Room Temperature Boron Diffusion in Amorphous Silicon

Abstract

As millisecond annealing is increasingly utilized, the as-implanted profile dominates the final dopant distribution. We characterized boron diffusion in amorphous silicon prior to post-implantation annealing. SIMS confirmed that both fluorine and germanium enhance boron motion in amorphous materials. The magnitude of boron diffusion in germanium amorphized silicon scales with increasing fluorine dose. Boron atoms are mobile at concentrations approaching 1x10¹⁹ atoms/cm3. It appears that defects inherent to the structure of amorphous silicon can trap and immobilize boron atoms at room temperature, but that chemical reactions involving Si-F and Si-Ge eliminate potential trapping sites. Sequential Ge+, F+, and B+ implants result in 80% more boron motion than do sequential Si+, F+, and B+ implants. The mobile boron dose and trapping site concentration change as functions of the fluorine dose through power law relationships. As the fluorine dose increases, the trapping site population decreases and the mobile boron dose increases. This reduction in trap density can result in as-implanted “junction depths” that are as much as 75% deeper (taken at 1x10¹⁸ atoms/cm^-3) for samples implanted with 500 eV, 1x10¹⁵ atoms/cm² boron.