Active worms spread in an automated fashion and can flood the Internet in a very short time. Modeling the spread of active worms can help us understand how active worms spread, and how we can monitor and defend against the propagation of worms effectively. In this paper, we present a mathematical model, referred to as the Analytical Active Worm Propagation (AAWP) model, which characterizes the propagation of worms that employ random scanning. We compare our model with the Epidemiological model and Weaver's simulator. Our results show that our model can characterize the spread of worms effectively. Taking the Code Red v2 worm as an example, we give a quantitative analysis for monitoring, detecting and defending against worms. Furthermore, we extend our AAWP model to understand the spread of worms that employ local subnet scanning. To the best of our knowledge, there is no model for the spread of a worm that employs the localized scanning strategy and we believe that this is the first attempt on understanding local subnet scanning quantitatively.
Silicon solar cell efficiencies of 16.9% have been achieved on 0.2 Ω cm float zone silicon, using a simplified cost effective rapid thermal process (RTP). Although the individual processing steps are not fully optimized yet, this represents the highest reported efficiency for solar cells processed with simultaneous front and back diffusion with no conventional high-temperature furnace steps. A diffusion temperature schedule coupled with an added short in situ slow cooling during RTP resulted in greater than 200 μm diffusion length and appropriate diffusion profiles for high efficiency cells. Plasma enhanced chemical vapor deposition (PECVD) of SiN/SiO2 was used for surface passivation and antireflection coating. Conventional cells fabricated by furnace diffusions and oxidations gave an efficiency of 18.8%. Process optimization can further reduce the gap between the conventional and RTP/PECVD cells.
The effectiveness of PECVD passivation of surface and bulk defects in Si, as well as phosphocout dfhrsed emitters, is investigated and quantihd. Significant hydrogen incorporation coupled with high posibive charge density in the PECVD SIN layer is found to play an important rde in bulk and surface paooivation. It b shown that photeassisted anneal in a forming gas ambient after PECVD depositions significantiy improvos the pass'V;dion of emnter and bulk defects. PECVD passivation of phosphorous doped emitters and boron doped bare Si surfaces is found to be a strong function of doping mncentration. Surface recombination velocily of lacs than 200 cm/s for 0.2 O h m and lesa than 1 cm/s for high resistivity substrates (-500 O h m) were achieved. PECVD passivation improved bulk l i i m e in the range of 30% to 70% in muHIcrystalline Si materials. However, the degree of the passivation was found to be highly material spedfic. Depending upon the passivation scheme, emitter saturation current density (&J can be reduced by a factor of 3 to 9.
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