Atomic diffusion induced by stress relaxation in InGaAs/GaAs epitaxial layers

The origin of the microscopic inhomogeneities in InxGa12xAs layers grown on GaAs by molecular beam epitaxy is analyzed through the optical absorption spectra near the band gap. It is seen that, for relaxed thick layers of about 2.8 mm, composition inhomogeneities are responsible for the band edge smoothing into the whole compositional range (0.05,x,0.8). On the other hand, in thin enough layers strain inhomogeneities are dominant. This evolution in line with layer thickness is due to the atomic diffusion at the surface during growth, induced by the strain inhomogeneities that arise from stress relaxation. In consequence, the strain variations present in the layer are converted into composition variations during growth. This process is energetically favorable as it diminishes elastic energy. An additional support to this hypothesis is given by a clear proportionality between the magnitude of the composition variations and the mean strain.

Roughness and light scattering of ion-beam-sputtered fluoride coatings for 193 nm

Scattering characteristics of multilayer fluoride coatings for 193 nm deposited by ion beam sputtering and the related interfacial roughnesses are investigated. Quarter- and half-wave stacks of MgF2 and LaF3 with increasing thickness are deposited onto CaF2 and fused silica and are systematically characterized. Roughness measurements carried out by atomic force microscopy reveal the evolution of the power spectral densities of the interfaces with coating thickness. Backward-scattering measurements are presented, and the results are compared with theoretical predictions that use different models for the statistical correlation of interfacial roughnesses.

Ultraviolet optical and microstructural properties of MgF2 and LaF3 coatings deposited by ion-beam sputtering and boat and electron-beam evaporation

Single layers of MgF2 and LaF3 were deposited upon superpolished fused-silica and CaF2 substrates by ion-beam sputtering (IBS) as well as by boat and electron beam (e-beam) evaporation and were characterized by a variety of complementary analytical techniques. Besides undergoing photometric and ellipsometric inspection, the samples were investigated at 193 and 633 nm by an optical scatter measurement facility. The structural properties were assessed with atomic-force microscopy, x-ray diffraction, TEM techniques that involved conventional thinning methods for the layers. For measurement of mechanical stress in the coatings, special silicon substrates were coated and analyzed. The dispersion behavior of both deposition materials, which was determined on the basis of various independent photometric measurements and data reduction techniques, is in good agreement with that published in the literature and with the bulk properties of the materials. The refractive indices of the MgF2 coatings ranged from 1.415 to 1.440 for the wavelength of the ArF excimer laser (193 nm) and from 1.435 to 1.465 for the wavelength of the F2 excimer laser (157 nm). For single layers of LaF3 the refractive indices extended from 1.67 to 1.70 at 193 nm to ~1.80 at 157 nm. The IBS process achieves the best homogeneity and the lowest surface roughness values (close to 1 nmrms) of the processes compared in the joint experiment. In contrast to MgF2 boat and e-beam evaporated coatings, which exhibit tensile mechanical stress ranging from 300 to 400 MPa, IBS coatings exhibit high compressive stress of as much as 910 MPa. A similar tendency was found for coating stress in LaF3 single layers. Experimental results are discussed with respect to the microstructural and compositional properties as well as to the surface topography of the coatings.

Atomic diffusion induced by stress relaxation in InGaAs/GaAs epitaxial layers

The origin of the microscopic inhomogeneities in InxGa1-xAs layers grown on GaAs by molecular beam epitaxy is analyzed through the optical absorption spectra near the band gap. It is seen that, for relaxed thick layers of about 2.8μm, composition inhomogeneities are responsible for the band edge smoothing into the whole compositional range (0.05atomic diffusion at the surface during growth, induced by the strain inhomogeneities that arise from stress relaxation. In consequence, the strain variations present in the layer are converted into composition variations during growth. This process is energetically favorable as it diminishes elastic energy. An additional support to this hypothesis is given by a clear proportionality between the magnitude of the composition variations and the mean strain

Nanotermoparells: heterounions de nanowires

Projecte de recerca elaborat a partir d’una estada a la University of California a Irvine, EEUU, entre juliol del 2007 i gener del 2008. Els termoparells són actualment els sensors de temperatura més populars i més utilitzats per a un ampli rang d’aplicacions: industrials, domèstiques, etc. Aconseguir miniaturar els dispositius fins a dimensions extremadament petites obra un ampli rang de noves aplicacions per aquests dispositius, per exemple, en el camp de la tecnologia lab-on-a-chip. En aquesta investigació, el concepte de termoparell, és a dir, dos cables de diferent metall connectats per un extrem s’ha extrapolat a l’escala nanomètrica, utilitzant nanowires com a element de construcció. Aquests nanowires s’han sintetitzat a través d’un nou procediment desenvolupat en el grup d’investigació de la Universitat de California, Irvine, que ha permès treballar amb nanowires de diferents dimensions (control independent de l’alçada i amplada) i un major grau d’èxit en la fabricació d’aquests termometres. El mètode també permet dipositar aquestes nanoestructures sobre substractes no conductors de manera controlable, simplificant notablement tot el procés de fabricació. L’obtenció d’aquests dispositius ha permès demostrar que, a part de ser bons sensors de temperatura a nivell macroscòpic (fonts de calor ambientals), també permet la determinació de temperatura a nivell microscòpic (fonts de calor focalitzada, com és el cas de feixos làser). Per a la seva caracterització ha estat necessari l’ús de tecnologia puntera (làsers, amplificadors, microscopis de forces atòmiques) i inclòs el disseny de nous dispositius. Aquests nanotermoparells presenten propietats extraordinàries, com una gran sensitivitat, gran velocitat de resposta a estímuls tèrmics, i un comportament estable vers l’ús i el temps.

Obtenció de capes primes d'alúmina mitjançant "Atomic Layer Deposition"

En aquest projecte s'ha estudiat la posada a punt d’un equip comercial ALD per a l’obtenció de capes primes d'alúmina a escala nanomètrica utilitzant vapor d’aigua i TMA com a precursors. Per tal de comprovar a bondat de les receptes experimentals aportades pel fabricant així com comprovar alguns aspectes de la teoria ALD s’han realitzat una sèrie de mostres variant els diferents paràmetres experimentals, principalment la temperatura de deposició, el nombre de cicles, la durada del cicle i el tipus de substrat. Per a la determinació dels gruixos nanomètrics de les capes i per tant dels ritmes de creixement s’ha utilitzat la el·lipsometria, una de les poques tècniques no destructives capaç de mesurar amb gran precisió gruixos de capes o interfases de pocs àngstroms o nanòmetres. En una primera etapa s'han utilitzat els valors experimentals donats pel fabricant del sistema ALD per determinar el ritme de creixement en funció de la temperatura de dipòsit i del numero de cicles, en ambdós casos sobre diversos substrats. S'ha demostrat que el ritme de creixement augmenta lleugerament en augmentar la temperatura de dipòsit, tot i que amb una variació petita, de l'ordre del 12% en variar 70ºC la temperatura de deposició. Així mateix s'ha demostrat la linealitat del gruix amb el número de cicles, tot i que no s’observa una proporcionalitat exacta. En una segona etapa s'han optimitzat els paràmetres experimentals, bàsicament els temps de purga entre pols i pols per tal de reduir considerablement les durades dels experiments realitzats a relativament baixes temperatures. En aquest cas s’ha comprovat que es mantenien els ritmes de creixement amb una diferencia del 3,6%, 4,8% i 5,5% en optimitzar el cicles en 6,65h, 8,31h, o 8,33h, respectivament. A més, per una d'aquestes condicions s’ha demostrat que es mantenia l’alta conformitat de les capes d’alúmina. A més, s'ha realitzat un estudi de l'homogeneïtat del gruix de les capes en tota la zona de dipòsit del reactor ALD. S’ha demostrat que la variació en gruix de les capes dipositades a 120ºC és com a màxim del 6,2% en una superfície de 110 cm2. Confirmant l’excepcional control de gruixos de la tècnica ALD.

Decentralized model reference control of flexible cable-stayed beam structures

A decentralized model reference controller is designed to reduce the magnitude of the transversal vibration of a flexible cable-stayed beam structure induced by a seismic excitation. The controller design is made based on the principle of sliding mode such that a priori knowledge

Studies about the atomic capabilities concept for linear control systems in physical multi-agent environments

This paper shows the impact of the atomic capabilities concept to include control-oriented knowledge of linear control systems in the decisions making structure of physical agents. These agents operate in a real environment managing physical objects (e.g. their physical bodies) in coordinated tasks. This approach is presented using an introspective reasoning approach and control theory based on the specific tasks of passing a ball and executing the offside manoeuvre between physical agents in the robotic soccer testbed. Experimental results and conclusions are presented, emphasising the advantages of our approach that improve the multi-agent performance in cooperative systems

Effective atomic orbitals for fuzzy atoms

The method of extracting effective atomic orbitals and effective minimal basis sets from molecular wave function characterizing the state of an atom in a molecule is developed in the framework of the "fuzzy" atoms. In all cases studied, there were as many effective orbitals that have considerable occupation numbers as orbitals in the classical minimal basis. That is considered to be of high conceptual importance

Slow recoveries: A structural interpretation

An analysis of the performance of GDP, employment and otherlabor market variables following the troughs in postwar U.S. businesscycles points to much slower recoveries in the three most recentepisodes, but does not reveal any significant change over time in therelation between GDP and employment. This leads us to characterizethe last three episodes as slow recoveries, as opposed to jobless recoveries.We use the estimated New Keynesian model in Galí-Smets-Wouters (2011) to provide a structural interpretation for the slowerrecoveries since the early nineties.

Statistical analysis and modelling of weather radar beam propagation conditions in the Po Valley (Italy)

Ground clutter caused by anomalous propagation (anaprop) can affect seriously radar rain rate estimates, particularly in fully automatic radar processing systems, and, if not filtered, can produce frequent false alarms. A statistical study of anomalous propagation detected from two operational C-band radars in the northern Italian region of Emilia Romagna is discussed, paying particular attention to its diurnal and seasonal variability. The analysis shows a high incidence of anaprop in summer, mainly in the morning and evening, due to the humid and hot summer climate of the Po Valley, particularly in the coastal zone. Thereafter, a comparison between different techniques and datasets to retrieve the vertical profile of the refractive index gradient in the boundary layer is also presented. In particular, their capability to detect anomalous propagation conditions is compared. Furthermore, beam path trajectories are simulated using a multilayer ray-tracing model and the influence of the propagation conditions on the beam trajectory and shape is examined. High resolution radiosounding data are identified as the best available dataset to reproduce accurately the local propagation conditions, while lower resolution standard TEMP data suffers from interpolation degradation and Numerical Weather Prediction model data (Lokal Model) are able to retrieve a tendency to superrefraction but not to detect ducting conditions. Observing the ray tracing of the centre, lower and upper limits of the radar antenna 3-dB half-power main beam lobe it is concluded that ducting layers produce a change in the measured volume and in the power distribution that can lead to an additional error in the reflectivity estimate and, subsequently, in the estimated rainfall rate.

Enhanced radar precipitation estimates using a combined clutter and beam blockage correction technique

Weather radar observations are currently the most reliable method for remote sensing of precipitation. However, a number of factors affect the quality of radar observations and may limit seriously automated quantitative applications of radar precipitation estimates such as those required in Numerical Weather Prediction (NWP) data assimilation or in hydrological models. In this paper, a technique to correct two different problems typically present in radar data is presented and evaluated. The aspects dealt with are non-precipitating echoes - caused either by permanent ground clutter or by anomalous propagation of the radar beam (anaprop echoes) - and also topographical beam blockage. The correction technique is based in the computation of realistic beam propagation trajectories based upon recent radiosonde observations instead of assuming standard radio propagation conditions. The correction consists of three different steps: 1) calculation of a Dynamic Elevation Map which provides the minimum clutter-free antenna elevation for each pixel within the radar coverage; 2) correction for residual anaprop, checking the vertical reflectivity gradients within the radar volume; and 3) topographical beam blockage estimation and correction using a geometric optics approach. The technique is evaluated with four case studies in the region of the Po Valley (N Italy) using a C-band Doppler radar and a network of raingauges providing hourly precipitation measurements. The case studies cover different seasons, different radio propagation conditions and also stratiform and convective precipitation type events. After applying the proposed correction, a comparison of the radar precipitation estimates with raingauges indicates a general reduction in both the root mean squared error and the fractional error variance indicating the efficiency and robustness of the procedure. Moreover, the technique presented is not computationally expensive so it seems well suited to be implemented in an operational environment.

The Coordinated Radio and Infrared Survey for High-Mass Star Formation (The CORNISH Survey). I. Survey Design

We describe the motivation, design, and implementation of the CORNISH survey, an arcsecondresolution radio continuum survey of the inner galactic plane at 5 GHz using the Very Large Array (VLA). It is a blind survey coordinated with the northern SpitzerGLIMPSE I region covering 10° beam across this survey, which covers a wide range of fairly low declinations. Two snapshots separated by 4 h kept the beam elongation to less that 1.5 over 75% of the survey area and less than 2 over 98% of the survey. The prime scientific motivation is to provide an unbiased survey for ultra-compact H II regions to study this key phase in massive star formation. A sensitivity around 2 mJy will allow the automatic distinction between radio-loud and radio-quiet midIR sources found in the Spitzersurveys. This survey has many legacy applications beyond star formation, including evolved stars, active stars and binaries, and extragalactic sources. The CORNISH survey for compact ionized sources complements other Galactic plane surveys that target diffuse and nonthermal sources, as well as atomic and molecular phases to build up a complete picture of the interstellar medium in the Galaxy.