Hole mobility enhancements in nanometer-scale strained-silicon heterostructures grown on Ge-rich relaxed Si1−xGex

Although strained-silicon (ε-Si) p-type metal–oxide–semiconductor field-effect transistors (p-MOSFETs) demonstrate enhanced hole mobility compared to bulk Si devices, the enhancement has widely been observed to degrade at large vertical effective fields. We conjecture that the hole wave function in ε-Si heterostructures spreads out over distances of ∼10 nm, even at large inversion densities, due to the strain-induced reduction of the out-of-plane effective mass. Relevant experimental and theoretical studies supporting this argument are presented. We further hypothesize that by growing layers thinner than the hole wave function itself, inversion carriers can be forced to occupy and hybridize the valence bands of different materials. In this article, we show that p-MOSFETs with thin (i.e., <3 nm) ε-Si layers grown on Ge-rich ${\mathrm{Si}}_{\text{1−x}}{\mathrm{Ge}}_x$ buffers exhibit markedly different mobility enhancements from prior ε-Si p-MOSFETs. Devices fabricated on a thin ε-Si layer grown on relaxed ${\mathrm{Si}}_{\text{0.3}}{\mathrm{Ge}}_{\text{0.7}}$ demonstrate hole mobility enhancements that increase with gate overdrive, peaking at a value of nearly 3 times. In other devices where the channel region consists of a periodic ε-Si/relaxed ${\mathrm{Si}}_{\text{0.3}}{\mathrm{Ge}}_{\text{0.7}}$ digital alloy, a nearly constant mobility enhancement of 2.0 was observed over inversion densities ranging from 3 to ${\text{14×10}}^{12}/{\mathrm{cm}}^2.$