Strain driven migration of In during the growth of InAs/GaAs quantum posts

Using the mechano-optical stress sensor technique, we observe a counter-intuitive reduction of the compressive stress when InAs is deposited on GaAs (001) during the growth of quantum posts. Through modelling of the strain fields, we find that such anomalous behaviour can be related to the strain-driven detachment of In atoms from the crystal and their surface diffusion towards the self-assembled nanostructures.We acknowledge the financial support by Spanish MINECO through Grant Nos. ENE2012-37804-C02-02 and TEC2011-29120-C05-04, and by Spanish CAM through Grant Nos. S2009/ESP-1503 and S2009/ENE-1477.Peer Reviewe

Spatiotemporal Characteristics of the Largest HIV-1 CRF02_AG Outbreak in Spain: Evidence for Onward Transmissions

Background and Aim: The circulating recombinant form 02_AG (CRF02_AG) is the predominant clade among the human immunodeficiency virus type-1 (HIV-1) non-Bs with a prevalence of 5.97% (95% Confidence Interval-CI: 5.41–6.57%) across Spain. Our aim was to estimate the levels of regional clustering for CRF02_AG and the spatiotemporal characteristics of the largest CRF02_AG subepidemic in Spain.Methods: We studied 396 CRF02_AG sequences obtained from HIV-1 diagnosed patients during 2000–2014 from 10 autonomous communities of Spain. Phylogenetic analysis was performed on the 391 CRF02_AG sequences along with all globally sampled CRF02_AG sequences (N = 3,302) as references. Phylodynamic and phylogeographic analysis was performed to the largest CRF02_AG monophyletic cluster by a Bayesian method in BEAST v1.8.0 and by reconstructing ancestral states using the criterion of parsimony in Mesquite v3.4, respectively.Results: The HIV-1 CRF02_AG prevalence differed across Spanish autonomous communities we sampled from (p < 0.001). Phylogenetic analysis revealed that 52.7% of the CRF02_AG sequences formed 56 monophyletic clusters, with a range of 2–79 sequences. The CRF02_AG regional dispersal differed across Spain (p = 0.003), as suggested by monophyletic clustering. For the largest monophyletic cluster (subepidemic) (N = 79), 49.4% of the clustered sequences originated from Madrid, while most sequences (51.9%) had been obtained from men having sex with men (MSM). Molecular clock analysis suggested that the origin (tMRCA) of the CRF02_AG subepidemic was in 2002 (median estimate; 95% Highest Posterior Density-HPD interval: 1999–2004). Additionally, we found significant clustering within the CRF02_AG subepidemic according to the ethnic origin.Conclusion: CRF02_AG has been introduced as a result of multiple introductions in Spain, following regional dispersal in several cases. We showed that CRF02_AG transmissions were mostly due to regional dispersal in Spain. The hot-spot for the largest CRF02_AG regional subepidemic in Spain was in Madrid associated with MSM transmission risk group. The existence of subepidemics suggest that several spillovers occurred from Madrid to other areas. CRF02_AG sequences from Hispanics were clustered in a separate subclade suggesting no linkage between the local and Hispanic subepidemics

Técnicas de preprocesado para la segmentación de imágenes

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Técnicas de preprocesado para la segmentación de imágenes

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Energy rehabilitation with external thermal insulation (ETICS) in Zaragoza (Spain): application to social housing in Saltillo, Mexico

The sustainability goals agreed by most countries have made them focus their attention on the high energy consumption of buildings and the weight within this consumption of the residential sector. This makes the energy rehabilitation of existing buildings a top priority. However, energy rehabilitation is not the same in all countries. In this study, we analyse two houses in Zaragoza, one rehabilitated with an external thermal insulation system (ETICS), while the other retained its original state. Their energy behaviour was monitored and the results of each case were analysed. The results obtained from this monitoring were used to propose rehabilitation strategies in single-family social housing in Saltillo, Mexico, where the residential sector is the third-largest energy consumer. This study demonstrates the interest in analysing and using successful real cases of energy rehabilitation interventions in similar climates as a useful knowledge transfer opportunity. In addition, the importance of this "technological know-how" is highlighted to carry out a more precise energy rehabilitation.Peer ReviewedPostprint (published version

Strain balanced epitaxial stacks of quantum dots and quantum posts

The self assembly of quantum dots by heteroepitaxy of lattice-mismatched semiconductors is based on elastic energy relaxation, which spontaneously occurs at the growth front when the largest atoms in the crystal cluster together. Because a larger covalent radius is related to weaker bonds, and this is in turn fundamentally related to smaller bandgaps, the formation of quantum dots leads to a confinement potential for electrons and/or holes. This effect has applications ranging from ultralow threshold diode lasers to highly efficient solar cells and usually requires the stacking of multiple quantum dots. The number of layers is limited by the stress accumulated during growth due to the larger covalent radius of the atoms that constitute the quantum dots. This accumulated stress can be relieved by introducing in the epitaxial layers compensating atomic species with a smaller covalent radius, enabling a reduction of spacer layer thickness. In the limit of sub-nanometer spacer thickness, the quantum dots, which have a tendency to line up vertically, fuse into a quantum post. The current efforts to optimize the properties of strain balanced quantum dot stacks and quantum posts are reviewed. The stacking of self-assembled epitaxial quantum dots with sub-nanometer spacer layers leads to the formation of quasi-1D nanostructures or quantum posts (QP). Using a matrix material with a small lattice parameter for strain compensation, the length of the QPs can be increased upwards of 100 nm. The resulting anisotropic strain field favors the luminescence emitted with dominant transverse magnetic component in the [1-10] direction. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.MICINN (TEC2008-06756-C03-01/03, ENE2009-14481-C02-02, CSD2006-0004, CSD2006-0019, TEC2008-06756-C03- 02/TEC and CSD2009-00013), CAM (S2009ESP-1503,4 S2009/ENE-1477), CSIC (PIF 200950I154) and J. And. (PAI research group TEP-120 and P08-TEP-03516).Peer Reviewe

Strain balanced quantum posts for intermediate band solar cells

Trabajo presentado al 35th PVSC, celebrado en Honolulu (US) del 20 al 25 de Junio de 2010.In this work we present strain balanced InAs quantum post of exceptional length in the context of photovoltaics. We discuss the general properties of these nanostructures and their impact in the practical implementation of an intermediate band solar cell. We have studied the photocurrent generated by strain balanced quantum posts embedded in a GaAs single crystal, and compared our results with quantum dot based devices. The incorporation of phosphorous in the matrix to partially compensate the accumulated stress enables a significant increase of the quantum post maximum length. The relative importance of tunneling and thermal escape processes is found to depend strongly on the geometry of the nanostructures.We acknowledge support by MICINN (NANOGEFFES ENE2009-14481-C02-02, TEC2008-06756-C03-02/TEC, Consolider GENESIS MEC CSD2006-0004, Consolider CSD2009-00013), CAM (Numancia 2 S2009/ENE-1477), CSIC (PIF 200950I154) and Junta de Andalucía (PAI research group TEP-120, project P08-TEP-03516).Peer Reviewe

Strain issues in stress compensated QDs for intermediate band solar cells

Trabajo presentado al "25th European Photovoltaic Solar Energy Conference and Exhibition" y al "5th World Conference on Photovoltaic Energy Conversion", celebrados en Valencia (España) del 6 al 10 de Septiembre de 2010.In this work we have investigated several problems that may appear during the growth of strain balanced quantum dots for high efficiency solar cells. We have used in situ stress measurements to evaluate the total stress introduced by the InAs quantum dots and the exact composition and thickness of the GaAsP stress compensating barrier. We have found that the quality of perfectly balanced samples can degrade as we stack the quantum dot layers (even for a reduced number of them) leading, eventually, to a complete suppression of the quantum dot formation. We attribute this effect to a strong strain modulation at the surface of the sample that leads to the migration of phosphorus adatoms away from the top of the quantum dots, increasing the inhomogeneity of the compensating barrier. We conclude that using a pure binary material as compensating layer would prevent this decomposition and lead to better material quality.Peer Reviewe

Epitaxial growth of strain balanced semiconductor nanostructures for solar cells

Trabajo presentado en el Nanomaterials for Energy and Environmental Applications, celebrado en España en abril de 201

Fabrication and characterization of strain balanced InAs quantum posts

Trabajo presentado a la "31st International Conference on the Physics of Semiconductors" celebrada en Zurich (Suiza) del 29 de julio al 3 de Agosto de 2012.Epitaxial quantum dot (QD) solar cells need tens if not hundreds of QD layers in order to have an appreciable absorption at long wavelengths. In the InAs/GaAs system, due to the larger covalent radius of InAs, increasing the number and stack density of QDs layers inevitably leads to an increase of the strain fields and eventually to the formation of dislocations. The strain balanced (SB) technique consists of compensating the compressive stress introduced by the QDs by a tensile stress in the spacer between layers. In this way, by alternating tensile-compressive regions, the strain in the material is minimized and the formation of dislocations is retarded. Several groups have implemented their own combinations of materials in an attempt to stack many layers of QDs, either using P or N compounds. We have used this technique before to stack 50 InAs quantum dot layers in a solar cell structure with improved crystal quality and optical properties. In this work, we report on the fabrication and optical characterization of InAs quantum posts (QPs) grown using the above approximation in the limit of negligible spacer layer thickness. In order to achieve proper balancing of the accumulated stress, the growth of the samples was characterized by in situ real-time measurements of the total accumulated stress during growth. This technique allows us to monitor the accumulated compressive stress arising from the InAs layer and the tensile stress introduced by the compensating layer (GaAsP of different compositions). The resulting InAs/GaAsP quantum posts (QPs) are 120-nm-long vertical nanostructures which show a record height/diameter aspect ratio >7.4 We have embedded these nanostructures in a p-i-n diode structure to investigate their electro-optical properties as a function of temperature and applied bias. Our study reveals weak localization of carriers along the QP vertical axis at low excitation power and a one-dimensional-like density of states at high excitation.Peer Reviewe