Epitaxy of layered compounds: GaSe on Si(111)

We investigated the interface between a three-dimensional substrate, Si, and a layered compound, GaSe, in the very first steps of the growth. Atomic models, related to deposits of 1.5 and 2.5 sheets, respectively, were derived from grazing incidence x-ray diffraction. They are based on the bulk phases, that is, on Ga₂Se₂ iono-covalent sheets, stacked with van der Waals bonding. Two orientations at 180^∘ from each other are found in nearly equal proportion. In addition, two different types of domains occur: one is strained on the substrate, the other one is strain relieved. The two types of domains are distinguished by a different treatment of their contributions to the diffracted intensity. The growth proceeds by complete sheets, preferably on the in-plane relaxed domains. As a consequence, for each sample, the final GaSe surface is expected to present steps, at least of one sheet height (7.95 {}).