Electron and hole capture in multiple-quantum-well structures

We have studied a series of modified GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple-quantum-well structures, where increased capture times are obtained owing to the engineering of the band configuration. Both electron and hole capture times have been measured as a function of well width. Hole capture times are fast (typically 10 ps) and depend weakly on the structure. Electron capture times are shown to vary between 2 and 120 ps for well widths covering the whole range between 30 and 100 Å. This variation and the short time observed at resonance, when the barrier and the well levels are separated by 1 LO-phonon energy, demonstrate the importance of quantum-mechanical electron capture processes.