A study was made of Ge diffusion in polysilicon emitters of SiGe heterojunction bipolar transistors made by Ge implantation. Implanted Ge was found to diffuse from the single-crystal Si substrate into deposited polysilicon emitter layers during rapid thermal annealing at 1045C. Measurements of Ge diffusivity in polycrystalline Si were reported for 800 to 900C, and could be described by:

D (cm2/s) = 2.6 x 10-2exp[-2.59(eV)/kT]

The measured diffusivity in polycrystalline Si was some 104 times larger than that reported for single-crystal Si. It was hypothesized that Ge diffusion in polysilicon occurred by diffusion along grain boundaries.

Germanium Diffusion in Polysilicon Emitters of SiGe Heterojunction Bipolar Transistors Fabricated by Germanium Implantation. M.J.Mitchell, P.Ashburn, P.L.F.Hemment: Journal of Applied Physics, 2002, 92[11], 6924-6