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El trabajo realizado es una contribución al desarrollo de diodos láser de pozo cuántico de InGaAs/GaAs(111)B para emisión en 1.06 -1.1 micras y de GaInNAs/GaAs para la región de 1.3 -1.55 micras. Se presenta un detallado estudio teórico que incluye el cálculo de la estructura de bandas y la ganancia material de los mencionados pozos cuánticos. En el cso del InGaAs, se demuestra emisión láser a temperatura ambiente hasta 1.11 micras y demuestra que el campo piezélectrico está sólo parcialmente apantallado en umbral . También se calcula por primera vez la renormalización del gap en estos dispositivos , resultados ser mayor que en el caso equivalente en substrato convencional (100). En cuanto al GalnNas, además de analizar de forma teórica los cambios que el N produce en la estructura de bandas y la ganancia del material , se realiza un estudio óptico y estructural de pozos cuánticos que permite relacionar el fenómeno de la localización de portadores con la morfología del pozo , así como aclarar el efecto de los tratamientos térmicos en la fotoluminiscencia. Se utilizan diodos electroluminiscentes para demostrar el enorme impacto de la recombinación no-radiativa y finalmente se demuestra emisión láser hasta 1.52 micras con valores record de densidd de corriente umbral.

Detecting the UV part of the spectrum is fundamental for a wide range of applications where ZnMgO has the potential to play a central role. The shortest achievable wavelength is a function of the Mg content in the films, which in turn is dependent on the growth technique. Moreover, increasing Mg contents lead to an electrical compensation of the films, which directly affects the responsivity of the photodetectors. In addition, the metal-semiconductor interface and the presence of grain boundaries have a direct impact on the responsivity through different gain mechanisms. In this work, we review the development of ZnMgO UV Schottky photodiodes using molecular beam epitaxy and spray pyrolysis, and we analyze and compare the physical mechanisms underlying the photodetector behavior. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Recently, quantum dots (QDs) have been drawing an increasing interest as an alternative approach to enhance solar cell efficiency. Multiple-exciton generation in zero-dimensional structures, the possibility to implement intermediate-band solar cells , or the introduction of sub-band-gap states are some of the benefits from using QDs. Indeed, the use of InAs QDs has been demonstrated to extend the photoresponse of bulk GaAs and higher short- circuit currents have already been obtained. The use of a modified capping layer (CL), particularly GaAs(Sb)(N), is expected to further extend the photoresponse from InAs QDs, allowing besides that an independent control of the hole and electron confinement and therefore of their respective escape times.

Different approaches have recently arisen aiming to exceed the Shockley-Queisser efficiency limit. Particularly, the use of self-organized quantum dots (QD) has been recently proposed in order to introduce new states within the barrier material, which enhances the subband gap absorption yielding a photocurrent increase. Stacking QD layers allows exploiting their unique properties for intermediate-band solar cells (SC) or tandem cells.In all these cases, tuning the QD properties by modifying the capping layer (CL) can be very useful.

The capping of epitaxially grown Quantum Dots (QD) is a key process in the fabrication of devices based on these nanostructures because capping can significantly affect the QDs morphology [3]. We have studied the QD morphology after capping in order to better understand the role of the capping process. We have grown real structures and compared the QD morphology obtained by cross-sectional Scanning Tunneling Microscopy (X-STM) with the morphology of QDs that were virtually grown in simulations based on a Kinetic Monte Carlo model (KMC) [1].

The origin of the modified optical properties of InAs/GaAs quantum dots (QD) capped with a thin GaAs1−xSbx layer is analyzed in terms of the band structure. To do so, the size, shape, and composition of the QDs and capping layer are determined through cross-sectional scanning tunnelling microscopy and used as input parameters in an 8 × 8 k·p model. As the Sb content is increased, there are two competing effects determining carrier confinement and the oscillator strength: the increased QD height and reduced strain on one side and the reduced QD-capping layer valence band offset on the other. Nevertheless, the observed evolution of the photoluminescence (PL) intensity with Sb cannot be explained in terms of the oscillator strength between ground states, which decreases dramatically for Sb > 16%, where the band alignment becomes type II with the hole wavefunction localized outside the QD in the capping layer. Contrary to this behaviour, the PL intensity in the type II QDs is similar (at 15 K) or even larger (at room temperature) than in the type I Sb-free reference QDs. This indicates that the PL efficiency is dominated by carrier dynamics, which is altered by the presence of the GaAsSb capping layer. In particular, the presence of Sb leads to an enhanced PL thermal stability. From the comparison between the activation energies for thermal quenching of the PL and the modelled band structure, the main carrier escape mechanisms are suggested. In standard GaAs-capped QDs, escape of both electrons and holes to the GaAs barrier is the main PL quenching mechanism. For small-moderate Sb (

Self-assembled InGaAs quantum dots show unique physical properties such as three dimensional confinement, high size homogeneity, high density and low number of dislocations. They have been extensively used in the active regions of laser devices for optical communications applications [1]. Therefore, buried quantum dots (BQDs) embedded in wider band gap materials have been normally studied. The wave confinement in all directions and the stress field around the dot affect both optical and electrical properties [2, 3]. However, surface quantum dots (SQDs) are less affected by stress, although their optical and electrical characteristics have a strong dependence on surface fluctuation. Thus, they can play an important role in sensor applications

Self-organized InGaAs QDs are intensively studied for optoelectronic applications. Several approaches are in study to reach the emission wavelengths needed for these applications. The use of antimony (Sb) in either the capping layer or into the dots is one example. However, these studies are normally focused on buried QD (BQD) where there are still different controversial theories concerning the role of Sb. Ones suggest that Sb incorporates into the dot [1], while others support the hypothesis that the Sb occupies positions surrounding the dot [2] thus helping to keep their shape during the capping growth.

The development of high efficiency laser diodes (LD) and light emitting diodes (LED) covering the 1.0 to 1.55 μm region of the spectra using GaAs heteroepitaxy has been long pursued. Due to the lack of materials that can be grown lattice-macthed to GaAs with bandgaps in the 1.0 to 1.55 μm region, quantum wells (QW) or quantum dots (QD) need be used. The most successful approach with QWs has been to use InGaAs, but one needs to add another element, such as N, to be able to reach 1.3/1.5μm. Even though LDs have been successfully demonstrated with the QW approach, using N leads to problems with compositional homogeneity across the wafer, and limited efficiency due to strong non-radiative recombination. The alternative approach of using InAs QDs is an attractive option, but once again, to reach the longest wavelengths one needs very large QDs and control over the size distribution and band alignment. In this work we demonstrate InAs/GaAsSb QDLEDs with high efficiencies, emitting from 1.1 to 1.52 μm, and we analyze the band alignment and carrier loss mechanisms that result from the presence of Sb in the capping layer.

The optical and structural properties of InAs/GaAs quantum dots (QD) are strongly modified through the use of a thin (~ 5 nm) GaAsSb(N) capping layer. In the case of GaAsSb-capped QDs, cross-sectional scanning tunnelling microscopy measurements show that the QD height can be controllably tuned through the Sb content up to ~ 14 % Sb. The increased QD height (together with the reduced strain) gives rise to a strong red shift and a large enhancement of the photoluminescence (PL) characteristics. This is due to improved carrier confinement and reduced sensitivity of the excitonic bandgap to QD size fluctuations within the ensemble. Moreover, the PL degradation with temperature is strongly reduced in the presence of Sb. Despite this, emission in the 1.5 !lm region with these structures is only achieved for high Sb contents and a type-II band alignment that degrades the PL. Adding small amounts of N to the GaAsSb capping layer allows to progressively reduce the QD-barrier conduction band offset. This different strategy to red shift the PL allows reaching 1.5 !lm with moderate Sb contents, keeping therefore a type-I alignment. Nevertheless, the PL emission is progressively degraded when the N content in the capping layer is increased

Diluted nitride self-assembled In(Ga)AsN quantum dots (QDs) grown on GaAs substrates are potential candidates to emit in the windows of maximum transmittance for optical fibres (1.3-1.55 μm). In this paper, we analyse the effect of nitrogen addition on the indium desorption occurring during the capping process of InxGa1−xAs QDs (x = l and 0.7). The samples have been grown by molecular beam epitaxy and studied through transmission electron microscopy (TEM) and photoluminescence techniques. The composition distribution inside the dots was determined by statistical moiré analysis and measured by energy dispersive X-ray spectroscopy. First, the addition of nitrogen in In(Ga)As QDs gave rise to a strong redshift in the emission peak, together with a large loss of intensity and monochromaticity. Moreover, these samples showed changes in the QDs morphology as well as an increase in the density of defects. The statistical compositional analysis displayed a normal distribution in InAs QDs with an average In content of 0.7. Nevertheless, the addition of Ga and/or N leads to a bimodal distribution of the Indium content with two separated QD populations. We suggest that the nitrogen incorporation enhances the indium fixation inside the QDs where the indium/gallium ratio plays an important role in this process. The strong redshift observed in the PL should be explained not only by the N incorporation but also by the higher In content inside the QDs

Quantum dot infrared photodetectors (QDIPs) are very attractive for many applications such as infrared imaging, remote sensing and gas sensing, thanks to its promising features such as high temperature operation, normal incidence response and low dark current [1]. However, the key issue is to obtain a high-quality active region which requires an optimization of the nanostructure. By using GaAsSb capping layer, InAs QDs have improved their optical emission in the range between 1.15 and 1.3 m (at Sb composition of 14 %), due to a reduction of a compressive strain in QD and an increment of a QD height [2]. In this work, we have demonstrated strong and narrow intraband photoresponses at ~ 5 m from GaAsSb-capped InAs/GaAs QDIPs under normal light-incidence.

Currently, dilute nitride III-N-As semiconductors, such as InGaAsN/GaAs quantum well material system, allow to develop very competitive lasers at long wavelength emission (1.3 µm). However, longer wavelengths, such as 1.55 µm, are very difficult to achieve without making worse the performance of the device. Alternatively, as it is well known, great efforts are being devoted to the study of dilute nitride III-N-As quantum dots (QDs) semiconductor [1]. Mainly, this is due to the attractive advantages that they show over other materials and structures: the strong reduction in the bandgap of the III-As semiconductor by adding even a few percent of nitrogen into them, and the interesting physical properties that the QDs offer to laser characteristics (e.g. low threshold current, etc)

Cross-sectional scanning tunneling microscopy is used to study at the atomic scale how the structural properties of InAsInGaAsPInP quantum dots _QDs_ are modified when an ultrathin _0–1.5 ML_ GaAs interlayer is inserted underneath the QDs. Deposition of the GaAs interlayer suppresses the influence of the AsP exchange reaction on QD formation and leads to a planarized QD growth surface. A shape transition from quantum dashes, which are strongly dissolved during capping, to well defined QDs takes place when increasing the GaAs interlayer thickness between 0 and 1.0 ML. Moreover, the GaAs interlayer allows the control of the AsP exchange reaction, reducing the QD height for increased GaAs thicknesses above 1.0 ML, and decreases the QD composition intermixing, producing almost pure InAs QDs.

Modelling of III-V semiconductor materials and nanostructures has been a very active field in the last 15 years. The rapid development in the material synthesis of low dimensional structures for optical applications has triggered a world wide interest for modelling methods capable of accurately describing systems comprising millions of atoms. With the development of empirical or semiempirical methods, together with the ever increasing computational power available to scientists, it is now possible to model e.g. quantum dots inside simulation boxes comprising 3 million atoms. In this talk we will review the most recent developments in the field of empirical atomistic methods, particularly the bond order potentials, and discuss its links and reliance on ab initio calculations. The links between these methods and modeling of segregation effect will also be discussed.

En la presente tesis doctoral se ha realizado un estudio utilizando diferentes técnicas de crecimiento (RPE-MOCVD y spray pyrolysis) y estructuras (nanohilos, pozos y puntos cuánticos y capas) con el objetivo de desarrollar dispositivos que cubran desde el rango visible hasta el ultravioleta. Es por esta razón por la que se han elegido materiales basados en ZnO, debido a la posibilidades que estos ofrecen para variar su bandgap en un amplio rango de energías. Prueba de ello es que en este estudio se ha conseguido cubrir un rango espectral desde 1.86 hasta 4.11 eV, estudiandose además fenómenos físicos como son la difusión e incorporaci ón de la aleación o la adsorción de gases en la super_cie, lo que ha permitido la fabricación de diferentes fotodetectores de gran sensibilidad. Por todo ello, los resultados obtenidos en esta tesis suponen una gran contribución al conocimiento de las propiedades físicas de las aleaciones de Zn(Cd)O y Zn(Mg)O para potenciales aplicaciónes en dispositivos que operen en el rango visible y ultravioleta del espectro, respectivamente. En esta memoria se da en primer lugar una visión de las propiedades de materiales basados en ZnO, entrando en detalle en una de las ventajas que este presenta, la facilidad que tiene este material para formar nanoestructuras. En el capítulo 3 se dan los conceptos teóricos necesarios para comprender las propiedades ópticas de este tipo de materiales, mostrando también los resultados más reseñables obtenidos en ZnO. En los capítulos referentes a los resultados se pueden diferenciar dos grandes bloques. En el primer bloque de resultados se han analizado nanohilos y pozos cuánticos de Zn(Cd)O crecidos por la técnica de RPE-MOCVD (Capítulos 4 y 5). En el segundo se expondrá el estudio realizado sobre capas y puntos cuánticos de Zn(Mg)O crecidos por la técnica spray pyrolysis como se describe en mayor detalle a continuación. Nanohilos y pozos cuánticos de Zn(Cd)O crecidos por RPE-MOCVD Teóricamente aleando el ZnO con CdO es posible disminuir el valor del band- gap desde 3.37 eV hasta 0.95 eV, cubriendo por completo el espectro visible. El desarrollo del ternario Zn(Cd)O permitiría la fabricación de heteroestructuras y pozos cuánticos, muy importantes en el desarrollo de dispositivos optoelectrónicos que cubran la parte visible del espectro. Sin embargo, la diferencia de estructura cristalina entre estos dos materiales junto a la baja solubilidad del Cd y su alta presión de vapor, di_culta la obtención de material de alta calidad cristalina con alto contenido en Cd. En esta tesis doctoral se ha realizado una completa caracterización óptica y estructural de nanohilos de Zn(Cd)O credidos por la técnica de RPE-MOCVD. Estos nanohilos tinene unas longitudes comprendidas entre 1 y 3 _m y diámetros entre 100 y 200 nm. La concentración máxima introducida de Cd en estas estructuras ha sido de hasta un 54% manteniendo la estructura wurtzita del ZnO, siendo este el mayor contenido de Cd introducido hasta la fecha en nanostructuras basada en ZnO. Este hecho se traduce en una variación de la energía de emisión entre 3.31 y 1.86 eV con el aumento en Cd. El uso de diferentes técnicas de alta resoluci ón de caracterización estructural ha permitido demostrar la presencia de una sola fase estructural wurtzita sin observarse ningún indicio de separación de fases ni acumulación de Cd a lo largo del nanohilo para todos los contenidos de Cd. Con el propósito de fabricar dispositivos en nanohilos individuales, parte de esta tesis doctoral ha estado dedicada a estudiar el impacto que el recocido térmico tiene en las propiedades ópticas y eléctricas de nanohilos de Zn(Cd)O. El recocido térmico es un proceso clave en la optimización de dispositivos, ya sea para la obtenci ón de contactos óhmicos, reducción de defectos o difusión de dopantes por ejemplo. En este estudio se ha observado una mejora muy signi_cativa de las propiedades de emisión de los nanohilos cuando estos eran recocidos a temperaturas mayores que la de crecimiento (300 oC). En las muestras con Cd se ha observado además que el recocido también produce un desplazamiento de la emisión hacia mayores energías debido a una reducción homogénea del contenido de Cd. Medidas de fotoluminiscencia con resolución temporal muestran el impacto que tiene la localización del excitón en las _uctuaciones de potencial, debidas a una distribución estadística del Cd, en la dinámica de los portadores. Comparando el tiempo de vida de los portadores entre los nanohilos recocidos y sin recocer se ha observado un aumento de este parámetro en las estructuras recocidas. Este aumento es fundamentalmente debido a una reducción de centros de recombinación no radiativa asociados a defectos presentes a lo largo del nanohilo. Además, se ha estudiado la evolución de los tiempos de vida de los portadores en función de la temperatura, registrándose una menor estabilidad con la temperatura de los tiempos de vida en las muestras recocidas. Este resultado sugiere que el recocido térmico consigue reducir parte del desorden de la aleación en la estructura. Tras haber caracterizados los nanohilos se desarrollaron una serie de procesa dos para la fabricación de dispositivos basados en nanohilos individuales. Se fabricaron en concreto fotodetectores sensibles al UV, en los que se observó también la alta sensibilidad que muestran a la adsorción de gases en la super_cie, incrementada por la gran relación super_cie/volúmen característica de las nanoestructuras. Estos procesos de adsorción observados tienen un impacto directo sobre las propiedades ópticas y electricas de los dispositivos como se ha demostrado. Por ello que en esta tesis se hayan estudiado en detalle este tipo de procesos, ideando maneras para tener un mayor control sobre ellos. Finalmente se crecieron estructuras de pozos cuántico de ZnCdO/ZnO en nanohilos con contenidos de Cd nominales de 54 %. Las medidas ópticas realizadas mostraron como al aumentar la anchura del pozo de 0.7 a 10 nm, la emisión relacionada con el pozo se desplazaba entre 3.30 y 1.97 eV. Este gran desplazamiento representa el mayor obtenido hasta la fecha en pozos cuánticos de ZnCdO/ZnO. Sin embargo, al caracterizar estructuralmente estas muestras se observó la presencia de procesos de difusión de Cd entre el pozo y la barrera. Como se ha podido medir, este tipo de procesos reducen sustancialmente la concentración de Cd en el pozo al difundirse parte a la barrera. cambiando completamente la estructura de bandas nominal de estas estructuras. Este estudio demuestra la importancia del impacto de los procesos de difusión en la interpretación de los efectos de con_namiento cuántico para este tipo de estructuras. Capas y puntos cuánticos de Zn(Mg)O crecidos por spray pyrolysis La técnica de spray pyrolysis, debido a su simplicidad, bajo coste y capacidad de crecer sobre grandes áreas conservando una alta calidad cristalina presenta un gran interés en la comunidad cientí_ca para el potencial desarrollo de dispositivos comerciales. En esta tesis se ha estudiado las propiedades ópticas y eléctricas de capas y puntos cuánticos de Zn(Mg)O crecidos por esta técnica. Al contrario que pasa con el Cd, al introducir Mg en la estructura wurtzita de ZnO se consigue aumentar el bandgap del semiconductor. Sin embargo, al igual que pasa con el CdO, la diferencia de estructura cristalina entre el ZnO y el MgO limita la cantidad de Mg que se puede incorporar, haciendo que para una cierta concentración de Mg aparezcan el fenómeno de separación de fases. En esta tesis se ha conseguido incorporar hasta un contenido de Mg del 35% en la estructura wurtzita del ZnO utilizando la técnica de spray pyrolysis, resultado que representa la mayor concentración de Mg publicada hasta la fecha. Este hecho ha posibilitado variar la energía del borde de absorción desde 3.30 a 4.11 eV. En estas capas se ha realizado una completa caracterización óptica observándose una diferencia entre las energías del borde de absorción y del máximo de emisión creciente con el contenido en Mg. Esta diferencia, conocida como desplazamiento de Stokes, es debida en parte a la presencia de _uctuaciones de potencial producidas por un desorden estadístico de la aleación. Se han fabricado fotodetectores MSM de alta calidad utilizando las capas de Zn(Mg)O previamente caracterizadas, observándose un desplazamiento del borde de absorción con el aumento en Mg desde 3.32 a 4.02 eV. Estos dispositivos muestran altos valores de responsividad (10-103 A/W) y altos contrastes entre la responsividad bajo iluminación y oscuridad (10-107). Estos resultados son en parte debidos a la presencia de mecanismos de ganancia y una reducción de la corriente de oscuridad en las muestras con alto contenido de Mg. Utilizando esta misma técnica de crecimiento se han crecido puntos de Zn(Mg)O con concentraciones nominales de Mg entre 0 y 100 %, con dimensiones medias entre 4 y 6 nm. Las medidas estructurales realizadas muestran que hasta un valor de Mg de 45 %, los puntos están compuestos por una única fase estructural, wurtzita. A partir de esa concentración de Mg aparece una fase cúbica en los puntos, coexistiendo con la fase hexagonal hasta una concentración nominales del 85 %. Para concentraciones mayores de Mg, los puntos muestran una única fase estructural cúbica. Medidas de absorción realizadas en estos puntos de Zn(Mg)O muestran un desplazamiento del borde de absorción entre 3.33 y 3.55 eV cuando la concentraci ón de Mg en los puntos aumenta hasta el 40 %. Este desplazamiento observado es debido solamente a la fase wurtzita del Zn(Mg)O donde se incorpora el Mg. ABSTRACT This PhD theis presents a study using di_erent growth techniques (RPEMOCVD and spray pyrolysis) and structures (nanowires, quantum dots and wells and layers) in order to develop devices that extend from the visible to the ultraviolet range. For this reason ZnO based materials have been choosen, because they o_er the possibility to tunne the bandgap in this energy range. Proof of this is that this study has managed to cover a spectral range from 1.86 to 4.11 eV, also being studied physical phenomena such as di_usion and incorporation of alloy or adsorption of gases on the surface, allowing the develop di_erent highly sensitive photodetectors. Therefore, the results obtained in this thesis are a great contribution two large blockso the knowledge of the physical properties of alloys Zn(Cd)O and Zn(Mg)O for potential applications in devices that operate in the visible and ultraviolet range, respectively. In the _rst chapter, the general properties of ZnO-based materials are presented, showing the facilities that these kind of materials o_er to obtain di_erent nanoestructures. In Chapter 3, optical theoretical concepts are given to understand the optical properties of these materials, also showing the most signi_cant results of ZnO. In the chapters related with the results, two blocks could be distinguish. In the _rst one, Zn(Cd)O nanowires and quantum wells grown by RPE-MOCVD have been analyzed (Chapters 4 and 5). The second block of results shows the study performed in Zn(Mg)O _lms and quantum dots grown by spray pyrolysis. Zn(Cd)O nanowires and quantum wells grown by RPE-MOCVD In summary, the results of the PhD thesis are a great contribution to the knowledge of the physical properties of Zn(Cd)O and Zn(Mg)O alloys and their application for high performance devices operating in the visible and UV ranges, respectively. The performance of the device is still limited due to alloy solubility and p-doping stability, which opens a door for future research in this _eld. Theoretically, annealing ZnO with CdO allows to reduce the bandgap from 3.37 to 0.95 eV, covering the whole visible spectrum. The development of ZnCdO alloys allows the fabrication of heterostructures and quantum wells, necessary for the development of high performance optoelectronic devices. However, the di_erent crystal structures between CdO and ZnO and the low solubility of Cd and its high vapor pressure, hinders the growth of ZnCdO alloys with high Cd contents. In this PhD thesis Zn(Cd)O nanowires have been optically and structurally characterized, obtaining a maximum Cd content of 54% while maintaining their wurtzite structure. This Cd content, which allows lowering the bandgap down to 1.86 eV, is the highest concentration ever reported in nanostructures based on ZnO. The combination of optical and structural characterization techniques used during this thesis has allowed the demonstration of the presence of a single wurtzite structure, without observing any indication of phase separation or Cd accumulation along the nanowire. Annealing processes are essential in the fabrication of optoelectronic devices. For this reason, a complete study of the annealing e_ects in the optical and electrical properties of Zn(Cd)O nanowires has been performed. In the _rst place, annealing nanowires at higher temperatures than their growth temperature (300 oC) allows a signi_cant improvement of their emission properties. However, in the samples that contain Cd a shift in the emission towards higher energies has been observed due to a homogeneous reduction of the Cd content in the nanowires. Time resolved photoluminescence measurements show the impact of the exciton localization in the potential _uctuations due to a statistical alloy disorder. An increase in the carrier lifetime has been obtained for the annealed nanowires. This increase is mainly due to the reduction of non-radiative recombination centers associated with the defects present in the material. Furthermore, temperature dependent time resolved photoluminescence measurements suggest a reduction of the alloy disorder in the annealed samples. In this thesis, single nanowire photodetectors with a high responsivity in the UV range have been demonstrated. Due to the high surface/volume ratio, these structures are very sensitive to gas adsorption at the surface, which largely de_nes the optical and electrical properties of the material and, therefore, of the device. With the aim of obtaining time stable devices, the dynamic adsorption-desorption processes have been studied, developing di_erent approaches that allow a higher control over them. Finally, ZnCdO/ZnO quantum wells have been grown with a nominal Cd concentration of 54% inside the well. The performed optical measurements show that increasing the well width from 0.7 to 10 nm, shifts the emission related with the well from 3.30 to 1.97 eV. This result represents the highest shift reported in the literature. However, a detailed structural characterization shows the presence of di_usion phenomena which substantially reduce the concentration of Cd in the well, while increasing it in the barrier. This type of phenomena should be considered when ac curately interpretating the quantum con_nement e_ects in Zn(Cd)O/ZnO quantum wells. Theoretically, annealing ZnO with CdO allows to decrease the bandgap from 3.37 to 0.95 eV, covering the whole visible spectrum. Zn(Mg)O _lms and quantum dots grown by spray pyrolysis Due to its simplicity, low-cost and capacity to grow over large areas conserving a high crystal quality, spray pyrolysis technique presents a great interest in the scienti_c community for developing comercial devices. In this thesis, a complete study of the optical and structural properties of Zn(Mg)O _lms and quantum dots grown by spray pyrolysis has been performed. Contrary to Zn(Cd)O alloys, when introducing Mg in the ZnO wurtzite structure an increase in the bandgap in obtained. Once again, the di_erence in the crystal structure of ZnO and MgO limits the amount of Mg that can be introduced before phase separation appears. In this PhD thesis, a maximum Mg content of 35% has been incorporated in the wurtzite structure using spray pyrolysis. This variation in the Mg content translates into an increase of the absorption edge from 3.30 to 4.11 eV. Up to this date, this result represents the highest Mg content introduced by spray pyrolysis in a ZnO wurzite structure reported in the literature. The comparison of the emission and absorption spectra shows the presence of an increasing Stokes shift with Mg content. This phenomenon is partialy related with the presence of potential _uctuations due to an statistic alloy disorder. MSM photodetectors have been processed on previously characterized Zn(Mg)O _lms. These devices have shown a shift in the absorption edge from 3.32 to 4.02 eV with the increase in Mg content, high responsivity values (10-103 A/W) and high contrast ratios between illuminated and dark responsivities (10-107). These values are explained by the presence of a gain mechanism and a reduction of dark current in the ZnMgO samples. Zn(Mg)O quantum dots have also been grown using spray pyrolysis with Mg concentrations between 0 and 100% and with average widths ranging 4 to 6 nm. Structural measurements show that at a Mg concentration of 45% the cubic phase appears, coexisting with the hexagonal phase up to an 85% concentration of Mg content. From 85% onwards the quantum dots show only the cubic phase. Absorption measurements performed in these structures reveal a shift in the absorption edge from 3.33 to 3.55 eV when the Mg content increases up to 40 %.

Las células solares de multi-unión basadas en aleaciones semiconductoras III-V han ostentado el récord de eficiencia de conversión energética durante muchos años, que actualmente se aproxima al 50 %. Cálculos teóricos de eficiencia límite, para los que se emplean diseños optimizados con la correcta combinación de parámetros de red y de energía de bandgap, determinan que es necesario obtener un material semiconductor de 1,0–1,15 eV ajustado en red al GaAs o el Ge. El uso de dicho material para formar una de las capas de una célula de 4 uniones podría permitir alcanzar una eficiencia del 60 % bajo concentración solar, lo que supondría un gran hito en el campo de la energía fotovoltaica. Por lo tanto, actualmente se están realizando grandes esfuerzos para investigar materiales con bandgap de 1,0–1,15 eV que puedan crecerse ajustados a la red del GaAs o del Ge. Los nitruros diluidos, como son el Ga1-xInxAs1-yNy o el GaAs1-x-ySbxNy, son candidatos prometedores: en ellos, la introducción de un pequeño porcentaje de N conlleva una fuerte reducción de la energía de bandgap, mientras que su constante de red puede ajustarse a la del GaAs o el Ge. Específicamente el GaAs1-x-ySbxNy presenta ciertas ventajas frente al Ga1-xInxAs1-yNy, como son una incorporación más eficiente del N y una menor formación de defectos relacionados con la presencia del N. Sin embargo, los nitruros diluidos, incluyendo el GaAs1-x-ySbxNy, se enfrentan a problemas inherentes a su crecimiento cuaternario que degradan seriamente la dinámica de sus portadores, lo que probablemente sea el motivo de la falta de éxito de las células solares basadas en GaAs1-x-ySbxNy que se ha observado hasta el momento. En esta tesis se demuestra que la sustitución de la aleación cuaternaria por una superred de GaAs1-xSbx/GaAs1-yNy con alineamiento tipo-II puede ser útil para obtener la capa de 1,0–1,15 eV ajustada en red que se desea implementar en la célula monolítica de multi-unión previamente diseñada. Por un lado, la separación espacial de los átomos de N y Sb evita los problemas de crecimiento del cuaternario, y además este tipo de estructura permite un control extra de la energía efectiva del bandgap a través del grosor de su periodo. Por otro lado, el alineamiento de tipo-II proporciona mayores tiempos de vida radiativos, que pueden llegar a reducirse a los tiempos propios del material cuaternario reduciendo el grosor de periodo. Además, la reducción del grosor de periodo en las superredes tipo-II conlleva mejoras en la absorción debido al solapamiento de las funciones de onda, así como el cambio en el régimen de transporte de portadores de difusivo a cuasi-balístico, lo que mejora la eficiencia de extracción de cargas. En estructuras con bajos contenidos de N y Sb (aquellos requeridos para obtener estructuras con energía de bandgap de ~1,15 eV) el desempeño como célula solar de una única unión de las superredes GaAs1-xSbx/GaAs1-yNy no supera al de células de capas gruesas de GaAs1-x-ySbxNy equivalentes (las cuales contienen el doble de N y Sb) en términos de eficiencia de conversión. Sin embargo, para contenidos de N y Sb mayores (aquellos requeridos para obtener estructuras con energía de bandgap de ~1,0 eV), las células solares de superred son claramente superiores a las células gruesas en términos de eficiencia de conversión. De hecho, superredes con un grosor de período de 3 nm muestran una mejora en eficiencia del 134 % comparadas con células gruesas equivalentes. La mejora se atribuye a la disminución de la recombinación no-radiativa y a la mayor homogeneidad en la composición de las superredes. Para explotar plenamente las ventajas fotovoltaicas de las superredes tipo-II, se ha aplicado un proceso de recocido térmico rápido a las diferentes estructuras. El recocido térmico reduce la densidad de estados de defectos radiativos inducidos por la presencia de N, lo que parece ser la causa de la mejora en el voltaje de circuito abierto (VOC) observada después del recocido, especialmente en las células solares de bajo contenido de N con bandgap de ~1,15 eV. Los resultados sugieren que la recombinación radiativa que proviene de estados de defectos de N es fuente de degradación para el VOC en las células solares basadas en GaAs(Sb)(N) sin recocer. En células solares con altos contenidos de N y Sb con bandgap de ~1,0 eV, no solo el VOC sino también la densidad de corriente de circuito abierto (JSC) se incrementa notablemente después del proceso de recocido, lo que conlleva una gran mejora en la eficiencia de conversión de dichas células. El gran incremento de JSC después del recocido térmico en células solares con energía de bandgap de ~1.0 eV es de especial interés, ya que podría mejorar el ajuste de corrientes entre sub-células al integrar estas estructuras en células solares de multi-unión. ----------ABSTRACT---------- III-V multi-junction solar cells (MJSCs) have hold record conversion efficiencies for many years, which is currently approaching 50 %. Theoretical efficiency limits are calculated using optimum designs with the right lattice constant-bandgap energy combination, which requires a 1.0–1.15 eV bandgap semiconductor material lattice-matched to GaAs/Ge. Insertion of such a material layer in a 4-junction MJSC could lead to an efficiency of 60 % under solar concentration, which would represent a significant breakthrough in photovoltaics. Therefore, 1.0–1.15 eV bandgap materials that can be grown lattice-matched to GaAs/Ge are being nowadays intensively researched. Dilute nitrides, such as Ga1-xInxAs1-yNy or GaAs1-x-ySbxNy, are the most suitable candidates: the introduction of a small amount of N in the GaAs matrix sharply reduces the bandgap energy, and at the same time the lattice constant can be adjusted to that of GaAs/Ge. In particular, GaAs1-x-ySbxNy has many potential advantages over Ga1-xInxAs1-yNy, such as promoting a more efficient N incorporation and reducing the formation of N-related defects. However, quaternary dilute nitrides, even GaAs1-x-ySbxNy, unavoidably suffer from inherent material problems that seriously degrade carrier dynamics, which are likely the reason for the lack of success of the GaAs1-x-ySbxNy based solar cells up to now. In this Thesis, we demonstrate that the substitution of the conventional quaternary alloy by a strain-balanced GaAs1-xSbx/GaAs1-yNy superlattice (SL) with a type-II band alignment is a suitable approach to form the lattice-matched 1.0–1.15 eV subcell to be implemented in the optimum monolithic multi-junction solar cell design. The spatial separation of Sb and N atoms avoids the ubiquitous growth problems, providing an accurate composition control and improving the crystal quality. Moreover, these new structures allow for additional control of the effective bandgap through the period thickness. The type-II band alignment provides long carrier lifetimes, which are also tunable down to the values of the bulk alloys by reducing the period thickness. A reduced period thickness in type-II SLs also results in enhanced absorption due to increased wavefunction overlap, as well as in a change in the transport regime from diffusive to quasi-ballistic, which provides improved carrier extraction efficiency. For the “low” N and Sb contents required in the ~1.15 eV structures, single-junction SL solar cells do not overcome the equivalent bulk devices (the latter having double amount of N and Sb) in terms of power conversion efficiency (PCE). Nevertheless, for the higher N and Sb contents required in the ~1.0 eV structures, the SL approach is advantageous in terms of solar cell PCE. Indeed, 3 nm period SL solar cells show an enhanced PCE of 134 % over the equivalent bulk devices. The improvement is attributed to a reduced non-radiative recombination and an improved composition homogeneity in the SLs. To fully exploit the potential of type-II SLs in photovoltaics, an adequate rapid thermal annealing (RTA) cycle might be applied to the structures. The RTA is shown to reduce the density of N-induced sub-bandgap radiative states, which seems to be the main reason for the enhancement of the open-circuit voltage (VOC) observed after RTA, particularly in the “low” N content ~1.15 eV structures. The results suggest that radiative recombination in a broad band of deep defect states is a source of VOC degradation in as-grown GaAs(Sb)(N)-based solar cells. In solar cells with higher N and Sb contents and ~1.0 eV bandgap, not only VOC but also short-circuit current density (JSC) is strongly increased after RTA, resulting in substantial enhancements of the PCE. The large increase of JSC after RTA in ~1.0 eV samples is particularly relevant since it could help to provide current matching when integrated in a MJSC.