Oxygen photo-adsorption related quenching of photoluminescence in group-III nitride nanocolumns

GaN and InGaN nanocolumns of various compositions are studied by room-temperature photoluminescence (PL) under different ambient conditions. GaN nanocolumns exhibit a reversible quenching upon exposure to air under constant UV excitation, following a t−1/2 time dependence and resulting in a total reduction of intensity by 85–90%, as compared to PL measured in vacuum, with no spectral change. This effect is not observed when exposing the samples to pure nitrogen. We attribute this effect to photoabsorption and photodesorption of oxygen that modifies the surface potential bending. InGaN nanocolumns, under the same experimental conditions do not show the same quenching features: The high-energy part of the broad PL line is not modified by exposure to air, whereas a lower-energy part, which does quench by 80–90%, can now be distinguished. We discuss the different behaviors in terms of carrier localization and possible composition or strain gradients in the InGaN nanocolumns.

Design, Technology and Characterization of Micromechanised Sensors and Actuators for Harsh Environments

Los sistemas micro electro mecánicos (MEMS) han demostrado ser una exitosa familia de dispositivos que pueden usarse como plataforma para el desarrollo de dispositivos con aplicaciones en óptica, comunicaciones, procesado de señal y sensorización. Los dispositivos MEMS estándar suelen estar fabricados usando tecnología de silicio. Sin embargo, el rendimiento de estos MEMS se puede mejorar si se usan otros materiales. Por ejemplo, el diamante nanocristalino (NCD) ofrece unas excelentes propiedades mecánicas, transparencia y una superficie fácil de funcionalizar. Por otro lado, el sistema de materiales (In; Ga; Al)N, los materiales IIIN, se pueden usar para producir estructuras monocristalinas con alta sensibilidad mecánica y química. Además, el AlN se puede depositar por pulverización catódica reactiva sobre varios substratos, incluyendo NCD, para formar capas policristalinas orientadas con alta respuesta piezoeléctrica. Adicionalmente, tanto el NCD como los materiales III-N muestran una gran estabilidad térmica y química, lo que los hace una elección idónea para desarrollar dispositivos para aplicaciones para alta temperatura, ambientes agresivos e incluso para aplicaciones biocompatibles. En esta tesis se han usado estos materiales para el diseño y medición de demostradores tecnológicos. Se han perseguido tres objetivos principales: _ Desarrollo de unos procesos de fabricación apropiados. _ Medición de las propiedades mecánicas de los materiales y de los factores que limitan el rendimiento de los dispositivos. _ Usar los datos medidos para desarrollar dispositivos demostradores complejos. En la primera parte de esta tesis se han estudiado varias técnicas de fabricación. La estabilidad de estos materiales impide el ataque y dificulta la producción de estructuras suspendidas. Los primeros capítulos de esta disertación se dedican al desarrollo de unos procesos de transferencia de patrones por ataque seco y a la optimización del ataque húmedo sacrificial de varios substratos propuestos. Los resultados de los procedimientos de ataque se presentan y se describe la optimización de las técnicas para la fabricación de estructuras suspendidas de NCD y materiales III-N. En un capítulo posterior se estudia el crecimiento de AlN por pulverización catódica. Como se ha calculado en esta disertación para obtener una actuación eficiente de MEMS, las capas de AlN han de ser finas, típicamente d < 200 nm, lo que supone serias dificultades para la obtención de capas orientadas con respuesta piezoeléctrica. Las condiciones de depósito se han mapeado para identificar las fronteras que proporcionan el crecimiento de material orientado desde los primeros pasos del proceso. Además, durante la optimización de los procesos de ataque se estudió un procedimiento para fabricar películas de GaN nanoporoso. Estas capas porosas pueden servir como capas sacrificiales para la fabricación de estructuras suspendidas de GaN con baja tensión residual o como capas para mejorar la funcionalización superficial de sensores químicos o biológicos. El proceso de inducción de poros se discutirá y también se presentarán experimentos de ataque y funcionalización. En segundo lugar, se han determinado las propiedades mecánicas del NCD y de los materiales III-N. Se han fabricado varias estructuras suspendidas para la medición del módulo de Young y de la tensión residual. Además, las estructuras de NCD se midieron en resonancia para calcular el rendimiento de los dispositivos en términos de frecuencia y factor de calidad. Se identificaron los factores intrínsecos y extrínsecos que limitan ambas figuras de mérito y se han desarrollado modelos para considerar estas imperfecciones en las etapas de diseño de los dispositivos. Por otra parte, los materiales III-N normalmente presentan grandes gradientes de deformación residual que causan la deformación de las estructuras al ser liberadas. Se han medido y modelado estos efectos para los tres materiales binarios del sistema para proporcionar puntos de interpolación que permitan predecir las características de las aleaciones del sistema III-N. Por último, los datos recabados se han usado para desarrollar modelos analíticos y numéricos para el diseño de varios dispositivos. Se han estudiado las propiedades de transducción y se proporcionan topologías optimizadas. En el último capítulo de esta disertación se presentan diseños optimizados de los siguientes dispositivos: _ Traviesas y voladizos de AlN=NCD con actuación piezoeléctrica aplicados a nanoconmutadores de RF para señales de alta potencia. _ Membranas circulares de AlN=NCD con actuación piezoeléctrica aplicadas a lentes sintonizables. _ Filtros ópticos Fabry-Pérot basados en cavidades aéreas y membranas de GaN actuadas electrostáticamente. En resumen, se han desarrollado unos nuevos procedimientos optimizados para la fabricación de estructuras de NCD y materiales III-N. Estas técnicas se han usado para producir estructuras que llevaron a la determinación de las principales propiedades mecánicas y de los parámetros de los dispositivos necesarios para el diseño de MEMS. Finalmente, los datos obtenidos se han usado para el diseño optimizado de varios dispositivos demostradores. ABSTRACT Micro Electro Mechanical Systems (MEMS) have proven to be a successful family of devices that can be used as a platform for the development of devices with applications in optics, communications, signal processing and sensorics. Standard MEMS devices are usually fabricated using silicon based materials. However, the performance of these MEMS can be improved if other material systems are used. For instance, nanocrystalline diamond (NCD) offers excellent mechanical properties, optical transparency and ease of surface functionalization. On the other hand, the (In; Ga; Al)N material system, the III-N materials, can be used to produce single crystal structures with high mechanical and chemical sensitivity. Also, AlN can be deposited by reactive sputtering on various substrates, including NCD, to form oriented polycrystalline layers with high piezoelectric response. In addition, both NCD and III-N materials exhibit high thermal and chemical stability, which makes these material the perfect choice for the development of devices for high temperatures, harsh environments and even biocompatible applications. In this thesis these materials have been used for the design and measurement of technological demonstrators. Three main objectives have been pursued: _ Development of suitable fabrication processes. _ Measurement of the material mechanical properties and device performance limiting factors. _ Use the gathered data to design complex demonstrator devices. In a first part of the thesis several fabrication processes have been addressed. The stability of these materials hinders the etching of the layers and hampers the production of free standing structures. The first chapters of this dissertation are devoted to the development of a dry patterning etching process and to sacrificial etching optimization of several proposed substrates. The results of the etching processes are presented and the optimization of the technique for the manufacturing of NCD and III-N free standing structures is described. In a later chapter, sputtering growth of thin AlN layers is studied. As calculated in this dissertation, for efficient MEMS piezoelectric actuation the AlN layers have to be very thin, typically d < 200 nm, which poses serious difficulties to the production of c-axis oriented material with piezoelectric response. The deposition conditions have been mapped in order to identify the boundaries that give rise to the growth of c-axis oriented material from the first deposition stages. Additionally, during the etching optimization a procedure for fabricating nanoporous GaN layers was also studied. Such porous layers can serve as a sacrificial layer for the release of low stressed GaN devices or as a functionalization enhancement layer for chemical and biological sensors. The pore induction process will be discussed and etching and functionalization trials are presented. Secondly, the mechanical properties of NCD and III-N materials have been determined. Several free standing structures were fabricated for the measurement of the material Young’s modulus and residual stress. In addition, NCD structures were measured under resonance in order to calculate the device performance in terms of frequency and quality factor. Intrinsic and extrinsic limiting factors for both figures were identified and models have been developed in order to take into account these imperfections in the device design stages. On the other hand, III-N materials usually present large strain gradients that lead to device deformation after release. These effects have been measured and modeled for the three binary materials of the system in order to provide the interpolation points for predicting the behavior of the III-N alloys. Finally, the gathered data has been used for developing analytic and numeric models for the design of various devices. The transduction properties are studied and optimized topologies are provided. Optimized design of the following devices is presented at the last chapter of this dissertation: _ AlN=NCD piezoelectrically actuated beams applied to RF nanoswitches for large power signals. _ AlN=NCD piezoelectrically actuated circular membranes applied to tunable lenses. _ GaN based air gap tunable optical Fabry-Pérot filters with electrostatic actuation. On the whole, new optimized fabrication processes has been developed for the fabrication of NCD and III-N MEMS structures. These processing techniques was used to produce structures that led to the determination of the main mechanical properties and device parameters needed for MEMS design. Lastly, the gathered data was used for the design of various optimized demonstrator devices.

Elastic (stress-strain) halo associated to ion-induced nano-tracks in lithium niobate: role of crystal anisotropy

The elastic strain/stress fields (halo) around a compressed amorphous nano-track (core) caused by a single high-energy ion impact on LiNbO3 are calculated. A method is developed to approximately account for the effects of crystal anisotropy of LiNbO3 (symmetry 3m) on the stress fields for tracks oriented along the crystal axes (X, Y or Z). It only considers the zero-order (axial) harmonic contribution to the displacement field in the perpendicular plane and uses effective Poisson moduli for each particular orientation. The anisotropy is relatively small; however, it accounts for some differential features obtained for irradiations along the crystallographic axes X, Y and Z. In particular, the irradiation-induced disorder (including halo) and the associated surface swelling appear to be higher for irradiations along the X- or Y-axis in comparison with those along the Z-axis. Other irradiation effects can be explained by the model, e.g. fracture patterns or the morphology of pores after chemical etching of tracks. Moreover, it offers interesting predictions on the effect of irradiation on lattice parameters

Depth dependent lattice disorder and strain in Mn-implanted and post-annealed InAs thin films

The lattice order degree and the strain in as-grown, Mn-implanted and post-implantedannealedInAsthinfilms were investigated with depth resolution by means of Rutherford backscattering spectrometry in channeling conditions (RBS/C). Three main crystallographic axes were analyzed for both In and As sublattices. The behaviour of the induced defects was evaluated in two regions with different native defects: the interface and the surface. The results show that Mn implantation and post-implantation annealing are anisotropic processes, affecting in a different way the In and As sublattices. The mechanisms influencing the enhancement and deterioration of the crystal quality during the implantation are discussed in relation to the as-grown defects and the segregation of the elements

Synchrotron strain scanning for residual stress measurement in cold-drawn steel rods

Cold-drawn steel rods and wires retain significant residual stresses as a consequence of the manufacturing process. These residual stresses are known to be detrimental for the mechanical properties of the wires and their durability in aggressive environments. Steel makers are aware of the problem and have developed post-drawing processes to try and reduce the residual stresses on the wires. The present authors have studied this problem for a number of years and have performed a detailed characterization of the residual stress state inside cold-drawn rods, including both experimental and numerical techniques. High-energy synchrotron sources have been particularly useful for this research. The results have shown how residual stresses evolve as a consequence of cold-drawing and how they change with subsequent post-drawing treatments. The authors have been able to measure for the first time a complete residual strain profile along the diameter in both phases (ferrite and cementite) of a cold-drawn steel rod.

Behaviour of steel prestressing wires under extreme conditions of strain rate and temperature

The purpose of this paper is to provide information on the behaviour of steel prestressing wires under likely conditions that could be expected during a fire or impact loads. Four loadings were investigated: a) the influence of strain rate – from 10–3 to 600 s–1 – at room temperature, b) the influence of temperature – from 24 to 600 °C – at low strain rate, c) the influence of the joint effect of strain rate and temperature, and d) damage after three plausible fire scenarios. At room temperature it was found that using “static” values is a safe option. At high temperatures our results are in agreement with design codes. Regarding the joint effect of temperature and strain rate, mechanical properties decrease with increasing temperature, although for a given temperature, yield stress and tensile strength increase with strain rate. The data provided can be used profitably to model the mechanical behaviour of steel wires under different scenarios.

Size, strain, and band offset engineering in GaAs(Sb)(N)-capped InAs quantum dots for 1.3 - 1.55 µm LEDs and LDs

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

Study of the evolution of the seismic cycle of stress and strain associated to the El Salvador Fault Zone

• Central America: – Regional studies in Central America (Seismic Hazard). – El Salvador Fault Zone (ESFZ). – Aguacaliente‐Navarro Fault Zone (ANFZ), Central Valley of Costa Rica. – Haiti (seismic hazard) • Spain: – Regional‐Nacional studies of seismic hazards (applications to building codes, eurocode, emergency plans, etc.) – Betic range zone, south of Spain. – Ibero‐Maghrebi region (collision zone)

A mathematical framework for finite strain elastoplastic consolidation. Part 1: Balance laws, variational formulation, and linearization

A mathematical formulation for finite strain elasto plastic consolidation of fully saturated soil media is presented. Strong and weak forms of the boundary-value problem are derived using both the material and spatial descriptions. The algorithmic treatment of finite strain elastoplasticity for the solid phase is based on multiplicative decomposition and is coupled with the algorithm for fluid flow via the Kirchhoff pore water pressure. Balance laws are written for the soil-water mixture following the motion of the soil matrix alone. It is shown that the motion of the fluid phase only affects the Jacobian of the solid phase motion, and therefore can be characterized completely by the motion of the soil matrix. Furthermore, it is shown from energy balance consideration that the effective, or intergranular, stress is the appropriate measure of stress for describing the constitutive response of the soil skeleton since it absorbs all the strain energy generated in the saturated soil-water mixture. Finally, it is shown that the mathematical model is amenable to consistent linearization, and that explicit expressions for the consistent tangent operators can be derived for use in numerical solutions such as those based on the finite element method.

Elastoplastic consolidation at finite strain. Part 2: finite element implementation and numerical examples

A mathematical model for finite strain elastoplastic consolidation of fully saturated soil media is implemented into a finite element program. The algorithmic treatment of finite strain elastoplasticity for the solid phase is based on multiplicative decomposition and is coupled with the algorithm for fluid flow via the Kirchhoff pore water pressure. A two-field mixed finite element formulation is employed in which the nodal solid displacements and the nodal pore water pressures are coupled via the linear momentum and mass balance equations. The constitutive model for the solid phase is represented by modified Cam—Clay theory formulated in the Kirchhoff principal stress space, and return mapping is carried out in the strain space defined by the invariants of the elastic logarithmic principal stretches. The constitutive model for fluid flow is represented by a generalized Darcy's law formulated with respect to the current configuration. The finite element model is fully amenable to exact linearization. Numerical examples with and without finite deformation effects are presented to demonstrate the impact of geometric nonlinearity on the predicted responses. The paper concludes with an assessment of the performance of the finite element consolidation model with respect to accuracy and numerical stability.

Divergence of the IGS rDNA in Fusarium proliferatum and Fusarium globosum reveals two strain specific non-orthologous types

A phylogenic analysis of Fusarium proliferatum and closely related species was performed using the most variable part within the intergenic spacer of the nuclear ribosomal DNA (IGS) and compared with a previously reported phylogeny performed in the same group of samples with a partial region of the nuclear single copy gene encoding the elongation factor 1α (EF-1α). The phylogenies from both genomic sequences were not concordant and revealed the presence of two nonorthologous IGS types, named types I and II, in F. proliferatum and Fusarium globosum. Two specific PCR assays designed to amplify either IGS type I or type II revealed that only one IGS type was present in each individual in these two species. The presence of both IGS types at the species level indicates that homogenization has not been achieved yet. This might be retarded if panmictic sexual reproduction was affected by certain levels of clonal reproduction and/or by the diverse hosts that these species are able to colonize. This study indicates that taxonomic studies carried out with the IGS rDNA, which has been widely used in Fusarium, should be undertaken with caution.

Flow and fracture behaviour of FV535 steel at different triaxialities, strain rates and temperatures

The new generation jet engines operate at highly demanding working conditions. Such conditions need very precise design which implies an exhaustive study of the engine materials and behaviour in their extreme working conditions. With this purpose, this work intends to describe a numerically-based calibration of the widely-used Johnson–Cook fracture model, as well as its validation through high temperature ballistic impact tests. To do so, a widely-used turbine casing material is studied. This material is the Firth Vickers 535 martensitic stainless steel. Quasi-static tensile tests at various temperatures in a universal testing machine, as well as dynamic tests in a Split Hopkinson Pressure Bar, are carried out at different triaxialities. Using ABAQUS/Standard and LS-DYNA numerical codes, experimental data are matched. This method allows the researcher to obtain critical data of equivalent plastic strain and triaxility, which allows for more precise calibration of the Johnson–Cook fracture model. Such enhancement allows study of the fracture behaviour of the material across its usage temperature range.

Spalling uniaxial strength of Al2O3 at high strain rates

In this article research into the uniaxial tensile strength of Al2O3 monolithic ceramic is presented. The experimental procedure of the spalling of long bars is investigated from different approaches. This method is used to obtain the tensile strength at high strain rates under uniaxial conditions. Different methodologies proposed by several authors are used to obtain the tensile strength. The hypotheses needed for the experimental set-up are also checked, and the requirements of the set-up and the variables are also studied by means of numerical simulations. The research shows that the shape of the projectile is crucial to achieve successfully tests results. An experimental campaign has been carried out including high speed video and a digital image correlation system to obtain the tensile strength of alumina. Finally, a comparison of the test results provided by three different methods proposed by different authors is presented. The tensile strength obtained from the three such methods on the same specimens provides contrasting results. Mean values vary from one method to another but the trends are similar for two of the methods. The third method gives less scatter, though the mean values obtained are lower and do not follow the same trend as the other methods for the different specimens.

A large strain anisotropic elastoplastic continuum theory for nonlinear kinematic hardening and texture evolution

In this paper we present a continuum theory for large strain anisotropic elastoplasticity based on a decomposition of the modified plastic velocity gradient into energetic and dissipative parts. The theory includes the Armstrong and Frederick hardening rule as well as multilayer models as special cases even for large strain anisotropic elastoplasticity. Texture evolution may also be modelled by the formulation, which allows for a meaningful interpretation of the terms of the dissipation equation

Effect of the temperature, strain rate and microstructure on flow and fracture characteristics of Ti-45Al-2Nb-2Mn+0.8vol.% TiB2 XD alloy

A series of quasi-static and dynamic tensile tests at varying temperatures were carried out to determine the mechanical behaviour of Ti-45Al-2Nb-2Mn+0.8vol.% TiB2 XD as-HIPed alloy. The temperature for the tests ranged from room temperature to 850  ∘C. The effect of the temperature on the ultimate tensile strength, as expected, was almost negligible within the selected temperature range. Nevertheless, the plastic flow suffered some softening because of the temperature. This alloy presents a relatively low ductility; thus, a low tensile strain to failure. The dynamic tests were performed in a Split Hopkinson Tension Bar, showing an increase of the ultimate tensile strength due to the strain rate hardening effect. Johnson-Cook constitutive relation was used to model the plastic flow. A post-testing microstructural of the specimens revealed an inhomogeneous structure, consisting of lamellar α2 + γ structure and γ phase equiaxed grains in the centre, and a fully lamellar structure on the rest. The assessment of the duplex-fully lamellar area ratio showed a clear relationship between the microstructure and the fracture behaviour.