Single-flux-quantum circuits utilizing self-shunted NbN/TaN/NbN Josephson junctions grown on silicon substrates

We report on the electrical properties of NbN/TaN/NbN Josephson junctions grown on thermally oxidized silicon substrates, along with the design and fabrication of superconducting single-flux-quantum (SFQ) circuits based on these NbN superconductor/normal metal/superconductor (SNS) junctions. The critical current density (Jc) of the junctions was found to be relatively sensitive to the barrier thickness, decreasing from 108.0 +/- 8.1 kA cm-2 for a 15 nm barrier to 12.8 +/- 1.9 kA cm-2 for a 30 nm barrier. For a Jc of approximately 24.5 +/- 2.1 kA cm-2 and a barrier thickness of 25 nm, the NbN SNS junctions are self-shunted and exhibit nonhysteretic current-voltage (I-V) characteristics. Especially for junctions with diameter (phi) ranging from 0.8 to 1.6 mu m, their critical current (Ic) falls within the range of 110-450 mu A, making them suitable for SFQ circuits. By considering the impact of excess current and incorporating it as an additional term in the conventional resistively and capacitively shunted junction model, the I-V curves of NbN SNS junctions can be precisely described, successfully minimizing the deviation between simulations and test results. The DC-SFQ and SFQ-DC interface circuits can both operate normally, and the bias margins of cell circuits such as Josephson transmission line, confluence buffer, D flip-flop, and splitter are greater than 40%. Compared to Nb superconductor/insulator/superconductor junctions, their self-shunting characteristics and relatively thick 25 nm barriers can also enhance the integration of circuits and increase the yield to complex circuits.