Origins of Twinned Microstructures in B 12 As 2 Epilayers Grown on (0001) 6H-SiC and Their Influence on Physical Propert
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1164-L09-10
Origins of Twinned Microstructures in B12As2 Epilayers Grown on (0001) 6H-SiC and Their Influence on Physical Properties Yu Zhang1, Hui Chen1, Ning Zhang1, Michael Dudley1, Yinyan Gong2，Martin Kuball2, Zhou Xu3, Yi Zhang3, James H. Edgar3, Lihua Zhang4, Yimei Zhu4 1
Department of Materials Science and Engineering, Stony Brook University, Stony Brook, NY 11794-2275 2 H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 3
Department of Chemical Engineering, Kansas State University, Manhattan, KS 4 Center for Functional Materials, Brookhaven National Laboratory, Upton, NY ABSTRACT The defect structure in B12As2 epitaxial layers grown at two different temperatures on (0001) 6H-SiC by chemical vapor deposition (CVD) was studied using synchrotron white beam x-ray topography (SWBXT) and high resolution transmission electron microscopy (HRTEM). The observed differences in microstructures were correlated with the differences in nucleation at the two growth temperatures. The effect of the difference in microstructure on macroscopic properties of the B12As2 was illustrated using the example of thermal conductivity which was measured using the 3-ω technique. The relationship between the measured thermal conductivity and observed microstructures is discussed. INTRODUCTION B12As2 is a member of the icosahedral boride family with a structure consisting of twelve boron atom icosahedra arranged at the corners of a rhombohedral unit cell with two-atom As-As chains along the body diagonal. It has a band gap of 3.47eV, and the material has the extraordinary ability to “self-heal” after radiation damage, making it potentially useful for devices operating in high electron radiation environments [1-6]. The absence of native substrates necessitates the heteroepitaxial growth of B12As2, typically on 6H-SiC substrates, often achieved using chemical vapor deposition [7]. Epitaxial growth on (0001) 6H-SiC is facilitated by the fact that the in-plane lattice constants of 6H-SiC are close to one half of those of B12As2. Gaining a detailed understanding of its microstructure is essential on the pathway to device demonstration. In this paper, we present studies of the influence of growth temperature on the microstructure of B12As2 thin films grown on (0001) 6H-SiC substrates. Implications of the microstructure on selected macroscopic physical properties are discussed. EXPERIMENT On axis, c-plane 6H-SiC wafers were used as substrates for the CVD growth of B12As2. The B12As2 films were synthesized by employing gaseous precursors of 1% B2H6 in H2 and 2% AsH3 in H2. The c-plane B12As2 was deposited at 1275°C and 1450°C, for samples, denoted here S1 and S2, respectively, with 500 Torr of reactor pressure. Non-destructive SWBXT was carried out
at the Stony Brook Topography station at the National Synchrotron Light Source, Brookhaven National Laboratory. Following this, cross-sectional TEM samples were made parallel to ( 1120 ) 6H-SiC. Conventional and high resolution TEM observation was performed using a JEOL 21
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