The impact of varying the thicknesses of InAlAs in the combined two-level InAlAs-InGaAs capping layer on the structure of InAs/GaAs quantum dots grown for advanced 1.3-mu m laser applications has been investigated. Scanning transmission electron microscopy and energy-loss filtered transmission electron microscopy have been employed to characterise the quantum dot morphology and to assess the nanoscale elemental distribution within the vicinity of quantum dots in order to determine the growth mechanism. An increase in the height of the quantum dots has been observed when the thickness of InAlAs capping layer is increased. In addition, there is evidence that the concentration of aluminium near the apex of the quantum dots is significantly reduced, with aluminium atoms migrating to the periphery of the quantum dots. The increased height of InAs quantum dots with increasing InAlAs capping layer thickness may be explained as a consequence of the suppression of the indium atom detachment rate from the InAs quantum dots during the capping process, resulting from the high indium adatom density on the surface of the capping layer. (c) 2005 Elsevier B.V. All rights reserved.