Sequential tunneling in doped superlattices: Fingerprints of impurity bands and photon-assisted tunneling

We report a combined theoretical and experimental study of electrical transport in weakly-coupled doped superlattices. Our calculations exhibit negative differential conductivity at sufficiently high electric fields for all dopings. In low-doped samples the presence of impurity bands modifies the current-voltage characteristics substantially and we find two different current peaks whose relative height is changing with the electron temperature. These findings can explain the observation of different peaks in the current-voltage characteristics with and without external THz irradiation in low-doped samples. From our microscopic transport model we obtain quantitative agreement with the experimental current-voltage characteristics without using any fitting parameters. Both our experimental data and our theory show that absolute negative conductance persists over a wide range of frequencies of the free-electron laser source.