Low-leakage superconducting tunnel junctions with a single-crystal Al2O3 barrier

We have developed a two-step growth scheme for single-crystal Al2O3 tunnel barriers. The barriers are epitaxially grown on single-crystal rhenium (Re) base electrodes that are grown epitaxially on a sapphire substrate, while polycrystalline Al is used as the top electrode. We show that by first growing an amorphous aluminium (Al) oxide layer at room temperature and crystallizing it at a high temperature in oxygen environment, a morphologically intact single-crystal Al2O3 layer is obtained. Tunnel junctions fabricated from these trilayers show very low subgap leakage current. This single-crystal Al2O3 junction may open a new venue for coherent quantum devices.