  We have studied a 50/50\AA superlattice of GaAs/Al$_{0.21}$Ga$_{0.79}$As
composition, modulation-doped with Si, to produce $n=1.4\times 10^{12}$
cm$^{-2}$ electrons per superlattice period. The modulation-doping was tailored
to avoid the formation of Tamm states, and photoluminescence due to interband
transitions from extended superlattice states was detected. By studying the
effects of a quantizing magnetic field on the superlattice photoluminescence,
the miniband energy width, the reduced effective mass of the electron-hole
pair, and the band gap renormalization could be deduced.
