Atomistic compositional details and their importance for spin qubits in isotope-purified silicon-germanium quantum wells

Understanding crystal characteristics down to the atomistic level increasingly emerges as a crucial insight for creating solid state platforms for qubits with reproducible and homogeneous properties. Here, isotope composition depth profiles in a SiGe/²⁸Si/SiGe heterostructure are analyzed with atom probe tomography (APT) and time-of-flight secondary-ion mass spectrometry. Spin-echo dephasing times T₂^echo=128 μs and valley energy splittings around 200 μeV have been observed for single spin qubits in this quantum well (QW) heterostructure, pointing towards the suppression of qubit decoherence through hyperfine interaction or via scattering between valley states. The concentration of nuclear spin-carrying ²⁹Si is 50 ppm in the ²⁸Si QW. APT allows to uncover that both the top SiGe/²⁸Si and the bottom ²⁸Si/SiGe interfaces of the QW are shaped by epitaxial growth front segregation signatures on a few monolayer scale. A subsequent thermal treatment broadens the top interface by about two monolayers, while the width of the bottom interface remains unchanged. Using a tight-binding model including SiGe alloy disorder, these experimental results suggest that the combination of the slightly thermally broadened top interface and of a minimal Ge concentration of 0.3 \% in the QW, resulting from segregation, is instrumental for the observed large valley splitting. Minimal Ge additions < 1 \%, which get more likely in thin QWs, will hence support high valley splitting without compromising coherence times. At the same time, taking thermal treatments during device processing as well as the occurrence of crystal growth characteristics into account seems important for the design of reproducible qubit properties.