Characterization of self-assembled Ge islands on Si(100) by atomic force microscopy and transmission electron microscopy

We present an alternative starting point of the fabrication of nanostructures for electronic devices by using self-assembling structures. One way for the growth of self-assembling structures as quantum dot (QD) arrays is based on the formation of coherently strained Ge islands on Si and requires controlling of a defined island growth (Stranski–Krastanov). For this reason we carried out systematic quantitative investigations of the growth of Ge islands. Stacks of two layers of Ge islands with a Si spacer were grown on a Si buffer and characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). In the first series the Ge layer thickness was varied at a constant growth temperature and in the second series the growth temperature was varied for a constant Ge layer thickness. Many of the results described in this paper confirm the expected growth behavior of Ge islands, i.e. the Stranski–Krastanov growth mode and the increasing island density with decreasing growth temperature. However, two new aspects of the island growth were found. Small Ge islands are already formed from 2.15 monolayers (ML) of Ge at high growth temperatures. At lower growth temperatures (≤645°C, 6.2 ML coverage), few large islands with defects and a high density of small coherent islands are observed simultaneously. TEM studies of cross-sectional and plan-view samples reveal that the small islands are elongated along (100) directions. Reasons for the formation of the two kinds of islands are discussed.