TEM study of homoepitaxial diamond layers scheduled for high power devices: FIB method of sample preparation

Homoepitaxial diamond structure observation by transmission electron microscopy (TEM) is still a very hard job due to the difficulty in preparing electron transparent samples for the further observation. The present contribution details the experimental operations with their respective conditions step by step. Finally high resolution TEM (HREM) observations of a CVD grown epilayer on a unnintentionally doped HPHT (001) oriented substrate are present to show the high quality of the sample preparation method.4 page

Boron concentration profiling by high angle annular dark field-scanning transmission electron microscopy in homoepitaxial delta-doped diamond layers

To develop further diamond related devices, the concentration and spatial location of dopants should be controlled down to the nanometer scale. Scanning transmission electron microscopy using the high angle annular dark field mode is shown to be sensitive to boron doping in diamond epilayers. An analytical procedure is described, whereby local boron concentrations above 1020 cm-3 were quantitatively derived down to nanometer resolution from the signal dependence on thickness and boron content. Experimental boron local doping profiles measured on diamond p-/p++/p- multilayers are compared to macroscopic profiles obtained by secondary ion mass spectrometry, avoiding reported artefacts.4 page

Influence of the substrate type on CVD grown homoepitaxial diamond layer quality by cross sectional TEM and CL analysis

To assess diamond-based semiconducting devices, a reduction of point defect levels and an accurate control of doping are required as well as the control of layer thickness. Among the analyses required to improve such parameters, cross sectional studies should take importance in the near future. The present contribution shows how FIB (focused ion beam) preparations followed by electron microscopy related techniques as TEM or CL allowed to performanalysis versus depth in the layer, doping and point defect levels. Three samples grown along the sameweek in the same machinewith identical growth conditions but on different substrates (CVD-IIIa (110) oriented, CVD-optical grade (100) oriented and a HPHT-Ib (100) oriented) are studied. Even though A-band is observed by CL, no dislocation is observed by CTEM. Point defect type and level are shown to substantially change with respect to the substrate type as well as the boron doping levels that vary within an order of magnitude. H3 present in the epilayer grown on HPHT type of substrate is replaced by T1 and NE3 point defects for epilayers grown on the CVD type one. An increase of excitonic transitions through LO phonons is also shown to take place near the surface while only TO ones are detected deeper in the epilayer. Such results highlight the importance of choosing the correct substrate.5 page

A microstructural study of superconductive nanocrystalline diamond

A transmission electron microscopy (TEM) study of superconducting nanocrystalline diamond (NCD) continuous layers is reported. The high resolution transmission electron microscopy (HREM) and the diffraction contrast modes of observations are used to reveal the nanograins configuration. Three types of them are observed: first, close to the interface with the Si/SiO2 substrate, 10 to 20 nm-sized diamond 16 seeds resulting from the 5nm size diamond powder deposition before growth that show some regrowth during CVD process, second a diamond overgrown layer, quasi-epitaxially by coalesced columnar NCD grains, and finally, up to the free surface, a thin disordered region composed of nanocrystallites smaller than 6 nm. This last layer was not nominally expected and is attributed to a renucleated-like (RND) diamond layer embedding ultra nanocrystalline grains. Diffraction contrast observations confirm this HREM observed behaviour.6 page

Cross sectional evaluation of boron doping and defect distribution in homoepitaxial diamond layers

In some diamond-based semiconducting devices, large variations of doping level are required over short distances. Tools to determine doping level and defects distribution should therefore be developed. The present contribution shows the capabilities of electron microscopy in this field. Focused ion beam (FIB-dual beam) cross section preparations allowed evaluating doping level in highly boron doped sample with doping transition down to some nm by diffraction contrast mode of transmission electron microscopy (CTEM) and by high angle annular dark field mode of scanning transmission electron microscopy (HAADF-STEM). The sensibility of the latter is around 1019cm-3 and, thus, cathodoluminescence (CL)is required for lower doping levels. Cross sectional analysis on FIB prepared lamella allowed to separate the epilayer behaviour from that of the substrate. Mid-gap centers involving boron, hydrogen and, for some peaks, also nitrogen are revealed. sp2 bonds are also present in the grown epilayer. These transitions make difficult the observation of excitonic recombinations in the cross section configuration.5 page

Ansiedad matemática de maestros en formación: un estudio de caso de alumnado de cuarto curso del Grado

El dominio afectivo juega un papel fundamental en el proceso de enseñanza y aprendizaje. Este ha empezado a cobrar importancia en la medida en que los educadores han apreciado la influencia que tiene sobre el aprendizaje escolar (Hernández, 1996). Autores como McLeod (1998) defienden la existencia de tres descriptores y define el dominio afectivo como un extenso rango de estados de ánimo que son generalmente considerados como algo diferente de la pura cognición, e incluye como componentes específicos el conjunto del sistema de creencias, las actitudes y las emociones

Dislocation generation mechanisms in heavily boron-doped diamond epilayers

Doping diamond layers for electronic applications has become straightforward during the last two decades. However, dislocation generation in diamond during the microwave plasma enhanced chemical vapor deposition growth process is still not fully understood. This is a truly relevant topic to avoid for an optimal performance of any device, but, usually, it is not considered when designing diamond structures for electronic devices. The incorporation of a dopant, here boron, into a lattice as close as that of diamond, can promote the appearance of dislocations in the epilayer. The present contribution analyzes the different processes that can take place in this epilayer and gives some rules to avoid the formation of dislocations, based on the comparison of the different dislocation generation mechanisms. Indeed, competitive mechanisms, such as doping atom proximity effect and lattice strain relaxation, are here quantified for heavily boron-doped diamond epilayers. The resulting growth condition windows for defect-free heavily doped diamond are here deduced, introducing the diamond parameters and its lattice expansion in several previously published critical thickness (h(c)) and critical doping level relationships for different doping levels and growth conditions. Experimental evidence supports the previously discussed thickness-doping-growth condition relationships. Layers with and without dislocations reveal that not only the thickness but also other key factors such as growth orientation and growth parameters are important, as dislocations are shown to be generated in epilayers with a thickness below the People and Bean critical thickness

Didácticas de aprendizaje: clases magistrales frente al aprendizaje basado en problemas en la Escuela Técnica Superior de Ingeniería de Algeciras

En el contexto de la asignatura de Ciencia e Ingeniería de los Materiales (CIM), común para todos los grados de ingeniería de la rama industrial de la Escuela Técnica Superior de Ingeniería de Algeciras (ETSIA), se comparan indicadores de calidad tras aplicar dos aproximaciones de evaluación continua: una más tradicional mediante exámenes escritos de bloques temáticos, y otra mediante una metodología docente con el objetivo de que los alumnos sean los responsables de la asimilación del conocimiento. En esta nueva metodología, el 100% de la evaluación es en forma de evaluación continua. Parte de esta evaluación es en forma de pruebas de progreso, donde cada tema lo han tenido que aplicar a un caso práctico, para el cuál se les daba una semana para resolverlo. Otra peculiaridad de esta segunda aproximación de evaluación continua es que la entrega de la prueba se realiza en distintos formatos: modelos CAD, hojas de cálculo, vídeos, exposiciones, etc. Con esta metodología se persigue hacer al alumnado responsable de su propio proceso de aprendizaje, siendo parte activa del mismo. De forma generalizada se puede concluir que el sistema de evaluación continua mejora, además de las calificaciones obtenidas en la asignatura, el rendimiento posterior de los alumnos y su actitud hacia el aprendizaje autónomo

Aplicación de metodologías de aprendizaje basado en casos prácticos para la enseñanza de asignaturas científico-tecnológicas

En el contexto de la asignatura de Ingeniería y Tecnología de los Materiales (ITM) de la Escuela Técnica Superior de Ingeniería de Algeciras (ETSIA), se compararon dos metodologías. Una metodología más tradicional basada en i) clases expositivas, ii) clases de problemas y iii) prácticas de laboratorio, y otra metodología docente basada en el aprendizaje en proyectos, con el objetivo de que los alumnos sean los responsables de la asimilación del conocimiento. Con esta nueva metodología, se eliminó la mayor parte de las clases expositivas y se sustituyó por trabajo de los alumnos en el aula, bajo supervisión del profesor, basado en casos prácticos. De forma generalizada se pudo concluir que con la metodología basada en casos prácticos mejoró, además de las calificaciones obtenidas en la asignatura, la tasa de éxito y de rendimiento

Epitaxial Growth of Boron Carbide on 4H-SiC

In this work, the successful heteroepitaxial growth of boron carbide (BxC) on 4H-SiC(0001) 4° off substrate using chemical vapor deposition (CVD) is reported. Towards this end, a two-step procedure was developed, involving the 4H-SiC substrate boridation under BCl3 precursor at 1200°C, followed by conventional CVD under BCl3 + C3H8 at 1600°C. Such a procedure allowed obtaining reproducibly monocrystalline (0001) oriented films of BxC with a step flow morphology at a growth rate of 1.9 µm/h. Without the boridation step, the layers are systematically polycrystalline. The study of the epitaxial growth mechanism shows that a monocrystalline BxC layer is formed after boridation but covered with a B- and Si-containing amorphous layer. Upon heating up to 1600°C, under pure H2 atmosphere, the amorphous layer was converted into epitaxial BxC and transient surface SiBx and Si crystallites. These crystallites disappear upon CVD growth