High-resolution photoluminescence and reflection studies of GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As multi-quantum-well structures grown by molecular-beam epitaxy: Determination of microscopic structural quality of interfaces

High-resolution photoluminescence and reflection measurements on two sets of GaAs-${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As multi-quantum-well (MQW) structures grown by molecular-beam epitaxy (MBE), are reported. Both the heavy-hole and the light-hole excitonic spectra reveal considerable structure with very sharp lines, some as narrow as 0.05 meV. We believe that these are the sharpest lines ever reported for such systems. Samples from different parts of the same wafer for each MQW system exhibit very different excitonic structure. To explain these observations a theoretical model is proposed which is based on our studies of the formation of heterointerfaces by MBE and the theory of the effects of interface roughness on the excitonic linewidths.