Solution Growth and Characterization of Icosahedral Boron Arsenide (B 12 As 2 )
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Solution Growth and Characterization of Icosahedral Boron Arsenide (B12As2) C.E. Whiteley1, Y. Zhang3, A. Mayo1, J.H. Edgar1, Y. Gong2, M. Kuball2, and M. Dudley3 1 Department of Chemical Engineering, Kansas State University, Manhattan, KS 66506 USA. 2 H.H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, UK. 3 Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY 11794, USA
ABSTRACT The crystallographic properties of bulk icosahedral boron arsenide (B12As2) crystals grown by precipitation from molten nickel solutions were characterized. Large crystals (5-8 mm) were produced by dissolving the boron in nickel at 1150°C for 48-72 hours, reacting with arsenic vapor, and slowly cooling to room temperature. The crystals varied in color from black and opaque to clear and transparent. Raman spectroscopy, x-ray topography (XRT), and defect selective etching revealed that the B12As2 single crystals were high quality with low dislocation densities. Furthermore, XRT results suggest that the major face of the plate-like crystals was (111) type, while (100), (010) and (001) type facets were also observed optically. The predominant defect in these crystals was edge character growth dislocations with a Burgers vector, and line direction. In short, XRT characterization shows that solution growth is a viable method for producing good quality B12As2 crystals.
I. INTRODUCTION Due to its unique chemical, physical, and electrical properties, icosahedral boron arsenide B12As2 is a potential candidate for radiation detection devices. The combination of the large capture cross-section for thermal neutrons (~3800 barns) of the 10B isotope with semiconducting properties makes efficient, compact, and robust neutron detectors possible. In addition, the relatively high hole mobility and the ability to self-heal from radiation damage are additional properties that make this semiconductor worthy of study [1,8]. B12As2 was previously deposited on several foreign substrates including silicon and silicon carbide (4H-SiC, 6H-SiC, 15R-SiC), but the mismatch of film-substrate properties created stress, crystalline defects and high residual impurity concentrations, which degrade the properties of the film [9-14]. High temperatures (>1300 °C) are generally necessary to produce good quality B12As2 by CVD, but at this temperature silicon substrates react with B12As2 films [2,12]. Similarly, B12As2 can react with SiC to form a boron carbide transition layer between the film and substrate at high temperatures [10-14]. These problems can be avoided in bulk crystals. The most extensive prior studies of the microstructure of B12As2 used synchrotron whitebeam x-ray topography (SWBXT) to characterize thin films deposited on m-plane 6H-SiC and 15R-SiC substrates [10-14]. Figure 1 depicts some common crystal planes in B12As2. This work revealed thin films that had multiple domains, including (1-21) B12As2, (2-12) B12As2, (353) B12As2 and their respective twins [10-14]. In B12As2 films grown on 3.5o off-axis and
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