Hole transport in SiGe channels on step-bunched vicinal Si surfaces

Hole transport in narrow modulation doped Si_1-xGe_x channels grown on vicinal Si(1 1 3) by molecular beam epitaxy is studied. Owing to the strong step-bunching properties of the Si(1 1 3) surface, the Si_1-xGe_x channels exhibit regular terraces with a width of typical 250 nm and a mean step height of 4 nm, corresponding to 25 monolayers, when appropriate growth conditions are used. Considerable amount of Si_1-xGe_x material accumulates at the step edges resulting in regular wire-like channel thickness variations. At low temperatures, we find a pronounced resistivity anisotropy for transport perpendicular and parallel to the step edges. The resistivity is maximum for the current flow perpendicular to the step edges. This can be explained by a lateral modulation of the hole confinement potential caused by the channel thickness variation near the step edges. Lowering the doping concentration, i.e. decreasing the effective carrier density in the channel, enhances the resistivity anisotropy up to ρ_perp;/ρ_|| ≈ 16. We attribute this to an increasing resistivity of depleted SiGe layer regions in between the wire-like structures, when the Fermi energy gets comparable to the lateral potential barriers.