Germanium diffusion in polysilicon emitters of SiGe heterojunction bipolar transistors fabricated by germanium implantation

Abstract

A study is made of germanium diffusion in polysilicon emitters of SiGe heterojunction bipolar transistors made by germanium implantation. Implanted Ge is found to diffuse from the single-crystal silicon substrate into deposited polysilicon emitter layers during rapid thermal anneal at 1045 °C. Measurements of germanium diffusivity in polycrystalline silicon are reported at temperatures between 800 and 900 °C and modeled by an Arrhenius relationship with a preexponential factor of Do=0.026±0.023 cm2/s and an activation energy of E=2.59±0.36 eV. The measured diffusivity in polycrystalline silicon is ≈104 times larger than that reported for single-crystal silicon. It is hypothesised that germanium diffusion in polysilicon occurs by diffusion along grain boundaries.