Surface termination control of charge transfer and band alignment across a semiconductor–crystalline-oxide heterojunction

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 $\mathrm{SrTi}{\mathrm{O}}₃$. Itinerant electrons in Si migrate across the interface toward the surface of $\mathrm{SrTi}{\mathrm{O}}₃$ 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 $\mathrm{SrTi}{\mathrm{O}}₃$ and Si. The modification in the dipole leads to a change in band alignment, in which the conduction band of $\mathrm{SrTi}{\mathrm{O}}₃$ moves from being above the valence band of Si in energy, to below it. By capping the $\mathrm{SrTi}{\mathrm{O}}₃$ surface with ultrathin ($\ensuremath{\le}1$ nm) layers of BaO, SrO, or $\mathrm{Ti}{\mathrm{O}}₂$, charge transfer across the interface can be weakened or inhibited. Ab initio modeling implicates the adsorption of oxygen associated with exposure to ambient conditions as driving the surface depletion in $\mathrm{SrTi}{\mathrm{O}}₃$. The electronic coupling between the surface and buried interface expands the functionality of semiconductor--crystalline-oxide heterojunctions.