Valley splitting and Density-driven scattering in undoped Si/SiGe two-dimensional electron system

Undoped Si/SiGe two-dimensional electron gas (2DEG) provides an ideal platform for hosting quantum-dot spin qubits owing to its enhanced spin dephasing times and compatibility with standard CMOS technology. The strained Si quantum well reduces the six-fold valley degeneracy of bulk silicon into two low-lying closely spaced degenerate states. The presence of a near-degenerate valley state acts as a leakage channel and compromises gate fidelity. A robust and uniform valley splitting across the entire chip is crucial for achieving scalability in the architecture and reliability in operation. Imperfections, such as broadened interfaces, alloy disorders, and atomic steps, significantly compromise valley splitting. The associated scattering mechanisms play a detrimental role in the performance of qubits. In this manuscript, exploiting low-temperature magnetotransport measurements, we investigate the scattering mechanisms and valley splitting in a high-mobility (μ=1.6×10^6 cm^2 〖 V〗^(-1) s^(-1)) undoped Si/SiGe 2DEG. At lower carrier densities, transport is limited by remote charged impurity scattering, whereas at higher densities, background impurity scattering near the quantum well dominates. Both the transport and quantum lifetimes of the charge carriers increase with carrier concentration due to the enhanced screening of the impurity potential. Furthermore, the magnetic-field-induced orbital confinement is found to enhance the valley splitting. The visibility of both spin and valley splitting is influenced by Landau level broadening due to electron-impurity scattering and temperature effects. Employing the temperature activation method, the valley splitting is found to be Δ_v=(95±4) μeV at a carrier density n=2.98×10^11 cm^(-2). These results provide critical insights into scattering-dominated regimes and valley splitting in undoped Si/SiGe, advancing its potential for silicon-based quantum devices.