  Large-scale arrays of quantum-dot spin qubits in Si/SiGe quantum wells
require large or tunable energy splittings of the valley states associated with
degenerate conduction band minima. Existing proposals to deterministically
enhance the valley splitting rely on sharp interfaces or modifications in the
quantum well barriers that can be difficult to grow. Here, we propose and
demonstrate a new heterostructure, the "Wiggle Well," whose key feature is Ge
concentration oscillations inside the quantum well. Experimentally, we show
that placing Ge in the quantum well does not significantly impact our ability
to form and manipulate single-electron quantum dots. We further observe large
and widely tunable valley splittings, from 54 to 239 ueV. Tight-binding
calculations, and the tunability of the valley splitting, indicate that these
results can mainly be attributed to random concentration fluctuations that are
amplified by the presence of Ge alloy in the heterostructure, as opposed to a
deterministic enhancement due to the concentration oscillations. Quantitative
predictions for several other heterostructures point to the Wiggle Well as a
robust method for reliably enhancing the valley splitting in future qubit
devices.
