1,721,041 research outputs found
Dechanneling by misfit dislocations in III-V semiconductor heterostructures
No full textThis work reports on the use of the dechanneling technique for the determination of the misfit dislocation distribution at the interface between mismatched III-V semiconductor heterostructures grown on (001) substrates. Planar channeling is employed because of the higher sensitivity with respect to the axial case. For low misfit heterostructures, dislocations are oriented along the [110] and [110BAR] and the depth distribution is nearly planar and close to the chemical interface. We show that, for this kind of structure, dechanneling measurements allow one to determine the dislocation densities along the two quoted directions. This is done by using a fitting procedure based on a numerical simulation program of the RBS-channeling spectra. As a result of this procedure, an estimation of the dislocation distribution thickness can also be obtained, besides the ratio of the energy loss under channeling conditions to that under non-channeling conditions. Experimental data concerning a set of InxGa1-xAs/GaAs single layer samples having different thicknesses and indium concentrations are presented and discussed in the framework of this model. The dechanneling probability due to the misfit dislocations is obtained from the RBS-channeling spectra at several beam energies for all the samples. In some cases we note two distinct dechanneling probabilities for the two {110} channeling planes perpendicular to the interface. In light of the dechanneling cross section properties, this fact reflects the difference between the dislocation densities in the two directions
Ion-beam analysis of mismatched epitaxial heterostructures
No full textThe structural characterization of mismatched epitaxial heterostructures requires at least the determination of the strain and of the dislocation density. In this paper it is shown that ion-channeling is a very suitable technique for measuring both of these properties. In particular an improved technique for measuring the absolute angular distance between any crystal direction is presented. It is shown that in the case of single layer heterostructures the achieved precision is comparable to or better than that of double crystal X-ray diffraction. Moreover by computing the lattice distortion caused by a misfit dislocation the dechanneling cross section and its dependence from the relative orientation of the channeling and the dislocation directions have been evaluated. The application of these techniques for studying the mechanisms of strain relaxation in the InxGa1-xAs/GaAs system is presented
Dechanneling cross-section for misfit dislocations
No full textThis work reports on the use of the dechanneling technique for the determination of the misfit dislocation distribution at the interface between mismatched III-V semiconductor heterostructures grown on (001) substrates. In this case the dislocations are oriented along the [110] and [110BAR] and the depth distribution is nearly planar and close to the chemical interface. The displacement field caused by one misfit dislocation is calculated in the frame of the linear elasticity theory. An analytical treatment of the dechanneling cross section dependence on both the dislocation line and Burgers vector orientation and on the beam direction in the channeling plane is performed within the Quere model. It is shown that (110) planar channeling is most sensitive to the dislocation orientation. Experimental data concerning a set of InxGa1-xAs/GaAs single layer samples are presented and analyzed. The two {110} planes perpendicular to the interface show distinct dechanneling probabilities. In the light of the dechanneling cross section properties this fact reflects the difference between the dislocation densities in the two directions. The obtained dislocation densities are in agreement with those derived indirectly from strain measurements. It is shown that the sensitivity of the dechanneling technique is of the order of 10(4) dislocation lines cm-1
Mechanisms of strain relaxation in III-V semiconductor heterostructures
Full text linkThe known models describing the breakdown of coherency between layer and substrate in mismatched heterostructures are based on the isotropic elastic continuum approximation. As a matter of fact an internal contribution to the misfit accommodation, that is a deviation from the so-called ''virtual crystal approximation'', is expected in ternary or more complex alloy structures. This effect is clearly seen in a set of InxGa1-xAs/GaAs low misfit samples in the presence of misfit dislocations. The complete structural characterisation including the elastic distortion field and the dislocation density and distribution has been performed by means of Rutherford backscattering based techniques and double crystal X-ray diffraction
Selective ion-channeling study of misfit dislocation grids in semiconductor heterostructures: theory and experiments
No full textPlanar dechanneling by networks of misfit dislocations was measured in a series of InxGa1-xAs/GaAs samples (001) grown by molecular-beam epitaxy. At the beginning of the strain-relaxation process the dechanneling probability exhibits different values for nominally equivalent (110) planes. At larger strain relaxation, the dechanneling probability saturates at a value around 1/2 as the beam-energy increases. In order to explain these results a new model for planar dechanneling by dislocations is proposed. This model is based on the harmonic approximation of the continuum potential but anharmonicity effects are taken into account. The perturbation to the harmonic oscillations caused by the lattice plane curvature around a dislocation is written in terms of a distortion function that depends only on the geometrical configuration of the channeling direction and of the dislocation line. This function is explicitly calculated for geometrical configurations relevant to the present samples allowing us then to solve the equation of motion. The results show that the dechanneling a probability saturates at a level sensibly lower than 100% due to the quasiplanar distribution of dislocations. Without any adjustable parameter, the comparison between computed and measured dechanneling probabilities supplies dislocation density values in excellent agreement with those measured by transmission electron microscopy and in good agreement with results deduced from previous strain-relaxation data
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Sub-micron photocurrent mapping of heterostructures by micro-probe optical-beam induced current
No full textElastic Distortion Field In Single Layer Heterostructures In the Presence of Misfit Dislocations
No full textVariations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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