The local microstructure parameters of Ge were determined in Ge films on Si(001) produced by molecular beam epitaxy process. Due to the 4% larger lattice constant and interatomic distances of Ge, strain evolving during growth of Ge films on Si(001) causes a series of critical morphological and some local spatial structure changes.
Spatial parameters of Ge atoms surrounding were determined from GeK EXAFS data using EXCURV92 program. XAFS measurements were performed at the VEPP-3 storage ring at the Budker Institute of Nuclear Physics in Novosibirsk. XAFS spectra were measured in surface sensitive mode based on total electron yield detection. The samples positions were kept to provide angles 0 and 90ƒ between the normal of the Si(001) plane and the electric field vector to determine structural parameters anisotropy.
Two structures were formed in parallel on two part of Si(001) substrate. Both type of structure composed of three Ge/Si bilayers. The first type of the structure contains three pseudo-amorphous 4-monolayer Ge (2D) films, the second structure type contains in addition to the flat critical thickness 4-monolayer Ge films - pyramid-like (3D) islands formed in Stranski-Krastanov growth in order to reduce a high strain energy. This self-organized uniform Ge nanostructures have lateral sizes ~ 15nm and high ~ 1.5nm. The local microstructure parameters (interatomic distances, Ge coordination numbers and their anisotropy) were linked to nanostructures morphology and adequate models were suggested and discussed. In particular, it was established that pseudo-amorphous 4-monolayer Ge (2D) films, as it is called, consist of 50%(Ge-Si) solid solution, in contrast with pyramid-like (3D) pure Ge islands formed in Stranski-Krastanov growth.
Simon B. Erenburg
Institute of Inorganic Chemistry RAS
Lavrentiev ave. 3, Novosibursk 630090, Russia
Phone: 7-3832-333166, Fax: 7-3832-344489