  We use electronic transport and atom probe tomography to study ZnO:Al / SiO2
/ Si Schottky junctions on lightly-doped n- and p-type Si. We vary the carrier
concentration in the the ZnO:Al films by two orders of magnitude but the
Schottky barrier height remains constant, consistent with Fermi level pinning
seen in metal / Si junctions. Atom probe tomography shows that Al segregates to
the interface, so that the ZnO:Al at the junction is likely to be metallic even
when the bulk of the ZnO:Al film is semiconducting. We hypothesize that Fermi
level pinning is connected to the insulator-metal transition in doped ZnO, and
that controlling this transition may be key to un-pinning the Fermi level in
oxide / Si Schottky junctions.
