Simultaneous Phosphorus and Si Self-Diffusion in Extrinsic, Isotopically Controlled Silicon Heterostructures
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Simultaneous Phosphorus and Si Self-Diffusion in Extrinsic, Isotopically Controlled Silicon Heterostructures Hughes H. Silvestri,a Hartmut A. Bracht,b Ian D. Sharp,a John Hansen,c Arne NylandstedLarsen,c and Eugene E. Hallera a Department of Materials Science and Engineering, University of California, Berkeley and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 b Institut für Materialphysik, Universität Münster, Germany c Institute of Physics and Astronomy, University of Aarhus, Denmark ABSTRACT We present experimental results of impurity and self-diffusion in an isotopically controlled silicon heterostructure extrinsically doped with phosphorus. As a consequence of extrinsic doping, the concentration of singly negatively charged native defects is enhanced and the role of these native defect charge states in the simultaneous phosphorus and Si self-diffusion can be determined. Multilayers of isotopically controlled 28Si and natural silicon enable simultaneous analysis of 30Si self-diffusion into the 28Si enriched layers and phosphorus diffusion throughout the multilayer structure. An amorphous 260 nm thick Si cap layer was deposited on top of the Si isotope heterostructure. The phosphorus ions were implanted to a depth such that all the radiation damage resided inside this amorphous cap layer, preventing the generation of excess native defects and enabling the determination of the Si self-diffusion coefficient and the phosphorus diffusivity under equilibrium conditions. These samples were annealed at temperatures between 950 and 1100 °C to study the diffusion. Detailed analysis of the diffusion process was performed on the basis of a P diffusion model which involves neutral and positively charged mobile P species and neutral and singly negatively charged self-interstitial. INTRODUCTION While dopant and self-diffusion in silicon are known to be mediated by interstitial silicon atoms and/or lattice vacancies [1], the precise role of native defects in dopant and self-diffusion in Si remains an unresolved aspect of the current understanding of diffusion in Si. The use of stable isotope heterostructures has been shown to be a valuable technique for accurately determining the temperature dependence of the Si self-diffusion coefficient, described by a single activation enthalpy [2]. While this result demonstrated the value of isotope multilayers in studying self-diffusion, it alone was unable to yield information on the role of native defects on self-diffusion in Si. In the case that these native point defects are charged, their concentration is affected by the position of the Fermi level [3]. In particular, extrinsic n-type (p-type) doping favors negatively (positively) charged defects. Therefore the observation of extrinsic dopant diffusion along with self-diffusion will shed light on the impact of doping and dopant diffusion on the formation and diffusion of native point defects. This interrelation between self- and dopant diffusion can be studied by utilizing isotopically en
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