Reduction of boron diffusion in silicon-germanium by fluorine implantation

This letter investigates the effect of a 185 keV, 2.3 × 10¹⁵ cm^-2 F⁺ implant on boron transient enhanced diffusion (TED) and boron thermal diffusion in SiGe by characterizing the diffusion of a boron marker layer in samples with and without a 288 keV, 6 × 10¹³ cm^-2 P⁺ implant. In samples implanted with F⁺ only, the fluorine suppresses boron thermal diffusion by 58%. In samples given both P⁺ and F⁺ implants, the fluorine completely eliminates boron transient enhanced diffusion caused by the P⁺ implant and also significantly reduces boron thermal diffusion. SIMS profiles after anneal show a fluorine concentration in the SiGe layer that is approximately 8 × higher than after implant, indicating that fluorine accumulates in the SiGe layer during anneal. A comparison with fluorine profiles in comparable silicon samples also shows that the fluorine concentration after anneal is dramatically higher in SiGe samples than in Si samples. This accumulation of fluorine in the SiGe layer during anneal will have major benefits for boron diffusion suppression in devices like SiGe HBTs, where boron must be kept within the SiGe layer.