We have systematically studied the effects of incorporation of Al in the embedding layers of self-organized InAs quantum dots (QDs) grown on GaAs(0 0 1) by molecular-beam epitaxy on their density and structural and optical properties. When using an In0.1AlxGa0.9-xAs lower embedding layer (x = 0.001-0.1) we observe a large increase in QD density with increasing x. The increase is much larger than when increasing the In content in a conventional InxGa1-xAs (x = 0-0.2) lower embedding layer. We attribute the differences to different nucleation mechanisms. Furthermore, introducing a thin InAlAs upper embedding layer results in strong 1.3 mum emission with a large energy separation between the ground and the first excited state transitions. (C) 2002 Elsevier Science B.V. All rights reserved.