Fabrication and characterization of Nb/Al-AlN /Nb superconducting tunnel junctions

Abstract

We report a Nb/Al-AlN /Nb superconducting tunnel junction process in which the AlN barrier is formed by plasma nitridation using a compact microwave electron-cyclotron-resonance (ECR) nitrogen plasma source integrated into a standard sputter cluster. This enables growth of uniform tunnel barriers across a broad range of specific resistances, with $R_n A$ down to $\approx 3,Ω,μ\mathrm{m}^2$. Junctions maintain excellent quality, exhibiting $R_j/R_n \ge 25$ at the highest barrier transparencies. We characterize resistivity, specific capacitance, and the evolution of junction parameters under room-temperature aging and thermal annealing. A consistent calibration of the junction specific capacitance $C_s$ versus $R_n A$ is established and independently validated by the performance of demonstrator SIS mixers designed using the extracted $C_s$.

Figures (6)

• Figure 1: (a) ADF-STEM micrograph of Nb/Al-AlOx/Nb tunneling structure characterized with $R_\mathrm{n}A$$\approx 30\,\Omega\cdot\mu m^2$ reproduced from Ref. aghdam2016dependence compared with (b) of Nb/Al-AlN/Nb tunneling structure with $R_\mathrm{n}A$$\approx 5\,\Omega\cdot\mu m^2$ . (c) low loss plasmon peak of AlN as recorded in the region 2 corresponding to the tunnel barrier layer seen on the micrograph (b) - blue curve, as compared to the database plasmon peak spectrum from Ref. serin1998eels - dotted curve.(d) low loss plasmon peak of Al as recorded in the region 1 of the micrograph (b) - red curve, as compared to the standard EELS plasmon spectra of Al taken from EELS atlas - dotted curve.
• Figure 2: Current-voltage characteristics of Nb/Al-AlN/Nb SIS junctions. (a) Junction with nominal area $0.8~\mu\mathrm{m}^2$) and $4~\mu\mathrm{m}^2$ and extracted $R_\mathrm{n}A \approx 3.6~\Omega\,\mu\mathrm{m}^2$ and $R_\mathrm{n}A \approx 13~\Omega\,\mu\mathrm{m}^2$, respectively. The $R_\mathrm{n}A$ values are determined following Eq.,(\ref{['eq:RnA']}). (b) shows the corresponding wafer-level statistics used to extract the $R_\mathrm{n}A$ values for the junctions displayed in (a).
• Figure 3: Aging and annealing behavior of Nb/Al-AlN/Nb junctions: (a) Thermal profile of the annealing process. (b) and (c) Evolution of normal resistance $R_n$ for junctions A and B, respectively. (d) junctions quality factor $R_j /R_n$. Results are shown for junctions batches with $\ R_nA\sim 15\,\Omega\cdot\mu m^2$ and $\ R_nA\sim 120\,\Omega\cdot\mu m^2$.
• Figure 4: Specific capacitance $C_S$ of Nb/Al-AlN/Nb junctions as a function of $R_\mathrm{n}A$. Results from direct probe measurements are shown as green triangle and values extracted from S-parameter measurements as light blue squares. For comparison, capacitance data reported by other groups are included (dark blue circles): (a) Ref. kojima2018, (b) Ref kawamura2000, (c) Ref. kooi2020, (d) Ref. khudchenko2015, (e) Ref. lodewijk2009. Data for Nb/Al-AlOX/Nb junctions are plotted as red diamonds aghdam2017Cs. The capacitance dependence is approximated by a semi-empirical relation belitsky1993Cs_vs_RnA$C_s = a / \ln(R_\mathrm{n}A)$ with $a = 211$ for AlOx-barriers junctions aghdam2017Cs and $a = 170$ for the present AlN-barrier junctions.
• Figure 5: (a) Mixer chip view. (b) Closeup view at the part of the chip containing SIS junctions: 1 -- $SiO_2$ layer between choke and the microstrip line around the twin junctions; 2 -- $SiO_2$ layer between choke and the microstrip line of the impedance transformer; 3 -- twin SIS junctions.