Ultrafast relaxation and transfer of charge carriers in type-II heterodoping (n-i-p-i) superlattices.

Abstract

In all-optical experiments, transient nonlinear absorption changes due to band filling and the Franz-Keldysh effect are used to study the ultrafast dynamics of charge carriers in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As type-II-hetero-n-i-p-i structures containing a single 20-nm-wide GaAs quantum well in each Ultrafast relaxation and transfer of charge carriers in type-II heterodoping superlattices intrinsic region. Pump pulses of 0.6 ps duration inject electron-hole pairs exclusively in the quantum wells located in the region with high electric field. We observe carrier cooling and thermionic transfer of the thermalized carriers perpendicular to the layers. The measured transfer times between \ensuremath{\sim}50 and \ensuremath{\sim}500 ps are found to increase with excitation power. The transfer process is followed by lateral carrier diffusion and, finally, by interband recombination which proceeds on a time scale from 100 \ensuremath{\mu}s to several seconds depending on the density of photoexcited carriers. The experimental results are compared with model calculations on Thomas-Fermi approximation.