Excitation spectroscopy of electron-hole plasma in cadmium selenide single crystals: hot electrons and phonons

The effects of concentration and energy factors on broadening of the recombination emission band of a nonequilibrium electron-hole plasma (EHP) can be studied separately by using a tunable laser for excitation of the semiconductor crystal. A comparison of the luminescence spectra under resonance excitation (hν0∼Eg; Eg is the band gap) and on ''hot'' (hν0>Eg) excitation of a CdSe crystal leads to the conclusion that broadening cannot be attributed to multiparticle effects but rather to transfer of the excess photoexcitation energy in the EHP. In this case broadening of the shortwave side of the band is due to photo-heating of the EHP, whereas the long-wave broodening is due to deviation of the longitudinal-optical (LO) system from equilibrium. A theoretical spectrum of LO vibration amplification is obtained by solving the balance equations for the EHP energy and for the number of noninteracting LO phonons. The spectra are in good agreement with the temperature of the ''hottest'' LO phonon modes (400 K for an excitation density of 6x1024 quant x cm-2s-1, hν0-Eg=0.4 eV and for EHP and lattice temperatures of 450 and 295 K respectively)