We have used photoluminescence spectroscopy to investigate the influence of interface roughness in GaAs/AlAs quantum wells on their optical properties over a wide continuous range of well thicknesses. In order to compare different correlation lengths of the in-plane disorder potential, the wells were fabricated with growth interruption at both, one, or neither of the interfaces. Growth-interruption increases the correlation length of the monolayer-island structure on the surface, which gives rise to a long-range interface roughness after overgrowth. The relation between the correlation lengths of the in-plane disorder potential and the exciton localization length determines the spectral shape of the exciton luminescence. When the correlation length of the in-plane disorder potential is larger than the exciton localization length, the excitonic spectrum splits up into discrete peaks, stemming from regions differing in effective thickness by an integral number of monolayers. The energies of monolayers peaks, taking into account the in-plane localization energy, are found to be reproducible in wafers grown under similar conditions. We conclude that atomically smooth growth islands are formed on both AlAs and GaAs surfaces after growth interruption. During overgrowth, surface segregation leads to the generation of an atomic-scale disorder in the first overgrown monolayers. This results in an additional in-plane disorder potential with a much shorter correlation length than the original surface. It also modifies the shape of the well potential in the growth direction, as we have modelled by growth simulations, blueshifting the excitonic transition energies with respect to a square-well model
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