Epitaxial aluminum nitride tunnel barriers grown by nitridation with a plasma source

Abstract

High critical current-density ͑10 to 420 kA/ cm 2 ͒ superconductor-insulator-superconductor tunnel junctions with aluminum nitride barriers have been realized using a remote nitrogen plasma from an inductively coupled plasma source operated in a pressure range of 10 −3 -10 −1 mbar. We find a much better reproducibility and control compared to previous work. From the current-voltage characteristics and cross-sectional transmission electron microscopy images it is inferred that, compared to the commonly used AlO x barriers, the polycrystalline AlN barriers are much more uniform in transmissivity, leading to a better quality at high critical current densities.

The ALMA Band 9 receiver

“…The noise in this figure is integrated over the full 4-12 GHz IF band. It is apparent from the figure that AlN junctions cover the band more effectively as they have higher critical current densities than AlO x junctions while still having DC I-V characteristics of sufficient quality (Zijlstra et al 2007). In Fig.…”

“…A more pressing problem, however, was that a better coverage of the atmospheric band would require even thinner tunnel barriers, which is not reliably achievable with aluminum oxide and therefore would cause a rapid decline in production yield. In view of some indications that aluminum nitride barriers might allow lower R n A values without deterioration of the quality of the nonlinear current-voltage characteristics, a program was started to develop such a tunnel barrier process (Zijlstra et al 2007) using a plasma source to create the chemically active nitrogen radicals. The success of this development led to the inclusion of this device technology in the remaining Band 9 cartridges.…”

“…The AlO x barrier SIS devices are fabricated on a 200 µm thick quartz substrate Mena et al 2011;Zijlstra et al 2007). First, a sacrificial Nb monitor layer is deposited, followed by an optically defined trilayer of Nb/Al/AlO x /Nb.…”

Bandwidth Limitations of Nb/AlN/Nb SIS Mixers Around 700 GHz

“…We have recently developed a technological route to AlN devices suitable for SIS mixers [15] using an ICP approach, which was also proposed in [16] and identified that in comparison to the AlN tunnel barriers have much more uniform tunneling properties. Mixing devices based on AlN barriers, using a different technological process, have recently been reported at frequencies from 275 to 425 GHz [17].…”

“…A set of SIS devices with a tuning circuit suitable for Band 9 of ALMA has been realized with AlN tunnel barriers, using our recently introduced method [15]. All devices contain a multisection microstripline, to tune out the capacitance of the SIS junction.…”