Strain balanced quantum posts

Quantum posts are assembled by epitaxial growth of closely spaced quantum dot layers, modulating the composition of a semiconductor alloy, typically InGaAs. In contrast with most self-assembled nanostructures, the height of quantum posts can be controlled with nanometer precision, up to a maximum value limited by the accumulated stress due to the lattice mismatch. Here we present a strain compensation technique based on the controlled incorporation of phosphorous, which substantially increases the maximum attainable quantum post height. The luminescence from the resulting nanostructures presents giant linear polarization anisotropy.Comment: Submitted to Applied Physics Letters (7th March 2011). 4 pages, 4 figure

Effect of an in-situ thermal annealing on the structural properties of self-assembled GaSb/GaAs quantum dots

In this work, the effect of the application of a thermal annealing on the structural properties of GaSb/GaAs quantum dots (QDs)1 is analyzed by aberration corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)2 and electron energy loss spectroscopy (EELS)3. Our results show that the GaSb/GaAs QDs are more elongated after the annealing, and that the interfaces are less abrupt due to the Sb diffusion. We have also found a strong reduction in the misfit dislocation density with the annealing. The analysis by EELS of a threading dislocation has shown that the dislocation core is rich in Sb. In addition, the region of the GaAs substrate delimited by the threading dislocation is shown to be Sb-rich as well. An enhanced diffusion of Sb due to a mechanism assisted by the dislocation movement is discussed

Atomic-column scanning transmission electron microscopy analysis of misfit dislocations in GaSb/GaAs quantum dots

The structural quality of GaSb/GaAs quantum dots (QDs) has been analyzed at atomic scale by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. In particular, we have studied the misfit dislocations that appear because of the high lattice mismatch in the heterostructure. Our results have shown the formation of Lomer dislocations at the interface between the GaSb QDs and the GaAs substrate, but also at the interface with the GaAs capping layer, which is not a frequent observation. The analysis of these dislocations point to the existence of chains of dislocation loops around the QDs. The dislocation core of the observed defects has been characterized, showing that they are reconstructed Lomer dislocations, which have less distortion at the dislocation core in comparison to unreconstructed ones. Strain measurements using geometric phase analysis (GPA) show that these dislocations may not fully relax the strain due to the lattice mismatch in the GaSb QDs

Bio-inspired computation: where we stand and what's next

In recent years, the research community has witnessed an explosion of literature dealing with the adaptation of behavioral patterns and social phenomena observed in nature towards efficiently solving complex computational tasks. This trend has been especially dramatic in what relates to optimization problems, mainly due to the unprecedented complexity of problem instances, arising from a diverse spectrum of domains such as transportation, logistics, energy, climate, social networks, health and industry 4.0, among many others. Notwithstanding this upsurge of activity, research in this vibrant topic should be steered towards certain areas that, despite their eventual value and impact on the field of bio-inspired computation, still remain insufficiently explored to date. The main purpose of this paper is to outline the state of the art and to identify open challenges concerning the most relevant areas within bio-inspired optimization. An analysis and discussion are also carried out over the general trajectory followed in recent years by the community working in this field, thereby highlighting the need for reaching a consensus and joining forces towards achieving valuable insights into the understanding of this family of optimization techniques

Delta doping and positioning effects of type II GaSb quantum dots in GaAs solar cell

GaSb quantum dot (QD) solar cell structures were grown by molecular beam epitaxy on GaAs substrates. We investigate the reduction in open-circuit voltage and study the influence of the location of QD layers and their delta doping within the solar cell. Devices with 5 layers of delta-doped QDs placed in the intrinsic, n- and p-regions of a GaAs solar cell are experimentally investigated, and the deduced values of Jsc, Voc, fill factor, efficiency (η) are compared. A trade-off is needed to minimize the Voc degradation while maximizing the short circuit current density (Jsc) enhancement due to sub-bandgap absorption. The voltage recovery is attributed to the removal of the QDs from the high-field region which reduces SRH recombination. The devices with p- or n-doped QDs placed in the flat band potential (p- or n-region) show a recovery in Jsc and Voc compared to devices with delta-doped QDs placed in the depletion region. However, there is less photocurrent arising from the absorption of sub-band gap photons. Furthermore, the long wavelength photoresponse of the n-doped QDs placed in the n-region shows a slight improvement compared to the control cell. The approach of placing QDs in the n-region of the solar cell instead of the depletion region is a possible route towards increasing the conversion efficiency of QD solar cells

Comportamiento de conectores de cortante tipo tornillo de resistencia grado dos para un sistema de sección compuesta

En Colombia, desde hace muchos años, uno de los tipos de conectores de cortante más usados son los tornillos, pero en la actualidad no existe ninguna metodología de diseño que justifique su uso en elementos de sección com-puesta, y la normatividad vigente indica que hay que realizar ensayos de los elementos que se pretendan emplear como conectores. Junto con los habituales detalles para diseño a flexión, y cortante vertical, es necesario especificar el diseño para el cortante horizontal generado en la interfase de la sección compuesta, y más aún, ajustar el diseño para las condiciones locales de construcción colombiana. Con esta finalidad se desarrolló un estudio en el cual se analizan los diversos efectos producidos en las secciones compuestas cuando se emplean tornillos como conectores de cortante. En esta investigación se estudiaron 18 modelos de sección compuesta fabricadas con perfiles de alma llena y losas de concreto de 21 MPa, en las cuales se manejaron configuraciones de 1, 2 ó 3 conectores de cortan-te tipo tornillo con diámetros de 1/2”, 5/8” ó 3/4"; y para separaciones de 0.08 m, 0.12 m o 0.14 m, por cada modelo se ensayaron tres probetas ante solicitación de corte directo (push-out). De acuerdo con los resultados obte-nidos en laboratorio, se efectuó el análisis correspondiente, evaluando la incidencia del diámetro y la separación de los conectores, en el comportamiento de los modelos, para posteriormente plantear una formulación de su dise-ño en secciones compuestas. Paralelamente, se analiza un modelo de los ensayados por medio de un programa que emplea el método de los elementos finitos, con el cual se pretende revisar en detalle aspectos poco apreciables en los ensayos físicos, entre ellos la degradación del concreto en la interfaz de la sección.Screw shear connectors have been most commonly used in Colombia for many years; however, there is no current design methodology justifying its use in composite sections and prevailing rules insist that elements used as shear connectors must be tested. Along with the usual details of bending design and vertical shear, horizontal shear design on the composite section interface must be specified, even more so in adjusting such design to Colombian construc-tion. A study was thus undertaken analysing effects on composite sections when screws were used as shear connec-tors. This research studied 18 composite section models having two 21MPa concrete slabs which had different confi-gurations with one, two or three 1/2", 5/8" or 3/4" diameter shear connector type screws, and 0.08m, 0.12m or 0.14m separations. Three specimens were tested for each model by direct shear or push-out method. The corres-ponding analysis was done according to laboratory results, assessing the influence of diameter and connector sepa-ration on the model’s behaviour; screw design in composite sections was then formulated. A model of the tests was analysed using a finite element method based-programme which reviewed in detail those aspects which had little a-ppreciable effect on the physical tests, including concrete degradation in the interface section

Explainable Artificial Intelligence (XAI): Concepts, Taxonomies, Opportunities and Challenges toward Responsible AI

In the last few years, Artificial Intelligence (AI) has achieved a notable momentum that, if harnessed appropriately, may deliver the best of expectations over many application sectors across the field. For this to occur shortly in Machine Learning, the entire community stands in front of the barrier of explainability, an inherent problem of the latest techniques brought by sub-symbolism (e.g. ensembles or Deep Neural Networks) that were not present in the last hype of AI (namely, expert systems and rule based models). Paradigms underlying this problem fall within the so-called eXplainable AI (XAI) field, which is widely acknowledged as a crucial feature for the practical deployment of AI models. The overview presented in this article examines the existing literature and contributions already done in the field of XAI, including a prospect toward what is yet to be reached. For this purpose we summarize previous efforts made to define explainability in Machine Learning, establishing a novel definition of explainable Machine Learning that covers such prior conceptual propositions with a major focus on the audience for which the explainability is sought. Departing from this definition, we propose and discuss about a taxonomy of recent contributions related to the explainability of different Machine Learning models, including those aimed at explaining Deep Learning methods for which a second dedicated taxonomy is built and examined in detail. This critical literature analysis serves as the motivating background for a series of challenges faced by XAI, such as the interesting crossroads of data fusion and explainability. Our prospects lead toward the concept of Responsible Artificial Intelligence, namely, a methodology for the large-scale implementation of AI methods in real organizations with fairness, model explainability and accountability at its core. Our ultimate goal is to provide newcomers to the field of XAI with a thorough taxonomy that can serve as reference material in order to stimulate future research advances, but also to encourage experts and professionals from other disciplines to embrace the benefits of AI in their activity sectors, without any prior bias for its lack of interpretability

Structural characterization of InAlAsSb/InGaAs/InP heterostructures for solar cells

In this work, we have characterized by transmission electron microscopy techniques the structural properties of InAlAsSb/InGaAs/InP heterostructures, with target applications in high efficiency solar cells. Previous photoluminescence (PL)1 analysis suggested the existence of compositional fluctuations in the active layer of these heterostructures. 220 bright field (BF)2 diffraction contrast micrographs have revealed strong strain contrast in the InGaAs buffer layer, related to the existence of these compositional fluctuations. The effect of a decomposed buffer on the growth of the InAlAsSb layer has been analyzed through the simulation of the strain fields in the heterostructure using the finite elements method (FEM).3 These simulations have shown that the strain in the buffer layer due to the compositional fluctuations only affects the first few nm of the InAlAsSb layer. The analysis by aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM)4and electron energy loss spectroscopy (EELS)5of the composition of the InAlAsSb layer reveals that any compositional fluctuation is only observed as an average effect, rather than in the form of clustering or atomically sharp transitions. The limitations of these techniques for the detection of small 3D compositional fluctuations are discussed

Modified qHAADF method for atomic column-by-column compositional quantification of semiconductor heterostructures

The qHAADF method allows the quantification of the composition at atomic column resolution in semiconductor materials by comparing the HAADF-STEM intensities between a region of interest to a region of the material of known composition. However, the application of this qHAADF approach requires both regions to be differentiable and included in the same micrograph at close proximity. This limits the application of this approach to certain materials and magnifications where this requirement is fulfilled. In this work, we extend the qHAADF method to analyses where the reference region is imaged in a separate micrograph. The validity of this modified method is proved by comparison to the original qHAADF approach using HAADF-STEM simulated images of the semiconductor heterostructure InSb/InAs. Additionally, the methods are applied successfully to experimental images both of a simple InSb/InAs interface and of a complex InSb/GaSb heterostructure, justifying the significance of the modified method over the original method

Desarrollo de catalizadores tiorresistentes para el acoplamiento de tecnologías deNOX en motorización diesel

En las últimas décadas, se han desarrollado y perfeccionado diferentes estrategias y tecnologías deNOx de elevada eficacia en respuesta a las regulaciones cada vez más estrictas de las emisiones en el sector del automóvil. Dos tecnologías catalíticas ampliamente utilizadas para este propósito son el Almacenamiento y Reducción de NOx (NSR) y la Reducción Catalítica Selectiva (SCR) en motorización diésel. El acoplamiento entre una trampa de NOx seguido de un catalizador NH3-SCR conduce a una mejora de la selectividad a N2, ya que el NH3 no deseado producido en el catalizador NSR se almacena y reacciona en el sistema SCR consecutivo, se habla de sistemas híbridos. Debido a la presencia de soot en los gases de escape o de azufre en los combustibles, es necesaria la búsqueda de materiales resistentes a este tipo de especies estudiando cómo estos contaminantes afectan a su actividad catalítica, en este sentido, el estudio de la tiorresistencia resulta clave en la determinación de una adecuada formulación del sistema híbrido catalítico, atendiendo a las condiciones reales de operación. El objetivo del presente estudio ha sido el desarrollo y mejora de un sistema catalítico híbrido, NSR-SCR, en forma de monolito, a partir de una formulación base.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech