Fabrication, Characterization, and Analysis of Ge/GeSn Heterojunction p-Type Tunnel Transistors

We present a detailed study on fabrication and characterization of Ge/GeSn heterojunction p-type tunnel-field-effect-transistors (TFETs). Critical process modules as high-k stack and p-i-n diodes are addressed individually. As a result an ultrathin equivalent oxide thickness of 0.84 nm with an accumulation capacitance of 3 μF/cm
₂
was achieved on an extremely scaled tri-layer stack of GeSnOx/Al
₂
O
₃
/HfO
₂
deposited by atomic-layer deposition monitored in situ by spectroscopic ellipsometry. Combining these process modules, Ge/GeSn heterojunction pTFETs are fabricated and characterized to demonstrate the best in-class pTFET performance in the GeSn material system. The transfer characteristics of the TFETs show signatures of the trap-assisted thermal generation in the subthreshold regime which is explained by a modified Shockley- Read-Hall model. For the ON-state current, we used band-to-band tunneling models calculated using parameters from the density functional theory. We then use the calibrated model to project performance of GeSn pTFETs with increased Sn content (lower bandgap), reduced trap density and ultrathin body geometry. Both experimental and projected results are benchmarked against state-of-the art III-V (e.g., In
_0.65
Ga
_0.35
/GaAs
_0.4
Sb
_0.6
) pTFETs. We demonstrate the ability of GeSn to achieve superior performance with both high ON-current and sub-60 mV/decade switching benefiting from the small and direct bandgap for higher Sn contents.