Reduced Parameter Spread in Arrays of Planar MgB2 Josephson Junctions Made by Focused Helium Ion Beam*

Series arrays of up to 100 planar Josephson junctions on MgB₂ film with less than 3 % spread in critical current at 10 K were successfully developed and characterized. This technique uses a 30 keV focused helium ion beam with nominal beam diameter less than 0.5 nm and range of dose from 0.9 to 3 × 10^16 (Ions/cm^2)[1] to locally damage the 25 nm-thick MgB₂ thin films grown by hybrid physical-chemical deposition (HPCVD) on SiC substrates to form the junction barriers. A typical single junction has an I_cR_n of 70 μV at 20 K and RSJ-like current-voltage characteristics. Under microwave radiation, flat Shapiro steps up to 150 μA width appear at voltages V_n = Nn(Φ₀) f, where N is the number of junction in the array, n is an integer representing Shapiro step index, and f is the applied microwave frequency. The greatly reduced spread in critical current is a significant improvement over the previous studies. This technique may lead to applications including Josephson voltage standards and arbitrary function generators that can work at around 20 K.

[1] L. Kasaei et al. AIP Advances 8, 075020 (2018); https://doi.org/10.1063/1.5030751