Determination of the binding energy of excitons to neutral donors located at the center or edge of the well or at the center of the barrier in ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As/GaAs multiple-quantum-well structures

The binding energy of excitons to neutral donors ($D^0$,X) in GaAs-${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As quantum wells is determined by high-resolution resonant-excitation photoluminescence and temperature-dependent photoluminescence measurements. Changes in the binding energy of excitons are observed when donors are located in the center of the well, at the edge of the well in the interface region, or in the center of the barrier. The variations in these binding energies are investigated as a function of well size from 75 to 350 Å. The binding energies are found to increase as well size was reduced until about 100 Å, after which they decreased. An additional transition is observed which is tentatively assigned to excitons bound to ionized donors located at the center or edges of the well.