Si layers were pseudomorphically grown by MBE on AlGaAs layers, the subsequent growth mode of GaAs was studied as a function of growth temperature, and of the Si layer thickness. At high temperatures (similar to 580degreesC), GaAs grows on Si/AlGaAs in a two-dimensional (2D) mode. The use of moderate temperatures (similar to 500degreesC) is effective to induce a change to a three-dimensional (3D) growth of GaAs. By employing a Si layer with a thickness of one monolayer, we succeeded to obtain GaAs self-assembled islands with an average lateral size of 20 nm and a height of 1.3 nm, these dimensions are appropriate for the synthesis of quantum dots. (C) 2002 Elsevier Science B.V. All rights reserved.