Coherent Diffraction Imaging of Three-dimensional strain fields in qubit devices

Electron spin qubits in (Si,Ge)/Si/(Si,Ge) heterostructures are based on quantum dots (QDs) defined by the strain of the Si quantum well (QW) plus electrostatic gates. Spin coherent shuttling over several microns is possible by applying time-dependent waveforms to an additional series of connecting clavier gates [1]. Strain fluctuations on the 100 nm level deteriorate the uniformity of electron potential and may lead to decoherence. On the other hand, engineering certain components of the strain tensor may be used to increase the valley splitting – the critical parameter defining the qubit operation temperature [2]. As we showed by Scanning X-ray Diffraction Microscopy (SXDM), a significant source of strain is the stress exerted by the gate electrodes themselves. Here, we propose to use coherent Bragg diffraction to investigate finely resolved 3D strain fields in shuttling devices (“QuBus”) with structure sizes smaller than the beam.