  Understanding band alignment and charge transfer at complex oxide interfaces
is critical to tailoring and utilizing their diverse functionality. Towards
this goal, we design and experimentally validate both Ohmic- and Schottky-like
charge transfers at oxide/oxide semiconductor/metal interfaces. We utilize a
method for predicting band alignment and charge transfer in ABO3 perovskites,
where previously established rules for simple semiconductors fail. The
prototypical systems chosen are the rare class of oxide metals, SrBO3 with
B=V-Ta, when interfaced with the multifaceted semiconducting oxide, SrTiO3. For
B=Nb and Ta, we confirm that a large accumulation of charge occurs in SrTiO3
due to higher energy Nb and Ta d-states relative to Ti; this gives rise to a
high mobility metallic interface, which is an ideal epitaxial oxide/oxide Ohmic
contact. On the other hand, for B=V, there is no charge transfer into the
SrTiO3 interface, which serves as a highly conductive epitaxial gate metal.
Going beyond these specific cases, this work opens the door to integrating the
vast phenomena of ABO3 perovskites into a wide range of practical devices.
