Increasing the valley splitting in Si/SiGe heterostructures by exploiting atomic concentration fluctuations

We present a theory of how alloy disorder affects the intervalley coupling and valley splitting in quantum dot qubits in Si/SiGe heterostructures. We show that this theory is in good agreement with experiments as well as the NEMO-3D 20-band strain-dependent sp³d⁵s^* tight-binding model. We find that, for realistic devices, alloy disorder is the dominant source of variation in the valley splitting.  Moreover, we show that increasing the alloy disorder increases the spread of the intervalley coupling, thereby increasing the average valley splitting. These results lead to a new and counterintuitive strategy for engineering quantum wells with large average valley splittings: increasing the alloy disorder. We find that adding just 5% Ge to the bottom of the quantum well enhances valley splittings substantially, achieving splittings larger than 100 μeV over 95% of the time. This strategy runs counter to the prevailing strategy of making devices with little disorder and very sharp interfaces.