Carrier-carrier scattering:: an experimental comparison of 5 and 3 nm AlxGa1-xAs/GaAs quantum wells

We have studied carrier dynamics among highly nonequilibrium carrier distributions generated with femtosecond laser pulses in Be:AlxGa1-xAs/GaAs quantum wells with different well width at hot carrier densities between 10(9) and 10(11) cm(-2). The spectra, at moderately high densities (greater than or equal to 10(10) cm(-2)) indicate that the initially narrow electron distribution is altered in a time less than or equal to the LO-phonon emission time, as a result of rapid carrier-carrier scattering. At an excitation density of about 10(10) cm(-2), the carrier-carrier scattering rate is faster in narrower quantum wells. By measuring the intensity of the unrelaxed peak, it is possible to determine the relative rates of carrier-carrier scattering and LO-phonon emission. Our results indicate that carrier-carrier scattering becomes as significant as LO-phonon emission at a density of about 1 x 10(10) cm(-2) in the 5 nm quantum wells. (C) 2000 Elsevier Science Ltd. All rights reserved.