Stacked GaAs(Sb)(N)-capped InAs/GaAs quantum dots for enhanced solar cell efficiency

Different approaches have arisen aiming to exceed the Shockley-Queisser efficiency limit of solar cells. Particularly, stacking QD layers allows exploiting their unique properties, not only for intermediate-band solar cells or multiple exciton generation, but also for tandem cells in which the tunability of QD properties through the capping layer (CL) could be very useful.

Stacked GaAsSbN-capped InAs/GaAs quantum dots for enhanced solar cell efficiency

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.

Height control of self-assembled quantum dots

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].

Stranski-Krastanov InN/InGaN quantum dots grown directly on Si(111)

The authors discuss and demonstrate the growth of InN surface quantum dots on a high-In-content In0.73Ga0.27N layer, directly on a Si(111) substrate by plasma-assisted molecular beam epitaxy. Atomic force microscopy and transmission electron microscopy reveal uniformly distributed quantum dots with diameters of 10–40 nm, heights of 2–4 nm, and a relatively low density of ∼7 × 109 cm−2. A thin InN wetting layer below the quantum dots proves the Stranski-Krastanov growth mode. Near-field scanning optical microscopy shows distinct and spatially well localized near-infrared emission from single surface quantum dots. This holds promise for future telecommunication and sensing devices.

Room temperature absorption in laterally biased Quantum Infrared Detectors fabricated by MBE regrowth

In this paper, we show room temperature operation of a quantum well infrared photodetector (QWIP) using lateral conduction through ohmic contacts deposited at both sides of two n-doped quantum wells. To reduce the dark current due to direct conduction in the wells, we apply an electric field between the quantum wells and two pinch-off Schottky gates, in a fashion similar to a field effect device. Since the normal incidence absorption is strongly reduced in intersubband transitions in quantum wells, we first analyze the response of a detector based on quantum dots (QD). This QD device shows photocurrent signal up to 150 K when it is processed in conventional vertical detector. However, it is possible to observe room temperature signal when it is processed in a lateral structure. Finally, the room temperature photoresponse of the QWIP is demonstrated, and compared with theory. An excellent agreement between the estimated and measured characteristics of the device is found

Absorption coefficient for the intraband transitions in quantum dot materials

In this paper, we present calculations of the absorption coefficient for transitions between the bound states of quantum dots grown within a semiconductor and the extended states of the conduction band. For completeness, transitions among bound states are also presented. In the separation of variables, single band k·p model is used in which most elements may be expressed analytically. The analytical formulae are collected in the appendix of this paper. It is concluded that the transitions are strong enough to provide a quick path to the conduction band for electrons pumped from the valence to the intermediate band

The influence of quantum dot size on the sub-bandgap intraband photocurrent in intermediate band solar cells

The effect of quantum dot (QD) size on the performance of quantum dot intermediate band solar cells is investigated. A numerical model is used to calculate the bound state energy levels and the absorption coefficient of transitions from the ground state to all other states in the conduction band. Comparing with the current state of the art, strong absorption enhancements are found for smaller quantum dots, as well as a better positioning of the energy levels, which is expected to reduce thermal carrier escape. It is concluded that reducing the quantum dot size can increase sub-bandgap photocurrent and improve voltage preservation.

InAs/AlGaAs quantum dot intermediate band solar cells with enlarged sub-bandgaps

In the last decade several prototypes of intermediate band solar cells (IBSCs) have been manufactured. So far, most of these prototypes have been based on InAs/GaAs quantum dots (QDs) in order to implement the IB material. The key operation principles of the IB theory are two photon sub-bandgap (SBG) photocurrent, and output voltage preservation, and both have been experimentally demonstrated at low temperature. At room temperature (RT), however, thermal escape/relaxation between the conduction band (CB) and the IB prevents voltage preservation. To improve this situation, we have produced and characterized the first reported InAs/AlGaAs QD-based IBSCs. For an Al content of 25% in the host material, we have measured an activation energy of 361 meV for the thermal carrier escape. This energy is about 250 meV higher than the energies found in the literature for InAs/GaAs QD, and almost 140 meV higher than the activation energy obtained in our previous InAs/GaAs QD-IBSC prototypes including a specifically designed QD capping layer. This high value is responsible for the suppression of the SBG quantum efficiency under monochromatic illumination at around 220 K. We suggest that, if the energy split between the CB and the IB is large enough, activation energies as high as to suppress thermal carrier escape at room temperature (RT) can be achieved. In this respect, the InAs/AlGaAs system offers new possibilities to overcome some of the problems encountered in InAs/GaAs and opens the path for QD-IBSC devices capable of achieving high efficiency at RT.

A numerical study into the influence of quantum dot size on the sub-bandgap interband photocurrent in intermediate band solar cells

A numerical study is presented of the sub-bandgap interband photon absorption in quantum dot intermediate band solar cells. Absorption coefficients and photocurrent densities are calculated for the valence band to intermediate band transitions using a four-band k · p method. It is found that reducing the quantum dot width in the plane perpendicular to the growth direction increases the photocurrent from the valence band to the intermediate-band ground state if the fractional surface coverage of quantum dots is conserved. This provides a path to increase the sub-bandgap photocurrent in intermediate band solar cells.

Realistic detailed balance study of the quantum efficiency of quantum dot solar cells

An attractive but challenging technology for high efficiency solar energy conversion is the intermediate band solar cell (IBSC), whose theoretical efficiency limit is 63%, yet which has so far failed to yield high efficiencies in practice. The most advanced IBSC technology is that based on quantum dots (QDs): the QD-IBSC. In this paper, k·p calculations of photon absorption in the QDs are combined with a multi-level detailed balance model. The model has been used to reproduce the measured quantum efficiency of a real QD-IBSC and its temperature dependence. This allows the analysis of individual sub-bandgap transition currents, which has as yet not been possible experimentally, yielding a deeper understanding of the failure of current QD-IBSCs. Based on the agreement with experimental data, the model is believed to be realistic enough to evaluate future QD-IBSC proposals.

Virtual-bound, filamentary and layered states in a box-shaped quantum dot of square potential form the exact numerical solution of the effective mass Schrodinger equation

The effective mass Schrodinger equation of a QD of parallelepipedic shape with a square potential well is solved by diagonalizing the exact Hamiltonian matrix developed in a basis of separation-of-variables wavefunctions. The expected below bandgap bound states are found not to differ very much from the former approximate calculations. In addition, the presence of bound states within the conduction band is confirmed. Furthermore, filamentary states bounded in two dimensions and extended in one dimension and layered states with only one dimension bounded, all within the conduction band which are similar to those originated in quantum wires and quantum wells coexist with the ordinary continuum spectrum of plane waves. All these subtleties are absent in spherically shaped quantum dots, often used for modeling.

Fotodiodos basados en (Zn,Mg)O para la detección de luz ultravioleta

La presente tesis fue ideada con el objetivo principal de fabricar y caracterizar fotodiodos Schottky en capas de ZnMgO y en estructuras de pozo cuántico ZnMgO/ZnO para la detección de luz UV. La elección de este material semiconductor vino motivada por la posibilidad que ofrece de detectar y procesar señales simultáneamente, en un amplio margen de longitudes de onda, al igual que su más directo competidor el GaN. En esta memoria se da en primer lugar una visión general de las propiedades estructurales y ópticas del ZnO, prestando especial atención a su ternario ZnMgO y a las estructuras de pozo cuántico ZnMgO/ZnO. Además, se han desarrollado los conocimientos teóricos necesarios para una mejor compresión y discusión de los resultados alcanzados. En lo que respecta a los resultados de esta memoria, en esencia, estos se dividen en dos bloques. Fotodiodos desarrollados sobre capas delgadas de ZnMgO no-polar, y sobre estructuras de pozo cuántico de ZnMgO/ZnO no-polares y semipolares Fotodiodos de capas delgadas de ZnMgO. Es bien conocido que la adición de Mg a la estructura cristalina del ZnO desplaza el borde de absorción hacia energías mayores en el UV. Se ha aprovechado esto para fabricar fotodiodos Schottky sobre capas de ZnMgO crecidas por MOCVD y MBE, los cuales detecten en un ventana de energías comprendida entre 3.3 a 4.6 eV. Sobre las capas de ZnMgO, con diferentes contenidos de Mg(5.6-18.0 %), crecidas por MOCVD se han fabricado fotodiodos Schottky. Se han estudiado en detalle las curvas corrientevoltaje (I-V). Seguidamente, se ha realizado un análisis de la respuesta espectral bajo polarización inversa. Tanto los valores de responsividad obtenidos como el contraste UV/VIS están claramente aumentados por la presencia de ganancia. Paralelamente, se han realizado medidas de espectroscopia de niveles profundos (DLOS), identificándose la presencia de dos niveles profundos de carácter aceptor. El papel desempeñado por estos en la ganancia ha sido analizado meticulosamente. Se ha demostrado que cuando estos son fotoionizados son responsables directos del gran aumento de la corriente túnel que se produce a través de la barrera Schottky, dando lugar a la presencia de la ganancia observada, que además resulta ser función del flujo de fotones incidente. Para extender el rango detección hasta 4.6 eV se fabricaron fotodiodos sobre capas de ZnMgO de altísima calidad cristalina crecidas por MBE. Sobre estos se ha realizado un riguroso análisis de las curvas I-V y de las curvas capacidad-voltaje (CV), para posteriormente identificar los niveles profundos presentes en el material, mediante la técnica de DLOS. Así mismo se ha medido la respuesta espectral de los fotodetectores, la cual muestra un corte abrupto y un altísimo contraste UV/VIS. Además, se ha demostrado como estos son perfectos candidatos para la detección de luz en la región ciega al Sol. Por otra parte, se han fabricado fotodiodos MSM sobre estas mismas capas. Se han estudiado las principales figuras de mérito de estos, observándose unas corrientes bajas de oscuridad, un contraste UV/VIS de 103, y la presencia de fotocorriente persistente. Fotodiodos Schottky de pozos cuánticos de ZnO/ZnMgO. En el segundo bloque de esta memoria, con el objeto final de clarificar el impacto que tiene el tratamiento del H2O2 sobre las características optoelectrónicas de los dispositivos, se ha realizado un estudio detallado, en el que se han analizado por separado fotodiodos tratados y no tratados con H2O2, fabricados sobre pozos cuánticos de ZnMgO/ZnO. Se ha estudiado la respuesta espectral en ambos casos, observándose la presencia de ganancia en los dos. A través de un análisis meticuloso de las características electrónicas y optoeletrónicas de los fotodiodos, se han identificado dos mecanismos de ganancia internos diferentes en función de que la muestra sea tratada o no-tratada. Se han estudiado fotodetectores sensibles a la polarización de la luz (PSPDs) usando estructuras de pozo cuántico no-polares y semipolares sobre sustratos de zafiro y sustratos de ZnO. En lo que respecta a los PSPDs sobre zafiro, en los cuales el pozo presenta una tensión acumulada en el plano, se ha visto que el borde de absorción se desplaza _E _21 meV con respecto a luz linealmente polarizada perpendicular y paralela al eje-c, midiéndose un contraste (RE || c /RE c)max _ 6. Con respecto a los PSPDs crecidos sobre ZnO, los cuales tienen el pozo relajado, se ha obtenido un 4E _30-40, y 21 meV para las heteroestructuras no-polar y semipolar, respectivamente. Además el máximo contraste de responsividad fue de (RE || c /RE c)max _ 6 . Esta sensibilidad a la polarización de la luz ha sido explicada en términos de las transiciones excitónicas entre la banda de conducción y las tres bandas de valencia. ABSTRACT The main goal of the present thesis is the fabrication and characterization of Schottky photodiodes based on ZnMgO layers and ZnMgO / ZnO quantum wells (QWs) for the UV detection. The decision of choosing this semiconductor was mainly motivated by the possibility it offers of detecting and processing signals simultaneously in a wide range of wavelengths like its main competitor GaN. A general overview about the structural and optical properties of ZnO, ZnMgO layers and ZnMgO/ZnO QWs is given in the first part of this thesis. Besides, it is shown the necessary theoretical knowledge for a better understanding of the discussion presented here. The results of this thesis may be divided in two parts. On the one hand, the first part is based on studying non-polar ZnMgO photodiodes. On the other hand, the second part is focused on the characterization of non-polar and semipolar ZnMgO / ZnO QWs Schottky photodiodes. ZnMgO photodiodes. It is well known that the addition of Mg in the crystal structure of ZnO results in a strong blue-shift of the ZnO band-gap. Taking into account this fact Schottky photodiodes were fabricated on ZnMgO layers grown by MOCVD and MBE. Concerning ZnMgO layers grown by MOCVD, a series of Schottky photodiodes were fabricated, by varying the Mg content from 5.6% to 18 %. Firstly, it has been studied in detail the current-voltage curves. Subsequently, spectral response was analyzed at reverse bias voltage. Both the rejection ratio and the responsivity are shown to be largely enhanced by the presence of an internal gain mechanism. Simultaneously, measurements of deep level optical spectroscopy were carried out, identifying the presence of two acceptor-like deep levels. The role played for these in the gain observed was studied in detail. It has been demonstrated that when these are photoionized cause a large increase in the tunnel current through the Schottky barrier, yielding internal gains that are a function of the incident photon flux. In order to extend the detection range up to 4.6 eV, photodiodes ZnMgO grown by MBE were fabricated. An exhaustive analysis of the both I-V and CV characteristics was performed. Once again, deep levels were identified by using the technique DLOS. Furthermore, the spectral response was measured, observing sharp absorption edges and high UV/VIS rejections ratio. The results obtained have confirmed these photodiodes are excellent candidates for the light detection in the solar-blind region. In addition, MSM photodiodes have also been fabricated on the same layers. The main figures of merit have been studied, showing low dark currents, a large UV/VIS rejection ratio and persistent photocurrent. ZnMgO/ZnO QWs photodiodes. The second part was focused on ZnMgO/ ZnO QWs. In order to clarify the impact of the H2O2 treatment on the performance of the Schottky diodes, a comparative study using treated and untreated ZnMgO/ZnO photodiodes has been carried out. The spectral response in both cases has shown the presence of gain, under reverse bias. Finally, by means of the analysis of electronic and optoelectronic characteristics, two different internal gain mechanisms have been indentified in treated and non-treated material. Light polarization-sensitive UV photodetectors (PSPDs) using non-polar and semipolar ZnMgO/ZnO multiple quantum wells grown both on sapphire and ZnO substrates have been demonstrated. For the PSPDs grown on sapphire with anisotropic biaxial in-plain strain, the responsivity absorption edge shifts by _E _21 meV between light polarized perpendicular and parallel to the c-axis, and the maximum responsivity contrast is (RE || c /RE c)max _ 6 . For the PSPDs grown on ZnO, with strain-free quantum wells, 4E _30-40, and 21 meV for non-polar and semipolar heterostructures, and maximum (R /R||)max _10. for non-polar heterostructure was achieved. These light polarization sensitivities have been explained in terms of the excitonic transitions between the conduction and the three valence bands.

Intermediate Band Solar Cell with Extreme Broadband Spectrum Quantum Efficiency

We report, for the first time, about an intermediate band solar cell implemented with InAs/AlGaAs quantum dots whose photoresponse expands from 250 to ~ 6000  nm. To our knowledge, this is the broadest quantum efficiency reported to date for a solar cell and demonstrates that the intermediate band solar cell is capable of producing photocurrent when illuminated with photons whose energy equals the energy of the lowest band gap. We show experimental evidence indicating that this result is in agreement with the theory of the intermediate band solar cell, according to which the generation recombination between the intermediate band and the valence band makes this photocurrent detectable. © 2015 American Physical Society

Propiedades ópticas y estructurales de nanoestructuras de Zn(Cd)(Mg)O

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 %.

Quantum dot parameters determination from quantum-efficiency measurements

The energy spectrum of the confined states of a quantum dot intermediate band (IB) solar cell is calculated with a simplified model. Two peaks are usually visible at the lowest energy side of the subbandgap quantum-efficiency spectrum in these solar cells. They can be attributed to photon absorption between well-defined states. As a consequence, the horizontal size of the quantum dots can be determined, and the conduction (valence) band offset is also determined if the valence (conduction) offset is known.