Superconductivity in nano- and micro-patterned high quality single crystalline boron-doped diamond films

We demonstrate nano- and micro-size patterning processes for high quality single crystal superconducting boron-doped diamond films with low damage using selective microwave plasma chemical vapour deposition and selective oxygen plasma etching. The offset critical temperature Tc(offset) of the micro strip is 10.2 K even if the strip width is 1 μm. However, we show that the critical temperature at zero resistivity Tc(zero) of several narrow strips is decreased owing to the unexpected defects induced by polishing damage and natural nanoscale scratch injury. These defects will induce a two-step superconducting transition. The probability of this tendency increases with decreasing the strip width. We also reveal that the critical current density Jc increases with decreasing the strip width, especially below 30 μm. By using a transport measurement, the maximum Jc is estimated to be 917,000 A/cm² at 2.0 K with a strip width of 2 μm, which is the highest value reported for a superconducting diamond film. The upper critical field is estimated to be 11.5 T by the WHH approximation and 9.3 T by BCS fitting. Such high values mean superconducting diamond wiring can be used under a high magnetic field. We also attempt to fabricate nano-patterning and reveal that the transition temperature suddenly decreases below 400 nm, and superconductivity is not observed below 300 nm. This work contributes to the future development of superconducting nano- and micro-electro mechanical systems by exploiting the excellent properties of robustness, processability, high transition temperature, critical current density, and critical field associated with diamond.