Ab-Initio Modeling of Arsenic Pile-Up and Deactivation at the Si/SiO 2 Interface
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1070-E06-03
Ab-Initio Modeling of Arsenic Pile-Up and Deactivation at the Si/SiO2 Interface Naveen Gupta, and Wolfgang Windl Materials Science and Engineering, The Ohio State University, Columbus, OH, 43210-1178 ABSTRACT Two recent papers by Pei et al. and Steen et al. have shown that the observed pile-up of arsenic at Si/SiO2 interfaces surprisingly does not seem to involve point defects as a major factor, causes local distortions that strain the Si in the pile-up region locally, and that the segregated arsenic atoms are deep donors. In this paper, we use ab-initio modeling to study possible configurations for high As concentrations that may fulfill these criteria. We find for a simple model structure that As nearest neighbors become stable in Si in the vicinity of the interface. We also have studied dopant deactivation using bulk-Si models. Even without invoking point defects explicitly and starting from a purely substitutional arrangement, we find that the energetically most favorable configurations are most stable in the neutral charge state, indicating that high enough concentrations of arsenic atoms make them electrically inactive and hence result in dopant dose loss. INTRODUCTION Arsenic is a group-V element commonly used as a donor dopant in silicon MOS (metal oxide semiconductor) devices, the currently most widely used technology for fabrication of integrated microelectronic circuits. Ion implantation is the preferred route for placing the As dopants into Si, which requires subsequent annealing to heal the implantation damage. During annealing of implanted Si, the As atoms diffuse and concentrate at the Si/SiO2 interface. Since it has been found experimentally that As diffuses in Si with the help of both interstitials and vacancies [1], it has been speculated that this trapping of As at the interface is due to a local supersaturation of point defects generated during the implantation process [2]. However, recent measurements involving anneals in oxidizing and inert ambients found negligible influence of the ambient on the segregated dose under near-equilibrium conditions, indicating that point defects may not play a dominating role in the segregation process [3]. Reference [3] also reported that the segregated arsenic atoms were deep donors with an electrical activity that increased eventually to full electrical activation for high sheet concentrations of the segregated atoms. Initially, it was believed that As piles up in a narrow region on the oxide side of the interface [5]. However, later characterization by x-ray photoelectron spectroscopy (XPS) [6] and medium energy ion scattering (MEIS) [7] experiments showed that at least half of the pileup is in the first monolayer on the Si side of the interface, while recent atomic-resolution electron-energy loss spectroscopy (EELS) and grazing-incidence X-ray fluorescence spectroscopy measurements [3,8] indicate that most or even all of the As is located on the Si side. The strain observed in the Z-contrast images of [8] also suggests a significant concentratio
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