Theoretical Study of Boron Clustering in Silicon

Abstract

Ion implantation is the method of choice to introduce dopants such as boron into silicon. Thermal anneals are used to heal the implant damage as well as to activate the dopant electrically. The implant-anneal cycle causes transient enhanced diffusion (TED) of boron and clustering of boron atoms at concentrations far below the solubility limit. The formation of these small immobile boron-interstitial clusters (BICs) causes the deactivation of boron. In this work, we use density-functional theory calculations to study the boron clustering process in Si. We determine the minimum-energy structures of these clusters at different sizes embedded in bulk Si and calculate the energy and charge state of each cluster within density-functional theory. Special care is taken with regard to structural minimization, charge state effects and energy corrections. In contrast to previous work, we argue that substitutional-boron clusters as defined previously are meaningless due to the high repulsive energy of nearest-neighbor boron atoms. We compare the larger clusters to the phase of precipitates at higher boron concentrations, Si1.8B5.2, and suggest the boron icosahedron as logical final BIC before the formation of more macroscopic precipitates in the absence of kinetic constraints. We also describe in detail the implementation of our ab-initio results into a continuum model, which we have used in previous work to simulate diffusion and deactivation of boron.