Microscopic theory of impurity states in coupled quantum wells and superlattices

We present a theory of impurity states in quantum wells and superlattices which treats the confining heterostructure potential and the random impurity potential on the same footing. The relevant three-dimensional Hamiltonian is diagonalized in the low-doping regime. The results are used to calculate infrared absorption spectra which contain contributions of impurity and intersubband transitions. We mainly discuss the excited impurity states, which are pinned to higher subbands and are resonant states in the continuum. After a detailed analysis of a coupled quantum well system, we study the transition to a superlattice. In particular, we are able to explain existing experimental data on a quadruple quantum well.