Origins of Twinned Microstructures in B₁₂As₂ Epilayers Grown on (0001) 6H-SiC and Their Influence on Physical Properties

Abstract

The defect structure in B 12 As 2 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 B 12 As 2 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. INTRODUCTIONB 12 As 2 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][2][3][4][5][6]. The absence of native substrates necessitates the heteroepitaxial growth of B 12 As 2 , 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 B 12 As 2 . 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 B 12 As 2 thin films grown on (0001) 6H-SiC substrates. Implications of the microstructure on selected macroscopic physical properties are discussed.