Development of high-Tc high-Bc contacts to Ge quantum wells

The discovery of quantum Hall ferromagnetic transition in Ge quantum wells (QWs) at B∼0.4 T enables formation of helical domain walls formed by counter-propagating chiral edge modes with opposite polarizations. If superconductivity is induced in such helical channels via proximity effect, it is expected to form a p-wave topological superconductor and support Majorana fermions. Fabricating transparent contacts to Ge QWs that remain superconducting at B∼1 T is thus crucial in demonstrating Majorana fermions in this system.

Here we report development of microcrystalline IrGe superconducting contacts to Ge/SiGe QWs with Tc up to 2.7 K and Bc up to 3.0 T. Properties of IrGe superconductor are characterized for different microcrystallization conditions of Ir thin films on Ge(001) and SiGe substrates. For optimal conditions, Tc reaches 3.2 K for Ir/Ge(001), 2.7 K for Ir/SiGe, and Bc reaches 2.4 T for Ir/Ge(001), 2.7 T for Ir/SiGe, much higher than 1.1 T for the bulk IrGe. TEM images of IrGe/Ge interface reveals a sharp boundary between IrGe microcrystallines and Ge. At optimized annealing conditions, we detect signatures of Andreev reflection in Ge deep QW in various Ir/Ge QW devices. Fabrication of transparent IrGe/Ge superconducting contacts is compatible with the mature CMOS technology, enabling hybrid superconductor-Ge QW a promising system towards error-tolerant quantum computing.