Growth Mechanisms and Defect Structures of B12As2 Epilayers

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Zhang, Yu
The Graduate School, Stony Brook University: Stony Brook, NY.
As a member of icosahedra boride family of materials, icosahedral boron arsenide (B<sub>12</sub>As<sub>2</sub>), with a wide band gap of 3.2eV at room temperature, possesses extraordinary resistance against radiation damage mediated via a self-healing mechanism which makes it attractive for applications in high radiation environments. Such properties could potentially be exploited in developing high-power beta-voltaic cells which are capable of converting nuclear power into electrical energy. In addition, B<sub>12</sub>As<sub>2</sub> has exceptional mechanical properties, high melting point and large Seebeck coefficient at high temperatures which make it promising for the fabrication of high temperature thermoelectronics. The absence of native substrates necessitates the growth of B<sub>12</sub>As<sub>2</sub> via heteroepitaxy on non-native substrates with compatible structural parameters. To date, growth on Si substrates with (100), (110) and (111) orientation, (11-20) and (0001) 6H-SiC substrates has been attempted. However, degenerate epitaxy, manifested by the presence of high densities of twin boundaries (rotational variants), was observed in the epilayers in all of these cases and is expected to have a detrimental effect on device performance which has severely hindered progress of this material to date. B<sub>12</sub>As<sub>2</sub> epilayers grown on a variety of 4H- and 6H-SiC substrates were studied using synchrotron white beam X-ray topography, high resolution transmission electron microscopy, scanning transmission electron microscopy as well as other characterization techniques. High quality single crystalline B<sub>12</sub>As<sub>2</sub> epilayers and the elimination of degenerate epitaxy in the growth of B<sub>12</sub>As<sub>2</sub> were achieved on 4H-SiC substrates intentionally misoriented from (0001) towards [1-100] and the growth mechanisms were proposed. The influence of the defect structures in B<sub>12</sub>As<sub>2</sub> films on their physical properties was also investigated. The goals of the studies are to understand the growth mechanisms and defects structures present in B<sub>12</sub>As<sub>2</sub> films so as to develop strategies to reduce defect densities and obtain single crystalline epilayers and better film quality for future device fabrication.