Experimental study of the Γ-Xelectron transfer in type-II (Al,Ga)As/AlAs superlattices and multiple-quantum-well structures

A detailed experimental study of the real-space Γ-X transfer in type-II GaAs/AlAs short-period superlattices and in type-II ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/AlAs multiple-quantum-well structures is presented. Transfer times on a subpicosecond and picosecond time scale are observed. The time constants critically depend on the thickness of the (Al,Ga)As layers, but not on the AlAs-layer thickness in the samples studied. The Γ-X transfer rate is determined by the spatial overlap of the Γ and X wave functions confined in the different layers. Intensity- and temperature-dependent measurements provide insight into the scattering mechanism. We conclude that electron–LO-phonon scattering is the dominant scattering process for samples with thick (Al,Ga)As layers (>100 Å). In contrast, interface scattering due to the interface mixing potential (Γ-${\mathit{X}}_{\mathit{z}}$ mixing) and/or due to potential fluctuations caused by interface roughness (Γ-${\mathit{X}}_{\mathit{x},}$y mixing) probably dominates for samples with thin (Al,Ga)As layers (<35 Å).