Nanoscale Tunnel Junctions and Metallic Single-Electron Transistors via Shadow Evaporation and In Situ Atomic Layer Deposition of Tunnel Barriers

Nanoscale metallic tunnel junctions are important elements of state-of-the-art technologies including superconducting qubits, single electronics, nanospintronics, and caloritronics. The most widely used versatile method of their fabrication is shadow evaporation through bilayer-resist masks patterned using electron-beam lithography. However, useable materials for the electrodes of junctions in this method were limited thus far because of the difficulty in forming good-quality tunnel barriers on them during the process. In this work, we have developed a process for fabricating nanoscale tunnel junctions in which the shadow evaporation method is combined with an atomic-layer-deposition (ALD)-grown Al2O3 tunnel barrier formed in situ. To realize this process, we constructed a vacuum evaporator in which the sample chamber is equipped with a room-temperature remote-plasma ALD apparatus. This method enables control of the thickness of the tunnel barrier with the precision of the order of 0.1 nm. We successfully fabricated nanoscale Ni/Al tunnel junctions and a Ni/Al/Ni single-electron transistor, both having Ni bottom electrodes, as well as nanoscale V/V double superconducting tunnel junctions using the developed process. This process extends the applicability of the shadow evaporation technique for fabricating nanoscale tunnel junctions in various metals for bottom electrodes, with tunnel barriers of good quality. As specific applications, it will improve the controllability of nanoscale tunnel spin injectors for spintronics, increase the design freedom of caloritronic devices, and introduce a possibility to use various superconducting metals in superconducting quantum circuits.