Intermediate Band to Conduction Band optical absorption in ZnTe:O

ZnTe doped with high concentrations of oxygen has been proposed in previous works as intermediate band (IB) material for photovoltaic applications. The existence of extra optical transitions related to the presence of an IB has already been demonstrated in this material and it has been possible to measure the absorption coefficient of the transitions from the valence band (VB) to the IB. In this work we present the first measurement of the absorption coefficient associated to transitions from the IB to the conduction band (CB) in ZnTe:O. The samples used are 4 ?m thick ZnTe layers with or without O in a concentration ~ 1019 cm-3, which have been grown on semi-insulating GaAs substrates by molecular beam epitaxy (MBE). The IB-CB absorption coefficient peaks for photon energies ~ 0.4 eV. It is extracted from reflectance and transmittance spectra measured using Fourier Transform Infrared (FTIR) spectroscopy. Under typical FTIR measurement conditions (low light intensity, broadband spectrum) the absorption coefficient in IB-to-CB transitions reaches 700 cm-1. This is much weaker than the one observed for VB-IB absorption. This result is consistent with the fact that the IB is expected to be nearly empty of electrons under equilibrium conditions in ZnTe(:O). The absorption for VB to IB transitions is also observed in the same samples through reflectance measurements performed in the visible range using a monochromator. These measurements are compared with the quantum efficiency (QE) from solar cells fabricated under similar conditions.

Providing 3D video services: the challenge from 2D to 3DTV quality of experience

Recently, three-dimensional (3D) video has decisively burst onto the entertainment industry scene, and has arrived in households even before the standardization process has been completed. 3D television (3DTV) adoption and deployment can be seen as a major leap in television history, similar to previous transitions from black and white (B&W) to color, from analog to digital television (TV), and from standard definition to high definition. In this paper, we analyze current 3D video technology trends in order to define a taxonomy of the availability and possible introduction of 3D-based services. We also propose an audiovisual network services architecture which provides a smooth transition from two-dimensional (2D) to 3DTV in an Internet Protocol (IP)-based scenario. Based on subjective assessment tests, we also analyze those factors which will influence the quality of experience in those 3D video services, focusing on effects of both coding and transmission errors. In addition, examples of the application of the architecture and results of assessment tests are provided.

Intermediate band to conduction band optical absorption in ZnTeO

ZnTe doped with high concentrations of oxygen has been proposed in previous works as an intermediate band (IB) material for photovoltaic applications. The existence of extra optical transitions related to the presence of an IB has already been demonstrated in this material and it has been possible to measure the absorption coefficient of the transitions from the valence band (VB) to the IB. In this study, we present the first measurement of the absorption coefficient associated with transitions from the IB to the conduction band (CB) in ZnTeO. The samples used are 4-mum-thick ZnTe layers with or without O in a concentration ~10 19 cm -3, which have been grown on semiinsulating GaAs substrates by molecular beam epitaxy (MBE). The IB-CB absorption coefficient peaks for photon energies ~0.4 eV. It is extracted from reflectance and transmittance spectra measured using Fourier transform infrared (FTIR) spectroscopy. Under typical FTIR measurement conditions (low light intensity, broadband spectrum), the absorption coefficient in IB-to-CB transitions reaches 700 cm -1. This is much weaker than the one observed for VB-IB absorption. This result is consistent with the fact that the IB is expected to be nearly empty of electrons under equilibrium conditions in ZnTe(O).

Interband absorption of photons by extended states in intermediate band solar cells

This paper considers sub-bandgap photon absorption in an InAs/GaAs quantum dot matrix. Absorption coefficients are calculated for transitions from the extended states in the valence band to confined states in the conduction band. This completes a previous body of work in which transitions between bound states were calculated. The calculations are based on the empirical k·p Hamiltonian considering the quantum dots as parallelepipeds. The extended states may be only partially extended?in one or two dimensions?or extended in all three dimensions. It is found that extended-to-bound transitions are, in general, weaker than bound-to-bound transitions, and that the former are weaker when the initial state is extended in more coordinates. This study is of direct application to the research of intermediate band solar cells and other semiconductor devices based on light absorption in semiconductors nanostructured with quantum dots.

Optical absorption analysis of quaternary molybdate- and tungstate-ordered double perovskites

Quaternary-ordered double perovskite A2MM’O6 (M=Mo,W) semiconductors are a group of materials with a variety of photocatalytic and optoelectronic applications. An analysis focused on the optoelectronic properties is carried out using first-principles density-functional theory with several U orbital-dependent one-electron potentials applied to different orbital subspaces. The structural non-equivalence of the atoms resulting from the symmetry has been taken in account. In order to analyze optical absorption in these materials deeply, the absorption coefficients have been split into inter- and intra-non-equivalent species contributions. The results indicate that the effect of the A and M’ atoms on the optical properties are minimal whereas the largest contribution comes from the non-equivalent O atoms to M transitions.

Nueva York : la revitalización del espacio público a partir de 1950

A partir de la segunda mitad del siglo XX, Nueva York experimenta una apertura al público de espacios nuevos, transformados u olvidados mediante la adaptación de mecanismos, ya sean formales o informales, de caracter permanente o temporal, para la incorporación de uso público en un contexto urbano limitado y congestionado. Estos recursos espaciales son mediadores entre el espacio público y el privado, y son el resultado físico de la negociación entre la legislación urbana, los interes privados de los promotores y las demandas de los ciudadanos. La tesis estudia una selección de obras, entre los años 1950 y 2015, que incorporan notables oportunidades para el uso colectivo, pero que han propiciado la creación de dos tipos de espacios: los “espaciosoasis”, aquellos que potencian una verdadera interacción social entre los usuarios; y los “espacios-vitrina”, para ver y no tocar, en los que el usuario participa indirectamente y, sin promover interacción alguna, atraen al público foráneo y rechazan al local. La diversidad de usos, la sensibilidad social, la supervisión cercana y la facilidad de mantenimiento de la pequeña escala posibilitan que los “espacios-oasis” sean claves a la hora de mantener el fragil y vulnerable equilibrio de la vida urbana. La tesis profundiza en la contextualización de las obras en relación con el apoyo de las políticas públicas y contexto cíclico de los constantes altibajos económicos. El papel del alcalde neoyorkino es clave a la hora decidir el mayor o mejor respaldo de la administración local a las obras que tienen una incidencia en el ámbito de lo público, por lo que la tesis estructura las obras en relación a las transiciones entre las alcaldías más importantes. La presión del mercado inmobiliario, los intereses privados y políticos, la excesiva comercialización y programación de estos espacios dificultan el lento y opaco proceso de la incorporación de uso público en la ciudad y compremeten el verdadero carácter cívico del espacio abierto urbano. La investigación estudia estos factores a través de una reflexión sobre el verdadero sentido de la revitalización de lo público, entendida en relación con las interacciones que fomenta y que se producen más allá de las intenciones del proyecto arquitectónico. El objetivo y el alcance de esta investigación permite reflexionar, discutir y explorar el uso y la participación cívica en el entorno construido, para de esta forma poder entender la evolución de las condiciones sociales, económicas, arquitectónicas y urbanas del espacio público. ABSTRACT During the second half of the twentieth century, New York experienced a launch of new spaces, changed or forgotten through the adoption of mechanisms, whether formal or informal, permanent or temporary, for the incorporation of spaces for public use in a limited and congested urban context. These assets are mediators between the public and private space, and are the physical result of negotiations between the urban legislation, the private interests of the developer and the demands of citizens. The dissertation examines a selection of projects, between 1950 and 2015, that incorporate remarkable spatial opportunities for collective use, which led to the creation of two types of spaces: “oasis-space”, those that enhance social interaction between patrons; and “vitrinespace”, space to be seen but not touched, where the user participates indirectly and does not promote any interaction, attracting an outside public and rejecting the local. Diversity of uses, social sensitivity, close supervision and ease of maintenance that enable small-scale “oasis spaces” are key when it comes to keeping the fragile and vulnerable balance of urban life. The investigation explains the contextualization of the projects in relation with the support of public politics and the cyclical context of constantly changing economics. The role of the New York mayor is key at the time of deciding the level of administrative support for the projects that occur at a point of overlap between the public and the private, which is why the thesis analyzes the projects in relationship to the transitions between the cities most influential mayors. Pressure from the housing market, private and political interests, excessive commercialization and programming of these spaces impede the slow and opaque process of incorporating them for public use in the city and compromise the true civic character open urban space. The research explores these factors by reflecting on the true meaning of the revitalization of the public, understood in relation to interactions it encourages and that occur outside of the intentions of the architectural project. The objective and scope of this study allows for reflecting on, discussing and exploring civic participation and use of the built environment, as a way to understand the evolution of social, economic, architectural and urban conditions of public space.

Engineering metal precipitate size distributions to enhance gettering in multicrystalline silicon

The extraction of metal impurities during phosphorus diffusion gettering (PDG) is one of the crucial process steps when fabricating high-efficiency solar cells using low-cost, lower-purity silicon wafers. In this work, we show that for a given metal concentration, the size and density of metal silicide precipitates strongly influences the gettering efficacy. Different precipitate size distributions can be already found in silicon wafers grown by different techniques. In our experiment, however, the as-grown distribution of precipitated metals in multicrystalline Si sister wafers is engineered through different annealing treatments in order to control for the concentration and distribution of other defects. A high density of small precipitates is formed during a homogenization step, and a lower density of larger precipitates is formed during extended annealing at 740º C. After PDG, homogenized samples show a decreased interstitial iron concentration compared to as-grown and ripened samples, in agreement with simulations.

A comprehensive diagram to grow metal-polarity InGaN alloys by molecular beam epitaxy

he composition, strain and surface morphology of (0001)InGaN layers are investigated as a function of growth temperature (460–645 °C) and impinging In flux. Three different growth regimes: nitrogen-rich, metal-rich and intermediate metal-rich, are clearly identified and found to be in correlation with surface morphology and strain relaxation. Best epilayers’ quality is obtained when growing under intermediate metal-rich conditions, with 1–2 monolayers thick In ad-coverage. For a given In flux, the In incorporation decreases with increasing growth temperature due to InN thermal decomposition that follows an Arrhenius behavior with 1.84±0.12 eV activation energy.

Self-organized colloidal quantum dots and metal nanoparticles for plasmon-enhanced intermediate-band solar cells

A colloidal deposition technique is presented to construct long-range ordered hybrid arrays of self-assembled quantum dots and metal nanoparticles. Quantum dots are promising for novel opto-electronic devices but, in most cases, their optical transitions of interest lack sufficient light absorption to provide a significant impact in their implementation. A potential solution is to couple the dots with localized plasmons in metal nanoparticles. The extreme confinement of light in the near-field produced by the nanoparticles can potentially boost the absorption in the quantum dots by up to two orders of magnitude. In this work, light extinction measurements are employed to probe the plasmon resonance of spherical gold nanoparticles in lead sulfide colloidal quantum dots and amorphous silicon thin-films. Mie theory computations are used to analyze the experimental results and determine the absorption enhancement that can be generated by the highly intense near-field produced in the vicinity of the gold nanoparticles at their surface plasmon resonance. The results presented here are of interest for the development of plasmon-enhanced colloidal nanostructured photovoltaic materials, such as colloidal quantum dot intermediate-band solar cells.

Phase transitions in number theory: from the birthday problem to Sidon sets

In this work, we show how number theoretical problems can be fruitfully approached with the tools of statistical physics. We focus on g-Sidon sets, which describe sequences of integers whose pairwise sums are different, and propose a random decision problem which addresses the probability of a random set of k integers to be g-Sidon. First, we provide numerical evidence showing that there is a crossover between satisfiable and unsatisfiable phases which converts to an abrupt phase transition in a properly defined thermodynamic limit. Initially assuming independence, we then develop a mean-field theory for the g-Sidon decision problem. We further improve the mean-field theory, which is only qualitatively correct, by incorporating deviations from independence, yielding results in good quantitative agreement with the numerics for both finite systems and in the thermodynamic limit. Connections between the generalized birthday problem in probability theory, the number theory of Sidon sets and the properties of q-Potts models in condensed matter physics are briefly discussed