The influence of annealing conditions on the crystallinity of Ge deposited on Si(001) is investigated. Ge deposited with postannealing at 800 °C and 850 °C, five cycles of postannealing at 750 °C and 850 °C (temperature-swing postannealing) and several cycles of annealing at 800 °C or 850 °C during the Ge growth by interrupting the deposition step (cyclic annealing) are compared. To check the threading dislocation density (TDD) of the deeper part of the Ge, thinning by HCl vapor phase etching (VPE) followed by Secco defect etching is performed for 5 μ m thick Ge of all three annealing variants. By comparing the TDD of the same Ge thickness with and without HCl VPE, TDD reduction by VPE is observed for the sample using the cyclic annealing process only. Lower TDD is observed at higher postannealing temperature. By the five cycles of temperature swinging, TDD becomes around a half compared to conventional postannealing at 850 °C. In the case of the cyclic annealing process significant improvement of TDD is observed with increasing Ge thickness. Even at a maximum temperature of 800 °C, the same or lower TDD levels were observed for higher than 2 μ m thick Ge compared to that with five cycles of temperature-swing postannealing. For the sample with cyclic annealing at 800 °C, a lower Si diffusion length into Ge is also observed for the cyclic annealing process indicating a lower thermal budget. A lower amount of tilted Ge planes at the interface is confirmed showing higher crystal quality also in the deeper part of the Ge layer.

The lattice strain induced by metallic electrodes can impair the functionality of advanced quantum devices operating with electron or hole spins. Here, we investigate the deformation induced by CMOS-manufactured titanium nitride electrodes on the lattice of a buried 10-nm-thick $\mathrm{Si}/{\mathrm{Si}}_{0.66}{\mathrm{Ge}}_{0.34}$ quantum well by means of nanobeam scanning x-ray diffraction microscopy. We are able to measure $\mathrm{Ti}\mathrm{N}$-electrode-induced local modulations of the strain tensor components in the range of 2–8 × ${10}^{-4}$ with about 60-nm lateral resolution. We evaluate that these strain fluctuations are reflected in local modulations of the potential of the $\mathrm{Si}$ conduction-band minimum larger than 2 meV, which is close to the orbital energy of an electrostatic quantum dot. We observe that the sign of the strain modulations at a given depth of the quantum-well layer depends on the lateral dimensions of the electrodes. Since our work explores the impact of device geometry on the strain-induced energy landscape, it enables further optimization of the design of scaled CMOS-processed quantum devices.

We study the critical thickness for the plastic relaxation of the Si quantum well layer embedded in a SiGe/Si/SiGe heterostructure for qubits by plan-view transmission electron microscopy and electron channeling contrast imaging. Misfit dislocation segments form due to the glide of pre-existing threading dislocations at the interface of the Si quantum well layer beyond a critical thickness given by the Matthews–Blakeslee criterion. Misfit dislocations are mostly ${60}^{°}$ dislocations (b=a/2 ) that are split into Shockely partials (b=a/6 ) due to the tensile strain field of the Si quantum well layer. By reducing the quantum well thickness below critical thickness, misfit dislocations can be suppressed. A simple model is applied to simulate the misfit dislocation formation and the blocking process. We discuss consequences of our findings for the layer stack design of SiGe/Si/SiGe heterostructures for usage in quantum computing hardware.

We present an SOI-waveguide coupled germanium photodiode with very high OE -3 dB bandwidth of ≥110 GHz at reverse bias of 2 V. This performance is achieved by a novel construction in that the germanium is sandwiched in between two in-situ doped silicon regions. This fabrication approach allows for avoiding ion-implantation into the germanium, which is certainly beneficial for the bandwidth as minority carrier diffusion effects are strongly suppressed. A responsivity of >0.6 A/W at 1550 nm (-2 V) is achieved, while the dark current of this device yields to about 300 nA (-2 V). To our knowledge, this is the most advanced germanium photo detector in terms of bandwidth combined with state-of-the-art responsivity as well as moderate dark currents. We demonstrate that the novel photodiodes can be fabricated with high yield.