Luminescence and Raman measurements on a new type of superlattice consisting of $n$- and $p$-type doped GaAs layers grown by molecular-beam epitaxy confirm crucial predictions of theory. A strongly tunable energy gap is found in luminescence. Raman experiments provide the first observation of electronic subbands in purely space-charge-induced quantum wells. A combined analysis of the luminescence and Raman data yields excellent agreement with self-consistent subband calculations based only on the design parameters of the sample.

• Received 16 June 1981

DOI:https://doi.org/10.1103/PhysRevLett.47.864