Selective exciton formation in thin GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells

We demonstrate experimentally that the exciton luminescence rise times in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells oscillate as a function of laser excess energy. We interpret these results as the occurrence of a selective optical-phonon assisted exciton formation. Experiments on doped quantum wells confirm our exciton formation model.