Atomic Layer Deposition of High-κ Dielectrics on Sulphur-Passivated Germanium

High mobility channels are currently being explored to replace the silicon channel in future CMOS technology nodes. However, until now the promising bulk properties are very difficult to translate into high transconductance due to a poor passivation of the interface between the gate dielectric and the channel. We have studied the S-passivation of the germanium surface combined with various high-permittivity dielectric gate stacks. (NH 4 ) 2 S is used to achieve a S-terminated Ge surface. We found that the Ge/S/Al 2 O 3 interface is superior to both the Ge/S/ZrO 2 and Ge/S/HfO 2 interfaces. Bi-layer stacks consisting of Ge/S/Al 2 O 3 /HfO 2 or Ge/S/Al 2 O 3 /ZrO 2 were built to achieve a gate stack with low EOT (Equivalent Oxide Thickness). In these bi-layer stacks, the Al 2 O 3 thickness is reduced to a minimum without degradation of the interface properties. Rather thick Al 2 O 3 interlayers (∼ 2 nm) are needed due to island growth on S-terminated Ge surface. A pMOSFET was built using a bi-layer gate stack (Ge/S/Al 2 O 3 /HfO 2 ). The peak mobility of this device is > 200 cm 2 /Vs at an EOT of 1.5 nm.