Two types of extended states in random dimer barrier superlattices

We study numerically the effects of short-range correlated disorder on the electronic and transport properties of intentionally disordered GaAs-AlxGa1−xAs superlattices. We consider layers having identical thickness where the Al concentration x takes at random two different values with the constraint that one of them appears only in pairs, i.e. the random dimer barrier. Various physical quantities such as the conductance, the universal fluctuation conductance, the localization length, the resistance and its probability distribution are statistically computed by means of the transfer matrix formalism to discriminate the nature of the electronic states. In spite of the presence of disorder, the system exhibits two kinds of sets of propagating states lying below the barrier due to the characteristic structure of the superlattice. The states close to the resonance can be viewed as consisting of weakly localized states with very large localization length. In the band tails, i.e. for vanishing conductance, the states are strongly localized. The nature of the transition between these two regimes is quantitatively investigated through relevant physical quantities.