  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. 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. One drawback of a directly grown tunnel barrier
compared to thermal oxidation or plasma nitridation is control of the barrier
thickness and uniformity across a substrate, leading to variations in current
density (Jc). Semiconductor barriers with smaller barrier heights enable
thicker tunnel barriers for a given Jc. 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.
