Femtosecond relaxation of carriers generated by near-band-gap optical excitation in compound semiconductors

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Abstract
A detailed examination of the physics underlying femtosecond relaxation of optically excited carriers in the near-band-gap regime of compound semiconductors with a special emphasis on band renormalization and Coulomb enhancement of the optical matrix elements is presented. This is done using a Monte Carlo formulation including Coulomb enhancement, band renormalization, and dynamic screening. The accuracy of the simulation has been verified through correlation with a series of experiments performed over a wide range of near-band-gap photon energies and pulse intensities. The results are found to differ greatly from those obtained for energy excitations far from the band edge. The observed carrier relaxation is found to be very insensitive to all relative scattering rates in contrast to excitation at high energies. Coulomb enhancement and band renormalization together are found to be important factors at both low and high excitation energies and should be important considerations in all efforts in the field. For a cold distribution, the effect of these processes is to accelerate the observed relaxation while for a hot distribution the opposite is found. These two processes are unimportant if the carriers are excited near thermal energies. The insensitivity of the simulation to relative contributions of the scattering processes, in combination with the strong distortions introduced by band renormalization and Coulomb enhancement makes the extraction of scattering-rate information difficult. Thus, near-band-gap relaxation should rather be considered a probe into band renormalization, Coulomb enhancement, and screening. The good correlation between measured and simulated data provides justification for extending the present semiclassical formulation to the computation of macroscopic physical observables dependent on near-band-gap femtosecond carrier relaxation.