Enhancing the performance of GepFETs using novel BF⁺ implantation

With the highest bulk hole mobility among the commonly used semiconductors, Ge is particularly promising as the enabler of high mobility channel for pFETs in future CMOS technology nodes. So far, ion-implanted B is typically used for the formation of S/D extensions or highly doped drains (HDD) in short channel Ge pFETs [1-3]. Although B is found to diffuse extremely slowly in Ge after annealing [4], facilitating the formation of ultra shallow junction, there are two drawbacks for implanting B⁺ in Ge. First, as shown in Fig. 1, in the low energy regime, the percentage of implanted B+ lost in Ge is >3X higher than in Si, partially negating the expected lower resistance in Ge for the same implant dose. Second, the activation of implanted B in crystalline Ge is reported to be much lower compared to that in a-Ge, which requires an additional pre-amorphization implantation (PAI) step [5]. It is worth noting that for Si pFETs, BF₂⁺ is usually favored for the formation of shallow S/D due to its higher ionization rate and heavier mass compared to B⁺, but in Ge, the understanding of diffusion and activation of B in the presence of F is still lacking. In this work, a comprehensive study of activation, diffusion as well as defect formation for B⁺, BF⁺, and BF₂⁺ implants in Ge is reported. For the first time, high performance Ge pFETs are fabricated using BF⁺ implant for S/D junctions, demonstrating a higher ON current as compared to pFETs using B⁺ and BF₂⁺ implants.