Jean-Pierre Gauthier

Synthesis and analysis of buried SiC layers in monocrystalline silicon

Abstract SiC layers were fabricated by implantation using 10–40 keV carbon ions in Si(100) and Si(111) wafers, with subsequent annealing between 800 and 1100°C. Computed and experimental IR curves were compared and the thickness of the film were determined from transmission data. The computed thickness were compared with twice the straggling width. The results lead to the conclusion that the fabricated films are continuous rather than SiC clusters in a silicon matrix.

Geochemical and petrological characterization of gem opals from Wegel Tena, Wollo, Ethiopia: opal formation in an Oligocene soil

Gem opals from Wegel Tena, Wollo Province, Ethiopia, occur in Oligocene rhyolitic ignimbrites. They display a unique geochemistry, with some samples yielding the highest Ba concentrations ever recorded. They are generally much richer in chemical impurities than opals from other localities. For example, the sum Al+Fe or the sum Na+Mg+Ca+K+Ba are often higher. These geochemical features make them easy to distinguish from other opals worldwide. We observed strong geochemical variations and some good positive correlations in our samples, such as Al+Fe vs. Na+Mg+Ca+K+Ba, Al vs Ca, or Ba vs Ca. This shows that the crystallography of opal has controlled, at least in part, the incorporation of chemical impurities, although opal is not well-crystallized. In addition, the multimodal distributions of several chemical impurities (e.g. U vs Sr, Al vs Ca, Ba vs Ca, etc.) suggest at least two origins of silica: weathering of feldspars and weathering of volcanic glass. In addition, opals from Wegel Tena contain numerous well-preserved microscopic plant fossils. Moreover, their host rock exhibits features typical of pedogenesis (abundant clays, desiccation cracks, and grain size sorting). We propose that the opals at Wegel Tena formed during the Oligocene period when volcanic emissions stopped for a time long enough to allow weathering of ingimbrites and therefore liberation of silica. This accompanied the formation of soil and development of plant life, and some plants were trapped in opal. Supplementary Material: The totality of the chemical analyses is available at http://www.geolsoc.org.uk/SUP18519.

Investigation of hidden periodic structures on SEM images of opal-like materials using FFT and IFFT

We have developed a method to use fast Fourier transformation (FFT) and inverse fast Fourier transformation (IFFT) to investigate hidden periodic structures on SEM images. We focused on samples of natural, play-of-color opals that diffract visible light and hence are periodically structured. Conventional sample preparation by hydrofluoric acid etch was not used; untreated, freshly broken surfaces were examined at low magnification relative to the expected period of the structural features, and, the SEM was adjusted to get a very high number of pixels in the images. These SEM images were treated by software to calculate autocorrelation, FFT, and IFFT. We present how we adjusted SEM acquisition parameters for best results. We first applied our procedure on an SEM image on which the structure was obvious. Then, we applied the same procedure on a sample that must contain a periodic structure because it diffracts visible light, but on which no structure was visible on the SEM image. In both cases, we obtained clearly periodic patterns that allowed measurements of structural parameters. We also investigated how the irregularly broken surface interfered with the periodic structure to produce additional periodicity. We tested the limits of our methodology with the help of simulated images.

Some observations on thin β-tin films by electron diffraction

Abstract Forbidden “110-type” reflections are present on some patterns of tin single-crystal films. Their intensities may vary in a large range and these reflections do not appear with polycrystalline samples. The other forbidden “002-type” reflections which were observed by other authors and ourselves are probably due to secondary elastic scattering, but the kind of “forbidden” spots studied here can be explained by dynamic interactions.