  Atomic Layer Deposition (ALD) is a promising technique for producing
Josephson junctions (JJs) with lower defect densities for qubit applications. A
key problem with using ALD for JJs is the interfacial layer (IL) that develops
underneath the tunnel barrier. An IL up to 2 nm forms between ALD Al2O3 and Al.
However, the IL thickness is unknown for ALD films less 1 nm. In this work,
Nb-Al-ALD-Al2O3-Nb trilayers with tunnel barriers from 0.6 - 1.6 nm were grown
in situ. Nb-Al-AlOx-Nb JJs with thermally oxidized tunnel barrier were produced
for reference. RN was obtained using a four-point method at 300 K. JC, and its
dependence on barrier thickness, was calculated from the Ambegaokar-Baratoff
formula. The Al surface was modeled using ab initio molecular dynamics to study
the nucleation of Al2O3 on Al. Current voltage characteristics were taken at 4
K to corroborate the room temperature measurements. Together, these results
suggest that ALD may be used to grow an ultrathin, uniform tunnel barrier with
controllable tunnel resistance and JC, but a thin IL develops during the
nucleation stage of ALD growth that may disqualify Al as a suitable wetting
layer for ALD JJ based qubits.
