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.

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 longerranged 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.

Dislocation free local SiGe-on-insulator virtual substrate is fabricated using lateral selective SiGe growth by reduced pressure chemical vapor deposition. The lateral selective SiGe growth is performed around ~1.25 µm square Si (001) pillar in a cavity formed by HCl vapor phase etching of Si at 850 °C from side of SiO2 / Si mesa structure on buried oxide. Smooth root mean square roughness of SiGe surface of 0.14 nm, which is determined by interface roughness between the sacrificially etched Si and the SiO2 cap, is obtained. Uniform Ge content of ~40% in the laterally grown SiGe is observed. In the Si pillar, tensile strain of ~0.65% is found which could be due to thermal expansion difference between SiO2 and Si. In the SiGe, tensile strain of ~1.4% along <010> direction, which is higher compared to that along <110> direction, is observed. The tensile strain is induced from both [110] and [-110] directions. Threading dislocations in the SiGe are located only ~400 nm from Si pillar and stacking faults are running towards <110> directions, resulting in wide dislocation-free area formation in SiGe along <010> due to horizontal aspect ratio trapping.

We describe the fabrication process and properties of an InP based quantum dot laser structure grown on a 5° off-cut silicon substrate. Several layers of quantum dot based dislocation filters embedded in GaAs and InP were used to minimize the defect density in the quantum dot active region which comprised eight emitting dot layers. The structure was analyzed using high resolution transmission electron microscopy, atomic force microscopy and photoluminescence. The epitaxial stack was used to fabricate optical amplifiers which exhibit electroluminescence spectra that are typical of conventional InAs quantum dot amplifiers grown on InP substrates. The amplifiers avail up to 20 dB of optical gain, which is equivalent to a modal gain of 46 cm-1.

Isotopically enriched 28Si quantum well layers in SiGe/Si/SiGe heterostructures are an excellent material platform for electron spin qubits. Here, we report the fabrication of 28SiGe/28Si/28SiGe heterostructures for qubits by a hybrid molecular beam epitaxy (MBE) / chemical vapour deposition (CVD) growth, where the thick relaxed SiGe substrates are realised by a reduced-pressure CVD and the 28SiGe/28Si/28SiGe stacks are grown by MBE. We achieve a fully strained 28Si quantum well layer in such heterostructures with a 29Si concentration as low as 200 ppm within the MBE grown layers and conclude that 29Si primarily originates from the residual natural Si vapour in the MBE chamber. A reliable surface preparation combining ex-situ wet chemical cleaning and in-situ annealing and atomic hydrogen irradiation offers epitaxy ready CVD grown SiGe substrates with low carbon and oxygen impurities. Furthermore, we also present our studies about the growth temperature effect on the misfit dislocation formation in this heterostructure. This shows that the misfit dislocation formation is significantly suppressed at a low MBE growth temperature, such as 350°C.