Peak thermal conductivity measurements of boron arsenide crystals

Recent experiments have validated prior theories of unusual high thermal conductivity $(\ensuremath{\kappa})$ in boron arsenide (BAs) and revealed large $\ensuremath{\kappa}$ variation associated with extended and point defects in the samples. The peak $\ensuremath{\kappa}$ provides valuable insights into the competition between intrinsic phonon-phonon scattering processes and extrinsic boundary and defect scattering processes in BAs. However, prior measurement methods have not been able to measure the peak $\ensuremath{\kappa}$ because of fundamental and technical limitations. Here, we report peak $\ensuremath{\kappa}$ measurements of BAs crystals synthesized under different conditions with source materials of different purities via a vapor transport method. For three representative samples, the measured $\ensuremath{\kappa}$ peak appears at temperatures between 120 and 150 K and varies from $410\ifmmode\pm\else\textpm\fi{}60\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ to $830\ifmmode\pm\else\textpm\fi{}100\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. The measured thermal conductivities agree with theoretical calculations across the full temperature range. The similar calculated and measured peak temperatures helps to narrow down the boundary scattering mean free path to be around 4 \ensuremath{\mu}m in two samples and 5 \ensuremath{\mu}m in another, while the variation of the peak magnitude reveals a one-order-of-magnitude difference in the strength of point defect scattering. The phonon-defect scattering behavior correlates well with the measured electronic Raman scattering background, the impurity concentrations revealed by secondary ion mass spectroscopy, and the Hall concentration and mobility of the $p$-type samples except for an anomalously high hole concentration that appears in one sample, which indicates nonuniform impurity distribution. The observed correlation clarifies the origins of extrinsic phonon scattering mechanisms in BAs crystals.