Franz-Keldysh oscillations at the miniband edge in a GaAs/AlxGa1-xAs superlattice

We have systematically measured the electroreflectance spectra of a GaAs (7.0 nm)/Al0.1Ga0.9As (3.5 nm) superlattice at various electric fields to investigate Franz-Keldysh (FK) oscillations. In the low-field regime, we clearly observe the FK oscillations toward the low-energy side of the M-1 critical point (mini-Brillouin-zone edge). As the electric field is increased, the direction of the FK oscillations is reversed, then the oscillations disappear. The change of the oscillation direction correlates with the transformation of the electronic structures from the miniband to the Stark-ladder states in the Wannier-Stark localization. We discuss these experimental results on the basis of a theory of the FK oscillations and envelope-function forms calculated by a transfer matrix method with Airy functions. (C) 1997 Academic Press Limited.