  Charge redistribution across heterojunctions has long been utilized to induce
functional response in materials systems. Here we examine how the composition
of the terminating surface affects charge transfer across a heterojunction
consisting of Si and the crystalline complex oxide SrTiO3. Itinerant electrons
in Si migrate across the interface toward the surface of SrTiO3 due to surface
depletion. The electron transfer in turn creates an electric field across the
interface that modifies the interfacial dipole associated with bonding between
SrTiO3 and Si, leading to a change in the band alignment from type-II to
type-III. By capping the SrTiO3 surface with ultra-thin layers of BaO, SrO or
TiO2, charge transfer across the interface can be weakened or inhibited. Ab
initio modeling implicates the adsorption of oxygen as driving surface
depletion in SrTiO3. The electronic coupling between the surface and buried
interface expands the functionality of semiconductor-crystalline oxide
heterojunctions.
