In this work we report a novel method for obtaining GaAs quantum dots (QDs) by molecular beam epitaxy (MBE) on an AlGaAs underlying film. We propose to use a Si monolayer (ML) grown on AlGaAs in order to induce a three-dimensional nucleation during the GaAs overgrowth. We observe that when 2.1 MLs of GaAs are grown on I ML of Si, the GaAs is self-assembled in islands. Islands of 15-Angstrom height and a lateral size of 200 Angstrom were clearly observed. 77 K photoluminescence and photoreflectance spectra of capped GaAs dots showed an additional signal at similar to 1.88 eV, presumably related to the islands emission, thereby verifying an efficient quantum confinement. (C) 2003 Elsevier Science B.V. All rights reserved.