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Strain relaxation of nonpolar GaN/Al0.6Ga0.4N multiple quantum wells grown in core–shell geometry by metal–organic vapor-phase epitaxy on GaN wires is investigated. Cracking along the a-direction i...

Heckel analysis is a widely used method for the characterisation of the compression behaviour of pharmaceutical samples during the preparation of solid dosage formulations. The present study introduces an optical version of the Heckel equation that is based on a combination of the conventional Heckel equation together with the linear relationship defined between the effective terahertz (THz) refractive index and the porosity of pharmaceutical tablets. The proposed optical Heckel equation allows us to, firstly, calculate the zero-porosity refractive index, and, secondly, predict the in-die development of the effective refractive index as a function of the compressive pressure during tablet compression. This was demonstrated for five batches of highly porous functionalised calcium carbonate (FCC) excipient compacts. The close match observed between the estimated in-die effective refractive index and the measured/out-of-die effective THz refractive index supports the validity of the proposed form of the equation. By comparing the measured and estimated in-die tablet properties, a clear change in the porosity and hence, the effective refractive index, due to post-compression elastic relaxation of the FCC compacts, has been observed. We have, therefore, proposed a THz-based compaction setup that will permit in-line monitoring of processes during tablet compression. We envisage that this new approach in tracking powder properties introduced in this preliminary study will lead to the onset of further extensive and detailed future studies.

Model description of patterns of atomic displacements in twisted bilayer systems has been proposed. The model is based on the consideration of several dislocation ensembles, employing a language that is widely used for grain boundaries and film/substrate systems. We show that three ensembles of parallel screw dislocations are sufficient both to describe the rotation of the layers as a whole, and for the vortex-like displacements resulting from elastic relaxation. The results give a clear explanation of the observed features of the structural state such as vortices, accompanied by alternating stacking., Comment: 6 pages, 5 figures

Mineral inclusions entrapped in other minerals may record the local stresses at the moment of their entrapment in the deep Earth. When rocks are exhumed to the surface of the Earth, residual stresses and strains may still be preserved in the inclusion. If measured and interpreted correctly through elastic geobarometry, they give us invaluable information on the pressures (P) and temperatures (T) of metamorphism. Current estimates of P and T of entrapment rely on simplified models that assumes that the inclusion is spherical and embedded in an infinite host, and that their elastic properties are isotropic. We report a new method for elastic geobarometry for anisotropic inclusions in quasi-isotropic hosts. The change of strain in the inclusion is modelled with the axial equations of state of the host and the inclusion. Their elastic interaction is accounted for by introducing a 4th rank tensor, the relaxation tensor, that can be evaluated numerically for any symmetry of the host and the inclusion and for any geometry of the system. This approach can be used to predict the residual strain/stress state developed in an inclusion after exhumation from known entrapment conditions, or to estimate the entrapment conditions from the residual strain measured in real inclusions. In general, anisotropic strain and stress states are developed in non-cubic mineral inclusions such as quartz and zircon, with deviatoric stresses typically limited to few kbars. For garnet hosts, the effect of the mutual crystallographic orientation between the host and the inclusion on the residual strain and stress is negligible when the inclusion is spherical and isolated. Assuming external hydrostatic conditions, our results suggest that the isotropic and the new anisotropic models give estimations of entrapment conditions within 2%.

International audience; In this paper, SiGe nano-heteroepitaxy on Si and SiGe nano-pillars was investigated in a 300 mm industrial reduced pressure-chemical vapour deposition tool. An integration scheme based on diblock copolymer patterning was used to fabricate nanometre-sized templates for the epitaxy of Si and SiGe nano-pillars. Results showed highly selective and uniform processes for the epitaxial growth of Si and SiGe nano-pillars. 200 nm thick SiGe layers were grown on Si and SiGe nano-pillars and characterised by atomic force microscopy, x-ray diffraction and transmission electron microscopy. Smooth SiGe surfaces and full strain relaxation were obtained in the 650 degrees C-700 degrees C range for 2D SiGe layers grown either on Si or SiGe nano-pillars.

We present a detailed experimental and theoretical analysis of the epitaxial stress relaxation process in micro-structured compositionally graded alloys. We focus on the pivotal SiGe/Si(001) system employing patterned Si substrates at the micrometre-size scale to address the distribution of threading and misfit dislocations within the heterostructures. SiGe alloys with linearly increasing Ge content were deposited by low energy plasma enhanced chemical vapour deposition resulting in isolated, tens of micrometre tall 3D crystals. We demonstrate that complete elastic relaxation is achieved by appropriate choice of the Ge compositional grading rate and Si pillar width. We investigate the nature and distribution of dislocations along the [001] growth direction in SiGe crystals by transmission electron microscopy, chemical defect etching and etch pit counting. We show that for 3 μm wide Si pillars and a Ge grading rate of 1.5% μm−1, only misfit dislocations are present while their fraction is reduced for higher Ge grading rates and larger structures due to dislocation interactions. The experimental results are interpreted with the help of theoretical calculations based on linear elasticity theory describing the competition between purely elastic and plastic stress relaxation with increasing crystal width and Ge compositional grading rate.

Dislocation networks are one of the most principle sources deteriorating the performances of devices based on lattice-mismatched heteroepitaxial systems. We demonstrate here a technique enabling fully coherent germanium (Ge) islands selectively grown on nanotip-patterned Si(001) substrates. The silicon (Si)-tip-patterned substrate, fabricated by complementary metal oxide semiconductor compatible nanotechnology, features ∼50-nm-wide Si areas emerging from a SiO2 matrix and arranged in an ordered lattice. Molecular beam epitaxy growths result in Ge nanoislands with high selectivity and having homogeneous shape and size. The ∼850 °C growth temperature required for ensuring selective growth has been shown to lead to the formation of Ge islands of high crystalline quality without extensive Si intermixing (with 91 atom % Ge). Nanotip-patterned wafers result in geometric, kinetic-diffusion-barrier intermixing hindrance, confining the major intermixing to the pedestal region of Ge islands, where kinetic diffusion barriers are, however, high. Theoretical calculations suggest that the thin Si/Ge layer at the interface plays, nevertheless, a significant role in realizing our fully coherent Ge nanoislands free from extended defects especially dislocations. Single-layer graphene/Ge/Si-tip Schottky junctions were fabricated, and thanks to the absence of extended defects in Ge islands, they demonstrate high-performance photodetection characteristics with responsivity of ∼45 mA W(-1) and an Ion/Ioff ratio of ∼10(3).

Biaxial tensile strain has been shown to greatly enhance the optoelectronic properties of epitaxial germanium (Ge) layers. As a result, tensile-Ge (and#949t-Ge) layers grown on larger lattice constant InGaAs or GeSn have attracted great research interest. However, no previous studies have investigated the plastic relaxation occurring in these and#949t-Ge layers. Here, we experimentally demonstrate that plastic relaxation occurs in nearly all and#949t-Ge epitaxial layers that are of practical interest for optoelectronic applications, even when layers may still exhibit strain-enhanced characteristics. We show arrays of misfit dislocations (MDs), which are mostly disassociated, form at the and#949t-Ge/InGaAs interface for and#949t-Ge layers as thin as 15 nm with less than 1% total mismatch. Wedge geometry of plain view transmission electron microscopy (PV-TEM) foils is utilized to carry out a depth dependent investigation MD spacing for a range of and#949t-Ge/InGaAs heterostructures. MD spacing measured by PV-TEM is correlated to and#949t-Ge layer relaxation measured by high-resolution x-ray diffraction. We confirm very low relaxation (< 10% relaxed) in and#949t-Ge layers does not imply they have been coherently grown. We demonstrate plastic relaxation in the and#949t-Ge layer is acutely sensitive to grown-in threading dislocations (TDs) in the template material, and that reducing TD density is critical for maximizing strain retention. Given that and#949t-Ge layer thicknesses of 150+ nm with greater than 1% tensile strain are desired for optoelectronic devices, this work suggests that MDs may inevitably be present at and#949t-Ge/InGaAs heterointerfaces in practical devices, and that the effect of MDs on optoelectronic performance must be better understood. Master of Science

6 páginas, 2 figuras, 1 tabla.-- Póster presentado al E-MRS 2001 Spring Meeting, Symposium M; Stress and Strain in Heteroepitaxy celebrado en Estrasburgo (Francia/2001)., We have followed, in situ and real-time, both the relaxation and morphological evolution along [1 1 0] direction during the growth of In0.2Ga0.8As/GaAs by molecular beam epitaxy (MBE) at low growth rates (0.2 and 0.5 monolayers per second, i.e. ML/s). The stress measurements were performed by optical monitorization of the strain-induced substrate curvature, and the morphology evolution was assessed by means of laser light scattering (LLS). The correlation of the real-time results obtained from both in situ techniques allowed us to detect the existence of a growth rate dependent initial elastic relaxation regime, which is associated with the development of a long-range ordered rippled-like morphology along [1 -^1 0] direction., This work was supported by Spanish "CICYT" under Project Nº MAT2000-1625. M.U. González thanks the Consejería de Educación y Cultura de la Comunidad de Madrid for financial support., Spanish "CICYT", Consejería de Educación y Cultura de la Comunidad de Madrid

Purpose: Amylodextrin, a starch-based controlled release excipient, spontaneously absorbs moisture during storage. The aim of this study was to investigate plasticisation of amylodextrin by moisture and its effect on compaction and tablet characteristics. Methods: The glass transition temperature (T-g) of amylodextrin powders with moisture fractions (T-g) 0.070

My Ph. D. work led to the development of numerical methods devoted to the study of elastic and plastic relaxation in SiGe/Si (001) heteroepitaxial nanostructures. The meth- ods were successfully applied to the study of self-assembled nanostructures arising from the heteroepitaxial growth of Ge (or SiGe alloys) on Si substrate (three dimensional islands and thin films). First of all, I studied the mechanisms allowing for elastic relaxation. Tuning the initial condition, it was possible to calculate with Finite Element Method the elas- tic field in SiGe islands. Then the variation of compositional profile, observed during the three dimensional growth, were determined by an ad hoc Monte Carlo algorithm coupled with FEM. My main contributions, however, were in the field of plastic relaxation and the study of dislocations in nanostructures. First I developed a novel approach to handle misfit dislocation in nanostructures in the FEM framework. The developed method allows for determining the elastic field of dislocations in small size object, where the extended defects deeply interact with free surfaces and interfaces. It was shown how my novel computational strategy can be applied to the study of the plastic relaxation onset (introduction of the first dislocations) in Ge/Si islands, finding an excellent agreement with experimental data. Fur- thermore we depicted how by proper patterning of the Si (001) substrate it is possible to predict dislocation confinement, as confirmed experimentally by AFM and TEM analysis, in prescribed areas of the heteroepitaxial system, opening a viable path for dislocation en- gineering. Then, I adapted the microMegas dislocation dynamics (DD) code to dislocation propagation in nanostructures. The DD code was originally designed to study the time evolution of extended defects in bulk system. I therefore introduced suitable modifications for treating free surfaces and for handling the propagation of threading arms (dislocation touching free surfaces) in the system. Simulations performed in overgrown islands, predict a dislocation pattern at the SiGe island extremely similar to the experimentally observed dislocation microstructure.

We demonstrate the possibility of growing SiGe islands on patterned Si(001) substrates with pits having a continuous variation of the sidewall inclination angle \ensuremath{\alpha} from \ensuremath{\alpha} \ensuremath{\sim} 4\ifmmode^\circ\else\textdegree\fi{} to \ensuremath{\alpha} \ensuremath{\sim} 54\ifmmode^\circ\else\textdegree\fi{}. Experiments show that the pit-inclination angle critically affects island positioning. While for shallow pits (pit-sidewall inclination angle \ensuremath{\alpha} \ensuremath{\sim}30\ifmmode^\circ\else\textdegree\fi{}) islands are observed solely within the pits, at higher angles islands grow outside the pits. In particular a progressive (complete at \ensuremath{\alpha} \ensuremath{\sim} 54\ifmmode^\circ\else\textdegree\fi{}) decoration of the pit rim by several islands is observed. We use elasticity theory to compare strain relaxation of a single island inside and outside the pit, as a function of \ensuremath{\alpha}. The theoretical results show that there exists an elastic driving force for island positioning inside shallow enough pits, which reaches its maximum at \ensuremath{\alpha} \ensuremath{\sim} 20\ifmmode^\circ\else\textdegree\fi{} and changes sign for \ensuremath{\alpha} g 40\ifmmode^\circ\else\textdegree\fi{}. At the same time, the calculations indicate a progressive relaxation of the wetting layer (WL) outside the pit with increasing \ensuremath{\alpha}. The consistency between numerical results and experimental observations gives a clear indication on the important role played by the elastic relaxation in this system.

A semianalytical iterative method for determining the three-dimensional concentration profile minimizing the elastic energy in alloyed heteroepitaxial islands is developed. The method is shown to converge after only a few iterations (solutions of the elastic problem). Exploiting the reduced computational effort, aspect-ratio-dependent and average-concentration-dependent Si/Ge intermixing in alloyed SiGe/Si(001) islands is systematically investigated, considering realistic shapes. Islands providing the better elastic relaxation are also the ones able to lower the elastic load through nonuniform alloying more significantly. Universal behavior in terms of relaxation vs average concentration is demonstrated. © 2009 The American Physical Society.

We report original finite element method simulations of the strain components at nanometric GeSi island on Si(0 0 1), for realistic shape, sizes and average composition, discussing the main mechanisms acting in the misfit strain relaxation. The tensile strain induced in a 30 nm Si capping layer and the one upon removing the island, after fixing the top part of the Si layer, is discussed in view of application as a field effect transistor channel, with high career mobility induced by the lattice deformation. The large shear components obtained for steeper island morphologies are predicted to be particularly performing, especially in comparison to one another strained-silicon configuration (totally top-down originated), recently developed by IBM corporation.

We compare elastic relaxation and Si-Ge distribution in epitaxial islands grown on both pit-patterned and flat Si(001) substrates. Anomalous x-ray diffraction yields that nucleation in the pits provides a higher relaxation. Using an innovative, model-free fitting procedure based on self-consistent solutions of the elastic problem, we provide compositional and elastic-energy maps. Islands grown on flat substrates exhibit stronger composition gradients and do not show a monotonic decrease of elastic energy with height. Both phenomena are explained using both thermodynamic and kinetic arguments.