Fermi Level Dependent Diffusion in Silicon SFR Workshop May 24, 2001 Ian D. Sharp, Hartmut Bracht, Eugene E. Haller Berkeley, CA 2001 GOAL: to conduct and analyze experiments on boron diffusion in silicon isotope hetero-structures. Develop a predictive boron diffusion model for various experimental conditions by 9/30/2001. 5/24/2001

Motivation Downscaling of device size is, in part, limited by diffusion of dopants. (P.A. Packan, Intel Corp., Symposium X3.2, MRS San Francisco, April 19, 2001) Si interstitials promote self- and dopant diffusion. The Si self-interstitial concentration is expected to be Fermi level position dependent. Need to develop a new, generalized diffusion model that incorporates physically justified parameters. 5/24/2001

Silicon Interstitials Below ~850 oC Si interstitials are the native defects controlling self-diffusion and the diffusion of most major dopants (e.g., B, P). To study equilibrium diffusion, it is necessary to avoid any excess Si interstitials, which may be generated by ion implantation, oxidation, or impurity clustering. Si interstitials have never been directly detected and their charge states and energy levels in the band gap are not known. The Si interstitial concentration can be determined from dopant diffusion coefficients and from Si self-diffusion. Si isotope multi-layer structures can be used to directly measure Si diffusion: highly enriched 28Si layers (99.95% 28Si), alternating with natural Si, reveal diffusion of 29Si and 30Si into the enriched regions. 5/24/2001

Experimental Approach Growth of isotopically enriched Si multi-layer structures. Deposition of a 260 nm thick amorphous Si cap layer by low temperature MBE. Boron dopant source: Boron ion implantation: 30 keV, 0.7x1016 cm-2 37 keV, 1.0x1016 cm-2 No implantation damage enters the isotopic multi-layer structure. Diffusion: samples sealed in silica ampoules backfilled with 180 Torr of Ar and thermally annealed in a temperature controlled tube furnace. Secondary Ion Mass Spectrometry (SIMS) used to determine concentration profiles of B, 28Si, and 30Si. Computer modeling of B and 30Si concentration profiles. 5/24/2001

As-Implanted Si Superlattice Sample 11B 5/24/2001

SIMS Profile: 15hr 57min Annealing at 950 oC B spike is due to oxygen at amorphous Si - crystalline Si interface 5/24/2001

Modeling Results for B and Si Concentration Profiles 30Si model fit 30Si SIMS data Si diffusion under intrinsic conditions 11B model fit Si diffusion under intrinsic conditions 11B SIMS data 5/24/2001

Self Diffusion in Si Self diffusion is enhanced as Fermi level is brought towards the valence band. T ( o C ) . 7 8 9 1 4 2 1 - 9 7 5 3 . 6 This work DSi(p++) ) 1 - s 2 m D c ( B D Bracht et al., Phys. Rev. Lett. 81 (1998) 393 D ( p ) S i DSi(ni) A n t o n i a d i s e t a l . , J . E l e c t r o c h e m . S o c . 1 2 5 ( 1 9 7 8 ) 8 1 3 . D ( n ) S i i 1 3 / T ( K - 1 ) 5/24/2001

Summary and Outlook Milestones First Fermi level dependant self-diffusion experiments in Si have been conducted. Si self-diffusion is enhanced by moving the Fermi level towards the valence band (p-type). The kick-out mechanism accurately describes the measured SIMS 30Si profiles assuming: Neutral boron interstitials: Singly negatively charged B on substitutional sites: Singly positively charged interstitial: The quantification of Si self-interstitial concentration and diffusion will allow for advanced predictive modeling of boron diffusion. Milestones Model diffusion of dopants in isotopic multi-layer structures. Study interference between diffusion and native defects by 9/30/2002. Comprehensive model on dopant- and self-diffusion in Si. Properties of dopants and native point defects in silicon by 9/30/2003. 5/24/2001