Josephson junctions coupled by an ion-implanted vertical semiconductor membrane

Superconducting devices with two niobium electrodes separated by a vertical silicon membrane have been fabricated. The silicon membrane typically of 100-300 nm thickness, is degenerately doped by arsenic ion implantation and capped by a thin layer of silicon dioxide. The small dimensions of the membrane are achieved by photolithography and controlled isotropic wet etching followed by CBrF3reactive ion etching. The electrodes of 200 nm thickness are formed by sputter deposition and photolithographic patterning. The metal film on top of the membrane is removed using a photoresist planarization technique, followed by CF4reactive ion etching. A typical device with a 300 nm-thick barrier and with 50 μm-wide electrodes displays Josephson junction characteristics with a critical current of about 8 μA at 4.2 K. The observed product of critical current and normal resistance is in the range of 0.4-0.8 mV at 4.2 K among the devices tested.