  We report on the study of the electrical current flowing in weakly coupled
superlattice (SL) structures under an applied electric field at very low
temperature, i.e. in the tunneling regime. This low temperature transport is
characterized by an extremely low tunneling probability between adjacent wells.
Experimentally, I(V) curves at low temperature display a striking feature, i.e
a plateau or null differential conductance. A theoretical model based on the
evaluation of scattering rates is developed in order to understand this
behaviour, exploring the different scattering mechanisms in AlGaAs alloys. The
dominant interaction in usual experimental conditions such as ours is found to
be the electron-ionized donors scattering. The existence of the plateau in the
I(V) characteristics is physically explained by a competition between the
electric field localization of the Wannier-Stark electron states in the weakly
coupled quantum wells and the electric field assisted tunneling between
adjacent wells. The influence of the doping concentration and profile as well
as the presence of impurities inside the barrier are discussed.
