High Hole-Mobility Strained- $\hbox {Ge/Si}_{0.6} \hbox {Ge}_{0.4}$ P-MOSFETs With High-K/Metal Gate: Role of Strained-Si Cap Thickness

Low-field effective hole mobility of highly strained (~2.4%, biaxial) germanium-channel (7.8 nm-thick) p-MOSFETs with high-K/metal gate stack has been experimentally investigated. Devices with various ultrathin strained-Si cap layer thicknesses, as thin as ~8 Å, show excellent capacitance-voltage characteristics with no hysteresis or frequency dispersion and hole mobility enhancement of more than 6.5X over Si universal and 2.3X over similar devices with no strained-Si cap, at E_eff = 0.6 MV/cm. The influence of the strained-Si cap thickness on the hole mobility is also studied. The mobility increases with increasing Si cap thickness up to ~1.8 nm (with a peak mobility of 940 cm²/Vs at this cap thickness) consistent with a reduction in remote Coulombic scattering.