Fluorine-enhanced boron diffusion in amorphous silicon

Silicon wafers were preamorphized with 70 keV Si+ at a dose of 1x1015 atoms/cm2, generating a deep amorphous layer of 1800 Aa. Implants of 500 eV 11B+, with and without 6 keV F+, followed at doses of 1x1015 atoms/cm2 and 2x1015 atoms/cm2, respectively. After annealing at 550 deg. C, secondary ion mass spectroscopy determined that the diffusivity of boron in amorphous silicon is significantly enhanced in the presence of fluorine. Ellipsometry and cross-sectional transmission electron microscopy indicate the enhanced diffusion only occurs in the amorphous layer. Fluorine increases the boron diffusivity by approximately five orders of magnitude at 550 deg. C. It is proposed that the ability of fluorine to reduce the dangling bond concentration in amorphous silicon may reduce the formation energy for mobile boron, enhancing its diffusivity