Atomic force microscopy is used to investigate a series of arsenic-mediated growth experiments in the molecular beam epitaxy of Ge on Si(0 0 1). The experiments are performed by first depositing Ge at a coverage of 6 ML (where three-dimensional islands form), followed by additional growth of Ge with coevaporation of As. During the As mediated second growth stage no new islands appear as expected of the surfactant effect. However, the shapes of the Ge islands formed in the first stage are observed to change from pyramids and domes to mesas that have different base orientations and flat (0 0 1) tops of significantly increased areas. Because the island facets can be viewed as stacks of atomic steps, the island shape changes suggest As-induced step movements during the Ge growth, which are important to understand the mechanisms of surfactant suppression in the formation of three-dimensional islands. (C) 2002 Elsevier Science B.V. All rights reserved.