InAs/GaAs quantum dots (QDs) in vertically correlated structures were studied by photoluminescence (PL) and atomic force microscopy (AFM). Samples were grown by low-pressure metalorganic vapor phase epitaxy (LP MOVPE). InAs QDs on the GaAs surface as well as GaAs hillocks covering the underlying QDs are elongated in the [-110] direction. This elongation is caused by higher lateral growth rates of both materials in the [-110] direction on the [100]-oriented substrates. The elongation and curvature of GaAs hillocks is probably the cause of the change of elongation direction of QDs in the upper layers of vertically stacked structures. With greater number of layers, QDs start to be circular. This is potentially a useful tool for controlling QD properties in laser structures. (c) 2006 Elsevier B.V. All rights reserved.