Competition between intrinsic and extrinsic phonon scatterings in cubic BP and BAs with point defects

Abstract

Point defects that exist widely in solid materials can strongly suppress thermal conductivity \ensuremath{\kappa}, a key property that determines the performance of electronic devices, photovoltaics, thermoelectric materials, etc. The \ensuremath{\kappa} of materials with point defects is essentially determined by the competition between the intrinsic and extrinsic phonon scattering. In this work, we have employed an ab initio Green's function method combined with the Boltzmann transport equation to investigate the influence of typical point defects on the \ensuremath{\kappa} and phonon-scattering landscape in cubic BP and BAs, which have recently attracted intense interest due to their high \ensuremath{\kappa}. We show the magnitude and temperature dependence of \ensuremath{\kappa} can be strongly suppressed by point defects in both materials due to the weak intrinsic phonon-phonon scattering, even for extremely low defect fractions at low temperatures, e.g., 0.001% P vacancy reduces the \ensuremath{\kappa} of BP by 23% at 100 K. The competition between phonon-defect and phonon-phonon scattering depends on the type and fraction of point defects, as well as temperature. For both materials, vacancies result in stronger phonon scattering than substitutions. For the ${\mathrm{Si}}_{\mathrm{B}}$ substitution in BP, the mass difference (${V}^{M}$) results in several times smaller phonon-scattering rates than force-constant perturbation (${V}^{K}$) for most phonons, while the ${V}^{M}$ produces comparable or even stronger phonon scattering than ${V}^{K}$ within 3.5--9.4 THz for ${\mathrm{Si}}_{\mathrm{As}}$ in BAs mainly due to the large As-to-Si mass ratio. Also, the frequency dependence of phonon-defect scattering rates by vacancies and substitutions follows $\ensuremath{\sim}{\ensuremath{\omega}}^{4}$ in the low-frequency range for both materials, a typical behavior of Rayleigh scattering. Furthermore, the scattering strength of different phonon branches by point defects varies and depends on the defect type. These results provide deep insights into phonon scattering in materials with point defects and will be helpful for manipulating the thermal properties of materials by defect engineering for relevant applications.