The effect of lateral confinement on valley splitting in a Si quantum well containing 5% Ge

Achieving large valley splitting in Si/SiGe quantum dots is essential for high-fidelity spin qubits and shuttling, and Si quantum wells containing Ge have been proposed as a route to mitigate the common challenge of small valley splitting energies. Here, we report valley splitting in a Si quantum well with on average 5% Ge, measured using pulsed-gate spectroscopy and Coulomb-diamond transport spectroscopy. In two samples and four different gate-voltage tunings, we find a strong dependence on lateral confinement through controlling the quantum dot location, shape, and size with barrier gates. In the large-dot regime, where the orbital splitting is lower than 0.5 meV, the valley splitting increases with orbital energy, reaching up to 300 µeV and remaining above 50 µeV at the weakest confinement. We discuss these results in light of theoretical calculations of the probability distribution for valley splitting in quantum wells containing Ge. The calculations show that the expected value of the distribution increases with increasing confinement, and the detailed behavior depends on the exact atomic distribution of Ge in the well.