Pedro Galindo - Academia.edu (original) (raw)

Papers by Pedro Galindo

It is well known that there is intense interest in expanding the usable wavelength for electronic... more It is well known that there is intense interest in expanding the usable wavelength for electronic devices. Research efforts are dedicated to develop GaAs technology in order to achieve emission at 1.3 and 1.55 µm, so GaAs could be used for telecommunication applications [1]. Ga(As)Sb on InAs/GaAs quantum dots (QDs) is a promising nanostructure to be used in telecommunications. The introduction of antimony to these nanostructures is an effective solution to obtain redshift emission, even at room temperature [2].

Science and Engineering of Composite Materials, 2002

Journal of Physics: Conference Series, 2010

We review in this communication our contribution to the structural characterisation of semiconduc... more We review in this communication our contribution to the structural characterisation of semiconductor quantum dots and wires by high resolution electron microscopy, both in phase-contrast and Z-contrast modes. We show how these techniques contribute to predict the preferential sites of nucleation of these nanostructures, and also determine the compositional distribution in 1D and 0D nanostructures. The results presented here were produced in the framework of the European Network of Excellence entitled "Self-Assembled semiconductor Nanostructures for new Devices in photonics and Electronics (SANDiE)".

Ultramicroscopy, 2014

In this work we examined MoS₂ sheets by aberration-corrected scanning transmission electron micro... more In this work we examined MoS₂ sheets by aberration-corrected scanning transmission electron microscopy (STEM) at three different energies: 80, 120 and 200 kV. Structural damage of the MoS₂ sheets has been controlled at 80 kV according a theoretical calculation based on the inelastic scattering of the electrons involved in the interaction electron-matter. The threshold energy for the MoS₂ material has been found and experimentally verified in the microscope. At energies higher than the energy threshold we show surface and edge defects produced by the electron beam irradiation. Quantitative analysis at atomic level in the images obtained at 80 kV has been performed using the experimental images and via STEM simulations using SICSTEM software to determine the exact number of MoS2₂ layers.

Physical Review Letters, 2009

We present experimental evidence of surface induced disordering at magnetic FeCoPd nanoparticles ... more We present experimental evidence of surface induced disordering at magnetic FeCoPd nanoparticles during the L1(0)-A1 phase transition using high-resolution aberration-corrected electron microscopy and strain mapping. In situ electron diffraction studies show a narrow temperature range of fully ordered L1(0) structure. The order-disorder transition is size dependent and induces strong lattice deformation in outer part of the nanocrystals. The formation of unusually large strain of 20% is discussed in terms of core-shell structure formation with surface disordered layer and ordered core.

Microscopy and Microanalysis, 2006

A systematic distortion in high-angle annular dark-field scanning transmission electron microscop... more A systematic distortion in high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images, which may be caused by residual electrical interference, has been evaluated. Strain mapping, using the geometric phase methodology, has been applied to images acquired in an aberration-corrected STEM. This allows this distortion to be removed and so quantitative analysis of HAADF-STEM images was enabled. The distortion is quantified by applying this technique to structurally perfect and strain-free material. As an example, the correction is used to analyse an InAs/GaAs dot-in-quantum well heterostructure grown by molecular beam epitaxy. The result is a quantitative measure of internal strain on an atomic scale. The measured internal strain field of the heterostructure can be interpreted as being due to variations of indium concentration in the quantum dot.

EPL (Europhysics Letters), 2009

Composition, doping, size, and strain distribution within quantum dots, and at the dots-substrate... more Composition, doping, size, and strain distribution within quantum dots, and at the dots-substrate interfaces, determine the confinement potential of electrons and holes creating a complex band structure. We use ultra-high vacuum Kelvin probe force microscopy to obtain the two-dimensional confinement potential in and around InAs and InSb dots epitaxially grown on GaAs. It is found that the potential manifests rich

Microscopy and …, 2007

In this article a method for determining errors of the strain values when applying strain mapping... more In this article a method for determining errors of the strain values when applying strain mapping techniques has been devised. This methodology starts with the generation of a thickness/defocus series of simulated high-resolution transmission electron microscopy images of InAsxP1-x/InP heterostructures and the application of geometric phase. To obtain optimal defocusing conditions, a comparison of different defocus values is carried out by the calculation of the strain profile standard deviations among different specimen thicknesses. Finally, based on the analogy of real state strain to a step response, a characterization of strain mapping error near an interface is proposed.

… of nanoscience and …, 2008

We determine the compositional distribution with atomic column resolution in a horizontal nanowir... more We determine the compositional distribution with atomic column resolution in a horizontal nanowire from the analysis of aberration-corrected high resolution Z-contrast images. The strain field in a layer capping the analysed nanowire is determined by anisotropic elastic theory from the resulting compositional map. The reported method allows preferential nucleation sites for epitaxial nanowires to be predicted with high spatial resolution, as required for accurate tuning of desired optical properties. The application of this method has been exemplified in this work for stacked InAs(P) horizontal nanowires grown on InP separated by 3 nm thick InP layers, but we propose it as a general method to be applied to other stacked nano-objects.

Atmospheric Environment, 2006

A complete competitive scheme is proposed in this work in order to perform a classification analy... more A complete competitive scheme is proposed in this work in order to perform a classification analysis of meteorological data in the 'Campo de Gibraltar' region (in the South of Spain) from 1999 to 2002. The main objectives of the study presented here have been the characterization of the meteorological conditions in the area, using a competitive neural network based on Kohonen learning rule. Standard Principal Component Analysis (PCA) and VARIMAX rotation have allowed interpreting the physical meaning of the classes obtained from the competitive scheme. Quantitative (using three quality indices) and qualitative (from the analysis of the data projection) criteria based on Fisher Discriminant Analysis were introduced to verify the results of the clustering. A randomized procedure is developed to assure the best performance of the models and to select the best model in the experiments. The different experiments developed extracted five classes, which were related to typical meteorological conditions in the area. r

We show in this article that it is possible to obtain elemental compositional maps and profiles w... more We show in this article that it is possible to obtain elemental compositional maps and profiles with atomic-column resolution across an InxGa1-xAs multilayer structure from 5th-order aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. The compositional profiles obtained from the analysis of HAADF-STEM images describe accurately the distribution of In in the studied multilayer in good agreement with Muraki's segregation model [Muraki, K., Fukatsu, S., Shiraki, Y. & Ito, R. (1992). Surface segregation of In atoms during molecular beam epitaxy and its influence on the energy levels in InGaAs/GaAs quantums wells. Appl Phys Lett 61, 557-559].

The 'Campo de Gibraltar' region is a very industrialized area where very few air pollution studie... more The 'Campo de Gibraltar' region is a very industrialized area where very few air pollution studies have been carried out. Up to date, no model has been developed in order to predict air pollutant levels in the different towns spread in the region. Carbon monoxide (CO), Sulphur dioxide (SO 2 ) and suspended particulate matter (SPM) series have been investigated (years 1999-2000-2001). Multilayer perceptron models (MLPs) with backpropagation learning rule have been used. A resampling strategy with two-fold crossvalidation allowed the statistical comparison of the different models considered in this study. Artificial neural networks (ANN) models were compared with Persistence and ARIMA models and also with models based on standard Multiple Linear Regression (MLR) over test sets with data that had not been used in the training stage. The models based on ANNs showed better capability of generalization than those based on MLR. The designed procedure of random resampling permits an adequate and robust multiple comparison of the tested models. Principal component analysis (PCA) is used to reduce the dimensionality of data and to transform exogenous variables into significant and independent components. Short-term predictions were better than medium-term predictions in the case of CO and SO 2 series. Conversely, medium-term predictions were better in the case of SPM concentrations. The predictions are significantly promising (e.g., d SPM 24-ahead =0.906, d CO 1-ahead =0.891, d SO2 1-ahead =0.851).

The distribution of Bi atoms in epitaxial GaAs ͑1−x͒ Bi x is analyzed through aberration-correcte... more The distribution of Bi atoms in epitaxial GaAs ͑1−x͒ Bi x is analyzed through aberration-corrected Z-contrast images. The relation between the atomic number and the intensity of the images allows quantifying the distribution of Bi atoms in this material. A bidimensional map of Bi atoms is extracted showing areas where nanoclustering is possible and evidencing the location of Bi at As-substitutional positions in the lattice. The distribution of Bi atoms differs from a random spatial pattern of Bi atoms in the material.

Applied Surface …, 2010

The compositional distribution of InAs quantum dots grown by molecular beam epitaxy on GaAs cappe... more The compositional distribution of InAs quantum dots grown by molecular beam epitaxy on GaAs capped InAs quantum dots has been studied in this work. Upper quantum dots are nucleated preferentially on top of the quantum dots underneath, which have been nucleated by droplet epitaxy. The growth process of these nanostructures, which are usually called as quantum dots molecules', has been explained. In order to understand this growth process, the analysis of the strain has been carried out from a 3D model of the nanostructure built from Transmission Electron Microscopy images sensitive to the composition.

Ultramicroscopy, 2007

Strain mapping is defined as a numerical image-processing technique that measures the local shift... more Strain mapping is defined as a numerical image-processing technique that measures the local shifts of image details around a crystal defect with respect to the ideal, defect-free, positions in the bulk. Algorithms to map elastic strains from high-resolution transmission electron microscopy (HRTEM) images may be classified into two categories: those based on the detection of peaks of intensity in real space and the Geometric Phase approach, calculated in Fourier space. In this paper, we discuss both categories and propose an alternative real space algorithm (Peak Pairs) based on the detection of pairs of intensity maxima in an affine transformed space dependent on the reference area. In spite of the fact that it is a real space approach, the Peak Pairs algorithm exhibits good behaviour at heavily distorted defect cores, e.g. interfaces and dislocations. Quantitative results are reported from experiments to determine local strain in different types of semiconductor heterostructures.

It is well known that there is intense interest in expanding the usable wavelength for electronic... more It is well known that there is intense interest in expanding the usable wavelength for electronic devices. Research efforts are dedicated to develop GaAs technology in order to achieve emission at 1.3 and 1.55 µm, so GaAs could be used for telecommunication applications [1]. Ga(As)Sb on InAs/GaAs quantum dots (QDs) is a promising nanostructure to be used in telecommunications. The introduction of antimony to these nanostructures is an effective solution to obtain redshift emission, even at room temperature [2].

Science and Engineering of Composite Materials, 2002

Journal of Physics: Conference Series, 2010

We review in this communication our contribution to the structural characterisation of semiconduc... more We review in this communication our contribution to the structural characterisation of semiconductor quantum dots and wires by high resolution electron microscopy, both in phase-contrast and Z-contrast modes. We show how these techniques contribute to predict the preferential sites of nucleation of these nanostructures, and also determine the compositional distribution in 1D and 0D nanostructures. The results presented here were produced in the framework of the European Network of Excellence entitled "Self-Assembled semiconductor Nanostructures for new Devices in photonics and Electronics (SANDiE)".

Ultramicroscopy, 2014

In this work we examined MoS₂ sheets by aberration-corrected scanning transmission electron micro... more In this work we examined MoS₂ sheets by aberration-corrected scanning transmission electron microscopy (STEM) at three different energies: 80, 120 and 200 kV. Structural damage of the MoS₂ sheets has been controlled at 80 kV according a theoretical calculation based on the inelastic scattering of the electrons involved in the interaction electron-matter. The threshold energy for the MoS₂ material has been found and experimentally verified in the microscope. At energies higher than the energy threshold we show surface and edge defects produced by the electron beam irradiation. Quantitative analysis at atomic level in the images obtained at 80 kV has been performed using the experimental images and via STEM simulations using SICSTEM software to determine the exact number of MoS2₂ layers.

Physical Review Letters, 2009

We present experimental evidence of surface induced disordering at magnetic FeCoPd nanoparticles ... more We present experimental evidence of surface induced disordering at magnetic FeCoPd nanoparticles during the L1(0)-A1 phase transition using high-resolution aberration-corrected electron microscopy and strain mapping. In situ electron diffraction studies show a narrow temperature range of fully ordered L1(0) structure. The order-disorder transition is size dependent and induces strong lattice deformation in outer part of the nanocrystals. The formation of unusually large strain of 20% is discussed in terms of core-shell structure formation with surface disordered layer and ordered core.

Microscopy and Microanalysis, 2006

A systematic distortion in high-angle annular dark-field scanning transmission electron microscop... more A systematic distortion in high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images, which may be caused by residual electrical interference, has been evaluated. Strain mapping, using the geometric phase methodology, has been applied to images acquired in an aberration-corrected STEM. This allows this distortion to be removed and so quantitative analysis of HAADF-STEM images was enabled. The distortion is quantified by applying this technique to structurally perfect and strain-free material. As an example, the correction is used to analyse an InAs/GaAs dot-in-quantum well heterostructure grown by molecular beam epitaxy. The result is a quantitative measure of internal strain on an atomic scale. The measured internal strain field of the heterostructure can be interpreted as being due to variations of indium concentration in the quantum dot.

EPL (Europhysics Letters), 2009

Composition, doping, size, and strain distribution within quantum dots, and at the dots-substrate... more Composition, doping, size, and strain distribution within quantum dots, and at the dots-substrate interfaces, determine the confinement potential of electrons and holes creating a complex band structure. We use ultra-high vacuum Kelvin probe force microscopy to obtain the two-dimensional confinement potential in and around InAs and InSb dots epitaxially grown on GaAs. It is found that the potential manifests rich

Microscopy and …, 2007

In this article a method for determining errors of the strain values when applying strain mapping... more In this article a method for determining errors of the strain values when applying strain mapping techniques has been devised. This methodology starts with the generation of a thickness/defocus series of simulated high-resolution transmission electron microscopy images of InAsxP1-x/InP heterostructures and the application of geometric phase. To obtain optimal defocusing conditions, a comparison of different defocus values is carried out by the calculation of the strain profile standard deviations among different specimen thicknesses. Finally, based on the analogy of real state strain to a step response, a characterization of strain mapping error near an interface is proposed.

… of nanoscience and …, 2008

We determine the compositional distribution with atomic column resolution in a horizontal nanowir... more We determine the compositional distribution with atomic column resolution in a horizontal nanowire from the analysis of aberration-corrected high resolution Z-contrast images. The strain field in a layer capping the analysed nanowire is determined by anisotropic elastic theory from the resulting compositional map. The reported method allows preferential nucleation sites for epitaxial nanowires to be predicted with high spatial resolution, as required for accurate tuning of desired optical properties. The application of this method has been exemplified in this work for stacked InAs(P) horizontal nanowires grown on InP separated by 3 nm thick InP layers, but we propose it as a general method to be applied to other stacked nano-objects.

Atmospheric Environment, 2006

A complete competitive scheme is proposed in this work in order to perform a classification analy... more A complete competitive scheme is proposed in this work in order to perform a classification analysis of meteorological data in the 'Campo de Gibraltar' region (in the South of Spain) from 1999 to 2002. The main objectives of the study presented here have been the characterization of the meteorological conditions in the area, using a competitive neural network based on Kohonen learning rule. Standard Principal Component Analysis (PCA) and VARIMAX rotation have allowed interpreting the physical meaning of the classes obtained from the competitive scheme. Quantitative (using three quality indices) and qualitative (from the analysis of the data projection) criteria based on Fisher Discriminant Analysis were introduced to verify the results of the clustering. A randomized procedure is developed to assure the best performance of the models and to select the best model in the experiments. The different experiments developed extracted five classes, which were related to typical meteorological conditions in the area. r

We show in this article that it is possible to obtain elemental compositional maps and profiles w... more We show in this article that it is possible to obtain elemental compositional maps and profiles with atomic-column resolution across an InxGa1-xAs multilayer structure from 5th-order aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. The compositional profiles obtained from the analysis of HAADF-STEM images describe accurately the distribution of In in the studied multilayer in good agreement with Muraki's segregation model [Muraki, K., Fukatsu, S., Shiraki, Y. & Ito, R. (1992). Surface segregation of In atoms during molecular beam epitaxy and its influence on the energy levels in InGaAs/GaAs quantums wells. Appl Phys Lett 61, 557-559].

The 'Campo de Gibraltar' region is a very industrialized area where very few air pollution studie... more The 'Campo de Gibraltar' region is a very industrialized area where very few air pollution studies have been carried out. Up to date, no model has been developed in order to predict air pollutant levels in the different towns spread in the region. Carbon monoxide (CO), Sulphur dioxide (SO 2 ) and suspended particulate matter (SPM) series have been investigated (years 1999-2000-2001). Multilayer perceptron models (MLPs) with backpropagation learning rule have been used. A resampling strategy with two-fold crossvalidation allowed the statistical comparison of the different models considered in this study. Artificial neural networks (ANN) models were compared with Persistence and ARIMA models and also with models based on standard Multiple Linear Regression (MLR) over test sets with data that had not been used in the training stage. The models based on ANNs showed better capability of generalization than those based on MLR. The designed procedure of random resampling permits an adequate and robust multiple comparison of the tested models. Principal component analysis (PCA) is used to reduce the dimensionality of data and to transform exogenous variables into significant and independent components. Short-term predictions were better than medium-term predictions in the case of CO and SO 2 series. Conversely, medium-term predictions were better in the case of SPM concentrations. The predictions are significantly promising (e.g., d SPM 24-ahead =0.906, d CO 1-ahead =0.891, d SO2 1-ahead =0.851).

The distribution of Bi atoms in epitaxial GaAs ͑1−x͒ Bi x is analyzed through aberration-correcte... more The distribution of Bi atoms in epitaxial GaAs ͑1−x͒ Bi x is analyzed through aberration-corrected Z-contrast images. The relation between the atomic number and the intensity of the images allows quantifying the distribution of Bi atoms in this material. A bidimensional map of Bi atoms is extracted showing areas where nanoclustering is possible and evidencing the location of Bi at As-substitutional positions in the lattice. The distribution of Bi atoms differs from a random spatial pattern of Bi atoms in the material.

Applied Surface …, 2010

The compositional distribution of InAs quantum dots grown by molecular beam epitaxy on GaAs cappe... more The compositional distribution of InAs quantum dots grown by molecular beam epitaxy on GaAs capped InAs quantum dots has been studied in this work. Upper quantum dots are nucleated preferentially on top of the quantum dots underneath, which have been nucleated by droplet epitaxy. The growth process of these nanostructures, which are usually called as quantum dots molecules', has been explained. In order to understand this growth process, the analysis of the strain has been carried out from a 3D model of the nanostructure built from Transmission Electron Microscopy images sensitive to the composition.

Ultramicroscopy, 2007

Strain mapping is defined as a numerical image-processing technique that measures the local shift... more Strain mapping is defined as a numerical image-processing technique that measures the local shifts of image details around a crystal defect with respect to the ideal, defect-free, positions in the bulk. Algorithms to map elastic strains from high-resolution transmission electron microscopy (HRTEM) images may be classified into two categories: those based on the detection of peaks of intensity in real space and the Geometric Phase approach, calculated in Fourier space. In this paper, we discuss both categories and propose an alternative real space algorithm (Peak Pairs) based on the detection of pairs of intensity maxima in an affine transformed space dependent on the reference area. In spite of the fact that it is a real space approach, the Peak Pairs algorithm exhibits good behaviour at heavily distorted defect cores, e.g. interfaces and dislocations. Quantitative results are reported from experiments to determine local strain in different types of semiconductor heterostructures.