We investigate by atomic force microscopy the changes in the morphology of InAs self-assembled quantum dots (QDs) following thermal annealing at growth temperature and capping with thin GaAs layers. The thermal annealing results in coalescence of QDs into larger islands. Capping with a GaAs layer produces a significant change in the shape of the largest islands within the dot ensemble, which evolve from "humped-back" island structures for thin (1 nm) cap layers into "ring-shaped" islands for thicker (> 4 nm) cap layers. We discuss the morphological changes in terms of indium diffusion from the dots into the cap layer. Finally, we demonstrate resonant tunnelling of electrons into this type of QD structure. (C) 2002 Elsevier Science B.V. All rights reserved.