First-principles and kinetic Monte Carlo simulation of dopant diffusion in strained Si and other materials

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Date

2006

Authors

Lin, Li, 1973-

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The TCAD tools of today are based on the atomic mechanisms underlying particular processes. This allows the simulators to be more predictive and to be utilized for a wider range of device architectures. The models in the simulators are also becoming increasingly sophisticated as new physics and processes are incorporated. Current diffusion simulators include models for ion implantation, dopant and defect diffusion, point defect/dopant /extended defect interactions, diffusion transients, stress, electric fields, charged defects and more. The study for this Ph.D. focuses on dopant diffusion in strained Si for metal oxide semiconductor field effect transistor (MOSFET) by using first-principles and Kinetic Monte Carlo simulation. Tensile-strained silicon is a promising candidate for the channel of MOSFET due to the high mobility of the carriers. We have performed density functional theory based first-principles calculations to investigate the effect of biaxial strain on boron diffusion in Si by a more direct approach than previous researchers. The results suggest that tensile biaxial strain lowers the activation enthalpy, and furthermore causes an anisotropy of boron diffusion. On the contrary, compressive biaxial strain increases the activation enthalpy. We study strain effect in different charge systems. Basically the trend of decreasing diffusion barrier persists. The analysis of charged system is much more complicated than neutral system due to the well-known reason that strain can change the band structure of Si. A lot of experiments show that fluorine can suppress boron transient enhanced diffusion (TED) if it is co-implanted with boron, but the reason of this effect is not very clear yet. We propose our own theory about the effect of fluorine on boron diffusion based on the fluorine-vacancy complex theory. Fluorine-vacancy complex is found to be a trap for mobile boron atoms as well as Si interstitials by ab initio calculations. The suppression of boron diffusion is due to not only the lower Si interstitial concentration, but also the direct interaction between fluorine-vacancy complex and boron. Kinetic Monte Carlo simulations are performed to confirm this effect and agree with the experimental results. Furthermore, the same theory also explains why boron can enhance the diffusion of fluorine.

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