Fabrication, characterization and mechanical loading of Si/SiGe membranes for spin qubit devices

Si/SiGe heterostructures on bulk Si substrates have been shown to host high fidelity electron spin qubits. Building a scalable quantum processor would, however, benefit from further improvement of critical material properties such as the valley-splitting landscape. Flexible control of the strain field and the out-of-plane electric field $\mathcal{E}_z$ may be decisive for valley splitting enhancement in the presence of alloy disorder. We envision the Si/SiGe membrane as a versatile scientific platform for investigating intervalley scattering mechanisms which have thus far remained elusive in conventional Si/SiGe heterostructures and have the potential to yield favourable valley-splitting distributions. Here, we report the fabrication of locally etched, suspended SiGe/Si/SiGe membranes from two different heterostructures and apply the process to realize a spin-qubit shuttling device on a membrane for future valley mapping experiments. The membranes have a thickness in the micrometer range and can be metallized to form a back-gate contact for extended control over the electric field. To probe their elastic properties, the membranes are stressed by loading with a profilometer stylus at room temperature. We distinguish between linear elastic and buckling modes, each offering mechanisms through which strain can be coupled to spin qubits.

💡 Research Summary

The authors present a comprehensive study on the fabrication, characterization, and mechanical loading of suspended Si/SiGe membranes designed for spin‑qubit applications. Recognizing that the valley‑splitting energy (E_VS) is a critical parameter for high‑fidelity silicon spin qubits, they propose a platform that allows independent control of the out‑of‑plane electric field (E_z) and in‑plane shear strain (ε_xy), both of which can enhance E_VS by modifying intra‑ and inter‑valley scattering mechanisms.

Two heterostructure wafers (designated A and B) are grown by CVD and MBE, each comprising a thick Si handle, a linearly graded Si₁₋ₓGeₓ buffer, a constant‑composition Si₇₀Ge₃₀ layer, and the active quantum‑well stack. After depositing a 1 µm SiO₂ hard mask on the backside, square patterns aligned along the