High transparency Josephson junctions on SiGe/Ge/SiGe heterostructures for quantum information

In recent years, there is increasing interest in hybrid superconductor-semiconductor (S-Sm) devices due to several possible applications in quantum information including gate-tunable transmons and parametric amplifiers. These realizations rely on S-Sm-S Josephson junctions for which the non-dissipative supercurrent can be tuned by an electrostatic gate creating a Josephson Field Effect Transistor (JoFET). Additionally, coherent double Cooper pairs transmission can occur in highly transparence S-Sm interface. The phenomenon corresponds to the cos(2φ) term in energy-phase relation which is necessary to implement the protected qubit.

Silicon-germanium (SiGe) heterostructures is one of the potential platforms to host the hybrid devices due to high hole mobility and the low Schottky barrier at the Ge-metal interface. Additionally, the compatibility with the silicon-based semiconductor industry is a capable advantage for the scaling-up qubit platform.

In this work, the exhaustive study of the properties of Ge-based JoFET is performed as a function of the gate voltage, temperature and magnetic field. Below the superconducting transition of the Al contacts, the devices exhibit a strong superconducting proximity effect due to a very high transparency at the S-Sm interfaces. The I_CR_N products extracted from the measurements exhibit the value up to 100µV for the shortest junction length (150 nanometers). The normalized excess current (I_excR_N/Δ) gives a transparency τ ≈ 0.9. The sub-gap anomaly features indicating multiple Andreev reflections (MAR) are observed up to the third order. For some devices, we also observed integer and half-integer Shapiro steps indicating the non-sinusoidal current-phase relation (CPR).