Dense electron-hole plasma cooling due to second nonequilibrium-phonon bottleneck in CdS crystallites

Abstract

Quantitative evidence on the existence of a second nonequilibrium-phonon bottleneck for dense electron-hole plasma cooling in a highly excited polar semiconductor is presented. The bottleneck is caused by recurrent fusion of nonequilibrium LO phonons from their decay products (daughter phonons). Carrier cooling was experimentally investigated in CdS, which offers a favorable phonon dispersion. Crystallites of 50-nm radius were utilized to prevent stimulated recombination and diffusion of photoexcited electron-hole plasma with a density around $1.5\ifmmode\times\else\texttimes\fi{}{10}^{19} {\mathrm{cm}}^{\ensuremath{-}3}.$ A transient of carrier effective temperature, deduced from time-resolved luminescence spectra, exhibit a slow-relaxation component with the time constant of 70 ps at room temperature. The transient was shown to be in quantitative consistence with the theoretical model based on Boltzmann equations for two generations of nonequilibrium phonons and degenerate-carrier energy rate equation with the energy income due to recombination effects (nonradiative capture via multiphonon emission, fermion self-heating, and band-gap renormalization) taken into account. The observed cooling rate agrees with two nonequilibrium-phonon bottlenecks with the depopulation time constants deduced from the available Raman data (0.5 ps for LO phonons and 12.4 ps for daughter phonons).