelet-like habit with width and thickness of 20-30 Ɋm and 5 Ɋm, respectively, and vapor-liquid-solid interface growth of hBN whiskers with lengths and diameters as large as 70 Ɋm and 5 Ɋm, respectively. Similar growth mechanisms were seen for samples enriched in either isotope. Isotopic analysis via secondary-ion mass spectrometry showed boron concentrations of 84.4 at% and 93.0 at% for the majority isotopes in the 10B-rich and 11B-rich samples, respectively. Raman spectroscopy showed an increase in peak Raman shift for the 10B-rich sample, having two barely resolved peaks at 1393.5 and 1388.8 cm-1, and a decrease for the 11B-rich sample, having peak at 1359.5 cm-1 (FWHM of 9.4 cm-1), compared to that of natural hBN, with its peak at 1365.8 cm-1 (FWHM of 10.3 cm-1). Raman shift showed a linear trend with increasing 10B concentration allowing for a calibration curve to be developed to estimate 10B enrichment in hBN using non-destructive methods. INTRODUCTION The enrichment of boron isotopes in hBN is beneficial for two main reasons. First, the efficiency of hBN neutron detectors is likely to increase with 10B concentration, as this increases the neutron capture cross-section of the material. Second, theory predicts the room temperature thermal conductivity of hBN to increase by 30% for isotopically pure hBN, which could help dissipate heat in devices [1]. This study used Raman spectroscopy coupled with isotopic concentration data obtained from secondary-ion mass spectroscopy (SIMS) to experimentally measure the isotopic effects in hBN. The isotope-enriched hBN samples grown in this study were analyzed along with an hBN sample of natural abundance, grown using the previously studied Ni-Cr solvent flux method, to allow for comparison of hBN at three different isotopic concentrations [2]. In this study, the isotopically-enriched hBN single crystals were grown by a high temperature reactive-precipitation method using a Ni-Cr-B alloy as the boron source material and N2 gas as a nitrogen source. This growth method is based on previous solvent-growth studies on hBN, which have highlighted Ni-Cr as an excellent solvent for boron and nitrogen [2,3]. The motivation for using the Ni-Cr-B alloy for enriched hBN growth is due to the lack of commercially-available isotopically enriched hBN powder. High purity enriched metallic boron sources can easily b
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