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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Fabrication and photon detection experiments of Nb/Al and Ta/Al superconducting tunnel junctions

Jang-Hyun Park, Young-Sik Park, Ho Seop Yoon, Sungho Lee, Yonuk Chong, Yong‐Ho Lee, Sang-Kil Lee, Jeon Kook Lee, Min Yang, Dong-Lak Kim, Sug-Whan Kim

Published Jun 24, 2008• 0 citations
Publisher

Abstract

We report on fabrication and photon detection experiments of Nb/Al and Ta/Al superconducting tunnel junctions (STJs). 5-layer STJ thin-films were fabricated using UV photolithography, DC magnetron sputtering, reactive ion etching, and chemical vapor deposition techniques. STJs with 4 different sizes (20, 40, 60 and 80 μm) were deposited on sapphire substrates and tested in a two stage adiabatic demagnetization refrigerator with an operating temperature ~ 50 mK. Photons from different light sources are injected into the junctions via an optical fiber in combination with a monochromator which can produce photons from 30 nm to 550 nm with 0.1 nm resolution. The junction is read out through a charge-sensitive preamplifier followed by a shaping stage. We have measured some performance indicators and quality factors of the junctions from resultant I-V curves.

DOI
10.1117/12.787727

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We report on fabrication and photon detection experiments of Nb/Al and Ta/Al superconducting tunnel junctions (STJs). 5-layer STJ thin-films were fabricated using UV photolithography, DC magnetron sputtering, reactive ion etching, and chemical vapor deposition techniques. STJs with 4 different sizes (20, 40, 60 and 80 μm) were deposited on sapphire substrates and tested in a two stage adiabatic demagnetization refrigerator with an operating temperature ~ 50 mK. Photons from different light sources are injected into the junctions via an optical fiber in combination with a monochromator which can produce photons from 30 nm to 550 nm with 0.1 nm resolution. The junction is read out through a charge-sensitive preamplifier followed by a shaping stage. We have measured some performance indicators and quality factors of the junctions from resultant I-V curves.
For high-quality superconducting tunnel junctions (STJs), it is necessary to reduce leakage current as much as possible. We describe the fabrication of niobium STJs using the selective niobium (Nb) etching process and various ways to minimize the leakage current. The experiment shows that the leakage current mainly comes from shorts in the tunnel barrier layer rather than those around the junction edges. Through systematic analysis of the thin film stress, surface morphology and modified junction structures, we fabricate high-quality Nb STJs with a gap voltage of 2.8 mV and a leakage current at 1 mV as low as 8.1% and 0.023% at 4.2 K and 0.3 K, respectively.
A novel process suitable for fabricating superconducting circuitry based on the all refractory material NbN is described. In this process, an in-situ trilayer film composed of NbN/MgO/NbN is used to fabricate Josephson tunnel junctions. Reactive ion etching processes are used to delineate devices and pattern insulators and metallizations. Fabricated junctions have yielded good tunneling characteristics with reasonable current density uniformity and reproducibility. Devices with gap voltages close to 5mV have been achieved for high quality junctions. In addition to the trilayer, there are two wiring layers, two resistor depositions, and two insulation layers, constituting a full NbN based fabrication technology. Using this process, we fabricated and successfully tested thin film DC SQUID and time domain reflectometer (TDR) circuits. Preliminary measurements suggest that the critical temperature of these circuits is well within the operating temperature of commercial two-stage closed cycle refrigerators.
Quasiparticle mixers have shown promise as millimetre and submillimetre wavelength detectors and mixers, with the potential for conversion gain and photon noise limited performance. We report on small area (0.1μm2- 1.0μm2) superconducting thin film edge junctions fabricated using a combination of magnetron sputter deposition and broad beam ion source technology for materials deposition, barrier formation, and electrode patterning. Devices fabricated on0.1 \times 0.4 \times4mm3quartz substrates with a wide range of normal state resistances,20 < R_{n} < 1000 \Omegaand critical current densities,10^{2} < j_{c} < 3 \times 10^{5}Acm-2, show steps due to photon assisted tunnelling with very small amounts of local oscillator power applied (10-8W). We have investigated a series of devices with various electrical characteristics and geometries all of which include integral superconducting choke structures. Results are presented and are compared with theoretical predictions.
We have evaluated the electrical characteristics of NbN/Al-AlNx/NbN tunnel junctions fabricated on sapphire-C or fused-silica substrates. NbN/Al-AlNx/NbN multilayers were prepared by reactive dc-magnetron sputtering deposition of lower and upper NbN layers, and rf-magnetron sputtering deposition of the Al layer, followed by radical-nitridation. The junctions showed low sub-gap leakage currents in the current-voltage characteristics at 4.2 K. The quality parameter, Rsg/Rn, where respective Rsgand Rnare the sub-gap resistance at 2 mV and junction resistance at 5 mV, was above 15. The gap voltage (Vg) was around 3.3 mV, which was reduced in comparison with Vgvalues of ~4.3 mV for junctions on MgO substrates. The reduced Vgwas caused by superconducting properties of lower NbN layers deposited on the sapphire or fused-silica substrates. Uniformity in critical current (Ic) was relatively good, and the maximum-to-minimum spread in Icwas ±1.7% for 200 junctions on a sapphire substrate and ±2.0% for the junctions on a fused-silica substrate.
We report on the fabrication and millikelvin char acterization of Nb/Al–AlOx/Nb Josephson junctions directly de posited on X-cut LiNbO3 substrates. Motivated by the strong electro-optic and piezoelectric properties of LiNbO3, such devices are of interest for future hybrid superconducting–electro-optic architectures. The junctions were fabricated using standard processes involving DC magnetron sputtering followed by controlled thermal oxidation of the aluminum layer to form an AlOx tunnel barrier with reproducible junction areas. Insulation of the junctions was achieved by anodization and SiOx deposition. Electrical measurements performed at 30 mK reveal high-quality Josephson tunnel junction behavior with a critical current density ofJc= 70 A/cm2and a characteristic voltage reaching values up toVm= 286 mV. Differential conductance measurements exhibit pronounced quasiparticle peaks corresponding to a sum gap energy of 2Δ = (2.73 ± 0.05) meV, confirming the good quality of the Nb electrodes. Magnetic-field-dependent measure ments yield a Josephson penetration depth of λJ= 45 μm, confirming operation in the small-junction regime, with deviations from the ideal Josephson interference pattern attributed to non-uniform current distribution across the junction area. Compared to our previous characterization at 4.2 K, this work demonstrates reliable Josephson junction operation on LiNbO3 at dilution-refrigerator temperatures relevant for quantum circuits, supporting the suitability of lithium niobate as a promising substrate for future hybrid superconducting–electro-optic device implementations.
A novel process suitable for fabrication superconducting circuitry based on the all refactory material NbN is described. In this process, and in-situ trilayer film composed of NbN/MgO/NbN is used to fabricate Josephson tunnel junctions. Reactive ion etching processes are used to delineate devices and pattern insulators and metallizations. Fabricated junctions have yielded good tunneling characteristics with reasonable current density uniformity and reproducibility. Devices with gap voltages close to 3mV have been achieved for high quality junctions (Vm 20 mV). In addition to the trilayer, there are two wiring layers, two resistor depositions, and two insulation layers, constituting a full NbN based fabrication technology. Using this process, we fabricated and successfully tested thin film DC SQUID and time domain reflectometer (TDR) circuits. Preliminary measurements suggest that the critical temperature of these circuits is well within the operating temperature of commercial two-stage closed cycle refrigerators. (Author)