Defect Structures of B 12 As 2 Epilayers Grown on c-plane and a-plane 6H-SiC Substrates
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Defect Structures of B12As2 Epilayers Grown on c-plane and a-plane 6H-SiC Substrates Hui Chen1, Guan Wang1, Michael Dudley1, Lihua Zhang2, Yimei Zhu2, Yi Zhang3, James H. Edgar3, and Martin Kuball4 1 Materials Science and Engineering, Stony Brook University, Stony Brook, NY, 11794 2 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY, 11973 3 Chemical Engineering, Kansas State University, Manhattan, KS, 66506 4 Physics, University of Bristol, Bristol, BS8 1TL, United Kingdom ABSTRACT B12As2 epitaxial layers grown on (0001) 6H-SiC and ( 1120 ) 6H-SiC substrates have been studied using scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and synchrotron white beam x-ray topography (SWBXT) and investigated with the aid of crystal structure visualization software. SWBXT showed that B12As2 adopted [111] growth orientation, parallel to [0001]SiC, on c-plane 6H-SiC and adopted [ 101 ] growth orientation, parallel to [ 1120 ]SiC, on a-plane 6H-SiC. However, SWBXT also revealed twins in both sets of the B12As2 films, consistent with the SEM observation of the surface morphology. Cross-sectional HRTEM also confirmed the presence of twins in both cases and also revealed the existence of an intermediate layer between the c-plane 6H-SiC and the B12As2 film, the latter being consistent with the result reported by Michael etc[1]. By correlating the HRTEM observations and crystal structure visualization, the atomic configurations across the twin boundaries in both samples as well as those in the intermediate layer in the c-plane sample are proposed. INTRODUCTION B12As2 is a member of icosahedral boride family and based on clusters of boron atoms and two-atom As-As chains. It has a wide bandgap of 3.47eV and is potentially useful for devices operating in high electron radiation environments [2-6]. A potential application of B12As2 is for the fabrication of beta cells capable of producing electrical energy by coupling a radioactive beta emitter to a semiconductor junction [7]. Recently chemical vapor deposition of B12As2 on 6H-SiC substrates has been developed to obtain high quality of thin films [7-9]. The in-plane lattice constants of 6H-SiC are closed to one half of those of B12As2 and thus can facilitate the epitaxial growth of B12As2 on (0001) 6H-SiC. However, there have been very few reports on the defect structures of B12As2 thin films grown on (0001) 6H-SiC and no reports on HRTEM images revealing the atomic configuration across the B12As2 twin boundaries. Also, so far there have been no publications reporting the epitaxial growth of B12As2 thin films on ( 1120 ) 6H-SiC. To better understand the defect structures and growth mechanisms of B12As2 thin films, we examined B12As2 epilayers grown on (0001) 6H-SiC and ( 1120 ) 6H-SiC, respectively. Our preliminary results indicate that twins are present in both sets of B12As2 epitaxial films. The B12As2 on (0001) 6H-SiC adopted the expected [111] orientation while that on ( 1120 ) 6H-SiC adopted [ 101 ]
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