Contribución al desarrollo y caracterización de fotodetectores avanzados basados en nitruros del grupo III

El objetivo de esta tesis es el desarrollo y la caracterización de detectores avanzados, basados en nitruros del grupo III. Es decir, se pretende la utilización de nitruros de galio, indio y aluminio, de manera que se aprovechen las cualidades específicas de estos materiales, para la fabricación de fotodetectores que destaquen y mejoren el actual estado de la técnica en diversos ámbitos o aplicaciones. Las principales líneas de actuación han sido tres: evaluación del comportamiento en alta temperatura, sensibilidad a la polarización de la luz, y aplicación en sistemas de fotoluminiscencia. Se ha realizado un estudio teórico y experimental del comportamiento de fotodetectores de tipo p-i-n en alta temperatura, centrando el interés en la evaluación de la detectividad específica en modo fotovoltaico. Se ha podido comprobar que el silicio, el material semiconductor más extendido en la actualidad, sufre una severa degradación de sus características en alta temperatura, debido a su gap relativamente estrecho. Los nitruros, sin embargo, al ser materiales de gap ancho, tienen mejores características en cuanto al ruido, siendo más ventajosa la diferencia en altas temperaturas. Su rendimiento se ve dificultado, sin embargo, por la peor calidad cristalina del material, lo cual supone un inconveniente más acusado en el caso de los ternarios, y en particular del InGaN. Se ha comprobado que la fabricación de fotodetectores de GaN con múltiple pozo cuántico de InGaN en su región intrínseca posibilita la detección en el visible, sin perjudicar las características eléctricas del fotodiodo. En este tipo de dispositivos, además, se ha llevado a cabo un estudio del ruido en alta y baja temperatura. Se han fabricado fotodetectores de tipo metal-semiconductor-metal con interdigitados sub-micrométricos en GaN crecido sobre el plano cristalino A. Se ha podido comprobar que estos dispositivos tienen una sensibilidad intrínseca a la polarización de la luz incidente para fotones de energías en torno al gap, debido a la anisotropía de la red cristalina en el plano de crecimiento. Además, se han caracterizado eléctricamente estos dispositivos, observando que tienen unos niveles muy bajos de corriente de oscuridad y ruido. Por último, se ha explorado de una manera más profunda la aplicación de los nitruros en sistemas de medida de fotoluminiscencia. Se ha demostrado la funcionalización covalente de superficies de GaN con complejos luminiscentes basados en rutenio, y su sensibilidad a la concentración ambiental de oxígeno. Se ha funcionalizado también un diodo emisor de luz basado en nitruros con este mismo complejo, demostrando la posibilidad de caracterizar el tiempo de vida de la luminiscencia, utilizando como excitación de la misma la luz emitida por este dispositivo. Dado que la simplificación de elementos ópticos dificulta la caracterización del tiempo de vida medio de la luminiscencia, se ha desarrollado un método para la medida del mismo en condiciones en las que no es posible eliminar la señal de fondo óptica debida a la excitación. Dicho método se ha validado experimentalmente realizando la medida de una membrana impregnada del complejo luminiscente de rutenio bajo este tipo de condiciones. Abstract The main purpose of this thesis is the development and characterization of advanced photodetectors, based on group-III nitrides. In other words, what is intended is the utilization of gallium, indium and aluminum nitrides, taking advantage of the specific characteristics of these materials, for the fabrication of photodetectors that stand out and improve the current state of the art for different applications. The three main lines of research have been: high-temperature behaviour assessment, sensitivity to the polarization of light, and application in photoluminescence measurement systems. A theoretical and experimental study on the behaviour of p-i-n photodetectors at high temperatures has been carried out, centered on the evaluation of specific detectivity in photovoltaic mode. Silicon, currently the most widely-used semiconductor, suffers a severe degradation of its characteristics at high temperature, due to its relatively thin bandgap. Nitrides, in contrast, being wide bandgap semiconductors, display better noise characteristics, with this difference being stronger at higher temperature. However, their performance is hindered by the lower crystal quality of the material, especially in the case of the ternary compounds, and for In- GaN in particular. It has been observed that the fabrication of GaN photodetectors with multiple quantum wells in their intrinsic region allows for light detection in the visible range, without sacrificing the electrical performance of the photodiode. In this kind of devices, a low- and high-temperature noise study has been carried out. Metal-semiconductor-metal photodetectors with sub-micron interdigitated contacts have been fabricated on A-plane GaN. It has been possible to confirm that these devices show an intrinsic sensitivity to the polarization of incident light at photon energies around the gap, due to the anisotropy on the crystal lattice. An electrical characterization has revealed that the devices present very low dark current and noise values. Finally, the application of nitrides in photoluminescence measurement systems has been explored. The covalent functionalization of GaN surfaces with rutheniumbased photoluminescent complexes and their sensitivity to the environmental oxygen concentration have been demonstrated. A nitride-based light emitting diode has also been functionalized with this same complex, and the feasibility of characterizing the luminescence lifetime using the light emitted by the device as excitation source has been proved. Since the simplification of optical elements makes the characterization of the luminescence lifetime difficult, a method has been developed for its measurement in conditions in which it is not possible to eliminate the background signal due to excitation. This method has been validated experimentally by measuring the luminescence of a membrane impregnated in luminescent ruthenium complex

Growth of Nitrogen-Doped Mg~xZn~1~-~xO for Use in Visible Rejection Photodetectors

Improvement in the Schottky behavior of metal (Au) contacts with Mg0.01Zn0.99O and Mg0.01Zn0.99O:N thin films were observed by treating the films with hydrogen peroxide (H2O2) (dipping of samples in H2O2 at 100 _C for 3 min). Contacts formed on untreated film showed Ohmic behavior in the current-voltage (I-V ) measurements. The H2O2 treatment led to a smooth surface morphology for the films and resulted in Schottky contact of Au fabricated on the treated films with barrier heights of 0.82≈0.85 eV. The absolute current density at a reverse bias of 3 V was 1≈6 × 10−6 A/cm2 for Au contacts on H2O2-treated films. The treated films showed lower electron concentration than the untreated films due to removal of the relatively high conducting top layers of the thin films. A metal-semiconductor-metal (MSM) detector was fabricated using a Mg0.05Zn0.95O:N film and was characterized for its spectral response

Clonal chromosomal mosaicism and loss of chromosome Y in elderly men increase vulnerability for SARS-CoV-2

The pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2, COVID-19) had an estimated overall case fatality ratio of 1.38% (pre-vaccination), being 53% higher in males and increasing exponentially with age. Among 9578 individuals diagnosed with COVID-19 in the SCOURGE study, we found 133 cases (1.42%) with detectable clonal mosaicism for chromosome alterations (mCA) and 226 males (5.08%) with acquired loss of chromosome Y (LOY). Individuals with clonal mosaic events (mCA and/or LOY) showed a 54% increase in the risk of COVID-19 lethality. LOY is associated with transcriptomic biomarkers of immune dysfunction, pro-coagulation activity and cardiovascular risk. Interferon-induced genes involved in the initial immune response to SARS-CoV-2 are also down-regulated in LOY. Thus, mCA and LOY underlie at least part of the sex-biased severity and mortality of COVID-19 in aging patients. Given its potential therapeutic and prognostic relevance, evaluation of clonal mosaicism should be implemented as biomarker of COVID-19 severity in elderly people. Among 9578 individuals diagnosed with COVID-19 in the SCOURGE study, individuals with clonal mosaic events (clonal mosaicism for chromosome alterations and/or loss of chromosome Y) showed an increased risk of COVID-19 lethality

Search for new particles in events with energetic jets and large missing transverse momentum in proton-proton collisions at root s=13 TeV

A search is presented for new particles produced at the LHC in proton-proton collisions at root s = 13 TeV, using events with energetic jets and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 101 fb(-1), collected in 2017-2018 with the CMS detector. Machine learning techniques are used to define separate categories for events with narrow jets from initial-state radiation and events with large-radius jets consistent with a hadronic decay of a W or Z boson. A statistical combination is made with an earlier search based on a data sample of 36 fb(-1), collected in 2016. No significant excess of events is observed with respect to the standard model background expectation determined from control samples in data. The results are interpreted in terms of limits on the branching fraction of an invisible decay of the Higgs boson, as well as constraints on simplified models of dark matter, on first-generation scalar leptoquarks decaying to quarks and neutrinos, and on models with large extra dimensions. Several of the new limits, specifically for spin-1 dark matter mediators, pseudoscalar mediators, colored mediators, and leptoquarks, are the most restrictive to date.Peer reviewe

Combined searches for the production of supersymmetric top quark partners in proton-proton collisions at root s=13 TeV

A combination of searches for top squark pair production using proton-proton collision data at a center-of-mass energy of 13 TeV at the CERN LHC, corresponding to an integrated luminosity of 137 fb(-1) collected by the CMS experiment, is presented. Signatures with at least 2 jets and large missing transverse momentum are categorized into events with 0, 1, or 2 leptons. New results for regions of parameter space where the kinematical properties of top squark pair production and top quark pair production are very similar are presented. Depending on themodel, the combined result excludes a top squarkmass up to 1325 GeV for amassless neutralino, and a neutralinomass up to 700 GeV for a top squarkmass of 1150 GeV. Top squarks with masses from 145 to 295 GeV, for neutralino masses from 0 to 100 GeV, with a mass difference between the top squark and the neutralino in a window of 30 GeV around the mass of the top quark, are excluded for the first time with CMS data. The results of theses searches are also interpreted in an alternative signal model of dark matter production via a spin-0 mediator in association with a top quark pair. Upper limits are set on the cross section for mediator particle masses of up to 420 GeV

Measurement of the top quark forward-backward production asymmetry and the anomalous chromoelectric and chromomagnetic moments in pp collisions at √s = 13 TeV

Abstract The parton-level top quark (t) forward-backward asymmetry and the anomalous chromoelectric (d̂ t) and chromomagnetic (μ̂ t) moments have been measured using LHC pp collisions at a center-of-mass energy of 13 TeV, collected in the CMS detector in a data sample corresponding to an integrated luminosity of 35.9 fb−1. The linearized variable AFB(1) is used to approximate the asymmetry. Candidate t t ¯ events decaying to a muon or electron and jets in final states with low and high Lorentz boosts are selected and reconstructed using a fit of the kinematic distributions of the decay products to those expected for t t ¯ final states. The values found for the parameters are AFB(1)=0.048−0.087+0.095(stat)−0.029+0.020(syst),μ̂t=−0.024−0.009+0.013(stat)−0.011+0.016(syst), and a limit is placed on the magnitude of | d̂ t| < 0.03 at 95% confidence level. [Figure not available: see fulltext.

Probing effective field theory operators in the associated production of top quarks with a Z boson in multilepton final states at root s=13 TeV

Peer reviewe

Measurement of B-c(2S)(+) and B-c*(2S)(+) cross section ratios in proton-proton collisions at root s=13 TeV

Peer reviewe

Measurement of the W gamma Production Cross Section in Proton-Proton Collisions at root s=13 TeV and Constraints on Effective Field Theory Coefficients

A fiducial cross section for W gamma production in proton-proton collisions is measured at a center-of-mass energy of 13 TeV in 137 fb(-1) of data collected using the CMS detector at the LHC. The W -> e nu and mu nu decay modes are used in a maximum-likelihood fit to the lepton-photon invariant mass distribution to extract the combined cross section. The measured cross section is compared with theoretical expectations at next-to-leading order in quantum chromodynamics. In addition, 95% confidence level intervals are reported for anomalous triple-gauge couplings within the framework of effective field theory.Peer reviewe

Search for dark photons in Higgs boson production via vector boson fusion in proton-proton collisions at √s = 13 TeV

A search is presented for a Higgs boson that is produced via vector boson fusion and that decays to an undetected particle and an isolated photon. The search is performed by the CMS collaboration at the LHC, using a data set corresponding to an integrated luminosity of 130 fb−1, recorded at a center-of-mass energy of 13 TeV in 2016–2018. No significant excess of events above the expectation from the standard model background is found. The results are interpreted in the context of a theoretical model in which the undetected particle is a massless dark photon. An upper limit is set on the product of the cross section for production via vector boson fusion and the branching fraction for such a Higgs boson decay, as a function of the Higgs boson mass. For a Higgs boson mass of 125 GeV, assuming the standard model production rates, the observed (expected) 95% confidence level upper limit on the branching fraction is 3.5 (2.8)%. This is the first search for such decays in the vector boson fusion channel. Combination with a previous search for Higgs bosons produced in association with a Z boson results in an observed (expected) upper limit on the branching fraction of 2.9 (2.1)% at 95% confidence level