Controlled threshold voltage of high-mobility Ge pMOSFETs with high-k/metal gate on epitaxial Ge films on Si substrates

To increase channel mobility beyond Si's physical limits, Ge is being intensively investigated as high- mobility channel material for potential high-speed circuit applications. Ge pMOSFETs have been demonstrated with an hole mobility enhancement by using stained or relaxed Ge-on-Si heterostructures. One of the issues, however, is the uncontrolled (normally positive) threshold voltages (Vt), which must be tuned to low negative values for CMOS applications. In this study, we investigate dependence of Vt shift on epi-Ge thickness, channel doping, and Si cap layers. The Vt shift is attributed to combined effect of Ge and Si properties in heterostructures.