Strained growth in surfactant-mediated heteroepitaxy

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.