Impact of point defect charge states on the thermal conductivity of AlN

Point defects, commonly introduced by growth procedures or intentionally by doping, can strongly suppress the thermal conductivity \ensuremath{\kappa} of AlN, a key property that affects the performance and lifetime of its application in electronics and optoelectronics. However, the influence of point defects on the \ensuremath{\kappa} of AlN has not been rigorously evaluated. Particularly, point defects often exist in varying charge states, posing more challenges in understanding the scattering behavior of phonons by point defects. Here, we have comparatively investigated the effect of neutral and charged point defects on the \ensuremath{\kappa} of wurtzite AlN using an ab initio Green's function approach combined with the linearized Peierls-Boltzmann transport equation. Specifically, we consider typical point defects including oxygen, carbon, and silicon substitutions as well as aluminum and nitrogen vacancies. We find that the neutral defects result in stronger phonon scattering and lower \ensuremath{\kappa} than the charged ones, and the charge states more strongly affect the strength of phonon scattering by substitutions than by vacancies. Particularly, compared with the charged ${\mathrm{C}}_{\mathrm{N}}^{1\ensuremath{-}}$, the neutral ${\mathrm{C}}_{\mathrm{N}}^{0}$ induces orders-of-magnitude-larger phonon scattering featured by a strong phonon resonance \ensuremath{\sim}4.5 THz, which originates from the remarkably large perturbations of interatomic force constants caused by the asymmetric lattice distortion of the nearest neighbors around the defect site. As a result, ${\mathrm{C}}_{\mathrm{N}}^{0}$ results in strongly suppressed \ensuremath{\kappa} comparable with the aluminum vacancy case. These results highlight the strong effect of the charge states of point defects on the \ensuremath{\kappa} and phonon scattering landscape in AlN, which deepens the understanding of phonon transport in defective materials and will be helpful for the manipulation of thermal properties through defect engineering in relevant applications.