Abstract

The ultrafast relaxation of carriers excited with moderate excess energies above the band edge of bulk GaAs is studied using degenerate pump-probe experiments with sub 100 fs optical pulses tunable in the near IR. Experiments are performed at room temperature both on bulk GaAs (Reflectivity) and on thin films (Transmission). The Ti-Sapphire laser is tuned over several wavelengths across its 80 nm bandwidth in order to differentiate between various scattering processes. Band-edge excited carriers are observed to thermalize completely with the lattice in about 2 ps, in good agreement with expected electron-phonon scattering times[1,2]. Hot carriers (E_ph-E_g>60 meV) show the usual behavior: an initial rise of reflectivity due to band-filling is observed during the pump pulse, energy relaxation results in a depletion of optically coupled states and reflectivity decay at later times. Whenever the average electron-hole excess energy is below lattice thermal energy, coupling of transientlyco/d carriers (Fig 1) with phonons is dominated by absorption processes, energy flow is reversed and goes from the lattice to the carriers[4]. Slow rise of reflectivity signal during thermalization is interpreted as a refilling of the optically coupled states by carriers which increase their average excess energy due to coupling with the lattice. A simple model based on quasi-thermalized Boltzman functions is here applied. Carrier induced bandgap shrinkage is as well kept into account and demonstrated to introduce a significant density dependence in the carrier average excess energy[3]. This is observed comparing excitations at different densities and fixed wavelength (Fig 2).

© 1996 IEEE