Stark-ladder behavior of theXlevels in a type-II GaAs/AlAs superlattice measured using electroreflectance spectroscopy

We have performed 17-K electroreflectance (ER) on a type-II (GaAs${)}_{10}$/(AlAs${)}_8$ superlattice (SL) as a function of electric field. We have resolved transitions to X-point levels confined in the AlAs, even though they are too weak to resolve in low-intensity photocurrent. The extra sensitivity of ER derives from the large field-induced energy shift of the transitions. Both the X-level Stark-ladder transitions increasing and decreasing in energy (m=±1/2) are clearly observed. As expected these have linear field evolution, with gradients in excellent agreement with the SL half-period.