Evolution of the electronic states of coupled (In,Ga)As-GaAs quantum wells into superlattice minibands

We have used photoluminescence excitation (PLE) spectroscopy to study the evolution of the electronic states, and associated optical transitions, as the number of wells is increased from 2 to 5 to 20 in the strained (In,Ga)As-GaAs quantum-well system. The indium fraction in the wells was nominally 0.12. The PLE spectra of the two-well and five-well samples show strong Δn=0 exciton transitions and further features associated with transitions from confined states to continuum states. When a superlattice (SL) is formed, in the 20-well sample, we see momentum-conserving optical transitions at the mini-Brillouin-zone center and edge. We have also identified transitions associated with ${\mathit{M}}_1$ critical points in the SL band structure. One of these is a Δn=0 exciton resonance below the saddle point while the other is a Δn≠0 exciton which has become allowed due to momentum mixing of the light- and heavy-hole bands in the plane perpendicular to the growth direction.