Coupling conduction-band valleys in modulated Si/SiGe heterostructures via shear strain*

Si/SiGe quantum dots are a promising platform for quantum computing. However, engineering a large and deterministic valley splitting remains a key practical challenge for Si-based spin qubits. Recent work [1] has shown that the most reliable method for enhancing the valley splitting is to introduce Ge concentration oscillations into the quantum well in a structure called a Wiggle Well. However, ultrashort oscillation periods are difficult to grow, while long oscillation periods do not provide useful improvements. Here, we show that the main benefits of short-wavelength oscillations can be achieved in long-wavelength λ≈1.7 nm structures through a second-order coupling process involving Brillouin-zone folding, induced by shear strain. Moreover, we find that the long-wavelength period also generates large spin-orbit coupling, unlike the short-period structure. Thus, the combination of shear strain and Ge concentration oscillations of wavelength λ≈1.7 nm both deterministically increases the valley splitting and generates sufficient spin-orbit coupling to remove the need for micromagnets.  We finally show that the required shear strain can be achieved using common fabrication techniques, making this an exceptionally promising system for scalable quantum computing. 

1. "SiGe quantum wells with oscillating Ge concentrations for quantum dot qubits." McJunkin, T., Harpt, B., Feng, Y. et al., Nat Commun 13, 7777 (2022).