Self-assembled nanostructures are grown by molecular beam epitaxy of InAs on AlGaInAs layers deposited on InP substrates and investigated by atomic force microscopy. It is found that for low InAs coverage above the critical thickness for two-dimensional growth, wire-like structures called dashes are formed, which take on the shape of quantum dots as the coverage is increased. The shape transformation is largely dependent on InAs coverage, and not on buffer layer composition or strain. Growth on vicinal surfaces shows that dot formation is favored when step edges run perpendicular to the direction of the dashes, but no change in the nanostructure occurs when the step edges are parallel to the dashes. Photoluminescence shows that the capped structures emit light at room temperature in the technologically important wavelength range from 1.5 to 2.1 mum. (C) 2003 Elsevier Science B.V. All-rights reserved.