  The discovery of a two-dimensional electron gas (2DEG) at the interface
between insulating oxides has led to a well-deserved level of excitement due to
possible applications as "in-plane" all-oxide nanoelectronics. Here we expand
the range of possibilities to the realm of "out-of-plane" nanoelectronics by
examining such all-oxide heterostructures as barriers in tunnel junctions. As
an example system we perform first-principles electronic structure and
transport calculations of a tunnel junction with a [SrTiO3]4/[LaO]1/[SrTiO3]4
heterostructure tunneling barrier embedded between SrRuO3 electrodes. The
presence of the LaO atomic layer induces the formation of a 2DEG within the
tunneling barrier which acts as an extended defect perpendicular to the
transport direction, providing a route for resonant tunneling. Our calculations
demonstrate that the tunneling conductance in this system can be strongly
enhanced compared to a pure SrTiO3 barrier due to resonant tunneling, but that
lattice polarization effects play a significant role in determining this
behavior. In addition we find that this resonant tunneling is highly selective
of the orbital symmetry of the tunneling states due to the "orbital
polarization" of the 2DEG. We also discuss how the properties of the 2DEG are
affected by the presence of metal electrodes.
