Atomistic modeling of Ge damage accumulation, amorphization and solid phase epitaxial regrowth

En los últimos años, el Ge ha ganado de nuevo atención con la finalidad de ser integrado en el seno de las existentes tecnologías de microelectrónica. Aunque no se le considera como un canddato capaz de reemplazar completamente al Si en el futuro próximo, probalemente servirá como un excelente complemento para aumentar las propiedades eléctricas en dispositivos futuros, especialmente debido a su alta movilidad de portadores. Esta integración requiere de un avance significativo del estado del arte en los procesos de fabricado. Técnicas de simulación, como los algoritmos de Monte Carlo cinético (KMC), proporcionan un ambiente atractivo para llevar a cabo investigación y desarrollo en este campo, especialmente en términos de costes en tiempo y financiación. En este estudio se han usado, por primera vez, técnicas de KMC con el fin entender el procesado “front-end” de Ge en su fabricación, específicamente la acumulación de dañado y amorfización producidas por implantación iónica y el crecimiento epitaxial en fase sólida (SPER) de las capas amorfizadas. Primero, simulaciones de aproximación de clisiones binarias (BCA) son usadas para calcular el dañado causado por cada ión. La evolución de este dañado en el tiempo se simula usando KMC sin red, o de objetos (OKMC) en el que sólamente se consideran los defectos. El SPER se simula a través de una aproximación KMC de red (LKMC), siendo capaz de seguir la evolución de los átomos de la red que forman la intercara amorfo/cristalina. Con el modelo de amorfización desarrollado a lo largo de este trabajo, implementado en un simulador multi-material, se pueden simular todos estos procesos. Ha sido posible entender la acumulación de dañado, desde la generación de defectos puntuales hasta la formación completa de capas amorfas. Esta acumulación ocurre en tres regímenes bien diferenciados, empezando con un ritmo lento de formación de regiones de dañado, seguido por una rápida relajación local de ciertas áreas en la fase amorfa donde ambas fases, amorfa y cristalina, coexisten, para terminar en la amorfización completa de capas extensas, donde satura el ritmo de acumulación. Dicha transición ocurre cuando la concentración de dañado supera cierto valor límite, el cual es independiente de las condiciones de implantación. Cuando se implantan los iones a temperaturas relativamente altas, el recocido dinámico cura el dañado previamente introducido y se establece una competición entre la generación de dañado y su disolución. Estos efectos se vuelven especialmente importantes para iones ligeros, como el B, el cual crea dañado más diluido, pequeño y distribuido de manera diferente que el causado por la implantación de iones más pesados, como el Ge. Esta descripción reproduce satisfactoriamente la cantidad de dañado y la extensión de las capas amorfas causadas por implantación iónica reportadas en la bibliografía. La velocidad de recristalización de la muestra previamente amorfizada depende fuertemente de la orientación del sustrato. El modelo LKMC presentado ha sido capaz de explicar estas diferencias entre orientaciones a través de un simple modelo, dominado por una única energía de activación y diferentes prefactores en las frecuencias de SPER dependiendo de las configuraciones de vecinos de los átomos que recristalizan. La formación de maclas aparece como una consecuencia de esta descripción, y es predominante en sustratos crecidos en la orientación (111)Ge. Este modelo es capaz de reproducir resultados experimentales para diferentes orientaciones, temperaturas y tiempos de evolución de la intercara amorfo/cristalina reportados por diferentes autores. Las parametrizaciones preliminares realizadas de los tensores de activación de tensiones son también capaces de proveer una buena correlación entre las simulaciones y los resultados experimentales de velocidad de SPER a diferentes temperaturas bajo una presión hidrostática aplicada. Los estudios presentados en esta tesis han ayudado a alcanzar un mejor entendimiento de los mecanismos de producción de dañado, su evolución, amorfización y SPER para Ge, además de servir como una útil herramienta para continuar el trabajo en este campo. In the recent years, Ge has regained attention to be integrated into existing microelectronic technologies. Even though it is not thought to be a feasible full replacement to Si in the near future, it will likely serve as an excellent complement to enhance electrical properties in future devices, specially due to its high carrier mobilities. This integration requires a significant upgrade of the state-of-the-art of regular manufacturing processes. Simulation techniques, such as kinetic Monte Carlo (KMC) algorithms, provide an appealing environment to research and innovation in the field, specially in terms of time and funding costs. In the present study, KMC techniques are used, for the first time, to understand Ge front-end processing, specifically damage accumulation and amorphization produced by ion implantation and Solid Phase Epitaxial Regrowth (SPER) of the amorphized layers. First, Binary Collision Approximation (BCA) simulations are used to calculate the damage caused by every ion. The evolution of this damage over time is simulated using non-lattice, or Object, KMC (OKMC) in which only defects are considered. SPER is simulated through a Lattice KMC (LKMC) approach, being able to follow the evolution of the lattice atoms forming the amorphous/crystalline interface. With the amorphization model developed in this work, implemented into a multi-material process simulator, all these processes can be simulated. It has been possible to understand damage accumulation, from point defect generation up to full amorphous layers formation. This accumulation occurs in three differentiated regimes, starting at a slow formation rate of the damage regions, followed by a fast local relaxation of areas into the amorphous phase where both crystalline and amorphous phases coexist, ending in full amorphization of extended layers, where the accumulation rate saturates. This transition occurs when the damage concentration overcomes a certain threshold value, which is independent of the implantation conditions. When implanting ions at relatively high temperatures, dynamic annealing takes place, healing the previously induced damage and establishing a competition between damage generation and its dissolution. These effects become specially important for light ions, as B, for which the created damage is more diluted, smaller and differently distributed than that caused by implanting heavier ions, as Ge. This description successfully reproduces damage quantity and extension of amorphous layers caused by means of ion implantation reported in the literature. Recrystallization velocity of the previously amorphized sample strongly depends on the substrate orientation. The presented LKMC model has been able to explain these differences between orientations through a simple model, dominated by one only activation energy and different prefactors for the SPER rates depending on the neighboring configuration of the recrystallizing atoms. Twin defects formation appears as a consequence of this description, and are predominant for (111)Ge oriented grown substrates. This model is able to reproduce experimental results for different orientations, temperatures and times of evolution of the amorphous/crystalline interface reported by different authors. Preliminary parameterizations for the activation strain tensors are able to also provide a good match between simulations and reported experimental results for SPER velocities at different temperatures under the appliance of hydrostatic pressure. The studies presented in this thesis have helped to achieve a greater understanding of damage generation, evolution, amorphization and SPER mechanisms in Ge, and also provide a useful tool to continue research in this field.

MOVPE growth of III-V semiconductors on silicon for third generation solar cells

Esta Tesis trata sobre el desarrollo y crecimiento -mediante tecnología MOVPE (del inglés: MetalOrganic Vapor Phase Epitaxy)- de células solares híbridas de semiconductores III-V sobre substratos de silicio. Esta integración pretende ofrecer una alternativa a las células actuales de III-V, que, si bien ostentan el récord de eficiencia en dispositivos fotovoltaicos, su coste es, a día de hoy, demasiado elevado para ser económicamente competitivo frente a las células convencionales de silicio. De este modo, este proyecto trata de conjugar el potencial de alta eficiencia ya demostrado por los semiconductores III-V en arquitecturas de células fotovoltaicas multiunión con el bajo coste, la disponibilidad y la abundancia del silicio. La integración de semiconductores III-V sobre substratos de silicio puede afrontarse a través de diferentes aproximaciones. En esta Tesis se ha optado por el desarrollo de células solares metamórficas de doble unión de GaAsP/Si. Mediante esta técnica, la transición entre los parámetros de red de ambos materiales se consigue por medio de la formación de defectos cristalográficos (mayoritariamente dislocaciones). La idea es confinar estos defectos durante el crecimiento de sucesivas capas graduales en composición para que la superficie final tenga, por un lado, una buena calidad estructural, y por otro, un parámetro de red adecuado. Numerosos grupos de investigación han dirigido sus esfuerzos en los últimos años en desarrollar una estructura similar a la que aquí proponemos. La mayoría de éstos se han centrado en entender los retos asociados al crecimiento de materiales III-V, con el fin de conseguir un material de alta calidad cristalográfica. Sin embargo, prácticamente ninguno de estos grupos ha prestado especial atención al desarrollo y optimización de la célula inferior de silicio, cuyo papel va a ser de gran relevancia en el funcionamiento de la célula completa. De esta forma, y con el fin de completar el trabajo hecho hasta el momento en el desarrollo de células de III-V sobre silicio, la presente Tesis se centra, fundamentalmente, en el diseño y optimización de la célula inferior de silicio, para extraer su máximo potencial. Este trabajo se ha estructurado en seis capítulos, ordenados de acuerdo al desarrollo natural de la célula inferior. Tras un capítulo de introducción al crecimiento de semiconductores III-V sobre Si, en el que se describen las diferentes alternativas para su integración; nos ocupamos de la parte experimental, comenzando con una extensa descripción y caracterización de los substratos de silicio. De este modo, en el Capítulo 2 se analizan con exhaustividad los diferentes tratamientos (tanto químicos como térmicos) que deben seguir éstos para garantizar una superficie óptima sobre la que crecer epitaxialmente el resto de la estructura. Ya centrados en el diseño de la célula inferior, el Capítulo 3 aborda la formación de la unión p-n. En primer lugar se analiza qué configuración de emisor (en términos de dopaje y espesor) es la más adecuada para sacar el máximo rendimiento de la célula inferior. En este primer estudio se compara entre las diferentes alternativas existentes para la creación del emisor, evaluando las ventajas e inconvenientes que cada aproximación ofrece frente al resto. Tras ello, se presenta un modelo teórico capaz de simular el proceso de difusión de fosforo en silicio en un entorno MOVPE por medio del software Silvaco. Mediante este modelo teórico podemos determinar qué condiciones experimentales son necesarias para conseguir un emisor con el diseño seleccionado. Finalmente, estos modelos serán validados y constatados experimentalmente mediante la caracterización por técnicas analíticas (i.e. ECV o SIMS) de uniones p-n con emisores difundidos. Uno de los principales problemas asociados a la formación del emisor por difusión de fósforo, es la degradación superficial del substrato como consecuencia de su exposición a grandes concentraciones de fosfina (fuente de fósforo). En efecto, la rugosidad del silicio debe ser minuciosamente controlada, puesto que éste servirá de base para el posterior crecimiento epitaxial y por tanto debe presentar una superficie prístina para evitar una degradación morfológica y cristalográfica de las capas superiores. En este sentido, el Capítulo 4 incluye un análisis exhaustivo sobre la degradación morfológica de los substratos de silicio durante la formación del emisor. Además, se proponen diferentes alternativas para la recuperación de la superficie con el fin de conseguir rugosidades sub-nanométricas, que no comprometan la calidad del crecimiento epitaxial. Finalmente, a través de desarrollos teóricos, se establecerá una correlación entre la degradación morfológica (observada experimentalmente) con el perfil de difusión del fósforo en el silicio y por tanto, con las características del emisor. Una vez concluida la formación de la unión p-n propiamente dicha, se abordan los problemas relacionados con el crecimiento de la capa de nucleación de GaP. Por un lado, esta capa será la encargada de pasivar la subcélula de silicio, por lo que su crecimiento debe ser regular y homogéneo para que la superficie de silicio quede totalmente pasivada, de tal forma que la velocidad de recombinación superficial en la interfaz GaP/Si sea mínima. Por otro lado, su crecimiento debe ser tal que minimice la aparición de los defectos típicos de una heteroepitaxia de una capa polar sobre un substrato no polar -denominados dominios de antifase-. En el Capítulo 5 se exploran diferentes rutinas de nucleación, dentro del gran abanico de posibilidades existentes, para conseguir una capa de GaP con una buena calidad morfológica y estructural, que será analizada mediante diversas técnicas de caracterización microscópicas. La última parte de esta Tesis está dedicada al estudio de las propiedades fotovoltaicas de la célula inferior. En ella se analiza la evolución de los tiempos de vida de portadores minoritarios de la base durante dos etapas claves en el desarrollo de la estructura Ill-V/Si: la formación de la célula inferior y el crecimiento de las capas III-V. Este estudio se ha llevado a cabo en colaboración con la Universidad de Ohio, que cuentan con una gran experiencia en el crecimiento de materiales III-V sobre silicio. Esta tesis concluye destacando las conclusiones globales del trabajo realizado y proponiendo diversas líneas de trabajo a emprender en el futuro. ABSTRACT This thesis pursues the development and growth of hybrid solar cells -through Metal Organic Vapor Phase Epitaxy (MOVPE)- formed by III-V semiconductors on silicon substrates. This integration aims to provide an alternative to current III-V cells, which, despite hold the efficiency record for photovoltaic devices, their cost is, today, too high to be economically competitive to conventional silicon cells. Accordingly, the target of this project is to link the already demonstrated efficiency potential of III-V semiconductor multijunction solar cell architectures with the low cost and unconstrained availability of silicon substrates. Within the existing alternatives for the integration of III-V semiconductors on silicon substrates, this thesis is based on the metamorphic approach for the development of GaAsP/Si dual-junction solar cells. In this approach, the accommodation of the lattice mismatch is handle through the appearance of crystallographic defects (namely dislocations), which will be confined through the incorporation of a graded buffer layer. The resulting surface will have, on the one hand a good structural quality; and on the other hand the desired lattice parameter. Different research groups have been working in the last years in a structure similar to the one here described, being most of their efforts directed towards the optimization of the heteroepitaxial growth of III-V compounds on Si, with the primary goal of minimizing the appearance of crystal defects. However, none of these groups has paid much attention to the development and optimization of the bottom silicon cell, which, indeed, will play an important role on the overall solar cell performance. In this respect, the idea of this thesis is to complete the work done so far in this field by focusing on the design and optimization of the bottom silicon cell, to harness its efficiency. This work is divided into six chapters, organized according to the natural progress of the bottom cell development. After a brief introduction to the growth of III-V semiconductors on Si substrates, pointing out the different alternatives for their integration; we move to the experimental part, which is initiated by an extensive description and characterization of silicon substrates -the base of the III-V structure-. In this chapter, a comprehensive analysis of the different treatments (chemical and thermal) required for preparing silicon surfaces for subsequent epitaxial growth is presented. Next step on the development of the bottom cell is the formation of the p-n junction itself, which is faced in Chapter 3. Firstly, the optimization of the emitter configuration (in terms of doping and thickness) is handling by analytic models. This study includes a comparison between the different alternatives for the emitter formation, evaluating the advantages and disadvantages of each approach. After the theoretical design of the emitter, it is defined (through the modeling of the P-in-Si diffusion process) a practical parameter space for the experimental implementation of this emitter configuration. The characterization of these emitters through different analytical tools (i.e. ECV or SIMS) will validate and provide experimental support for the theoretical models. A side effect of the formation of the emitter by P diffusion is the roughening of the Si surface. Accordingly, once the p-n junction is formed, it is necessary to ensure that the Si surface is smooth enough and clean for subsequent phases. Indeed, the roughness of the Si must be carefully controlled since it will be the basis for the epitaxial growth. Accordingly, after quantifying (experimentally and by theoretical models) the impact of the phosphorus on the silicon surface morphology, different alternatives for the recovery of the surface are proposed in order to achieve a sub-nanometer roughness which does not endanger the quality of the incoming III-V layers. Moving a step further in the development of the Ill-V/Si structure implies to address the challenges associated to the GaP on Si nucleation. On the one hand, this layer will provide surface passivation to the emitter. In this sense, the growth of the III-V layer must be homogeneous and continuous so the Si emitter gets fully passivated, providing a minimal surface recombination velocity at the interface. On the other hand, the growth should be such that the appearance of typical defects related to the growth of a polar layer on a non-polar substrate is minimized. Chapter 5 includes an exhaustive study of the GaP on Si nucleation process, exploring different nucleation routines for achieving a high morphological and structural quality, which will be characterized by means of different microscopy techniques. Finally, an extensive study of the photovoltaic properties of the bottom cell and its evolution during key phases in the fabrication of a MOCVD-grown III-V-on-Si epitaxial structure (i.e. the formation of the bottom cell; and the growth of III-V layers) will be presented in the last part of this thesis. This study was conducted in collaboration with The Ohio State University, who has extensive experience in the growth of III-V materials on silicon. This thesis concludes by highlighting the overall conclusions of the presented work and proposing different lines of work to be undertaken in the future.

Understanding phosphorus diffusion into silicon in a MOVPE environment for III-V on silicon solar cells

Dual-junction solar cells formed by a GaAsP or GaInP top cell and a silicon bottom cell seem to be attractive candidates to materialize the long sought-for integration of III?V materials on silicon for photovoltaic applications. When manufacturing a multi-junction solar cell on silicon, one of the first processes to be addressed is the development of the bottom subcell and, in particular, the formation of its emitter. In this study, we analyze, both experimentally and by simulations, the formation of the emitter as a result of phosphorus diffusion that takes place during the first stages of the epitaxial growth of the solar cell. Different conditions for the Metal-Organic Vapor Phase Epitaxy (MOVPE) process have been evaluated to understand the impact of each parameter, namely, temperature, phosphine partial pressure, time exposure and memory effects in the final diffusion profiles obtained. A model based on SSupremIV process simulator has been developed and validated against experimental profiles measured by ECV and SIMS to calculate P diffusion profiles in silicon formed in a MOVPE environment taking in consideration all these factors.

Recrystallization of amorphous nano-tracks and uniform layers generated by swift-ion-beam irradiation in lithium niobate.

The thermal annealing of amorphous tracks of nanometer-size diameter generated in lithium niobate (LiNbO3) by Bromine ions at 45 MeV, i.e., in the electronic stopping regime, has been investigated by RBS/C spectrometry in the temperature range from 250°C to 350°C. Relatively low fluences have been used (<1012 cm−2) to produce isolated tracks. However, the possible effect of track overlapping has been investigated by varying the fluence between 3×1011 cm−2 and 1012 cm−2. The annealing process follows a two-step kinetics. In a first stage (I) the track radius decreases linearly with the annealing time. It obeys an Arrhenius-type dependence on annealing temperature with activation energy around 1.5 eV. The second stage (II) operates after the track radius has decreased down to around 2.5 nm and shows a much lower radial velocity. The data for stage I appear consistent with a solid-phase epitaxial process that yields a constant recrystallization rate at the amorphous-crystalline boundary. HRTEM has been used to monitor the existence and the size of the annealed isolated tracks in the second stage. On the other hand, the thermal annealing of homogeneous (buried) amorphous layers has been investigated within the same temperature range, on samples irradiated with Fluorine at 20 MeV and fluences of ∼1014 cm−2. Optical techniques are very suitable for this case and have been used to monitor the recrystallization of the layers. The annealing process induces a displacement of the crystalline-amorphous boundary that is also linear with annealing time, and the recrystallization rates are consistent with those measured for tracks. The comparison of these data with those previously obtained for the heavily damaged (amorphous) layers produced by elastic nuclear collisions is summarily discussed.

Performance analysis of AlGaAs/GaAs tunnel junctions for ultra-high concentration photovoltaics

An n(++)-GaAs/p(++)-AlGaAs tunnel junction with a peak current density of 10 100Acm(-2) is developed. This device is a tunnel junction for multijunction solar cells, grown lattice-matched on standard GaAs or Ge substrates, with the highest peak current density ever reported. The voltage drop for a current density equivalent to the operation of the multijunction solar cell up to 10 000 suns is below 5 mV. Trap-assisted tunnelling is proposed to be behind this performance, which cannot be justified by simple band-to-band tunnelling. The metal-organic vapour-phase epitaxy growth conditions, which are in the limits of the transport-limited regime, and the heavy tellurium doping levels are the proposed origins of the defects enabling trap-assisted tunnelling. The hypothesis of trap-assisted tunnelling is supported by the observed annealing behaviour of the tunnel junctions, which cannot be explained in terms of dopant diffusion or passivation. For the integration of these tunnel junctions into a triple-junction solar cell, AlGaAs barrier layers are introduced to suppress the formation of parasitic junctions, but this is found to significantly degrade the performance of the tunnel junctions. However, the annealed tunnel junctions with barrier layers still exhibit a peak current density higher than 2500Acm(-2) and a voltage drop at 10 000 suns of around 20 mV, which are excellent properties for tunnel junctions and mean they can serve as low-loss interconnections in multijunction solar cells working at ultra-high concentrations.

Development of One Meter Long Double-Sided CeO2 Buffered Ni-5at.W Templates by Reel-to-Reel Chemical Solution Deposition Route

High performance long-length coated conductors fabricated using various techniques have attracted a lot of interest recently. In this work, a reel-to-reel design for depositing double-sided coatings on long-length flexible metallic tapes via a chemical solution method is proposed and realized. The major achievement of the design is to combine the dip coating and drying processes in order to overcome the technical difficulties of dealing with the wet films on both sides of the tape. We report the successful application of the design to fabricate a one-meter-long double side coated CeO2/Ni-5at%W template. The CeO2 films on both sides exhibit a dense, crack-free morphology, and a high fraction of cube texture on the surface. Homogeneity studies on global texture over the length also reveal that the average full width at half maximum values of the in-plane and out-of-plane orientation on the CeO2 layer are 7.2 ° and 5.8° with standard deviation of 0.26° and 0.34°, respectively, being indicative of the high quality epitaxial growth of the films prepared in the continuous manner. An all chemical solution derived YBCOLow-TFA/Ce0.9La0.1O2 /Gd2Zr2O7/CeO2 structure is obtained on a short sample, demonstrating the possibility of producing long-length texture templates for coated conductors by this low cost deposition route.

Strategies to unblock the n-GaAs surface when electrodepositing Bi from acidic solutions

Bismuth ultra-thin films grown on n-GaAs electrodes via electrodeposition are porous due to a blockade of the electrode surface caused by adsorbed hydrogen when using acidic electrolytes. In this study, we discuss the existence of two sources of hydrogen adsorption and we propose different routes to unblock the n-GaAs surface in order to improve Bi films compactness. Firstly, we demonstrate that increasing the electrolyte temperature provides compact yet polycrystalline Bi films. Cyclic voltammetry scans indicate that this low crystal quality might be a result of the incorporation of Bi hydroxides within the Bi film as a result of the temperature increase. Secondly, we have illuminated the semiconductor surface to take advantage of photogenerated holes. These photocarriers oxidize the adsorbed hydrogen unblocking the surface, but also create pits at the substrate surface that degrade the Bi/GaAs interface and prevent an epitaxial growth. Finally, we show that performing a cyclic voltammetry scan before electrodeposition enables the growth of compact Bi ultra-thin films of high crystallinity on semiconductor substrates with a doping level low enough to perform transport measurements.

Si(100) versus Ge(100): Watching the interface formation for the growth of III-V-based solar cells on abundant substrates

We investigated the atomic surface properties of differently prepared silicon and germanium (100) surfaces during metal-organic vapour phase epitaxy/chemical vapour deposition (MOVPE/MOCVD), in particular the impact of the MOVPE ambient, and applied reflectance anisotropy/difference spectroscopy (RAS/RDS) in our MOVPE reactor to in-situ watch and control the preparation on the atomic length scale for subsequent III-V-nucleation. The technological interest in the predominant opto-electronic properties of III-V-compounds drives the research for their heteroepitaxial integration on more abundant and cheaper standard substrates such as Si(100) or Ge(100). In these cases, a general task must be accomplished successfully, i.e. the growth of polar materials on non-polar substrates and, beyond that, very specific variations such as the individual interface formation and the atomic step structure, have to be controlled. Above all, the method of choice to grow industrial relevant high-performance device structures is MOVPE, not normally compatible with surface and interface sensitive characterization tools, which are commonly based on ultrahigh vacuum (UHV) ambients. A dedicated sample transfer system from MOVPE environment to UHV enabled us to benchmark the optical in-situ spectra with results from various surfaces science instruments without considering disruptive contaminants. X-ray photoelectron spectroscopy (XPS) provided direct observation of different terminations such as arsenic and phosphorous and verified oxide removal under various specific process parameters. Absorption lines in Fourier-transform infrared (FTIR) spectra were used to identify specific stretch modes of coupled hydrides and the polarization dependence of the anti-symmetric stretch modes distinguished different dimer orientations. Scanning tunnelling microscopy (STM) studied the atomic arrangement of dimers and steps and tip-induced H-desorption proved the saturation of dangling bonds after preparati- n. In-situ RAS was employed to display details transiently such as the presence of H on the surface at lower temperatures (T <; 800°C) and the absence of Si-H bonds at elevated annealing temperature and also surface terminations. Ge buffer growth by the use of GeH4 enables the preparation of smooth surfaces and leads to a more pronounced amplitude of the features in the spectra which indicates improvements of the surface quality.

Single phase a-plane MgZnO epilayers for UV optoelectronics: substitutional behaviour of Mg at large contents

High quality 1 μm thick a-plane MgxZn1−xO layers were produced by molecular beam epitaxy with Mg contents higher than 50%. Resonant Rutherford backscattering spectrometry combined with ion channeling revealed a uniform growth in both composition and atomic order. The lattice-site location of Mg, Zn and O elements was determined independently, proving the substitutional behaviour of Mg in Zn-sites of the wurtzite lattice. X-Ray diffraction pole figure analysis also confirms the absence of phase separation. Optical properties at such high Mg contents were studied in Schottky photodiodes.

Epitaxial self-alignment: A new route to hybrid active plasmonic nanostructures

Concepts of lateral ordering of epitaxial semiconductor quantum dots (QDs) are for the first time transferred to hybrid nanostructures for active plasmonics. We review our recent research on the self-alignment of epitaxial nanocrystals of In and Ag on ordered one-dimensional In(Ga)As QD arrays and isolated QDs by molecular beam epitaxy. By changing the growth conditions the size and density of the metal nanocrystals are easily controlled and the surface plasmon resonance wavelength is tuned over a wide range in order to match the emission wavelength of the QDs. Photoluminescence measurements reveal large enhancement of the emitted light intensity due to plasmon enhanced emission and absorption down to the single QD level.

In situ study of Ge(100) surfaces with tertiarybutylphosphine supply in vapor phase epitaxy ambient

GaInP nucleation on Ge(100) often starts by annealing of the Ge(100) substrates under supply of phosphorus precursors. However, the influence on the Ge surface is not well understood. Here, we studied vicinal Ge(100) surfaces annealed under tertiarybutylphosphine (TBP) supply in MOVPE by in situ reflection anisotropy spectroscopy (RAS), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). While XPS reveals a P termination and the presence of carbon on the Ge surface, LEED patterns indicate a disordered surface probably due to by-products of the TBP pyrolysis. However, the TBP annealed Ge(100) surface exhibits a characteristic RA spectrum, which is related to the P termination. RAS allows us to in situ control phosphorus desorption dependent on temperature.

Influence of crude protein content, ingredient complexity, feed form, and duration of feeding of the phase I diets on productive performance and nutrient digestibility of Iberian pigs.

The influence of CP content and ingredient complexity, feed form, and duration of feeding of the Phase I diets on growth performance and total tract apparent digestibility -TTAD- of energy and nutrients was studied in Iberian pigs weaned at 28 d of age. There were 12 dietary treatments with 2 type of feeds -high-quality, HQ; and low-quality, LQ-, 2 feed forms -pellets vs. mash-, and 3 durations -7, 14, and 21 d- of supply of the Phase I diets.

Influence of source and micronization of soya bean meal on growth performance, nutrient digestibility and ileal mucosal morphology of Iberian piglets

The effects of inclusion in the diet of different sources of soya bean meal (SBM) on growth performance, total tract apparent digestibility (TTAD) and apparent ileal digestibility (AID) of major dietary components and mucosal ileum morphology were studied in Iberian pigs weaned at 30 days of age. From 30 to 51 days of age (phase I), there was a control diet based on regular soya bean meal (R-SBM; 44% CP) of Argentina (ARG) origin and five extra diets in which a high-protein soya bean meal (HP-SBM; 49% CP) of the USA or ARG origin, either ground (990 μm) or micronized (60 μm), or a soya protein concentrate (SPC; 65% CP) substituted the R-SBM. From 51 to 61 days of age (phase II), all pigs were fed a common commercial diet in mash form. The following pre-planned orthogonal contrasts were conducted: (1) R-SBM v. all the other diets, (2) SPC v. all the HP-SBM diets, (3) micronized HP-SBM v. ground HP-SBM, (4) HP-SBM of ARG origin v. HP-SBM of US origin and (5) interaction between source and the degree of grinding of the HP-SBM. Dietary treatment did not affect growth performance of the pigs at any age but from 30 to 51 days of age, post weaning diarrhoea (PWD) was higher (P<0.001) and the TTAD and AID of all nutrients were lower for pigs fed the R-SBM diet than for pigs fed the HP-SBM or the SPC diets. However, no differences between the HP-SBM and the SPC containing diets were detected for any trait. The TTAD of organic matter (P=0.07) and gross energy (GE) (P=0.05) tended to be higher for the micronized HP-SBM than for the ground HP-SBM and that of GE was higher (P<0.05) for US meal than for the ARG meal. Pigs fed R-SBM had lower villus height (P<0.01) than pigs fed HP-SBM or SPC but no differences in ileal mucosal morphology were detected between SPC and HP-SBM containing diets. It is concluded that feeding the HP-SBM or SPC-reduced PWD and improved nutrient digestibility and ileal morphology as compared with feeding the R-SBM, but had no effect on pig performance. The inclusion in the diet of added value soya products (micronized SBM or SPC) in substitution of the R-SBM increased the TTAD of all nutrients and reduced PWD but had no advantage in terms of growth performance over the use of ground HP-SBM.

A simple mathematical model that describes the growth of the area and the number of total and viable cells in yeast colonies

We propose a model, based on the Gompertz equation, to describe the growth of yeasts colonies on agar medium. This model presents several advantages: (i) one equation describes the colony growth, which previously needed two separate ones (linear increase of radius and of the squared radius); (ii) a similar equation can be applied to total and viable cells, colony area or colony radius, because the number of total cells in mature colonies is proportional to their area; and (iii) its parameters estimate the cell yield, the cell concentration that triggers growth limitation and the effect of this limitation on the specific growth rate. To elaborate the model, area, total and viable cells of 600 colonies of Saccharomyces cerevisiae, Debaryomyces fabryi, Zygosaccharomyces rouxii and Rhodotorula glutinis have been measured. With low inocula, viable cells showed an initial short exponential phase when colonies were not visible. This phase was shortened with higher inocula. In visible or mature colonies, cell growth displayed Gompertz-type kinetics. It was concluded that the cells growth in colonies is similar to liquid cultures only during the first hours, the rest of the time they grow, with near-zero specific growth rates, at least for 3 weeks.

Influence of source and micronization of soya bean meal on growth performance, nutrient digestibility and ileal mucosal morphology of Iberian piglets

The effects of inclusion in the diet of different sources of soya bean meal (SBM) on growth performance, total tract apparent digestibility (TTAD) and apparent ileal digestibility (AID) of major dietary components and mucosal ileum morphology were studied in Iberian pigs weaned at 30 days of age. From 30 to 51 days of age (phase I), there was a control diet based on regular soya bean meal (R-SBM; 44% CP) of Argentina (ARG) origin and five extra diets in which a high-protein soya bean meal (HP-SBM; 49% CP) of the USA or ARG origin, either ground (990 ?m) or micronized (60 ?m), or a soya protein concentrate (SPC; 65% CP) substituted the R-SBM. From 51 to 61 days of age (phase II), all pigs were fed a common commercial diet in mash form.