Aggregation of SiC-X Grains in Supernova Ejecta

We present a model for the formation of silicon carbide aggregates within the expanding and cooling supernova remnant. Many SiC-X grains have been found to be aggregates of smaller crystals which are isotopically homogenous. The initial condensation of SiC in the ejecta occurs within a interior dense shell of material which is created by a reverse shock which rebounds from the core-envelope interface. A subsequent reverse shock accelerates the grains forward, but the gas drag from the ejecta on the rapidly moving particles limits their travel distance. By observing the effects of gas drag on the travel distance of grains, we propose that supernova grain aggregates form from material that condensed in a highly localized region, which satisfies the observational evidence of isotopic homogeneity in SiC-X grains.Comment: 9 pages, 5 figures, To be published in the Astrophysical Journa

New attempts to understand nanodiamond stardust

We report on a concerted effort aimed at understanding the origin and history of the pre-solar nanodiamonds in meteorites including the astrophysical sources of the observed isotopic abundance signatures. This includes measurement of light elements by secondary ion mass spectrometry (SIMS), analysis of additional heavy trace elements by accelerator mass spectrometry (AMS) and dynamic calculations of r-process nucleosynthesis with updated nuclear properties. Results obtained indicate: a) there is no evidence for the former presence of now extinct 26Al and 44Ti in our diamond samples other than what can be attributed to silicon carbide and other "impurities"; this does not offer support for a supernova (SN) origin but neither does it negate it; b) analysis by AMS of platinum in "bulk diamond" yields an overabundance of r-only 198Pt that at face value seems more consistent with the neutron burst than with the separation model for the origin of heavy trace elements in the diamonds, although this conclusion is not firm given analytical uncertainties; c) if the Xe-H pattern was established by an unadulterated r-process, it must have been a strong variant of the main r-process, which possibly could also account for the new observations in platinum.Comment: Workshop on Astronomy with Radioactvities VII; Publications of the Astronomical Society of Australia, accepte

Supernova Reverse Shocks and SiC Growth

We present new mechanisms by which the isotopic compositions of X-type grains of presolar SiC are altered by reverse shocks in Type II supernovae. We address three epochs of reverse shocks: pressure wave from the H envelope near t = 10$^6$s; reverse shock from the presupernova wind near 10$^8-10^9$s; reverse shock from the ISM near 10$^{10}$s. Using 1-D hydrodynamics we show that the first creates a dense shell of Si and C atoms near 10$^6$s in which the SiC surely condenses. The second reverse shock causes precondensed grains to move rapidly forward through decelerated gas of different isotopic composition, during which implantation, sputtering and further condensation occur simultaneously. The third reverse shock causes only further ion implantation and sputtering, which may affect trace element isotopic compositions. Using a 25M$_{\odot}$ supernova model we propose solutions to the following unsolved questions: where does SiC condense?; why does SiC condense in preference to graphite?; why is condensed SiC $^{28}$Si-rich?; why is O richness no obstacle to SiC condensation?; how many atoms of each isotope are impacted by a grain that condenses at time t$_0$ at radial coordinate r$_0$? These many considerations are put forward as a road map for interpreting SiC X grains found in meteorites and their meaning for supernova physics.Comment: 28 pages, 14 figures, animation for Figure 3 and machine-readable Table 3 can be found at http://antares.steelangel.com/~edeneau/supernova/DHC_2003, Submitted to Ap

Low Temperature Deposition of High-k/Metal Gate Stacks on High-Sn Content (Si)GeSn-Alloys

(Si)GeSn is an emerging group IV alloy system offering new exciting properties, with great potential for low power electronics due to the fundamental direct band gap and prospects as high mobility material. In this Article, we present a systematic study of HfO2/TaN high-k/metal gate stacks on (Si)GeSn ternary alloys and low temperature processes for large scale integration of Sn based alloys. Our investigations indicate that SiGeSn ternaries show enhanced thermal stability compared to GeSn binaries, allowing the use of the existing Si technology. Despite the multielemental interface and large Sn content of up to 14 atom %, the HfO2/(Si)GeSn capacitors show small frequency dispersion and stretch-out. The formed TaN/HfO2/(Si)GeSn capacitors present a low leakage current of 2 × 10(-8) A/cm(2) at -1 V and a high breakdown field of ∼8 MV/cm. For large Sn content SiGeSn/GeSn direct band gap heterostructures, process temperatures below 350 °C are required for integration. We developed an atomic vapor deposition process for TaN metal gate on HfO2 high-k dielectric and validated it by resistivity as well as temperature and frequency dependent capacitance-voltage measurements of capacitors on SiGeSn and GeSn. The densities of interface traps are deduced to be in the low 10(12) cm(-2) eV(-1) range and do not depend on the Sn-concentration. The new processes developed here are compatible with (Si)GeSn integration in large scale applications

A NanoSIMS study of Si- and Ca-Ti-isotopic compositions of presolar silicon carbide grains from supernovae

We report results from NanoSIMS isotopic measurements on 37 presolar silicon carbide grains of type X which are believed to have formed in the ejecta of supernova explosions. Isotopic data were obtained for Si and Ca-Ti (all grains), C and N (two grains), and Ti (one grain). All X grains exhibit large enrichments in Si-28 (up to 5X solar), in agreement with previously studied X grains. On a scale of 200 nm, the Si-isotopic ratios do not vary by more than the analytical uncertainties of several percent in all but one X grain. This implies that most X grains formed from well-mixed regions in supernova ejecta. X grain M9-68-3 is characterized by two regions with distinct Si- and Ti-isotopic signatures which may either represent two distinct grains or overgrowth of matter from two different mixtures in the supernova ejecta. Most of the Ca in the X grains is most likely contamination as indicated by close to normal Ca-42/Ca-40 ratios. Seven X grains show enhanced Ca-44/Ca-40 ratios of up to 6X the solar ratio. Spatial distributions of Ca-44 excesses and Ti are positively correlated, giving strong support to the view that excesses in Ca-44 are due to the decay of radioactive Ti-44. Inferred initial Ti-44/Ti-48 ratios are between 0.01 and 0.28 and are correlated with Si-isotopic ratios. Radiogenic Ca-44 is widely distributed in six X grains. X grain M9-132-4 exhibits a pronounced heterogeneity in the distribution of radiogenic Ca-44 and Ti-48 as well as in Ti-44/Ti-48, pointing to presence of a small Ti-rich subgrain or heterogeneous loss of Ca and Ti after grain formation. This grain has a unique Si-isotopic composition with Si-30/Si-29 = 2.2X the solar ratio and C- and N-isotopic compositions as typically observed in X grains. Copyright (C) 2003 Elsevier Ltd