Deep levels in a-plane, high Mg-content MgxZn1-xO epitaxial layers grown by molecular beam epitaxy

Deep level defects in n-type unintentionally doped a-plane MgxZn1−xO, grown by molecular beam epitaxy on r-plane sapphire were fully characterized using deep level optical spectroscopy (DLOS) and related methods. Four compositions of MgxZn1−xO were examined with x = 0.31, 0.44, 0.52, and 0.56 together with a control ZnO sample. DLOS measurements revealed the presence of five deep levels in each Mg-containing sample, having energy levels of Ec − 1.4 eV, 2.1 eV, 2.6 V, and Ev + 0.3 eV and 0.6 eV. For all Mg compositions, the activation energies of the first three states were constant with respect to the conduction band edge, whereas the latter two revealed constant activation energies with respect to the valence band edge. In contrast to the ternary materials, only three levels, at Ec − 2.1 eV, Ev + 0.3 eV, and 0.6 eV, were observed for the ZnO control sample in this systematically grown series of samples. Substantially higher concentrations of the deep levels at Ev + 0.3 eV and Ec − 2.1 eV were observed in ZnO compared to the Mg alloyed samples. Moreover, there is a general invariance of trap concentration of the Ev + 0.3 eV and 0.6 eV levels on Mg content, while at least and order of magnitude dependency of the Ec − 1.4 eV and Ec − 2.6 eV levels in Mg alloyed samples.

Deep level analysis of homoepitaxial ZnO doped with N

High intrinsic carrier concentration (n-type) • Efforts to reduce this effect: • Homoepitaxy1 • Non-polar orientations • Similar samples exhibit residual doping as low as ~1014 cm-3 (2) The path to p-type doping • Many dopants proposed • N is a promising candidate • Simple NO is a deep level • Complex levels have shallower energies • N-related levels observed near the VB by many groups • Energies between 130 meV and 160 meV from VBM

Obtaining an intermediate band photovoltaic material through the Bi insertion in CdTe

Defect interaction can take place in CdTe under Te and Bi rich conditions. We demonstrate in this work through first principles calculations, that this phenomenon allows a Jahn Teller distortion to form an isolated half-filled intermediate band in the host semiconductor band-gap. This delocalized energy band supports the experimental deep level reported in the host band-gap of CdTe at a low bismuth concentration. Furthermore, the calculated optical absorption of CdTe:Bi in this work shows a significant subband-gap absorption that also supports the enhancement of the optical absorption found in the previous experimental results.

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 cells. The transition metal supersaturated Silicon approach

Within the framework of the third solar cell generation some new ideas to enlarge the spectral response of the solar cells toward the infrared have been proposed. Among them the inclusion of an Intermediate Band (IB) seems to be very promising. This paper will deal with one of the ways to generate the IB namely the deep level center approach. We will discuss not only its existence but also the carriers lifetime recovery which is necessary to obtain the expected increase of the solar cell efficiency.

Review of experimental results related to the operation of intermediate band solar cells

The intermediate band solar cell (IBSC) has drawn the attention of the scientific community as a means to achieve high-efficiency solar cells. Complete IBSC devices have been manufactured using quantum dots, highly mismatched alloys, or bulk materials with deep-level impurities. Characterization of these devices has led, among other experimental results, to the demonstration of the two operating principles of an IBSC: the production of the photocurrent from the absorption of two below bandgap energy photons and the preservation of the output voltage of the solar cell. This study offers a thorough compilation of the most relevant reported results for the variety of technologies investigated and provides the reader with an updated record of IBSC experimental achievements. A table condensing the reported experimental results is presented, which provides information at a glance about achievements, as well as pending results, for every studied technology.

Novel Gravity Distortion Evaluation on Large Antenna Array for Deep Space Communication

This paper presents a simple gravity evaluation model for large reflector antennas and the experimental example for a case study of one uplink array of 4x35-m antennas at X and Ka band. This model can be used to evaluate the gain reduction as a function of the maximum gravity distortion, and also to specify this at system designer level. The case study consists of one array of 35-m antennas for deep space missions. Main issues due to the gravity effect have been explored with Monte Carlo based simulation analysis.

Direct estimation wave setup as a medium level in swash

The extreme runup is a key parameter for a shore risk analysis in which the accurate and quantitative estimation of the upper limit reached by waves is essential. Runup can be better approximated by splitting the setup and swash semi-amplitude contributions. In an experimental study recording setup becomes difficult due to infragravity motions within the surf zone, hence, it would be desirable to measure the setup with available methodologies and devices. In this research, an analysis is made of evaluated the convenience of direct estimation setup as the medium level in the swash zone for experimental runup analysis through a physical model. A physical mobile bed model was setup in a wave flume at the Laboratory for Maritime Experimentation of CEDEX. The wave flume is 36 metres long, 6.5 metres wide and 1.3 metres high. The physical model was designed to cover a reasonable range of parameters, three different slopes (1/50, 1/30 and 1/20), two sand grain sizes (D50 = 0.12 mm and 0.70 mm) and a range for the Iribarren number in deep water (ξ0) from 0.1 to 0.6. Best formulations were chosen for estimating a theoretical setup in the physical model application. Once theoretical setup had been obtained, a comparison was made with an estimation of the setup directly as a medium level of the oscillation in swash usually considered in extreme runup analyses. A good correlation was noted between both theoretical and time-averaging setup and a relation is proposed. Extreme runup is analysed through the sum of setup and semi-amplitude of swash. An equation is proposed that could be applied in strong foreshore slope-dependent reflective beaches.

About the origin of low wafer performance and crystal defect generation on seed-cast growth of industrial mono-like silicon ingots

The era of the seed-cast grown monocrystalline-based silicon ingots is coming. Mono-like, pseudomono or quasimono wafers are product labels that can be nowadays found in the market, as a critical innovation for the photovoltaic industry. They integrate some of the most favorable features of the conventional silicon substrates for solar cells, so far, such as the high solar cell efficiency offered by the monocrystalline Czochralski-Si (Cz-Si) wafers and the lower cost, high productivity and full square-shape that characterize the well-known multicrystalline casting growth method. Nevertheless, this innovative crystal growth approach still faces a number of mass scale problems that need to be resolved, in order to gain a deep, 100% reliable and worldwide market: (i) extended defects formation during the growth process; (ii) optimization of the seed recycling; and (iii) parts of the ingots giving low solar cells performance, which directly affect the production costs and yield of this approach. Therefore, this paper presents a series of casting crystal growth experiments and characterization studies from ingots, wafers and cells manufactured in an industrial approach, showing the main sources of crystal defect formation, impurity enrichment and potential consequences at solar cell level. The previously mentioned technological drawbacks are directly addressed, proposing industrial actions to pave the way of this new wafer technology to high efficiency solar cells.

Design and Simulation of Deep Nanometer SRAM Cells under Energy, Mismatch, and Radiation Constraints

La fiabilidad está pasando a ser el principal problema de los circuitos integrados según la tecnología desciende por debajo de los 22nm. Pequeñas imperfecciones en la fabricación de los dispositivos dan lugar ahora a importantes diferencias aleatorias en sus características eléctricas, que han de ser tenidas en cuenta durante la fase de diseño. Los nuevos procesos y materiales requeridos para la fabricación de dispositivos de dimensiones tan reducidas están dando lugar a diferentes efectos que resultan finalmente en un incremento del consumo estático, o una mayor vulnerabilidad frente a radiación. Las memorias SRAM son ya la parte más vulnerable de un sistema electrónico, no solo por representar más de la mitad del área de los SoCs y microprocesadores actuales, sino también porque las variaciones de proceso les afectan de forma crítica, donde el fallo de una única célula afecta a la memoria entera. Esta tesis aborda los diferentes retos que presenta el diseño de memorias SRAM en las tecnologías más pequeñas. En un escenario de aumento de la variabilidad, se consideran problemas como el consumo de energía, el diseño teniendo en cuenta efectos de la tecnología a bajo nivel o el endurecimiento frente a radiación. En primer lugar, dado el aumento de la variabilidad de los dispositivos pertenecientes a los nodos tecnológicos más pequeños, así como a la aparición de nuevas fuentes de variabilidad por la inclusión de nuevos dispositivos y la reducción de sus dimensiones, la precisión del modelado de dicha variabilidad es crucial. Se propone en la tesis extender el método de inyectores, que modela la variabilidad a nivel de circuito, abstrayendo sus causas físicas, añadiendo dos nuevas fuentes para modelar la pendiente sub-umbral y el DIBL, de creciente importancia en la tecnología FinFET. Los dos nuevos inyectores propuestos incrementan la exactitud de figuras de mérito a diferentes niveles de abstracción del diseño electrónico: a nivel de transistor, de puerta y de circuito. El error cuadrático medio al simular métricas de estabilidad y prestaciones de células SRAM se reduce un mínimo de 1,5 veces y hasta un máximo de 7,5 a la vez que la estimación de la probabilidad de fallo se mejora en varios ordenes de magnitud. El diseño para bajo consumo es una de las principales aplicaciones actuales dada la creciente importancia de los dispositivos móviles dependientes de baterías. Es igualmente necesario debido a las importantes densidades de potencia en los sistemas actuales, con el fin de reducir su disipación térmica y sus consecuencias en cuanto al envejecimiento. El método tradicional de reducir la tensión de alimentación para reducir el consumo es problemático en el caso de las memorias SRAM dado el creciente impacto de la variabilidad a bajas tensiones. Se propone el diseño de una célula que usa valores negativos en la bit-line para reducir los fallos de escritura según se reduce la tensión de alimentación principal. A pesar de usar una segunda fuente de alimentación para la tensión negativa en la bit-line, el diseño propuesto consigue reducir el consumo hasta en un 20 % comparado con una célula convencional. Una nueva métrica, el hold trip point se ha propuesto para prevenir nuevos tipos de fallo debidos al uso de tensiones negativas, así como un método alternativo para estimar la velocidad de lectura, reduciendo el número de simulaciones necesarias. Según continúa la reducción del tamaño de los dispositivos electrónicos, se incluyen nuevos mecanismos que permiten facilitar el proceso de fabricación, o alcanzar las prestaciones requeridas para cada nueva generación tecnológica. Se puede citar como ejemplo el estrés compresivo o extensivo aplicado a los fins en tecnologías FinFET, que altera la movilidad de los transistores fabricados a partir de dichos fins. Los efectos de estos mecanismos dependen mucho del layout, la posición de unos transistores afecta a los transistores colindantes y pudiendo ser el efecto diferente en diferentes tipos de transistores. Se propone el uso de una célula SRAM complementaria que utiliza dispositivos pMOS en los transistores de paso, así reduciendo la longitud de los fins de los transistores nMOS y alargando los de los pMOS, extendiéndolos a las células vecinas y hasta los límites de la matriz de células. Considerando los efectos del STI y estresores de SiGe, el diseño propuesto mejora los dos tipos de transistores, mejorando las prestaciones de la célula SRAM complementaria en más de un 10% para una misma probabilidad de fallo y un mismo consumo estático, sin que se requiera aumentar el área. Finalmente, la radiación ha sido un problema recurrente en la electrónica para aplicaciones espaciales, pero la reducción de las corrientes y tensiones de los dispositivos actuales los está volviendo vulnerables al ruido generado por radiación, incluso a nivel de suelo. Pese a que tecnologías como SOI o FinFET reducen la cantidad de energía colectada por el circuito durante el impacto de una partícula, las importantes variaciones de proceso en los nodos más pequeños va a afectar su inmunidad frente a la radiación. Se demuestra que los errores inducidos por radiación pueden aumentar hasta en un 40 % en el nodo de 7nm cuando se consideran las variaciones de proceso, comparado con el caso nominal. Este incremento es de una magnitud mayor que la mejora obtenida mediante el diseño de células de memoria específicamente endurecidas frente a radiación, sugiriendo que la reducción de la variabilidad representaría una mayor mejora. ABSTRACT Reliability is becoming the main concern on integrated circuit as the technology goes beyond 22nm. Small imperfections in the device manufacturing result now in important random differences of the devices at electrical level which must be dealt with during the design. New processes and materials, required to allow the fabrication of the extremely short devices, are making new effects appear resulting ultimately on increased static power consumption, or higher vulnerability to radiation SRAMs have become the most vulnerable part of electronic systems, not only they account for more than half of the chip area of nowadays SoCs and microprocessors, but they are critical as soon as different variation sources are regarded, with failures in a single cell making the whole memory fail. This thesis addresses the different challenges that SRAM design has in the smallest technologies. In a common scenario of increasing variability, issues like energy consumption, design aware of the technology and radiation hardening are considered. First, given the increasing magnitude of device variability in the smallest nodes, as well as new sources of variability appearing as a consequence of new devices and shortened lengths, an accurate modeling of the variability is crucial. We propose to extend the injectors method that models variability at circuit level, abstracting its physical sources, to better model sub-threshold slope and drain induced barrier lowering that are gaining importance in FinFET technology. The two new proposed injectors bring an increased accuracy of figures of merit at different abstraction levels of electronic design, at transistor, gate and circuit levels. The mean square error estimating performance and stability metrics of SRAM cells is reduced by at least 1.5 and up to 7.5 while the yield estimation is improved by orders of magnitude. Low power design is a major constraint given the high-growing market of mobile devices that run on battery. It is also relevant because of the increased power densities of nowadays systems, in order to reduce the thermal dissipation and its impact on aging. The traditional approach of reducing the voltage to lower the energy consumption if challenging in the case of SRAMs given the increased impact of process variations at low voltage supplies. We propose a cell design that makes use of negative bit-line write-assist to overcome write failures as the main supply voltage is lowered. Despite using a second power source for the negative bit-line, the design achieves an energy reduction up to 20% compared to a conventional cell. A new metric, the hold trip point has been introduced to deal with new sources of failures to cells using a negative bit-line voltage, as well as an alternative method to estimate cell speed, requiring less simulations. With the continuous reduction of device sizes, new mechanisms need to be included to ease the fabrication process and to meet the performance targets of the successive nodes. As example we can consider the compressive or tensile strains included in FinFET technology, that alter the mobility of the transistors made out of the concerned fins. The effects of these mechanisms are very dependent on the layout, with transistor being affected by their neighbors, and different types of transistors being affected in a different way. We propose to use complementary SRAM cells with pMOS pass-gates in order to reduce the fin length of nMOS devices and achieve long uncut fins for the pMOS devices when the cell is included in its corresponding array. Once Shallow Trench isolation and SiGe stressors are considered the proposed design improves both kinds of transistor, boosting the performance of complementary SRAM cells by more than 10% for a same failure probability and static power consumption, with no area overhead. While radiation has been a traditional concern in space electronics, the small currents and voltages used in the latest nodes are making them more vulnerable to radiation-induced transient noise, even at ground level. Even if SOI or FinFET technologies reduce the amount of energy transferred from the striking particle to the circuit, the important process variation that the smallest nodes will present will affect their radiation hardening capabilities. We demonstrate that process variations can increase the radiation-induced error rate by up to 40% in the 7nm node compared to the nominal case. This increase is higher than the improvement achieved by radiation-hardened cells suggesting that the reduction of process variations would bring a higher improvement.