Metal-Nitride-oxide-semiconductor light-emitting devices for general lighting.

The potential for application of silicon nitride-based light sources to general lighting is reported. The mechanism of current injection and transport in silicon nitride layers and silicon oxide tunnel layers is determined by electro-optical characterization of both bi- and tri-layers. It is shown that red luminescence is due to bipolar injection by direct tunneling, whereas Poole-Frenkel ionization is responsible for blue-green emission. The emission appears warm white to the eye, and the technology has potential for large-area lighting devices. A photometric study, including color rendering, color quality and luminous efficacy of radiation, measured under various AC excitation conditions, is given for a spectrum deemed promising for lighting. A correlated color temperature of 4800K was obtained using a 35% duty cycle of the AC excitation signal. Under these conditions, values for general color rendering index of 93 and luminous efficacy of radiation of 112 lm/W are demonstrated. This proof of concept demonstrates that mature silicon technology, which is extendable to lowcost, large-area lamps, can be used for general lighting purposes. Once the external quantum efficiency is improved to exceed 10%, this technique could be competitive with other energy-efficient solid-state lighting options. ©2011 Optical Society of America OCIS codes: (230.2090) Electro-optical devices; (150.2950) Illumination.

Intense green-yellow electroluminescence from Tb+-implanted silicon-rich silicon nitride/oxide light emitting devices

High optical power density of 0.5 mW/cm2, external quantum efficiency of 0.1%, and population inversion of 7% are reported from Tb+-implanted silicon-rich silicon nitride/oxide light emitting devices. Electrical and electroluminescence mechanisms in these devices were investigated. The excitation cross section for the 543 nm Tb3+ emission was estimated under electrical pumping, resulting in a value of 8.2 × 10−14 cm2, which is one order of magnitude larger than one reported for Tb3+:SiO2 light emitting devices. These results demonstrate the potentiality of Tb+-implanted silicon nitride material for the development of integrated light sources compatible with Si technology.

Microstructure of highly oriented, hexagonal, boron nitride thin films grown on crystalline silicon by radio frequency plasma-assisted chemical vapor deposition

We present a high‐resolution electron microscopy study of the microstructure of boron nitride thin films grown on silicon (100) by radio‐frequency plasma‐assisted chemical vapor deposition using B2H6 (1% in H2) and NH3 gases. Well‐adhered boron nitride films grown on the grounded electrode show a highly oriented hexagonal structure with the c‐axis parallel to the substrate surface throughout the film, without any interfacial amorphous layer. We ascribed this textured growth to an etching effect of atomic hydrogen present in the gas discharge. In contrast, films grown on the powered electrode, with compressive stress induced by ion bombardment, show a multilayered structure as observed by other authors, composed of an amorphous layer, a hexagonal layer with the c‐axis parallel to the substrate surface and another layer oriented at random

Plasma-enhanced chemical vapor deposition of boron nitride thin films from B2H6-H2-NH3 and B2H6-N2 gas mixtures

Highly transparent and stoichiometric boron nitride (BN) films were deposited on both electrodes (anode and cathode) of a radio-frequency parallel-plate plasma reactor by the glow discharge decomposition of two gas mixtures: B2H6-H2-NH3 and B2H6-N2. The chemical, optical, and structural properties of the films, as well as their stability under long exposition to humid atmosphere, were analyzed by x-ray photoelectron, infrared, and Raman spectroscopies; scanning and transmission electron microscopies; and optical transmittance spectrophotometry. It was found that the BN films grown on the anode using the B2H6-H2-NH3 mixture were smooth, dense, adhered well to substrates, and had a textured hexagonal structure with the basal planes perpendicular to the film surface. These films were chemically stable to moisture, even after an exposition period of two years. In contrast, the films grown on the anode from the B2H6-N2 mixture showed tensile stress failure and were very unstable in the presence of moisture. However, the films grown on the cathode from B2H6-H2-NH3 gases suffered from compressive stress failure on exposure to air; whereas with B2H6-N2 gases, adherent and stable cathodic BN films were obtained with the same crystallographic texture as anodic films prepared from the B2H6-H2-NH3 mixture. These results are discussed in terms of the origin of film stress, the effects of ion bombardment on the growing films, and the surface chemical effects of hydrogen atoms present in the gas discharge.

Correlation between charge transport and electroluminescence properties of Si-rich oxide/nitride/oxide-based light emitting capacitors.

The electrical and electroluminescence (EL) properties at room and high temperatures of oxide/ nitride/oxide (ONO)-based light emitting capacitors are studied. The ONO multidielectric layer is enriched with silicon by means of ion implantation. The exceeding silicon distribution follows a Gaussian profile with a maximum of 19%, centered close to the lower oxide/nitride interface. The electrical measurements performed at room and high temperatures allowed to unambiguously identify variable range hopping (VRH) as the dominant electrical conduction mechanism at low voltages, whereas at moderate and high voltages, a hybrid conduction formed by means of variable range hopping and space charge-limited current enhanced by Poole-Frenkel effect predominates. The EL spectra at different temperatures are also recorded, and the correlation between charge transport mechanisms and EL properties is discussed.

Efficiency and reliability enhancement of silicon nanocrystal field-effect luminescence from nitride-oxide gate stacks

We report on a field-effect light emitting device based on silicon nanocrystals in silicon oxide deposited by plasma-enhanced chemical vapor deposition. The device shows high power efficiency and long lifetime. The power efficiency is enhanced up to 0.1 %25 by the presence of a silicon nitride control layer. The leakage current reduction induced by this nitride buffer effectively increases the power efficiency two orders of magnitude with regard to similarly processed devices with solely oxide. In addition, the nitride cools down the electrons that reach the polycrystalline silicon gate lowering the formation of defects, which significantly reduces the device degradation.

Fabricació de Lents Zonals de Fresnel per aplicació als raigs X tous

Estudi elaborat a partir d’una estada al Paul Scherrer Institut del Maig a l’Octubre del 2006 amb l’ajuda i supervisió dels Dr. Konstantins Jefimovs i Dr. Christian David. Focalitzar raigs X tous és una necessitat essencial per al microanàlisis, la microscopia, i fer imatges en moltes Instal·lacions de Radiació Sincrotró. Les Lents Zonals de Fresnel (FZP, de la denominació anglesa “Fresnel Zone Plates”) han demostrat donar uns punts focals amb una resolució espacial destacada i una baixa il·luminació de fons. Tanmateix, la fabricació de FZP és complexa i no totalment reproduïble. A més a més, el temps de vida de les FZP és força curt, ja que estant situades sobre membranes de nitrur de silici molt fines i altament absorbents. Per tant, hem fet esforços per implementar FZP de silici, que s’espera que siguin més resistents. L’element està fet d’una oblia de cristall de silici poc absorbent, i no presenta cap interfase entre materials. Així doncs, aquestes lents són especialment adequades per a aguantar les extremes càrregues de radiació de les fonts de raigs X més brillants. Particularment, això és molt important per a les aplicacions a les pròximes generacions de fonts de raigs X, com els Làsers d’Electrons Lliures (FEL, de la denominació anglesa “Free Electron Laser”). El silici també garanteix que no hi hagi cap banda d’absorció en el rang d’energies de la finestra de l’aigua (200-520 eV), fent aquestes lents ideals per a fer imatges de mostres biològiques. En aquest informe, hi ha una descripció detallada de tots els passos involucrats en la fabricació de les Lents Zonals de Fresnel de silici. En resum, les estructures de FZP es modelen sobre una resina utilitzant litografia per feix d’electrons i llavors el patró es transmet al silici mitjançant un gravat d’ions reactius (RIE, de la denominació anglesa ‘Reactive Ion Etching’) utilitzant una fina (20 nm) màscara de Crintermitja. Les membranes de silici es poden aprimar després de la fabricació de les estructures per a garantir una transmissió suficient fins i tot a baixes energies. Aquest informe també inclou l’anàlisi i la discussió d’alguns experiments preliminars per avaluar el rendiment de les Si FZPs fets a la línia de llum PolLux del Swiss Ligth Source amb l’ajuda dels Dr. Jörg Raabe i Dr. George Tzvetkov.

Flexible employment and cross- regional adjustment

Employment flexibility is commonly associated to greater labour mobility and thus faster cross-regional adjustments. The literature however offers very little hard evidence on this and quite limited theoretical guidance. This paper examines empirically the relationship between employment flexibility and cross-regional adjustment (migration) at the regional and local levels in the UK. Employment flexibility is associated to higher labour mobility (but only at a rather localised scale) and at the same time seems to reduce the responsiveness of migration to unemployment. This suggest that rising flexibility may be linked to higher persistence in spatial disparities, as intra-regional adjustments are strengthened while extraregional adjustments weakened. Keywords: Employment flexibility, regional migration, labour market adjustment JEL Codes: R11, R23, J08, J61

Bibliotecas y centros de documentación en un entorno virtual

Les Biblioteques i Centres de Documentació han exercit sempre un paper important en el desenvolupament de l'aprenentatge, de la docència i de la investigació, però el canvi que s'ha experimentat i s'està experimentant en el camp docent en aquests últims anys, ha potenciat molt més la funció d'aquest tipus de centres, convertint-los en eixos bàsics del sistema educatiu, passant a formar part, en molts casos, del model dissenyat per exercir la funció educativa i investigadora a través d'un entorn virtual.

Hacer castells, construir nación: castells, modelo festivo y catalanismo

Este trabajo propone un análisis del proceso del catalanismo, los modelos festivos y los castells para interpretar el modo en que esta relación es histórica y ha pasado por diferentes etapas. Si bien el catalanismo se había aliado con las sardanas hasta los años cincuenta, lo que se ha denominado el modelo festivo de la Catalunya Vella, a partir de este momento y con el inicio de la modernidad castellera, las alianzas se afianzan con los castells, el modelo festivo de la Catalunya Nova. Esto se produce porque los castells pueden hacer frente a los retos de la nueva realidad española y del nuevo nacionalismo que guiará el proceso de construcción de la Cataluña en que vivimos actualmente

Growth and characterization of CoO ultra thin films

In this report we present the growth process of the cobalt oxide system using reactive electron beam deposition. In that technique, a target of metallic cobalt is evaporated and its atoms are in-flight oxidized in an oxygen rich reactive atmosphere before reaching the surface of the substrate. With a trial and error procedure the deposition parameters have been optimized to obtain the correct stoichiometry and crystalline phase. The evaporation conditions to achieve the correct cobalt oxide salt rock structure, when evaporating over amorphous silicon nitride, are: 525 K of substrate temperature, 2.5·10-4 mbar of oxygen partial pressure and 1 Å/s of evaporation rate. Once the parameters were optimized a set of ultra thin film ranging from samples of 1 nm of nominal thickness to 20nm thick and bulk samples were grown. With the aim to characterize the samples and study their microstructure and morphology, X-ray diffraction, transmission electron microscopy, electron diffraction, energy dispersive X-ray spectroscopy and quasi-adiabatic nanocalorimetry techniques are utilised. The final results show a size dependent effect of the antiferromagnetic transition. Its Néel temperature becomes depressed as the size of the grains forming the layer decreases.

Full Exploration of the Diels–Alder Cycloaddition on Metallofullerenes M3N@C80 (M=Sc, Lu, Gd): The D5h versus Ih Isomer and the Influence of the Metal Cluster

In this work a detailed investigation of the exohedral reactivity of the most important and abundant endohedral metallofullerene (EMF) is provided, that is, Sc3N@Ih-C80 and its D5h counterpart Sc3N@D5h-C80, and the (bio)chemically relevant lutetium- and gadolinium-based M3N@Ih/D5h-C80 EMFs (M=Sc, Lu, Gd). In particular, we analyze the thermodynamics and kinetics of the Diels–Alder cycloaddition of s-cis-1,3-butadiene on all the different bonds of the Ih-C80 and D5h-C80 cages and their endohedral derivatives. First, we discuss the thermodynamic and kinetic aspects of the cycloaddition reaction on the hollow fullerenes and the two isomers of Sc3N@C80. Afterwards, the effect of the nature of the metal nitride is analyzed in detail. In general, our BP86/TZP//BP86/DZP calculations indicate that [5,6] bonds are more reactive than [6,6] bonds for the two isomers. The [5,6] bond D5h-b, which is the most similar to the unique [5,6] bond type in the icosahedral cage, Ih-a, is the most reactive bond in M3N@D5h-C80 regardless of M. Sc3N@C80 and Lu3N@C80 give similar results; the regioselectivity is, however, significantly reduced for the larger and more electropositive M=Gd, as previously found in similar metallofullerenes. Calculations also show that the D5h isomer is more reactive from the kinetic point of view than the Ih one in all cases which is in good agreement with experiments

The European Union and refugees: towards more restrictive asylum policies in the European Union?

Several scholars have argued that European countries have decided to cooperate on asylum and migration matters at the EU level in order to develop more restrictive policies. In particular, it has been argued that European states have ‘venue-shopped’ to a new policy-venue in order to escape national constraints. This paper puts this argument to the test by assessing the extent to which the development of EU cooperation on asylum matters has indeed led to the adoption of more restrictive asylum standards. The paper argues that, actually, EU asylum cooperation has led to an overall increase in protection standards for asylum-seekers and refugees. This outcome is explained by two main factors: the increasing ‘judicialisation’ of asylum in the EU and institutional changes in the EU asylum policy area that have strengthened the role of more ‘refugee-friendly’ institutions.

Regional economic dynamics and convergence in the european union

Deepening in the European Union (EU) integration process has enhanced the question of economic disparities at a regional level. Theconvergence process observed until the late seventies was exhausted onwards incoincidence with important changes in the economic activity. The paper showshow these factors would have provoked a regional differenciated response that,despite being important, would have not strengthened the decrease in regionalinequalities. We use an alternative and (in our opinion) richer approach to thetraditional convergence analysis, where the evolution of the whole regionaldistribution is what matters and not that of a representative economy. Moreover,when analysing inequalities among regional economies, the geographical spaceacquire an outstanding role. Hence, we apply spatial association tests and relatethem to the convergence analysis