We have developed an all-NbN integrated circuit technology for future integration of single flux quantum (SFQ) circuit with superconducting single-photon detector array. The all-NbN integrated circuits consist of epitaxial NbN/AlN/NbN tunnel junctions fabricated on single-crystal MgO substrates, Mo shunt resistors and 500-nm-thick NbN ground plane on the top. Critical current density and sheet resistance of the Mo resistor were set at 2.5 kA/cm2and 1.2 Ω, respectively. Current uniformity of the NbN/AlN/NbN tunnel junctions was measured to be σ = 3% for 3 μm × 3 μm junction size. The NbN-based SFQ logic cells were designed for and tested at 4.2 K. We actually measured circuit inductances of the fabricated SFQ cells. Circuit simulation revealed that the measured circuit inductances in the fabricated SFQ cells should allow operating margins of more than +/-20%.
We have developed an all-NbN integrated circuit technology for future integration of single flux quantum (SFQ) circuit with superconducting single-photon detector array. The all-NbN integrated circuits consist of epitaxial NbN/AlN/NbN tunnel junctions fabricated on single-crystal MgO substrates, Mo shunt resistors and 500-nm-thick NbN ground plane on the top. Critical current density and sheet resistance of the Mo resistor were set at 2.5 kA/cm2and 1.2 Ω, respectively. Current uniformity of the NbN/AlN/NbN tunnel junctions was measured to be σ = 3% for 3 μm × 3 μm junction size. The NbN-based SFQ logic cells were designed for and tested at 4.2 K. We actually measured circuit inductances of the fabricated SFQ cells. Circuit simulation revealed that the measured circuit inductances in the fabricated SFQ cells should allow operating margins of more than +/-20%.
We report on the fabrication and the operation of all-NbN rapid single flux quantum (RSFQ) circuits. The NbN integrated circuit consists of NbN/AlN/NbN tunnel junctions, Mo resistors, sputtered SiO/sub 2/ insulating layers, and 400-nm-thick NbN ground plane and wiring layer. The circuits were fabricated with the minimum junction size of 2 /spl mu/m/spl times/2 /spl mu/m and the alignment margin of +/-0.5 /spl mu/m. We designed RSFQ logic cells under critical current density of 2.5 kA/cm/sup 2/, sheet resistance of 2 /spl Omega/ and sheet inductance of 1.4 pH. We experimentally investigated dc bias margin of the fabricated RSFQ cells at 9 K. We observed relatively large dc bias margin of more than +/-25% for a pulse splitter, a confluence buffer and an RS flip-flop, while the dc bias margin of a T flip-flop was less than +/-5%. We succeeded to demonstrate the operation of 16-bit concurrent flow shift register at 9 K.
We developed NbN-based SFQ circuits consisting of high-quality epitaxial NbN/AlN/NbN trilayer grown on MgO (100) substrate. The SFQ logic cells such as JTL, DC/SFQ converter, voltage driver, pulse splitter, TFF, and CB were designed by setting the critical current density JCat 2.5 kA/cm2. We succeeded in demonstrating correct operations of all the designed SFQ cells, but the JCrange allowing the operation was quite narrow as around 70% for the designed value. The circuit simulation revealed that the TFF cell requires the JCrange from 65% to 115% for the correct operation, while the DC/SFQ converter was not operational for the JCof more than 70%.
We developed a process for fabricating nitride superconductor-based superconducting qubits monolithically integrated with a single flux quantum (SFQ)-based readout and control circuits. Since we used a Josephson junction (JJ) made of an NbN/AlN/NbN tri-layer, including a critical current density (Jc) of 50 A/cm2as the suitable material for superconducting qubits and SFQ readout/control circuits operating at 20 mK, the JJs for both the qubits and the SFQ circuits could simultaneously be fabricated. The device structure was also composed of three superconductive layers, comprising two superconductive layers as wiring layers and other layers as the top ground plane. Then, we used a Pd film as the resistive layer, which is required as the shunt resistors and the bias resistors for the SFQ control circuits, followed by an investigation of the properties of NbN/AlN/NbN epitaxial junctions and evaluation of the sheet inductance of the base NbN and wiring NbTiN layer. Our results showed the successful fabrication of a JJ with a sufficiently low leakage current and aJcof around 50 A/cm2, which was close to the target value. Furthermore, the evaluated sheet inductance exceeded 1 pH, which was several times larger than the values obtained for Nb-integrated circuits.
We have developed resistively shunted NbN junctions to realize superconducting single flux quantum circuits operating at 10 K and/or high speed. The junctions consist of epitaxial NbN/AlN/NbN tunnel junctions and molybdenum (Mo) resistors fabricated on single-crystal MgO substrates. The junction qualities are systematically investigated in a wide range of critical current density (Jc). The gap voltage and the ratio of Rsg/RN were about 5.6 mV and 11 for the junctions with the Jc of 10 kA/cm2, respectively. The overdamped Josephson junctions with parallel Mo resistors having a nominal sheet resistance showed non-hysteretic current-voltage characteristics for the junctions with Jc of 10 kA/cm2.
Abstract 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 ( J c ) 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 J c 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 ( φ ) ranging from 0.8 to 1.6 μ m, their critical current ( I c ) falls within the range of 110–450 μ 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.