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

Quantitative evidence on the existence of a second nonequilibrium-phonon bo ttleneck for dense electron-hole plasma cooling in a highly excited polar s emiconductor is presented. The bottleneck is caused by recurrent fusion of nonequilibrium LO phonons from their decay products (daughter phonons). Car rier cooling was experimentally investigated in CdS, which offers a favorab le phonon dispersion. Crystallites of 50-nm radius were utilized to prevent stimulated recombination and diffusion of photoexcited electron-hole plasm a with a density around 1.5x10(19) cm(-3) A transient of carrier effective temperature, deduced from time-resolved luminescence spectra, exhibit a slo w-relaxation component with the time constant of 70 ps at room temperature. The transient was shown to be in quantitative consistence with the theoret ical model based on Boltzmann equations for two generations of nonequilibri um phonons and degenerate-carrier energy rate equation with the energy inco me due to recombination effects (nonradiative capture via multiphonon emiss ion, fermion self-heating, and band-gap renormalization) taken into account . The observed cooling rate agrees with two nonequilibrium-phonon bottlenec ks with the depopulation time constants deduced from the available Raman da ta (0.5 ps for LO phonons and 12.4 ps for daughter phonons). [S0163-1829(98 )08543-9].