Carrier capture in ultrathin InAs/GaAs quantum wells

The carrier capture into ultrathin InAs layers embedded in a GaAs matrix ha s been investigated by time-resolved two-wavelength pump-probe phototransmi ssion at 4.2 K. Using an InAs thickness of 1.2 monolayers, we observe switc hing of the carrier relaxation from optical to acoustic phonon emission. At the light-hole (lh) exciton transition we find a constant capture time of 20 ps. In contrast, the capture time decreases abruptly from 50 ps to 22 ps within the heavy-hole (hh) exciton transition as the energy separation bet ween Ih and hh states exceeds the threshold for GaAs LO phonon emission. Th e combination of both characteristics provides strong evidence for a two-st ep capture process of the holes. First the holes are captured by the weakly confined Ih state and then they cool down to the hh state. We calculated t he transient bleaching of the excitonic absorption considering both phase-s pace filling and exciton screening. The calculations show in agreement with the measurements that the phototransmission transients directly reflect th e population of the confined InAs states only at excitation densities below 3 x 10(8) cm(-2). At larger excitation densities, the phototransmission ri se time becomes significantly smaller than the capture times whereas its de cay time appears longer than the carrier lifetime.