Carrier capture in ultrathin InAs/GaAs quantum wells

The carrier capture into ultrathin InAs layers embedded in a GaAs matrix has been investigated by time-resolved two-wavelength pump-probe phototransmission at 4.2 K. Using an InAs thickness of 1.2 monolayers, we observe switching 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 between lh and hh states exceeds the threshold for GaAs LO phonon emission. The combination of both characteristics provides strong evidence for a two-step capture process of the holes. First the holes are captured by the weakly confined lh state and then they cool down to the hh state. We calculated the transient bleaching of the excitonic absorption considering both phase-space filling and exciton screening. The calculations show in agreement with the measurements that the phototransmission transients directly reflect the population of the confined InAs states only at excitation densities below $3\times {10}^8{\mathrm{cm}}^{\mathrm{-}2}.$ At larger excitation densities, the phototransmission rise time becomes significantly smaller than the capture times whereas its decay time appears longer than the carrier lifetime.