Investigation of the properties of SiGe islands by selective wet chemical etching and scanning probe microscopy

Abstract

In this work self-assembled Ge islands grown on Si(001) are investigated by means of selective wet chemical etching and scanning probe microscopy.<br />By means of selective wet chemical etching and atomic force microscopy the compositional profile of free standing islands, which are grown by deposition of Ge on Si(001) at various temperatures and growth rates, is investigated. It is shown that for low growth temperatures (560-600oC) Si is mainly located at the periphery of the islands. At higher temperatures also the center of the islands has a higher Si content. The experiments furthermore demonstrate, that by increasing the islands growth rate their Si content is decreasing. By performing simple simulations the obtained experimental results can be reproduced and it is shown that the compositional profiles of the islands can be explained by means of a simple kinetic model, which is based just on surface diffusion processes.<br />While for as-grown islands there is a four-fold symmetry in their compositional profile, this is not any more true for annealed islands. The islands are not any more etched symmetrically; instead, one side is etched faster. This is the result of a lateral motion, taking place during post-growth annealing and which is driven by an energetically favorable intermixing between Si and Ge. Ge-rich material diffuses from one side of the island towards the other, where it gets intermixed with Si arriving from the substrate. During this lateral motion the islands become bigger and Si-richer.<br />Furthermore, by scanning tunneling microscopy the shrinking of islands, on as-grown as well as on post-growth annealed samples, is investigated. Islands shrink by following the inverse path observed during their growth, demonstrating thus that the island shape is thermodynamically determined.<br />By means of selective etching also Ge islands, which were capped by Si are investigated. The removal of the Si cap allows the investigation of both the morphology and the composition of the buried islands. It is shown that the morphology of the Si cap does not always correspond to that of the buried islands. Furthermore, it is shown that the compositional profile of the islands, which are capped at the temperature range 300-450oC is the same as that of the as-grown islands. For islands, which are overgrown at 580oC it is shown that although their morphology is changing drastically, the Ge-rich part does still exist. This indicates that at least up to 580oC no bulk diffusion is taking place.