Anisotropy in the hole mobility measured along the [110] and [1¯10] orientations in a strained Ge quantum well

In this paper, we report on anisotropic transport properties of strained germanium (sGe) quantum wells grown on Si (001) substrates with p-type doping beneath the sGe channel. Mobility measurements were made along orthogonal [110] directions. The level of measured resistivity anisotropy in the [110] and [1¯10] orientations was found to vary between 2 and 9 for different samples. This corresponds to an actual mobility anisotropy ratio of between 1.3 and 2, values that are significantly higher than previously found for sGe. From modeling of the low temperature (12 K) mobility, using the relaxation time approach, the anisotropy in mobility was accounted for by a difference in interface roughness scattering between the two orientations. For the [110] orientation, a step height of Δ = 0.28 nm and interface roughness periodicity of λ = 7 nm were found while for the [1¯10] orientation, λ reduced to 4 nm and Δ increased to 0.42 nm. High-resolution X-ray diffraction and transmission electron microscopy confirmed a 1° off-cut in the wafer towards the [1¯10] direction