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

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

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

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

Psychological and socio-demographic variables associated with sexual risk behavior for sexually transmitted infections/HIV

New diagnoses of sexually transmitted infections (STIs) and HIV are rising in the adult population. The main objective of this study was to analyze whether knowledge of STIs/HIV, worry about STIs/HIV and pregnancy, and self-efficacy to refuse sex are predictors of sexual risk behaviors among Spanish young and adults. The study sample was composed of 1,106 young and adults of both sexes aged between 17 and 55 years. Results showed that being single, homosexual, having been tested for HIV, having previously contracted an STI, having a college education and earning a monthly income of €900 or more were the characteristics associated with higher scores in knowledge of STIs/HIV. Self-efficacy to refuse sex predicted most vaginal and anal sexual behaviors (i.e., age at vaginal and anal sex initiation and the number of couples that have remained vaginal sex). We also found that participants with greater knowledge of STIs/HIV reported older age at vaginal sex initiation and higher condom use in the first vaginal sexual contact. We consider that these findings can be useful for the development of STI/HIV prevention programs.La población adulta es un grupo ascendente en los nuevos diagnósticos de las infec de transmisión sexual (ITS) y el VIH. El objetivo principal es analizar si el conocimiento sobre el ITS/VIH, la preocupación por las ITS/VIH y el embarazo y la autoeficacia para rechazar relaciones sexuales son predictoras de las conductas sexuales de riesgo en jóvenes y adultos españoles. Participaron 1.106 jóvenes y adultos de ambos sexos, entre los 17 y los 55 años. Los resultados mostraron que ser soltero, homosexual, haberse hecho la prueba del VIH, haber tenido una ITS, tener estudios universitarios e ingresos económicos mensuales de 900 a más de 1.200 euros tenían las puntuaciones más altas en el conocimiento sobre el ITS/VIH. La autoeficacia predijo gran parte de las conductas sexuales vaginales y anales (la edad de inicio del sexo vaginal y anal y el número de parejas con las que se ha mantenido sexo vaginal). Además, se halló que a mayor conocimiento sobre ITS/VIH, mayor edad de inicio del sexo vaginal y mayor uso del preservativo en la primera relación sexual vaginal. Se espera que estos resultados sean útiles para el desarrollo de programas de prevención de ITS/VIH

Cambios temporales en el origen de la materia orgánica en las marismas del Río Miño (NW Península Ibérica) mediante marcadores lipídicos. Temporal changes in the organic matter sources in the Minho River tidal marshes (NW Iberian Península): A lipid biomarker approach.

[EN]: The lipid biomarker distribution in a sediment core fromMinho river tidal-marshes (NW Iberian Peninsula) was studiedusing gas chromatography-mass spectrometry (GC/MS) in order to evaluate changes in the sources and distribution of organic matter (OM) in the estuary during the last centuries. The distribution of terrestrial and phytoplankton biomarker inventory reflected an heterogeneous mixture of OM from marine and terrestrial sources. Lignin derived phenols, triterpenoids and long chain n-alkanes with an odd number of carbons are indicators of soil-and vascular plant-derived continental OM, being more abundant on the topmost sections of the core. In addition, a conspicuous contribution from plankton-derived OM (e.g. short chain fatty acids and alcohols, phytosterols) is observed in the deepest sections of the core. However, in general the lipid biomarker assemblage found indicate the existence of a complex input of organic matter (OM) to this estuarine area, from both river run-off and marine depositions. Temporal variations observed could also be attributed to terrigenous contributions after flooding events from Miño river, although a differential degradation of individual biomarkers should not be neglected when interpreting biomarker assemblages in this geodynamically active environment. [ES]: Con el objetivo de evaluar las variaciones en las fuentes y distribución de la matería orgânica (MO) durante los últimos siglos, se estudia la distribución de biomarcadores lipídicos en una secuencia sedimentaria de las marismas del río Miño (NW Península Ibérica) mediante Cromatografía de Gases¿Espectrometria de Masas (CG/MS) . La distribución de biomarcadores terrestres y marinos muestra una mezcla heterogénea de MO con diferente orígen. Los fenoles derivados de la lignina, triterpenoides y n-alcanos de cadena larga con número impar de carbonos proceden de plantas superiores, indicadores de MO de origen continental , se encuentran com mayor abundancia en las secciones superiores del sondeo. Los lípidos derivados del plancton marino (ej., acidos grasos y alcoholes de cadena corta, fitosteroles) se encontraron con mayor abundancia relativa en las secciones mas profundas del sondeo. Sin embargo, la distribución global de los biomarcadores lipídicos estudiados muestra la complejidad de las fuentes de MO en este estuario, y que incluye las descargas del río y deposiciones de sedimentos marinos. Algunas variaciones observadas pueden deberse a contribuciones de MO terrígena debidas a descargas elevadas procedentes del río Miño. No obstante, a la hora de interpretar los cámbios observados en la distribución de determinados biomarcadores en este ambiente geodinámicamente activo, se debe tener en cuenta la posible ocurrencia de fenómenos de degradación preferencial o selectiva en el sedimento.Peer Reviewe

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

Election turnout statistics in many countries: similarities, differences, and a diffusive field model for decision-making

We study in details the turnout rate statistics for 77 elections in 11 different countries. We show that the empirical results established in a previous paper for French elections appear to hold much more generally. We find in particular that the spatial correlation of turnout rates decay logarithmically with distance in all cases. This result is quantitatively reproduced by a decision model that assumes that each voter makes his mind as a result of three influence terms: one totally idiosyncratic component, one city-specific term with short-ranged fluctuations in space, and one long-ranged correlated field which propagates diffusively in space. A detailed analysis reveals several interesting features: for example, different countries have different degrees of local heterogeneities and seem to be characterized by a different propensity for individuals to conform to the cultural norm. We furthermore find clear signs of herding (i.e. strongly correlated decisions at the individual level) in some countries, but not in others.Comment: 15 pages, 9 figures, 7 table