Second nonequilibrium-phonon bottleneck for carrier cooling in highly excited polar semiconductors

Cooling of high-density electron-hole plasma excited in direct-gap polar semiconductors by a picosecond pulse of an extreme intensity is considered. Convenient models of carrier coupling with LO phonons via screened Fröhlich interaction, nonequilibrium LO-phonon creation by free-carrier intraband relaxation, and fermion recombination-induced heating were combined with a description of second-generation nonequilibrium phonons produced by anharmonic decay of the nonequilibrium LO phonons. An additional source of nonequilibrium LO phonons due to nonradiative capture by deep centers via multiphonon emission was also introduced. A numerical simulation for a CdS crystal was performed. Even at room temperature, the carrier cooling is shown to exhibit a noticeable slow component for plasma densities above ${10}^{19}{\mathrm{cm}}^{\mathrm{-}3}.$ At the late stage of the relaxation, the relevant time constant $\left(\sim 100\mathrm{ps}\right)$ is found to be comparable with the carrier lifetime, while the magnitude of the excess effective temperature is determined by nonequilibrium population of the second-generation phonons. The “width” of the second nonequilibrium-phonon bottleneck is demonstrated to be determined by peculiarities of the lattice vibration spectrum.