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IEEE Transactions on Applied Superconductivity

Optical Spectroscopic Analysis of ICP Nitridation for SIS Junctions With AlN Barriers

Michael E. Cyberey, David Noah Oakland, Jianzhong Zhang, Anthony R. Kerr, Shing-Kuo Pan, Arthur W. Lichtenberger

Published Jan 7, 2015• 5 citations

Abstract

High quality, low-leakage superconductor-insulator-superconductor (SIS) junctions with Nb electrodes and aluminum oxide barriers are widely reported in the literature, and have become integral in the design and fabrication of various superconducting circuits. However, as current densities are increased, aluminum oxide based tunnel barriers show excess leakage currents due to defects and pinholes in the barrier layer. First reported by our group in 2007, AlN tunnel barriers grown via inductively coupled plasma (ICP) nitridation of Al overlayers are a promising alternative, producing low-leakage Nb/Al-AlN/Nb SIS junctions, with current densities in excess of 30 kA/cm2. A correlation between junction quality and plasma dissociation has been reported for Nb/Al-AlN/Nb junctions produced via an electron cyclotron resonance (ECR) plasma. In this work, a quantitative measure of the relative dissociation (RD) of an ICP is determined through the use of a commercially available Ocean Optics USB4000 Optical Spectrometer. The effects of various ICP parameters on the RD, and a correlation between the RD and quality of the resulting Nb/Al-AlN/Nb junctions are reported.

DOI: 10.1109/tasc.2015.2388594

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High quality, low-leakage superconductor-insulator-superconductor (SIS) junctions with Nb electrodes and aluminum oxide barriers are widely reported in the literature, and have become integral in the design and fabrication of various superconducting circuits. However, as current densities are increased, aluminum oxide based tunnel barriers show excess leakage currents due to defects and pinholes in the barrier layer. First reported by our group in 2007, AlN tunnel barriers grown via inductively coupled plasma (ICP) nitridation of Al overlayers are a promising alternative, producing low-leakage Nb/Al-AlN/Nb SIS junctions, with current densities in excess of 30 kA/cm2. A correlation between junction quality and plasma dissociation has been reported for Nb/Al-AlN/Nb junctions produced via an electron cyclotron resonance (ECR) plasma. In this work, a quantitative measure of the relative dissociation (RD) of an ICP is determined through the use of a commercially available Ocean Optics USB4000 Optical Spectrometer. The effects of various ICP parameters on the RD, and a correlation between the RD and quality of the resulting Nb/Al-AlN/Nb junctions are reported.

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

.

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 ≃ 3 Ω μ m 2 . Junctions maintain excellent quality, exhibiting R j / R n ⩾ 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 .

Nb-based circuits have broad applications in quantum-limited photon detectors, low-noise parametric amplifiers, superconducting digital logic circuits, and low-loss circuits for quantum computing. The current state-of-the-art approach for superconductor-insulator-superconductor (SIS) junction material is the Gurvitch trilayer process based on magnetron sputtering of Nb electrodes with Al-Oxide or AlN tunnel barriers grown on an Al overlayer [1]. However, a current limitation of elemental Nb-based circuits is the low-loss operation of THz circuits operating above the 670 GHz gap frequency of Nb and operation at higher temperatures for projects with a strict power budget, such as space-based applications. NbTiN is an alternative higher energy gap material and we have previously reported on the first NbTiN/AlN/NbTiN superconducting-insulating-superconducting (SIS) junctions with an epitaxially grown AlN tunnel barrier [2]. One drawback of a directly grown tunnel barrier compared to thermal oxidation is control of the barrier thickness and the uniformity of thickness across a substrate, leading to variations in current density (Jc). Semiconductor barriers with smaller barrier heights enable thicker tunnel barriers for a given Jc[3]. GaN is an alternative semiconductor material with a closed-packed Wurtzite crystal structure similar to AlN and it can be epitaxially grown as a tunnel barrier using the Reactive Bias Target Ion Beam Deposition (RBTIBD) technique. This work presents the preliminary results of the first reported high-quality NbTiN/GaN/NbTiN heterojunctions with underdamped SIS I(V) characteristics

We investigate the influence of substrate bias on the plasma nitridation process to form tunnel barriers in superconducting tunnel junctions, especially focusing on the bias effects on the barrier transparency. In addition, we assess the role of substrate bias in the nitridation process in the enhancement of the reproducibility of AlN-SIS fabrication process. We find that this parameter is critical to optimize the fabrication of high-performance tunnel-junctions for superconducting mixer technology.

Nb/Al–AlN x /Nb SIS junctions have been fabricated by various RF plasma nitridation techniques. The reproducibility of the R N A product from batch to batch was improved by diluting the N 2 plasma with He or Ar. For the He+N 2 process, a power-law relation between the R N A product and the nitridation time was found to hold along 10 0 -10 6 Ω μm 2 , which implies that the thickness of the barrier grows logarithmically during the time it is exposed to the plasma. By applying a pulse-time-modulated RF power, we were able to slow down the nitridation significantly. Finally, we present the results of preliminary RF measurements. The Nb/Al–AlN x /Nb SIS mixers were tested at around 150GHz and 350GHz, and showed noise performances comparable to those of Nb/Al–AlO x /Nb SIS mixers.

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.