Long-wavelength nonequilibrium optical phonon dynamics in cubic and hexagonal semiconductors

We have investigated the relaxation of zone-center nonequilibrium optical phonons in bulk cubic and hexagonal semiconductors. Our theory is based on the application of Fermi's golden rule formula involving anharmonic interaction between phonons. Calculations of the lifetimes of the zone-center longitudinal optical (LO) and transverse optical (TO) phonons in cubic materials have been performed by modeling acoustic phonon modes within Debye's isotropic continuum scheme. Lifetimes of A₁(LO), E₁(TO), A₁(TO), E₂² and E₂¹ modes in wurtzite semiconductors have also been calculated by employing Debye's scheme within a semi-isotropic continuum model for acoustic modes. We have obtained a few trends in the lifetime results within a crystal phase (cubic or hexagonal), and between the two crystal phases. Our results support and explain available experimental data over a large temperature range, and make some predictions.