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IEEE Transactions on Applied Superconductivity

Fabrication of NbTiN/Al–AlNX/NbTiN Josephson Junctions for Superconducting Circuits Operating Around 10 K

Hiroyuki Akaike, Shunya Sakamoto, Kentaro Munemoto, Akira Fujimaki

Published May 10, 2016• 8 citations
Publisher

Abstract

We have fabricated NbTiN/Al-AlNX/NbTiN Josephson tunnel junctions on thermally oxidized Si substrates for their applications to single-flux-quantum circuits operating around 10 K. NbTiN films with a critical temperature Tcof 14.2 K and a root-mean-square value of their surface roughness of 0.45 nm were used as junction electrodes. For a junction barrier, radical-nitrided AlNXwas used to realize high uniformity in the critical current Icof the junctions. We obtained NbTiN junctions with a gap voltage Vg of 4.3 mV and an IcRn. product of 2.6 mV at 4.2 K, where R. is the junction resistance. The critical current density Jc, at 4.2 K was obtained in the range between 0.2 and 6.2 kA/cm2by changing a nitridation time for the AlNXbarrier formation. A series array of 200 junctions with an area of 10 μm × 10 μm exhibited a small I', spread, and the standard deviation 1-σ in Icuniformity was less than 1.6% in the temperature range between 4 K and 10 K.

DOI
10.1109/tasc.2016.2565613

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We have fabricated NbTiN/Al-AlNX/NbTiN Josephson tunnel junctions on thermally oxidized Si substrates for their applications to single-flux-quantum circuits operating around 10 K. NbTiN films with a critical temperature Tcof 14.2 K and a root-mean-square value of their surface roughness of 0.45 nm were used as junction electrodes. For a junction barrier, radical-nitrided AlNXwas used to realize high uniformity in the critical current Icof the junctions. We obtained NbTiN junctions with a gap voltage Vg of 4.3 mV and an IcRn. product of 2.6 mV at 4.2 K, where R. is the junction resistance. The critical current density Jc, at 4.2 K was obtained in the range between 0.2 and 6.2 kA/cm2by changing a nitridation time for the AlNXbarrier formation. A series array of 200 junctions with an area of 10 μm × 10 μm exhibited a small I', spread, and the standard deviation 1-σ in Icuniformity was less than 1.6% in the temperature range between 4 K and 10 K.
We fabricated NbTiN Josephson junctions (JJs) with HfOxbarriers on thermally oxidized Si substrates. Hf overlayers with different resistivities were used for the HfOxbarrier formation. The Hf overlayer with low resistivity produced junctions with high quality parameters. The critical current density (Jc) was obtained in the range of 0.01-0.4 kA/cm2by changing the thermal oxidation conditions of the Hf layer. The critical current uniformity was estimated at a maximum-to-minimum spread of ±10.1% for 85 of the 100 JJs. We evaluated the specific capacitance, which was approximately 110 fF/μm2for the Jcranging from 0.1 to 0.4 kA/cm2. The quasiparticle tunnel characteristics were improved by using the NbN electrodes instead of the NbTiN electrodes.
We describe the fabrication processes and electrical characteristics of two types of NbN junctions. One is a self-shunted NbN/NbNx/AlN/NbN Josephson junction, which is expected to improve the density of integrated circuits; the other is an underdamped NbN/AlNx/NbN tunnel junction with radical-nitride AlNx barriers, which has highly controllable junction characteristics. In the former, the junction characteristics were changed from underdamped to overdamped by varying the thickness of the NbNx layer. Overdamped junctions with a 6-nm-thick NbNx film exhibited a characteristic voltage of Vc =0.8mV and a critical current density of Jc =22A/cm2 at 4.2K. In the junctions with radical-nitride AlNx barriers, Jc could be controlled in the range 0.01-3kA/cm2 by varying the process conditions, and good uniformity of the junction characteristics was obtained.
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.
We present electrical characterization data of sputtered Nb/a-Si/Nb Josephson junctions (JJs) for high-speed and high-density superconducting circuits. Junctions were studied with critical current densities (Jc) ranging from 0.01 to 3 mA/μm2 at 4 K. For junctions deposited at room temperature and processed to a maximum temperature of 150 °C, the dependence of Jc on barrier thickness d is exponential, Jc∝exp (−d/d0), with d0 constant over the entire range of Jc values studied. Junctions were annealed at temperatures up to 300 °C to study changes in their electrical properties and possible compatibility with high temperature fabrication processes. Current–voltage characteristics, critical current uniformity, critical current modulation with in-plane magnetic field, and sub-gap resistance behavior of these junctions were measured at 4 K and demonstrate that the junction properties do not degrade with annealing. These data indicate that Nb/a-Si/Nb JJs are a potential candidate for higher speed and higher density superconducting circuits.
Niobium/aluminum-oxide/niobium Josephson tunnel junctions have been fabricated using electron beam evaporation to form all-metal films in a high vacuum system. A quality factor, V m (the product of the maximum dc Josephson current and the subgap resistance defined at 2 mV) between 36–37 mV, has been obtained for the best junctions of a 6 nm-thick Al layer by cooling down the surface of the base electrode for 100 minutes under a high background pressure, before the deposition of aluminum, after the formation of the base niobium electrode. An array of 80 series-connected junctions has also been fabricated using the selective niobium anodization process and the planarization process. The current-voltage characteristics of the array showed a standard deviation of the maximum dc Josephson current of 3.3%.
A description of the fabrication technology of high-quality tunnel SIS junctions is presented, with following characteristics: energy gap in superconductors Vg=3.2-3.4 mV, tunnel current density up to J 35 kA/cm2, quality factor Rj/Rn (ratio of subgap resistance to normal-state resistance) up to 30, junction area up to 1 μm2. The SIS junctions are integrated into the NbTiN|SiO2|Al microstrip line. Keywords: superconducting devices, superconductor--insulator--superconductor tunnel junction, plasma etching.