Picosecond band filling in highly excited In-Ga-As-P films

The dynamics of dense electron-hole plasmas in submicrometer-thick In-Ga-As-P films, subsequent to photoexcitation by a 0.5-ps optical pulse, are studied by measuring time-resolved changes in the absorbance of the films at an energy 1 eV above the band gap. Initial electron-hole pair densities are between 6.5×1018 and 1.0×1020 cm-3. Extensive band filling is observed as a partial bleaching of absorption at the probe wavelength. The bleaching scales in magnitude with the initial photoexcited carrier density, and is due to a plasma which has a temperature between 300 and 600 K, at a delay after photoexcitation of 0.5-1 ps. The bleaching decays into an induced absorption, due to instantaneous band-gap renormalization, in a time that decreases from 20 ps at 1.0×1020 carriers/cm3 to ∼5 ps at 6.5×1018 carriers/cm3. However, the initial decay time of the bleaching, 2.8±0.7 ps, is independent of the initial excitation density. The initial decay process could be due to either carrier cooling or plasma expansion. Radiative or Auger band-to-band recombination, or capture by surface or bulk traps, cannot explain the initial decay process of the bleaching.