An Investigation into the Characteristics of the Superconducting Phases in Heavily P-doped Germanium*

Realization of superconductivity in group IV semiconductors is anticipated to be a key in the scalability of the qubit devices. Such materials are promising candidates for fault-tolerant hybrid semiconductor-superconductor quantum systems due to their high purity and ease of processing. In this study, superconducting Ge is realized via ion implantation of Ga, followed by activation annealing. For all annealing temperatures, transport measurements showed an abrupt normal-superconductor transition at 2.5–3.5 K, with residual resistances < 50 mΩ at 20 mK. Typical values for critical magnetic fields were about 0.6 to 0.8 T, corresponding to coherence lengths of 20 to 25 nm. Microscopy measurements revealed the presence of a 20nm thick polycrystalline Ga-rich Ge layer near the top surface. To localize the superconducting region, implantation energy was reduced. The resulting superconducting layer showed no grain structure in micrometer scale, but Raman measurements confirmed its nanocrystalline nature. By reducing the implantation energy and the annealing temperatures, coherence length only decreased to ~18 nm. To further characterize the superconducting Ge films, strategies for their integration into all-Ge Josephson junctions and transistors will be discussed.