Coexistence of the Franz-Keldysh and Wannier-Stark effect in semiconductor superlattices

The optical absorption of short-period semiconductor superlattices with an electric field perpendicular to the layer plane is studied. The applied fields cover the range from the ''miniband regime'' (low fields), where Franz-Keldysh (FK) oscillations can be observed, up to the regime of Wannier-Stark (WS) transitions (high fields). Special emphasis is devoted to the investigation of the intermediate-field regime where both effects are present. We give a thorough theoretical treatment which is the basis for results published recently. It is shown that the crystal momentum representation can correctly describe the absorption behavior over the whole held range. For intermediate fields one obtains complicated but regular structures due to the coexistence of WS transitions and FK oscillations. Using double differential photocurrent spectroscopy, a technique of modulation spectroscopy, we have investigated a strongly coupled GaAs/AlAs superlattice. A symmetric fan of negative and positive WS transitions and a series of FK oscillations both near the lower and the upper combined miniband edge could be observed with unprecedented resolution. The theoretical results are confirmed excellently.