Exciton localization and interface roughness in growth-interrupted GaAs/AlAs quantum wells

We have used photoluminescence spectroscopy to investigate the influence of interface roughness in GaAs/ AlAs quantum wells on their optical propertie s over a wide continuous range of well thicknesses. In order to compare dif ferent correlation lengths of the in-plane disorder potential, the wells we re fabricated with growth interruption at both, one, or neither of the inte rfaces. Growth-interruption increases the correlation length of the monolay er-island structure on the surface, which gives rise to a long-range interf ace roughness after overgrowth. The relation between the correlation length s of the in-plane disorder potential and the exciton localization length de termines the spectral shape of the exciton luminescence. When the correlati on length of the in-plane disorder potential is larger than the exciton loc alization length, the excitonic spectrum splits up into discrete peaks, ste mming from regions differing in effective thickness by an integral number o f monolayers. The energies of monolayers peaks, taking into account the in- plane localization energy, are found to be reproducible in wafers grown und er similar conditions. We conclude that atomically smooth growth islands ar e formed on both AlAs and GaAs surfaces after growth interruption. During o vergrowth, surface segregation leads to the generation of an atomic-scale d isorder in the first overgrown monolayers. This results in an additional in -plane disorder potential with a much shorter correlation length than the o riginal surface. It also modifies the shape of the well potential in the gr owth direction, as we have modelled by growth simulations, blueshifting the excitonic transition energies with respect to a square-well model.