2023, Corley-Wiciak, Cedric, Richter, Carsten, Zoellner, Marvin H., Zaitsev, Ignatii, Manganelli, Costanza L., Zatterin, Edoardo, Schülli, Tobias U., Corley-Wiciak, Agnieszka A., Katzer, Jens, Reichmann, Felix, Klesse, Wolfgang M., Hendrickx, Nico W., Sammak, Amir, Veldhorst, Menno, Scappucci, Giordano, Virgilio, Michele, Capellini, Giovanni

A strained Ge quantum well, grown on a SiGe/Si virtual substrate and hosting two electrostatically defined hole spin qubits, is nondestructively investigated by synchrotron-based scanning X-ray diffraction microscopy to determine all its Bravais lattice parameters. This allows rendering the three-dimensional spatial dependence of the six strain tensor components with a lateral resolution of approximately 50 nm. Two different spatial scales governing the strain field fluctuations in proximity of the qubits are observed at <100 nm and >1 μm, respectively. The short-ranged fluctuations have a typical bandwidth of 2 × 10-4 and can be quantitatively linked to the compressive stressing action of the metal electrodes defining the qubits. By finite element mechanical simulations, it is estimated that this strain fluctuation is increased up to 6 × 10-4 at cryogenic temperature. The longer-ranged fluctuations are of the 10-3 order and are associated with misfit dislocations in the plastically relaxed virtual substrate. From this, energy variations of the light and heavy-hole energy maxima of the order of several 100 μeV and 1 meV are calculated for electrodes and dislocations, respectively. These insights over material-related inhomogeneities may feed into further modeling for optimization and design of large-scale quantum processors manufactured using the mainstream Si-based microelectronics technology.

2023, Talamas Simola, Enrico, Montanari, Michele, Corley-Wiciak, Cedric, Di Gaspare, Luciana, Persichetti, Luca, Zöllner, Marvin H., Schubert, Markus A., Venanzi, Tommaso, Trouche, Marina Cagnon, Ortolani, Michele, Mattioli, Francesco, Sfuncia, Gianfranco, Nicotra, Giuseppe, Capellini, Giovanni, Virgilio, Michele, De Seta, Monica

The fabrication of complex low-dimensional quantum devices requires the control of the heteroepitaxial growth at the subnanometer scale. This is particularly challenging when the total thickness of stacked layers of device-active material becomes extremely large and exceeds the multi-μm limit, as in the case of quantum cascade structures. Here, we use the ultrahigh-vacuum chemical vapor deposition technique for the growth of multi-μm-thick stacks of high Ge content strain-balanced Ge/SiGe tunneling heterostructures on Si substrates, designed to serve as the active material in a THz quantum cascade laser. By combining thorough structural investigation with THz spectroscopy absorption experiments and numerical simulations we show that the optimized deposition process can produce state-of-the-art threading dislocation density, ultrasharp interfaces, control of dopant atom position at the nanoscale, and reproducibility within 1% of the layer thickness and composition within the whole multilayer. We show that by using ultrahigh-vacuum chemical vapor deposition one achieves simultaneously a control of the epitaxy down to the sub-nm scale typical of the molecular beam epitaxy, and the high growth rate and technological relevance of chemical vapor deposition. Thus, this technique is a key enabler for the deposition of integrated THz devices and other complex quantum structures based on the Ge/SiGe material system.