Enhanced Boron Diffusion in Amorphous Silicon

Abstract

In prior works, we demonstrated the phenomenon of fluorine-enhanced boron diffusion within self-amorphized silicon. Present studies address the process dependencies of low temperature boron motion within ion implanted materials utilizing a germanium amorphization. Silicon wafers were preamorphized with either 60 keV or 80 keV Ge⁺ at a dose of 1×10¹⁵ atoms/cm². Subsequent 500 eV, 1×10¹⁵ atoms/cm²¹¹B⁺ implants, as well as 6 keV F⁺ implants with doses ranging from 1×10¹⁴ atoms/cm² to 5×10¹⁵ atoms/cm² were also done. Furnace anneals were conducted at 550°C for 10 minutes under an inert N₂ ambient. Secondary Ion Mass Spectroscopy (SIMS) was utilized to characterize the occurrence of boron diffusion within amorphous silicon at room temperature, as well as during the Solid Phase Epitaxial Regrowth (SPER) process. Amorphous layer depths were verified through Cross-Sectional Transmission Electron Microscopy (XTEM) and Variable Angle Spectroscopic Ellipsometry (VASE). Boron motion within as-implanted samples is observed at fluorine concentrations greater than 1×10²⁰ atoms/cm³. The magnitude of the boron motion scales with increasing fluorine dose and concentration. During the initial stages of SPER, boron was observed to diffuse irrespective of the co-implanted fluorine dose. Fluorine enhanced diffusion at room temperature does not appear to follow the same process as the enhanced diffusion observed during the regrowth process.