We identify the surface morphology of the buffer layer as key parameter for the formation of InAs quantum dots (QDs) or dashes (QDashes) by chemical beam epitaxy (CBE) on lattice-matched InGaAsP on InP (1 0 0) substrates. Growth conditions leading to the formation of QDashes are always accompanied by a rough buffer layer morphology. Although other growth parameters such as higher growth temperature, larger As flux, and compressive buffer layer strain favor the formation of QDs, once, the buffer layer has a rough morphology, QDashes are formed during InAs growth. On smooth buffer layers we always find well-shaped and symmetric QDs. Hence, we conclude that not the growth conditions during InAs deposition, but rather the related surface morphology of the buffer layer determines the formation of QDs or QDashes, which exhibit both high optical quality. (c) 2007 Elsevier B.V. All rights reserved.