X-ray analysis of strain, composition and elastic energy in Ge islands on Si(001) (original) (raw)
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Physical Review B, 2002
The growth of strained epitaxial self assembled nanocrystals is comprised of a variety of kinetic and thermodynamic factors that determine their morphology and size. Some of the significant factors to their stability are strain and interdiffusion. Here we directly measure the gradient of composition and strain in Ge nanocrystals grown on Si͑001͒ using anomalous x-ray scattering. By combining our x-ray results, where we relate strain, interdiffusion, and shape with atomic force microscopy measurements, we have been able to determine the complete strain configuration of these islands. We show that the amount of elastic energy in pyramids and domes can be evaluated. The transition from pyramids to domes is accompanied by an increase of lattice parameter and enhancement of interdiffusion, both leading to a drastic decrease of the elastic energy stored per atom.
Nanotechnology
The effect of Ge deposition rate on the morphology and structural properties of self-assembled Ge/Si(001) islands was studied. Ge/Si(001) layers were grown by solid-source molecular-beam epitaxy at 500 °C. We adjusted the Ge coverage, 6 monolayers (ML), and varied the Ge growth rate by a factor of 100, R = 0.02-2 ML s(-1), to produce films consisting of hut-shaped Ge islands. The samples were characterized by scanning tunnelling microscopy, Raman spectroscopy, and Rutherford backscattering measurements. The mean lateral size of Ge nanoclusters decreases from 14.1 nm at R = 0.02 ML s(-1) to 9.8 nm at R = 2 ML s(-1). The normalized width of the size distribution shows non-monotonic behaviour as a function of R and has a minimum value of 19% at R = 2 ML s(-1). Ge nanoclusters fabricated at the highest deposition rate demonstrate the best structural quality and the highest Ge content (∼0.9).
Alloying in Ge(Si)∕Si(001) self-assembled islands during their growth and capping: XPS and AFM study
Physical Review B, 2008
In this paper, we present a study on the Ge composition and shape evolution of self-assembled Ge/ Si͑001͒ islands during the island growth and the subsequent Si capping at 750°C. By combining atomic force microscope images and x-ray photoemission spectroscopy data, we quantitatively determine the Ge distribution in the wetting layer and in the islands, separately. We found that in as-grown sample, the wetting layer is substantially Si-richer than the islands, its average composition being independent of the growth rate. Upon capping, the islands proceed to a reverse Stranski-Krastanov shape evolution, with a progressive Si enrichment of both the wetting layer and the islands. We demonstrate that this evolution occurs at constant island volume. The observed behavior indicates the suppression of the lateral diffusion of both Ge and Si atoms from the wetting layer to the surface of the enlarging islands, and vice versa.
Evolution of strained Ge islands grown on Si(111): a scanning probe microscopy study
Solid State Communications, 1999
We have followed the evolution of strained Ge/Si(111) Stranski-Krastanov islands by atomic force and scanning tunneling microscopies. Following the morphological evolution during the annealing of the samples we were able to recognize the key features of the relaxation process in these structures. The introduction of edge misfit dislocations after a critical thickness, and the inhomogeneous strain field inside the islands, lead to an intra-island ripening mechanism. We show that this mechanism changes the island shape from truncated tetrahedron to "atoll-like". ᭧
Strain and composition profiles of self-assembled Ge∕Si(001) islands
Journal of Applied Physics, 2005
Epitaxial growth of Ge/ Si͑001͒ in the Stranski-Krastanow regime results in the formation of island ensembles with various sizes and morphologies. During formation there is generally a strain-driven Si diffusion into the Ge islands. We investigate this issue in an epilayer grown by molecular-beam epitaxy containing pyramids, domes, and superdomes. A series of samples obtained by wet chemical etching of the original layer for different times in diluted hydrogen peroxide was evaluated by atomic force microscopy, spectroscopic ellipsometry, and Raman scattering. The average island composition as etching proceeds becomes Si richer, changing from about Si 0.2 Ge 0.8 to Si 0.35 Ge 0.65 , whereas the lattice strain increases, in particular, the material at the island summits is essentially relaxed. The composition of the wetting layer is nearly Si 0.45 Ge 0.55 . The results also reveal relatively Si-rich nuclei of a uniform size of Ӎ100 nm for all domes and superdomes, in accordance with a dislocation-induced growth mechanism of superdomes.
Reversible Shape Evolution of Ge Islands on Si(001)
Physical Review Letters, 2001
The evolution of strained Ge͞Si(001) islands during exposure to a Si flux was investigated by scanning tunneling microscopy. Dome islands display appreciable shape changes at Si coverages as low as 0.5 monolayer. With increasing coverage, they transform into ͕105͖-faceted pyramids, and eventually into stepped mounds with steps parallel to the ͗110͘ directions. These morphological changes are induced by alloying and represent a complete reversal of those previously observed during Ge island growth. The results are interpreted with a simple model in which the equilibrium shape of an island mainly depends on its volume and composition.
Role of Strain-Dependent Surface Energies in Ge/Si(100) Island Formation
Physical Review Letters, 2005
Formation energies for Ge=Si100 pyramidal islands are computed combining continuum calculations of strain energy with first-principles-computed strain-dependent surface energies. The strain dependence of surface energy is critically impacted by the presence of strain-induced changes in the Ge f100g surface reconstruction. The appreciable strain dependencies of rebonded-step f105g and dimer-vacancy-linereconstructed f100g surface energies are estimated to give rise to a significant reduction in the surface contribution to island formation energies.
Strain relief mechanisms in Ge/Si(100) islands
Materials Science and Engineering: B, 2003
The shape evolution and the effect of deposition temperature on the strain status of Ge/Si(100) islands have been investigated in the deposition temperature range 450 Á/750 8C. The island crystallographic structure was investigated by transmission electron microscopy. The analysis of the Moiré patterns reveals that the island lattice deformation and the elastic energy per unit volume stored in the islands decrease with increasing island size in quantitative agreement with theoretical results. Furthermore, we evidenced that the effective mismatch o between the silicon substrate and the island epilayer decreases upon increasing the deposition temperature. This misfit reduction is fully accounted by the amount of SiGe intermixing in the epilayer. #
Vertical and lateral ordering of Ge islands grown on Si(001): theory and experiments
Journal of Physics: Condensed Matter, 2007
A set of recent results concerning lateral and vertical ordering of Ge islands grown on Si(001) is reviewed. Experimental data generated by chemical vapour deposition and analysed by atomic force microscopy and photoelectron spectroscopy are compared with computer simulations and modelling based on atomistic approaches and continuum theory. In particular, we show that it is possible to probe experimentally the detailed strain field generated by buried Ge islands at the surface of the Si capping layer. The observed arrangement of small Ge islands grown over the capping layer is demonstrated to be very close to the one predicted by a simple model where the local chemical potential is inferred from the strain field at the atomic scale, as given by Tersoff-potential molecular dynamics simulations. Moreover, we review recent experimental evidence for lateral ordering, triggered by partial Si capping, in the first layer of Ge islands on Si(001). Theoretical support is given by showing that when two islands lie in close proximity the elastic field is likely to generate a flow of atoms leading to an effective gliding motion along opposite directions of both islands, eventually stopped by the presence of further neighbouring islands.