Franz-Keldysh oscillations and Wannier-Stark localization in GaAs/AlAs superlattices with single-monolayer AlAs barriers

Using photocurrent spectroscopy, we have studied the absorption properties of GaAs/AlAs superlattices with only 1-monolayer (ML) and 2-ML wide AlAs barriers in an electric field F parallel to the growth direction. Wannier-Stark localization of the carrier states takes place at large electric fields, even for the case of 1-ML AlAs barriers. In the low-field regime, Franz-Keldysh oscillations are observed at fields up to 60 kV/cm. The oscillation period shows an ${\mathit{F}}^{2/3}$ behavior, as theoretically predicted, and is larger than expected from a one-electron model.