Identification of a Mobility-Limiting Scattering Mechanism in Modulation-Doped Si/SiGe Heterostructures

High electron mobilities in modulation-doped Si/SiGe in the range of (1.5-1.8) × ${10}^5$ ${\mathrm{cm}}^2$/V s at 0.4-1.4 K have been achieved by several groups. Those numbers fall short of the expected theoretical value. We have examined how strain in the Si channel affects the low temperature electron mobility by systematically varying the Ge content in the relaxed buffer underneath, and by changing the Si channel thickness. A clear reduction in mobility is observed at a critical layer thickness, which is identified as the thickness at which remaining threading dislocations glide in the Si channel, resulting in misfit dislocations at the bottom interface. Understanding and control of this mechanism have led to the growth of samples with mobility values in the range of (3-4) × ${10}^5$ ${\mathrm{cm}}^2$/V s.