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Strain engineering in Sn-rich group IV semiconductors is a key enabling factor to exploit the direct bandgap at mid-infrared wavelengths. Here, we investigate the effect of strain on the growth of GeSn alloys in a Ge/GeSn core/shell nanowire geometry by controlling the Ge core diameter and correlating the results with theoretical strain calculations. Incorporation of the Sn content in the 10-20 at. % range is achieved with Ge core diameters ranging from 50 nm to 100 nm. While the smaller cores lead to the formation of a regular and homogeneous GeSn shell, larger cores lead to the formation of multifaceted sidewalls and broadened segregation domains, inducing the nucleation of defects. This behavior is rationalized in terms of the different residual strain, as obtained by realistic finite element method simulations. The extended analysis of the strain relaxation as a function of core and shell sizes, in comparison with the conventional planar geometry, provides a deeper understanding of the role of strain in the epitaxy of metastable GeSn semiconductors.

We discuss the rather scattered predictions on the orthorhombic lattice parameters for the C49 TiSi2 phase, which are reported in the literature. New and accurate x-ray diffraction measurements and ab initio calculations are presented. Both agree in indicating that the density is higher than the one commonly assumed and it is close to the stable C54 phase. An unexpected tetragonalization of the orthorhombic cell has been observed and the theoretical analysis shows that this phenomenon could be originated by the massive presence of stacking faults. (C) 2001 American Institute of Physics

Through a tight-binding rigid-band approach we show that changes in the relative stability of the C54, C49, and C40 phases of TiSi2, with electrons per atom ratio, are produced by the corresponding differences in the electronic density of states at the Fermi level. In particular, by increasing this ratio the stable phase evolves from C49 to C54, and then to C40. Our microscopic model provides a straightforward interpretation of very recent experimental findings concerning the sizeable variations in the transition temperature between C49 and C54 TiSi2 in the presence of Al or Mo layers. (C) 1998 American Institute of Physics. [S0003-6951(98)03740-1].

We present a total-energy stability diagram of FeSi2 phases which explains the recent experimental findings of pseudomorphic epitaxy on Si(111), depending on film thickness and growth conditions. In particular, our tight binding calculations point out that the appearance of the fluorite structure (gamma) at very low coverages can only be attributed to a binding energy with the silicon substrate sizeably larger than tetragonal (alpha) and CsCl-defected phases. (C) 1995 American Institute of Physics.

The nucleation and the evolution of dislocations in SiGe/Si(001) films can be controlled and confined along stripes aligned along pits carved in the substrate, leaving micrometric coherent areas free of dislocations. In this work, we have addressed the stability of such metastable areas versus, film thickness, different Ge contents (xGe = 10%-30%) and larger pit-pattern periods, revealing the flexibility and effectiveness of this method even for coherent areas of about 64 μm2. The thermal stability of such configuration has been finally verified by post-growth annealing treatment, in order to simulate device processing. Finally, l Raman spectroscopy and X-ray nanodiffraction have been used to characterize the periodic strain variations across the pattern.

Taboada, A. G. et al., We report on the mask-less integration of GaAs crystals several microns in size on patterned Si substrates by metal organic vapor phase epitaxy. The lattice parameter mismatch is bridged by first growing 2-μm-tall intermediate Ge mesas on 8-μm-tall Si pillars by low-energy plasma enhanced chemical vapor deposition. We investigate the morphological evolution of the GaAs crystals towards full pyramids exhibiting energetically stable {111} facets with decreasing Si pillar size. The release of the strain induced by the mismatch of thermal expansion coefficients in the GaAs crystals has been studied by X-ray diffraction and photoluminescence measurements. The strain release mechanism is discussed within the framework of linear elasticity theory by Finite Element Method simulations, based on realistic geometries extracted from scanning electron microscopy images. © 2014 AIP Publishing LLC., Financial support by the Swiss Federal Program Nano-Tera through projects NEXRAY and COSMICMOS and Spanish MINECO and CAM through projects EPIC-NANOTICS and Q&C Light are gratefully acknowledged.

We present the calculation of the Raman spectra in backscattering configuration for Al(x)Ga(1-x)As (001) superlattices and modulated alloys, based on the bond-charge-model dynamics, the average t-matrix approximation for compositional disorder and the bond polarizability model for the Raman tensor. The features of Raman spectra are shown to reflect the Al concentration profile along the growth axis, hence providing a valuable tool for compositional characterization of superlattices.

We report the observation of large scale self-assembly of long horizontal nanowires into orthogonally oriented bundles, during in situ annealing of a few monolayers of Ge on Si(001). Results are interpreted in terms of a collective wave-propagation mechanism, previously suggested for interpreting ripple faceting on Ge/Si(1 1 10) surfaces. Quantitative agreement between experiments and theory is found. The onset of the mechanism, the number of wires in the bundles, and their total density can be controlled by carefully tuning the growth parameters.

We show that suitable pit-patterning of a Si(001) substrate can strongly influence the nucleation and the propagation of dislocations during epitaxial deposition of Si-rich Si1-xGex alloys, preferentially gettering misfit segments along pit rows. In particular, for a 250 nm layer deposited by molecular beam epitaxy at x(Ge) - 15%, extended film regions appear free of dislocations, by atomic force microscopy, as confirmed by transmission electron microscopy sampling. This result is quite general, as explained by dislocation dynamics simulations, which reveal the key role of the inhomogeneous distribution in stress produced by the pit-patterning.

The onset of plastic relaxation in SiGe islands grown on pit-patterned Si(001) substrates is investigated using elasticity theory solved by finite element methods. Larger critical island volumes with respect to the unpatterned case are predicted. A justification based on the qualitatively different stressors acting on the substrate in the presence of pits is provided. Experimental results in terms of critical SiGe-island volumes as a function of the Ge content are nicely reproduced by the model.