Elimination of Degenerate Epitaxy in the Growth of High Quality B 12 As 2 Single Crystalline Epitaxial Films
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Elimination of Degenerate Epitaxy in the Growth of High Quality B12As2 Single Crystalline Epitaxial Films Yu Zhang1, Hui Chen1, Michael Dudley1, Yi Zhang2, J. H. Edgar2, Yinyan Gong3, Silvia Bakalova3, Martin Kuball3, Lihua Zhang4, Dong Su4, Yimei Zhu4 1
Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY, 11794-2275, U.S.A. 2 Department of Chemical Engineering, Kansas State University, Manhattan, KS 66506, U.S.A. 3 H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 4 Center for Functional Materials, Brookhaven National Laboratory, Upton, NY 119735000, U.S.A. ABSTRACT Elimination of degenerate epitaxy in the growth of icosahedral boron arsenide (B12As2, abbreviated as IBA) was achieved on m-plane 15R-SiC substrates and 4H-SiC substrates intentionally misoriented by 7 degrees from (0001) towards [1-100]. Synchrotron white beam x-ray topography (SWBXT) revealed that only single orientation IBA was present in the epitaxial layers demonstrating the absence of twin variants which dominantly constitute the effects of degenerate epitaxy. Additionally, low asterism in the IBA diffraction spots compared to those grown on other SiC substrates indicates a superior film quality. Cross-sectional high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) both confirmed the absence of twins in the IBA films and their high quality. The ease of nucleation on the ordered step structures present on these unique substrates overrides symmetry considerations that drive degenerate epitaxy and dominates the nucleation process of the IBA. INTRODUCTION As a member of icosahedra borides, IBA (with a wide band gap of 3.2eV at room temperature [1, 2]) possesses extraordinary resistance against radiation damage mediated via a “self-healing” mechanism which makes it attractive for applications in high radiation environments [3-8]. Such properties could potentially be exploited in developing high-power beta-voltaic cells which are capable of converting nuclear power into electrical energy [9, 10]. In addition, IBA 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 [11]. Lastly, IBA has also attracted considerable attention as a potential material for compact solid-state neutron detectors due to the neutron-absorbing ability of the 10B isotope [12, 13]. The distinctive properties of IBA are associated with its unusual crystal structure which is based on a three-fold symmetric structure with twelve-boron-atom icosahedra residing at the corners of an α-rhombohedral unit cell and As-As chains lying along the rhombohedral [111] axis. Each boron atom occupies a vertex of an icosahedron, and is stiffly bonded to five other B atoms as well as either an As atom or another icosahedron
[3, 7]. (Figure 1)
(a) (b) Figure 1. Atomic configurations of IBA unit cell from side view (a) and plan view (b).
The a
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