Higher-order lattice anharmonicity reshaping non-equilibrium carrier dynamics in wide-phonon-gap semiconductors

In semiconductors where three-phonon decay channels are suppressed, a mechanism termed "hot-phonon delocalization" induced by higher-order anharmonic decays emerges as the dominant process governing carrier thermalization. By combining ab initio solutions of the time-dependent coupled electron-phonon Boltzmann transport equations with femtosecond stimulated Raman spectroscopy, this study demonstrates that four-phonon couplings can substantially reshape the non-equilibrium carrier dynamics in semiconductors with significant phonon gaps, such as BAs and BSb. The results show that momentum-redistribution channels, specifically o+o→o+o and o+a→o+a, effectively delocalize hot phonons that initially accumulate in long-wavelength states by spreading them across the Brillouin zone. This behavior is fundamentally different from the conventional picture, where hot-phonon relaxation is assumed to be governed primarily by emission processes. This mechanism suppresses hot-phonon accumulation and enhances the efficiency of three-phonon Ridley- and Vallée-Bogani-type decays, mitigating the phonon bottleneck effect and ultimately improving Joule heating efficiency. In BAs, four-phonon coupling increases the energy dissipation rate of optical phonons nearly 70-fold, reducing phonon reabsorption and enabling high-energy electrons and holes to continuously emit optical phonons, as supported by our proposed phenomenological model. Consequently, the onset of phonon reabsorption heating is delayed from 6.0 to 12.9 ps. These findings provide a comprehensive understanding of carrier thermalization, revealing that hot-phonon delocalization governs energy-exchange pathways in wide-phonon-gap semiconductors on the picosecond timescale.