Effects of surface tunneling of two-dimensional hole gases in undoped Ge/GeSi heterostructures

We investigate the effect of surface tunneling on charge distributions of two-dimensional hole gases (2DHGs) in undoped Ge/GeSi heterostructures. As in the electron channel case, the 2DHG density saturates at a high gate voltage. As the channel depth of 2DHGs increases, a crossover of charge distributions in the system from equilibrium to nonequilibrium is observed at a depth of ∼50 nm. A surface tunneling model is proposed to explain the density crossover. Magnetotransport analysis is performed to investigate the limiting scattering mechanisms. The power law dependence of mobility on density suggests that the dominant scattering mechanisms for the shallow- and deep-channel 2DHGs are remote impurity and background impurity scattering, respectively. Clear quantum Hall plateaus and vanishing longitudinal magnetoresistance are observed in the 2DHG device of channels as shallow as 9 nm.