Franz-Keldysh oscillations and Wannier-Stark localization in GaAs/AlAs superlattices.

We have studied the electroabsorption properties of GaAs/AlAs superlattices with 1 monolayer (ML) and 2 ML wide AlAs barriers. At large electric fields F parallel to the growth direction, Wannier-Stark localization of the carrier states is observed, even for the case of 1 ML AlAs barriers. In the low-field regime (up to 60 kV/cm), Franz-Keldysh oscillations are found. These oscillations are associated with the miniband dispersion of the superlattice. The oscillation period shows an Fhigh2/3 behavior, as theoretically expected. Numerical calculations on electroabsorption in strongly coupled superlattices are also presented.