DC SQUIDS made of NbN/a-Si/NbN tunnel junctions

We have fabricated planar dc-SQUIDs using all hard metal NbN/a-Si/NbN tunnel junctions. The a-Si barrier is oxidized to block pinholes in an rf plasma. The SQUIDs have a square-washer coil with an inductance of 0.2-0.3 nH; two 4 μm × 4 μm junctions are shunted by resistors of a-Nb3Ge films. An Auger analysis has shown that the a-Si is fully oxidized by the plasma oxidation, but the barrier in the junction is a mixture of Si and Si oxide, indicating a reaction between oxygen of the barrier and Nb of the counter NbN film. We have operated the SQUIDs in a flux-locked loop at a modulation frequency of 92 kHz. The power spectrum of the flux noise has a strong 1/f component which agrees with calculation assuming the temperature fluctuation of the critical current as a main source. At high frequencies the noise spectrum is white at 10^-10φ0²/Hz including an electronics system noise.