Phase separation of Ni germanide formed on a Ge-on-Si structure for Ge MOSFETs

Phase separation of Ni germanide due to the penetration of Ni atoms from Ni germanide in the Ge layer into the bottom Si substrate is reported. The orthorhombic-structure Ni germanide is formed in a Ge-on-Si substrate, which is promising for high-performance Ge p-channel metal-oxide-semiconductor field effect transistor (PMOSFETs). However, the Ni penetration and resulting Ni-rich Ni silicide formation happens when the Ni germanide becomes thick enough to touch the bottom Si substrate. The phase separation or Ni penetration observed in this work is believed to be due to the lower heat of formation of NiSi than NiGe. The Ni penetration must be controlled in fabricating source/drain regions for high-mobility Ge-on-Si MOSFETs.