Performance Optimization of Lifetime Sensitive Devices Based on Coupled Optical, Process, and Device Modeling
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The market for silicon electronic devices such as photovoltaics and image sensors has been experiencing explosive growth in recent years. Silicon solar cells are gathering increasing attention as promising means of satisfying part of the growing need for green energy supply and recent advances in the design of complementary-metal-oxide-semiconductor (CMOS) image sensors have led to their adoption in several high volume products incorporating mobile imaging and digital still and video cameras. Such devices are sensitive to carrier lifetime and their fabrication requires precise control of impurities and defects present in the silicon wafers. Failure to do so not only degrades the performance and efficiency, it also poses a great threat to the survival of commercial electronics companies in today’s competitive market. Particularly, in order to enable solar cells to significantly contribute to the world’s energy resources further cost reduction must be accomplished by enhancing their efficiencies through development of new technologies and optimization of their device structures and processes. Complex modern processes involving multiple thermal steps make this impossible without the aid of computational models which help us gain better understanding of the involved atomic processes and their impact on the device performance. In this dissertation a reliable and comprehensive TCAD framework is developed establishing the connection between processing conditions and the resulting device performance. It also provides us with optimization tools in a cost-effective way simply because the cost of experiments are increasing as process equipment becomes more expensive and complex. The focus of this dissertation, in the process modeling, was on the gettering of transition metals. The competitive gettering of metal impurities (Cu, Ni, Fe, Mo, and W) by boron doped, phosphorus doped regions, and dislocation loops was modeled. Ab initio density functional theory calculations were first performed to determine the binding energies of metals to the gettering sites, and based on that, continuum models were developed to model the redistribution and trapping of the metals. Critical model parameters were calibrated against experimental measurements. It was found that Fe is most strongly trapped by the dislocation loops while Cu and Ni are most strongly trapped by the P4V clusters formed in high phosphorus concentrations. In addition, it is found that none of the mentioned gettering sites are effective in trapping Mo and W. Finally, the calculated metal redistribution was coupled with device simulation via Shockley-Read-Hall recombination model to calculate carrier lifetime and the resulting device performance. Thereby, processing conditions and performance of a generic image sensing photodiode was optimized. The TCAD framework can be extended to other ULSI devices, as well. Also, the performance of a textured metal-wrap-through solar cell were analyzed using coupled optical and device 3D numerical simulations. All of the models and parameters in the simulation were calibrated based on experimental measurements. The simulation results were very close to the measurements done on fabricated devices, demonstrating the reliability of the developed TCAD framework for solar cell optimization. The opportunities to attain efficiencies exceeding 20% were investigated.
- Electrical engineering