Dopant and Self-Diffusion in Extrinsic n-Type Silicon Isotopically Controlled Heterostructures
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Dopant and Self-Diffusion in Extrinsic n-Type Silicon Isotopically Controlled Heterostructures Hughes H. Silvestri,a,b Ian D. Sharp,a,b Hartmut A. Bracht,c Sam P. Nicols,a,b Jeff W. Beeman,b John Hansen,d Arne Nylandsted-Larsen,d and Eugene E. Hallera,b a Department of Materials Science and Engineering, University of California, Berkeley, CA94720 b Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 c Institut für Materialphysik, Universität Münster, Germany d Institute of Physics and Astronomy, University of Aarhus, Denmark ABSTRACT We present experimental results of dopant- and self-diffusion in extrinsic silicon doped with As. Multilayers of isotopically controlled 28Si and natural silicon enable simultaneous analysis of 30Si diffusion into the 28Si enriched layers and dopant diffusion throughout the multilayer structure. In order to suppress transient enhanced self- and dopant diffusion caused by ion implantation, we adopted a special approach to dopant introduction. First, an amorphous 250-nm thick Si layer was deposited on top of the Si isotope heterostructure. Then the dopant ions were implanted to a depth such that all the radiation damage resided inside this amorphous cap layer. These samples were annealed for various times and temperatures to study the impact of As diffusion and doping on Si self-diffusion. The Si self-diffusion coefficient and the dopant diffusivity for various extrinsic n-type conditions were determined over a wide temperature range. We observed increased diffusivities that we attribute to the increase in the concentration of the native defect promoting the diffusion. INTRODUCTION The current solution for increased speed in CMOS transistors is the reduction in device size. The main size reduction comes in the form of a shorter channel length. In order to facilitate shorter channel lengths and prevent device breakdown, shallower source and drain regions are needed [1]. Ion implantation of dopants is used to create the very shallow, heavily doped source and drain regions. However, further thermal processing of the device leads to diffusion of the dopant and results in greater junction depth. This problem has highlighted the need for a better understanding of the diffusion of dopants in silicon under extrinsic conditions. An improved understanding of diffusion phenomena in silicon will help to develop physically reasonable models to accurately predict junction depths after thermal processing. As a step in this direction, we study the diffusion of n-type dopants in Si. Dopant diffusion in silicon is known to be mediated by interstitial silicon atoms and/or lattice vacancies [2]. 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. In order to understand dopant diffusion in detail, the impact of doping and dopant diffusion on the formation and diffusion of native point defects must be known
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