Ge quantum well structure on Si with reduced thickness using growth and high-temperature diffusion

The epitaxial growth of a strained Ge quantum well on Si with low defect density and low interface roughness has historically necessitated microns of metamorphic growth. To reduce microwave loss from the epilayer, superconducting qubits made from 2D super-semi Josephson junctions require a mesa design. With devices fabricated from micron-scale heterostructures, connectivity between on-mesa and off-mesa features is poor and lithography in the vicinity of the mesa edge is obstructed. Here, an alternative heterostructure is presented: 100 nm of Ge is grown by molecular beam epitaxy on a Si substrate and subsequently heated to activate diffusion and form a Si_0.2Ge_0.8 virtual substrate before proceeding with further growth. In a Hall bar produced from this heterostructure, the hole mobility associated with its quantum well exceeds 4.4×10⁴ cm²/V∙s with a sheet carrier density of 5.7×10¹¹ cm^-2 at a temperature of 2 K. These metrics indicate that the mean free path of 550 nm is large enough for lateral Josephson junctions to create gate-tunable transmon qubits.