It was shown recently that in the submonolayer regime the shape of islands
of exchanged adatoms can be changed substantially by temperature, depositio
n flux, or coverage in the surfactant-mediated heteroepitaxial growth syste
ms. The shape transition from fractal islands to compact ones can be induce
d by decreasing temperature or increasing flux. This shape transition is co
mpletely contrary to the classic diffusion-limited aggregate (DLA) theory,
but can be explained in the frame of a reaction-limited aggregate (RLA) mod
el, in which a stable island consists of the exchanged (or dead) adatoms on
ly, an adatom must overcome a large energy barrier to become the seed of a
stable island, or overcome another little smaller barrier to join an existi
ng island. We propose that the strain due to the mismatch in the heteroepit
axy plays the key role in the coverage-induced shape change. Our simulation
shows that the strain always makes the islands more compact. With the stra
in taken into account, there is indeed an island shape change to more compa
ct islands. Applied to the growth of Ge on the Si(1 1 1) substrate pre-cove
red by a monolayer of Pb, the coverage-induced shape change of Ge islands c
an be explained. (C) 2001 Elsevier Science Ltd. All rights reserved.