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We show that for a mismatched heteroepitaxial layer with linear compositional grading, the first misfit dislocations will be introduced at a finite distance yC from the substrate interface. This is of practical as well as fundamental importance; it alters the value of the critical layer thickness for lattice relaxation and it moves the misfit dislocations away from the interface, where contaminants and defects may cause dislocation pinning or mobility reduction. We have calculated the position of the initial misfit dislocations yC for linearly graded Si1-xGex/Si(001) heteroepitaxial layers with lattice mismatch given by f=Cfy, where Cf is the grading coefficient and y is the distance from the interface. The distance of the first misfit dislocations from the interface yC decreases with increasing grading coefficient but can exceed 40 nm in layers with shallow grading (|Cf|<12 cm-1). For the range of grading coefficients investigated, yC varies from 6% to 11% of the critical layer thickness. Based on the model presented here it is possible to choose the grading coefficient to achieve the desired separation of the misfit dislocations from the substrate interface. [ABSTRACT FROM AUTHOR]

We have calculated the equilibrium strain and misfit dislocation density profiles for heteroepitaxial Si1-xGex/Si (001) with convex exponential grading of composition. A graded layer of this type exhibits two regions free from misfit dislocations, one near the interface of thickness y1 and another near the free surface of thickness h-yd, where h is the layer thickness. The intermediate region contains an exponentially tapered density of misfit dislocations. We report approximate analytical models for the strain and dislocation density profile in exponentially graded Si1-xGex/Si (001) which may be used to calculate the effective stress and rate of lattice relaxation. The results of this work are readily extended to other semiconductor material systems and may be applied to the design of exponentially graded buffer layers for metamorphic device structures including transistors and light emitting diodes. [ABSTRACT FROM AUTHOR]

We present a computational approach for the determination of the equilibrium misfit dislocation density and strain in a semiconductor heterostructure with an arbitrary compositional profile. We demonstrate that there is good agreement between our computed results and known analytical solutions for heterostructures containing a single linearly graded layer or a single uniform composition layer. We have calculated the dislocation density and strain profiles in Si1-xGex/Si(001), InxGa1-xAs/GaAs(001), and ZnSySe1-y/GaAs(001) heterostructures, each containing a uniform composition layer (uniform layer) on a linearly graded buffer layer (graded layer). The density of misfit dislocations in the graded layer is inversely proportional to its grading coefficient and is unchanged by the presence of the uniform layer, but the dislocated thickness increases with the uniform layer thickness. If the uniform layer is sufficiently thick, misfit dislocations will exist throughout the graded layer, but additional misfit dislocations are not produced in the uniform layer. The biaxial strain in the uniform layer is inversely proportional to its thickness and is unchanged by the graded layer. We have also calculated the equilibrium configuration in a convex, exponentially graded Si1-xGex/Si(001) layer, for which the misfit dislocation density is tapered with distance from the interface. Other nonlinear grading profiles offer opportunities to tailor the misfit dislocation density and strain profile. [ABSTRACT FROM AUTHOR]

The equilibrium strain and misfit dislocation density profiles for heteroepitaxial [Si.sub.1-x][Ge.sub.x]/Si(001) with convex exponential grading of composition. Appropriate analytical models are described for the strain and dislocation density profile in exponentially graded [Si.sub.1-x][Ge.sub.x]/Si(001) that is used for calculating the effective stress and rate of lattice relaxation.

We present design equations for error function (or “S-graded”) graded buffers for use in accommodating lattice mismatch of heteroepitaxial semiconductor devices. In an S-graded metamorphic buffer layer the composition and lattice mismatch profiles follow a normal cumulative distribution function. Minimum-energy calculations suggest that the S-graded profile may be beneficial for control of defect densities in lattice-mismatched devices because they have several characteristics which enhance the mobility and glide velocities of dislocations, thereby promoting long misfit segments with relatively few threading arms. First, there is a misfit-dislocation-free zone (MDFZ) adjacent to the interface, which avoids dislocation pinning defects associated with substrate defects. Second, there is another MDFZ near the surface, which reduces pinning interactions near the device layer which will be grown on top. Third, there is a large built-in strain in the top MDFZ, which enhances the glide of dislocations to sweep out threading arms. In this paper we present approximate design equations for the widths of the MDFZs, the built-in strain, and the peak misfit dislocation density for a general S-graded semiconductor with diamond or zincblende crystal structure and (001) orientation, and show that these design equations are in fair agreement with detailed numerical energy-minimization calculations for ZnS ySe 1−y/GaAs (001) heterostructures. [ABSTRACT FROM AUTHOR]

The design of lattice-mismatched semiconductor devices requires a predictive model for strains and threading dislocation densities. Previous work enabled modeling of uniform layers but not the threading dislocations in device structures with arbitrary compositional grading. In this work we present a kinetic model for lattice relaxation which includes misfit-threading dislocation interactions, which have not been considered in previous annihilation-coalescence models. Inclusion of these dislocation interactions makes the kinetic model applicable to compositionally graded structures, and we have applied it to ZnSe/GaAs (001) and ZnSSe/GaAs (001) heterostructures. The results of the kinetic model are consistent with the observed threading dislocation behavior in ZnSe/GaAs (001) uniform layers, and for graded ZnSSe/GaAs (001) heterostructures the kinetic model predicts that the threading dislocation density may be reduced by the inclusion of grading buffer layers employing compositional overshoot. This 'dislocation compensation' effect is consistent with our high-resolution x-ray diffraction experimental results for graded ZnSSe/GaAs (001) structures grown by photoassisted metalorganic vapor-phase epitaxy. [ABSTRACT FROM AUTHOR]

We report an experimental and modeling study of ZnS ySe1− y/GaAs (001) structures, all of which comprised a uniform top layer of ZnS0.014Se0.986 grown on a compositionally graded buffer layer or directly on the GaAs substrate. High-resolution x-ray diffraction was used to estimate dislocation densities on type A slip systems, with misfit dislocation (MD) line segments oriented along the $$ [1\bar{1}0] $$ direction, and type B slip systems, with MD line segments oriented along a [110] direction. A control sample having no graded buffer exhibits equal dislocation densities on the two types of slip systems ( DA ≈ DB ≈ 1.5 × 108 cm−2), but a forward-graded (FG) structure (grading coefficient of 27 cm−1) exhibits 20% more dislocations on the type B slip systems ( DA ≈ 1.6 × 108 cm−2 and DB ≈ 1.9 × 108 cm−2) and a steep forward-graded structure (grading coefficient of 54 cm−1) exhibits 50% more type B dislocations ( DA ≈ 2 × 108 cm−2 and DB ≈ 3 × 108 cm−2). The insertion of an overshoot interface reduced the dislocation densities in the uniform top layer by promoting annihilation and coalescence reactions, but type B dislocations were removed more effectively. Based on equilibrium calculations the overshoot graded layer in the steep graded overshoot structure is expected to exhibit large compressive and tensile strains, with a reversal in the sign of the strain near its middle, which may promote annihilation and coalescence reactions between threading dislocations. [ABSTRACT FROM AUTHOR]

Introduction: Endoscopic procedures are expanding and have been evolving in recent years, increasing their volume along with the development of new devices. This is due to the low morbidity and complication rate, the reversibility of the procedures, and the positive impact on patients. Among the endoscopic procedures gaining interest is sleeve gastroplasty. It emerged as a feasible and safe alternative to traditional bariatric surgery. There are no guidelines available on the indications and use of endoscopic gastroplasty. The aim of this study is to present preliminary results of a case series of endoscopic gastric plication procedures performed for different options: as a primary endoscopic sleeve, as revision for sleeve gastrectomy, and as revision for gastric bypass., Materials and Methods: A retrospective analysis was performed on a prospective database collecting data on all patients with obesity treated with endoscopic gastroplasty with the Overstitch™ (Apollo Endosurgery, Inc., Austin, Texas) device from 2022 to 2023 in the bariatric surgery unit., Results: Twenty-three patients were treated from May 2022 to July 2023 with endoscopic gastric plication. Ten patients (43%) were submitted to primary endoscopic sleeve gastroplasty, three patients to revision of sleeve gastrectomy, one patient to revision of one anastomosis gastric bypass, and eight patients received a revision of Roux-en-Y gastric bypass. The body mass index (BMI) of patients submitted to primary ESG ranged from 33 to 42kg/m2, with a mean BMI of 37kg/m2. The age of the patients ranged from 22 to 70 years, with a mean age of 45. In one case, we registered a Clavien-Dindo 2 complication-an inflammatory perigastric reaction without a collection occurred 15 days after a primary ESG., Conclusion: Endoscopic gastric plication is emerging as a safe, mini-invasive, and effective procedure alternative to sleeve gastrectomy in patients with I or II class obesity or for those unfit for bariatric surgery. The endoscopic suturing device can be used both as a primary procedure or as a revisional option after failure of the primary surgery, thus proving to be a versatile option to provide to bariatric patients.

In Italy, access to the degree course in veterinary medicine is regulated each academic year by a government decree that sets the maximum student intake number for each of the 14 existing faculties. Candidates are selected by means of a multiple-choice test on the following subjects: logic and general knowledge, biology, chemistry, physics, and mathematics. Data for the 2005/2006 academic year are presented here. Overall, 4,495 candidates took the test and 1,415 (31.5%) qualified. The questions relating to physics and mathematics were more difficult than those in other subjects (p < 0.001). Logic and general knowledge was the subject in which candidates' knowledge was best. Separate data for each of the 14 Italian faculties are presented, along with the cut-off score for admission. In addition, a "difficulty admission index" has been calculated for each faculty centre.