Elastic Constants of Boron Arsenide and Boron Phosphide Crystals

We report experimental results for picosecond ultrasonics (PU) studies of boron arsenide and boron phosphide crystals with zinc blend structure. These materials have coefficients of thermal expansion similar to commonly used semiconductors like Si and GaAs, and the synthesis of BAs and BP single crystals with thermal conductivity as high as 1000 and 540 Wm^-1K^-1 respectively has made them into promising materials for thermal management applications. Using PU, we measure Brillouin scattering frequencies and calculate longitudinal and shear velocities along several directions of the BAs and BP single crystals. The crystal directions are determined using electron backscattering diffraction and associated Christoffel equations are used to solve for the elastic constants of the crystals. We report C₁₁, C₁₂, and C₄₄ values of 289.4, 113.5 and 159.0 GPa for BAs and 310, 87, and 160 GPa for BP, respectively. Furthermore, we also demonstrate the utility of PU in studying optical and acoustic attenuation in these crystals. The elastic constants and the attenuation measurements will help to predict thermal transport behavior in these materials.