Mechanisms Limiting EOT Scaling and Gate Leakage Currents of High- $k$ /Metal Gate Stacks Directly on SiGe

This letter addresses mechanisms responsible for a high gate leakage current (Jg) and a thick interfacial layer in the surface channel SiGe pFET enabling transistor fabrication with sub-1-nm equivalent-oxide-thickness (EOT) high-k /metal gate stack. The primary mechanism limiting EOT scaling is Ge-enhanced Si oxidation resulting in a thick (1.4-nm) SiOx interface layer. A secondary mechanism, i.e., Ge diffusion (ges3%) into high- k, results in high Jg. In the framework of this understanding, we optimized a high-k nitridation process to form as an efficient diffusion barrier, which reduces both O and Ge diffusion resulting in the total gate stack EOT ~ 0.9 nm with J _g comparable to that of bulk Si substrate samples. The proposed plasma nitridation process enables fabrication of the sub-1-nm EOT gate-first gate stack with HfSiON dielectric directly on SiGe without Si cap.