and Silicon

Heterojunctions between semiconductors and oxides are fundamental to field-effect devices that have revolutionized information technology. However, their principal functionality remains as capacitors within the metal-insulator-semiconductor (MIS) paradigm. Here we report charge transfer and the formation of built-in electric fields across heterojunctions between single-crystalline SrNb_xTi_1-xO_3-δ and Si(001). A non-monotonic anomaly in the sheet resistance is observed, accompanied by a crossover in sign of the Hall resistance which indicates the formation of a hole gas in the Si and the presence of strong built-in fields. Hard X-ray photoelectron spectroscopy measurements reveal pronounced asymmetric features in both the SrNb_xTi_1-xO_3-δ and Si core-level spectra that we show arise from built-in fields. Analysis of these unprecedented asymmetries enables band-bending to be spatially mapped across the heterojunction. Control of charge transfer and built-in fields opens a pathway to realize hybrid pn-junctions, isotype junctions etc., that elevate the functionality of semiconductor-oxide heterojunctions beyond the MIS paradigm.