  Many nanoelectronic devices rely on thin dielectric barriers through which
electrons tunnel. For instance, aluminium oxide barriers are used as Josephson
junctions in superconducting electronics. The reproducibility and drift of
circuit parameters in these junctions are affected by the uniformity,
morphology, and composition of the oxide barriers. To improve these circuits
the effect of the atomic structure on the electrical response of aluminium
oxide barriers must be understood. We create three-dimensional atomistic models
of aluminium oxide tunnel junctions and simulate their electronic transport
properties with the non-equilibrium Green's function formalism. Increasing the
oxide density is found to produce an exponential increase in the junction
resistance. In highly oxygen-deficient junctions we observe metallic channels
which decrease the resistance significantly. Computing the charge and current
density within the junction shows how variation in the local potential
landscape can create channels which dominate conduction. An atomistic approach
provides a better understanding of these transport processes and guides the
design of junctions for nanoelectronics applications.
