TRANSIENT SCANNING ELECTRON BEAM ANNEALING METHODS USED TO STUDY DIFFUSION AND DEFECTS IN IMPLANTED SILICON.

Rapid thermal annealing of arsenic and boron difluoride implants, such as those used for source/drain regions in CMOS, has been carried out using a scanning electron beam annealer, as part of a study of transient diffusion effects. Three types of e-beam anneal have been performed, with peak temperatures in the range 900 -1200 degree C; the normal isothermal e-beam anneals, together with sub-second fast anneals and 'dual-pulse' anneals, in which the sample undergoes an isothermal pre-anneal followed by rapid heating to the required anneal temperature is less than 0. 5s. The diffusion occuring during these anneal cycles has been modelled using SPS-1D, an implant and diffusion modelling program developed by one of the authors. This has been modified to incorporate simulated temperature vs. time cycles for the anneals. Results are presented applying the usual equilibrium clustering model, a transient point-defect enhancement to the diffusivity proposed recently by Fair and a new dynamic clustering model for arsenic. Good agreement with SIMS measurements is obtained using the dynamic clustering model, without recourse to a transient defect model.

Plasma stabilisation of metallic nanoparticles on silicon for the growth of carbon nanotubes

Ammonia (NH 3) plasma pretreatment is used to form and temporarily reduce the mobility of Ni, Co, or Fe nanoparticles on boron-doped mono- and poly-crystalline silicon. X-ray photoemission spectroscopy proves that NH 3 plasma nitrides the Si supports during nanoparticle formation which prevents excessive nanoparticle sintering/diffusion into the bulk of Si during carbon nanotube growth by chemical vapour deposition. The nitridation of Si thus leads to nanotube vertical alignment and the growth of nanotube forests by root growth mechanism. © 2012 American Institute of Physics.

Light absorption and electrical transport in Si:O alloys for photovoltaics

Thin films (100-500 nm) of the Si:O alloy have been systematically characterized in the optical absorption and electrical transport behavior, by varying the Si content from 43 up to 100 at. %. Magnetron sputtering or plasma enhanced chemical vapor deposition have been used for the Si:O alloy deposition, followed by annealing up to 1250 °C. Boron implantation (30 keV, 3-30× 1014 B/cm2) on selected samples was performed to vary the electrical sheet resistance measured by the four-point collinear probe method. Transmittance and reflectance spectra have been extracted and combined to estimate the absorption spectra and the optical band gap, by means of the Tauc analysis. Raman spectroscopy was also employed to follow the amorphous-crystalline (a-c) transition of the Si domains contained in the Si:O films. The optical absorption and the electrical transport of Si:O films can be continuously and independently modulated by acting on different parameters. The light absorption increases (by one decade) with the Si content in the 43-100 at. % range, determining an optical band gap which can be continuously modulated into the 2.6-1.6 eV range, respectively. The a-c phase transition in Si:O films, causing a significant reduction in the absorption coefficient, occurs at increasing temperatures (from 600 to 1100 °C) as the Si content decreases. The electrical resistivity of Si:O films can be varied among five decades, being essentially dominated by the number of Si grains and by the doping. Si:O alloys with Si content in the 60-90 at. % range (named oxygen rich silicon films), are proved to join an appealing optical gap with a viable conductivity, being a good candidate for increasing the conversion efficiency of thin-film photovoltaic cell. © 2010 American Institute of Physics.

Thermal evolution of Er silicate thin films grown by rf magnetron sputtering

Stoichiometric Er silicate thin films, monosilicate (Er2SiO 5) and disilicate (Er2Si2O7), have been grown on c-Si substrates by rf magnetron sputtering. The influence of annealing temperature in the range 1000-1200 °C in oxidizing ambient (O 2) on the structural and optical properties has been studied. In spite of the known reactivity of rare earth silicates towards silicon, Rutherford backscattering spectrometry shows that undesired chemical reactions between the film and the substrate can be strongly limited by using rapid thermal treatments. Monosilicate and disilicate films crystallize at 1100 and 1200 °C, respectively, as shown by x-ray diffraction analysis; the crystalline structures have been identified in both cases. Moreover, photoluminescence (PL) measurements have demonstrated that the highest PL intensity is obtained for Er2Si2O7 film annealed at 1200 °C. In fact, this treatment allows us to reduce the defect density in the film, in particular by saturating oxygen vacancies, as also confirmed by the increase of the lifetime of the PL signal. © 2008 IOP Publishing Ltd.

Optimization of burnable poison design for Pu incineration in fully fertile free PWR core

The design challenges of the fertile-free based fuel (FFF) can be addressed by careful and elaborate use of burnable poisons (BP). Practical fully FFF core design for PWR reactor has been reported in the past [1]. However, the burnable poison option used in the design resulted in significant end of cycle reactivity penalty due to incomplete BP depletion. Consequently, excessive Pu loading were required to maintain the target fuel cycle length, which in turn decreased the Pu burning efficiency. A systematic evaluation of commercially available BP materials in all configurations currently used in PWRs is the main objective of this work. The BP materials considered are Boron, Gd, Er, and Hf. The BP geometries were based on Wet Annular Burnable Absorber (WABA), Integral Fuel Burnable Absorber (IFBA), and Homogeneous poison/fuel mixtures. Several most promising combinations of BP designs were selected for the full core 3D simulation. All major core performance parameters for the analyzed cases are very close to those of a standard PWR with conventional UO2 fuel including possibility of reactivity control, power peaking factors, and cycle length. The MTC of all FFF cores was found at the full power conditions at all times and very close to that of the UO2 core. The Doppler coefficient of the FFF cores is also negative but somewhat lower in magnitude compared to UO2 core. The soluble boron worth of the FFF cores was calculated to be lower than that of the UO2 core by about a factor of two, which still allows the core reactivity control with acceptable soluble boron concentrations. The main conclusion of this work is that judicial application of burnable poisons for fertile free fuel has a potential to produce a core design with performance characteristics close to those of the reference PWR core with conventional UO2 fuel.

Recycle of transuranium elements in light water reactors for reduction of geological storage requirements

This paper presents results of a feasibility study aimed at developing a zero-transuranic-discharge fuel cycle based on the U-Th-TRU ternary cycle. The design objective is to find a fuel composition (mixture of thorium, enriched uranium, and recycled transuranic components) and fuel management strategy resulting in an equilibrium charge-discharge mass flow. In such a fuel cycle scheme, the quantity and isotopic vector of the transuranium (TRU) component is identical at the charge and discharge time points, thus allowing the whole amount of the TRU at the end of the fuel irradiation period to be separated and reloaded into the following cycle. The TRU reprocessing activity losses are the only waste stream that will require permanent geological storage, virtually eliminating the long-term radiological waste of the commercial nuclear fuel cycle. A detailed three-dimensional full pressurized water reactor (PWR) core model was used to analyze the proposed fuel composition and management strategy. The results demonstrate the neutronic feasibility of the fuel cycle with zero-TRU discharge. The amount of TRU and enriched uranium loaded reach equilibrium after about four TRU recycles. The reactivity coefficients were found to be within a range typical for a reference PWR core. The soluble boron worth is reduced by a factor of ∼2 from a typical PWR value. Nevertheless, the results indicate the feasibility of an 18-month fuel cycle design with an acceptable beginning-of-cycle soluble boron concentration even without application of burnable poisons.

Novel nanostructured biomaterials: implications for coronary stent thrombosis.

BACKGROUND: Nanomedicine has the potential to revolutionize medicine and help clinicians to treat cardiovascular disease through the improvement of stents. Advanced nanomaterials and tools for monitoring cell-material interactions will aid in inhibiting stent thrombosis. Although titanium boron nitride (TiBN), titanium diboride, and carbon nanotube (CNT) thin films are emerging materials in the biomaterial field, the effect of their surface properties on platelet adhesion is relatively unexplored. OBJECTIVE AND METHODS: In this study, novel nanomaterials made of amorphous carbon, CNTs, titanium diboride, and TiBN were grown by vacuum deposition techniques to assess their role as potential stent coatings. Platelet response towards the nanostructured surfaces of the samples was analyzed in line with their physicochemical properties. As the stent skeleton is formed mainly of stainless steel, this material was used as reference material. Platelet adhesion studies were carried out by atomic force microscopy and scanning electron microscopy observations. A cell viability study was performed to assess the cytocompatibility of all thin film groups for 24 hours with a standard immortalized cell line. RESULTS: The nanotopographic features of material surface, stoichiometry, and wetting properties were found to be significant factors in dictating platelet behavior and cell viability. The TiBN films with higher nitrogen contents were less thrombogenic compared with the biased carbon films and control. The carbon hybridization in carbon films and hydrophilicity, which were strongly dependent on the deposition process and its parameters, affected the thrombogenicity potential. The hydrophobic CNT materials with high nanoroughness exhibited less hemocompatibility in comparison with the other classes of materials. All the thin film groups exhibited good cytocompatibility, with the surface roughness and surface free energy influencing the viability of cells.