Unified Model for Laser Doping of Silicon from Precursors

Laser doping of silicon with the help of precursors is well established in photovoltaics. Upon illumination with the constant or pulsed laser beam, the silicon melts and doping atoms from the doping precursor diffuse into the melted silicon. With the proper laser parameters, after resolidification, the silicon is doped without any lattice defects. Depending on laser energy and on the kind of precursor, the precursor either melts or evaporates during the laser process. For high enough laser energies, even parts of the silicon’s surface evaporate. Here, we present a unified model and simulation program, which considers all these cases. We exemplify our model with experiments and simulations of laser doping from a boron oxide precursor layer. In contrast to previous models, we are able to predict not only the width and depth of the patterns on the deformed silicon surface but also the doping profiles over a wide range of laser energies. In addition, we also show that the diffusion of the boron atoms in the molten Si is boosted by a thermally induced convection in the silicon melt: the Gaussian intensity distribution of the laser beam increases the temperature-gradient-induced surface tension gradient, causing the molten Si to circulate by Marangoni convection. Laser pulse energy densities above H > 2.8 J/cm2 lead not only to evaporation of the precursor, but also to a partial evaporation of the molten silicon. Without considering the evaporation of Si, it is not possible to correctly predict the doping profiles for high laser energies. About 50% of the evaporated materials recondense and resolidify on the wafer surface. The recondensed material from each laser pulse forms a dopant source for the subsequent laser pulses.

Enhanced Mechanical Property of RMI C/SiC Composites with Optimized Porous Carbon Structure by Heat Treatment

C/SiC composites were fabricated by reactive melt infiltration (RMI) using porous C/C preforms as the skeleton, followed by the infiltration of molten silicon. A convenient technique of heat treatment in the range of 1500 to 2400 oC was applied to modify the porous carbon structure. The effects of heat treatment on the porous C/C preforms and the as-synthesized C/SiC composites were investigated in detail. The results show that the optimal porous carbon structure could be obtained after heat treatment at 1500 oC. After 1500 oC heat treatment, the median pore size and porosity of the porous C/C preform were 9.3 µm and 13.65% respectively, which were in favor of the subsequent infiltration of molten Si. The C/SiC composites with the optimized porous carbon structure showed a dense and uniform morphology without obvious cracks. Their bending strength could be up to 276 MPa, which was 28% higher than that of the C/SiC composites with untreated C/C preforms. However, with the increasing heat treatment temperature (2400 oC), the bending strength of the as obtained composites began to decrease because of the degradation of in-situ fiber strength. The optimized C/SiC composites exhibited a typical pseudo-plastic fracture behavior with obvious fiber pull-out. The improved mechanical property could be ascribed to the lower porosity of composites, the higher in-situ strength of fibers and the reduced matrix cracks.

Mathematical Modeling оf the Silicon Production Process from Pelletized Charge

The paper considers the problem of recycling the dust waste resulting from metallurgical silicon production; such dust contains considerable amounts of valuable silica. The problem is solved by redirecting this byproduct to the silicon smelting process. We herein propose using the dust left in silicon and aluminum production as a component of pelletized charge, used for silicon smelting in ore-thermal furnaces (OTF). Mathematical (physico-chemical) modeling was applied to study the behavior of pelletized-charge components, in order to predict the chemical composition of smelting-produced silicon. We generated a model that simulated the four temperature zones of a furnace, as well as the crystalline-silicon phase (25°С). The model contained 17 elements entering the furnace, due to being contained in raw materials, electrodes, and the air. Modeling produced molten silicon, 91.73 wt% of which was the target product. Modeling showed that, when using the proposed combined charge, silicon extraction factor would amount to 69.25%, which agrees well with practical data. Results of modeling the chemical composition of crystalline silicon agreed well with the chemical analysis of actually produced silicon.