Shape transition of InAs quantum dots on the GaAs(001)

InAs quantum dots (QD's) grown by molecular-beam epitaxy on GaAs(001) substrates were studied by atomically resolved in situ (without breaking a ultrahigh vacuum) scanning tunneling microscopy. A detailed sequence of shapes, appearing during the growth process, is evaluated: The smallest QD is terminated by {137}A bounding facets with a rhombic base. With increasing size, {135}B and {112}B facets appear in succession besides {137}A, making the QD more rounded. At a critical diameter of the base, the QD undergoes a shape transition from flat to steep by the formation of steeper {110} and {111} facets at the sides and gradually achieves a squared base. For all QD's the {137}A facets remain on top. Almost all shapes that have been observed for InAs QD's on GaAs(001) in earlier publications are covered