SiGe quantum wells with oscillating Ge concentrations for quantum dot qubits

Large-scale arrays of quantum-dot spin qubits in Si/SiGe quantumwells 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, whosekey feature isGe 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 mu eV. 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 theWiggleWell as a robust method for reliably enhancing the valley splitting in future qubit devices.

Automated summary

This study proposes a new heterostructure, the Wiggle Well, which uses concentration oscillations of Ge in the quantum well to enhance the valley splitting of quantum-dot spin qubits. The experimental results show that the presence of Ge in the quantum well does not affect the formation and manipulation of single-electron quantum dots. It is suggested that the enhancement of the valley splitting is mainly attributed to random concentration fluctuations and the presence of Ge alloy.