Tunable absorption coefficient in GaAs doping superlattices

The optical-absorption coefficient $\ensuremath{\alpha}(\ensuremath{\omega})$ of GaAs "NIPI" crystals [a new type of semiconductor superlattice consisting of $n$- and $p$-doped layers, possibly separated by intrinsic ($i$) regions, in an otherwise homogeneous bulk] is measured at photon energies below the forbidden gap of the unmodulated material. We use the extremely strong photoconductive response of these NIPI crystals as a very sensitive method for the detection of very weak absorption signals in the tail far below the gap of bulk GaAs. Another peculiarity of NIPI crystals expected from theory is the tunability of $\ensuremath{\alpha}(\ensuremath{\omega})$ by variation of the energy gap, which is no longer a constant quantity for these systems. The experimental results on molecular-beam-epitaxy-grown GaAs NIPI crystals reported in this paper show that both the frequency dependence and the dependence on modulation of the band gap are in excellent agreement with theory.