Direct evaluation of composition profile, strain relaxation, and elastic energy of Ge:Si(001) self-assembled islands by anomalous x-ray scattering (original) (raw)
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X-ray analysis of strain, composition and elastic energy in Ge islands on Si(001)
International Journal of Nanotechnology, 2008
X-ray diffraction techniques have been extensively employed to study several structural and chemical properties of self-assembled islands. In this review we discuss recent results on Ge islands grown on Si(001). Grazing incidence diffraction is used to map the strain distribution of Ge pyramids and domes. By tuning the X-ray energy near the Ge K edge-to perform anomalous diffraction measurements-the chemical compositions of both types of islands were obtained. The data allow for direct evaluation of the elastic energy that is one of the crucial components for morphological evolution in this system. We then present a method to map out three-dimensional chemical and structural parameters in Ge domes. Finally, we discuss how such results can be combined to give new insights on growth mechanisms.
Strain and correlation of self-organized Ge 1-xMn x nanocolumns embedded in Ge (001)
Physical Review B - Condensed Matter and Materials Physics, 2010
We report on the structural properties of Ge1−xM nx layers grown by molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray scattering, atomic force and transmission electron microscopy to study the structural properties of the columns. We demonstrate how the elastic deformation of the matrix (as calculated using atomistic simulations) around the columns, as well as the average inter-column distance can account for the shape of the diffusion around Bragg peaks.
Physical Review Letters, 2003
Three-dimensional composition maps of nominally pure Ge domes grown on Si(001) at 600 C were obtained from grazing incidence anomalous x-ray scattering data at the Ge K edge. The data were analyzed in terms of a stack of layers with laterally varying concentration. The results demonstrated that the domes contained a Si-rich core covered by a Ge-rich shell and were independently supported by selective etch experiments. The composition profile resulted from substrate Si alloying into the Ge during growth to partially relax the stress in and under the domes.
2011
Plastic relaxation and formation of defects are crucial issues in the epitaxial growth of nanoparticles and thin films. Indeed, defects generate local stress in the crystalline lattice, which affects their surroundings and may lead to undesired effects such as reduced charge-carrier lifetime or nonradiative recombinations. Here, we use a nondestructive method based on x-ray diffuse scattering close to forbidden reflections to identify the defect types with a high sensitivity and quantify their average size and strain field. Combined with transmission electron microscopy, it offers opportunities to track both ensemble average and single defects inside three-dimensional structures. These techniques have been applied to partially embedded and high-Ge-content (x Ge = 0.87 ± 0.06) dots selectively grown in 20-nm-sized pits on Si(001) surfaces through openings in a SiO 2 template. The stress in the 20-nm-wide Ge islands is relaxed not only by interfacial dislocations but also by microtwins and/or stacking faults located at the interface, proving the importance of {111} planes and twinning in the relaxation process of nanometer-size Ge dots.
Fully coherent growth of Ge on free-standing Si(001) nanomesas
Physical Review B, 2014
We investigate the structural properties of Ge nanostructures selectively grown on Si. Defect-free nanostructures with a lateral size of 100 nm and surrounded by a thick (ß20 times larger than the coherent-film limit) Ge layer are achieved as demonstrated by transmission electron microscopy. As demonstrated by modeling based on elasticity theory solved by finite element methods, the peculiar combination of morphology and chemical composition of the nanostructures allows for a very efficient elastic relaxation of the heteroepitaxial strain. We demonstrate that, despite the relatively large size of the nanostructures, even a single dislocation would raise the energy of the system. A direct comparison between the strain field predicted by modeling and measured by energy-dispersive synchrotron-radiation grazing incidence x-ray diffraction shows substantial agreement.
Residual strain in Ge pyramids on Si(111) investigated by x-ray crystal truncation rod scattering
2000
Epitaxial growth of germanium on boron-terminated Si͑111͒ results in the formation of triangular pyramidal Ge islands which are partially relaxed. We show that the termination of the Si͑111͒ surface with 1/3 ML of boron is essential for the formation of faceted islands. We have investigated the residual strain in the Ge islands using x-ray crystal truncation rod scattering, and developed an analytical expression for the scattered intensity from islands with a nonuniform lattice parameter. We compare the measured intensity to x-ray scattering profiles calculated on the basis of different strain models. It is found that the Ge lateral lattice parameter changes linearly from the bottom to the top of the islands.
Philosophical Magazine, 2006
Germanium quantum dots were grown by ultra-high vacuum chemical vapour deposition on {100} silicon substrates. Two types of heterostructures were studied: single islands randomly grown on silicon as well as stacking islands grown in between silicon spacers. {110} cross-sections were prepared by mechanical polishing for high resolution electron microscopy observation (HREM) and X-ray nanoanalysis with a 200-kV transmission electron microscope equipped with a field emission gun. Two methods were considered for measuring the local displacements from the HREM images. The first consists of localizing the centre of the atomic columns with sub-pixel resolution by image processing mostly in real space. In the second method, the displacements were measured through the local Fourier components of the lattice fringes, by calculating two-phase images from two reciprocal vectors. Strains, stresses and germanium concentrations were then deduced from the displacements. Only calculations leading to...
Lattice bending in three-dimensional Ge microcrystals studied by X-ray nanodiffraction and modelling
Journal of Applied Crystallography, 2016
Extending the functionality of ubiquitous Si-based microelectronic devices often requires combining materials with different lattice parameters and thermal expansion coefficients. In this paper, scanning X-ray nanodiffraction is used to map the lattice bending produced by thermal strain relaxation in heteroepitaxial Ge microcrystals of various heights grown on high aspect ratio Si pillars. The local crystal lattice tilt and curvature are obtained from experimental three-dimensional reciprocal space maps and compared with diffraction patterns simulated by means of the finite element method. The simulations are in good agreement with the experimental data for various positions of the focused X-ray beam inside a Ge microcrystal. Both experiment and simulations reveal that the crystal lattice bending induced by thermal strain relaxation vanishes with increasing Ge crystal height.
One-dimensional Ge nanostructures on Si(001) and Si(1 1 10): Dominant role of surface energy
Ge/Si(001) is a prototypical system for investigating three-dimensional island self-assembly owed to the Stranski–Krastanow growth mode. More than twenty years of research have produced an impressive amount of results, together with various theoretical interpretations. It is commonly believed that lattice-mismatch strain relief is the major driving force leading to the formation of these islands. However, a set of recent results on Si(001) and vicinals point out that, under suitable conditions, this is not the case. Indeed, we here review experimental and theoretical results dealing with nanostructures mainly determined by surface-energy minimization. Results are intriguing, as they reveal the existence of magic sizes, show the presence of very peculiar morphologies, such as micron-long wires, and distinguish among attempts to facet the wetting-layer and true SK islands.