Predictive Model for B Diffusion in Strained SiGe Based on Atomistic Calculations
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0913-D05-07
Predictive Model for B Diffusion in Strained SiGe Based on Atomistic Calculations Chihak Ahn1, Jakyoung Song2, and Scott T. Dunham1,2 1 Physics, University of Washington, Seattle, WA, 98195 2 Electrical Engineering, University of Washington, Seattle, WA, 98195
ABSTRACT Using an extensive series of first principles calculations, we have developed general models for the change in energy of boron migration state via interstitial mechanism as a function of local alloy configuration. The model is based on consideration of global strain compensation as well as local effects due to nearby arrangement of Ge atoms. We have performed KLMC (Kinetic Lattice Monte Carlo) simulations based on change in migration energy to explain the reduced B diffusion in strained SiGe and compared our results to experimental observations. These models include significant effects due to both global stress and local chemical effects, and accurately predict the B diffusivity measured experimentally in strained SiGe on Si as a function of Ge content. INTRODUCTION There is great interest in utilizing SiGe for enhanced mobility, increased activation, and reduced contact resistance. In order to control device structures at the nanoscale, a fundamental understanding of the effects of alloy concentration and associated strain is critical. Even though there have been many experiments showing that boron diffusion is retarded in strained SiGe, the physical mechanism is not well understood. Kuo et al. concluded that stress effects are not significant [1], but Zangenberg et al. reported very strong stress dependence of B diffusivity [2]. And, although Lever et al. attributed diffusivity reduction to B-Ge pairing [3], Hattendorf et al. found that there is no significant binding between B and Ge using β-NMR technique [4]. Previous ab-initio calculations by Wang et al. suggested that the presence of Ge increases the migration energy and reduces concentration of Si interstitials [5]. In this paper we investigated B diffusion mechanism in strained SiGe to solve the controversy, considering both global strain compensation and local Ge configuration. SIMULATION AND RESULTS B diffuses in Si lattice mainly via interstitial mechanism [6], and previous research indicates that boron migration occurs via a two step process: from substitutional B with neighboring tetrahedral Si (BsSiiT) to B in one of 6 hexagonal sites (the subset of 12 hexagonal sites away from the given SiiT site) and then back to one of 6 substitutional sites [7,8]. The B transition state is located between substitutional site and hex site along direction. We calculated the change in interstitial-mediated boron transition state energy in 64 (or 65 with I) atom supercell subject to periodic boundary condition under various Ge configurations, using
Figure 1.The energy along one step of boron diffusion path (Bihex ⇒ BsSiiT) in pure Si and Si63Ge. The highest barrier is for Ge in the first nearest neighbor (1NN) site to final Bs (square symbols), followed by Ge at 2NN (triangles). Note that
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