  Built-in electric fields across heterojunctions between semiconducting
materials underpin the functionality of modern device technologies.
Heterojunctions between semiconductors and epitaxially grown crystalline oxides
provide a rich setting in which built-in fields can be explored. Here, we
present an electrical transport and hard X-ray photoelectron spectroscopy study
of epitaxial SrNbxTi1-xO3-{\delta} / Si heterojunctions. A non-monotonic
anomaly in the sheet resistance is observed near room temperature, which is
accompanied by a crossover in sign of the Hall resistance. The crossover is
consistent with the formation of a hole gas in the Si and the presence of a
built-in field. Hard X-ray photoelectron spectroscopy measurements reveal
pronounced asymmetric features in both the SrNbxTi1-xO3-{\delta} and Si
core-level spectra that we show arise from built-in fields. The extended probe
depth of hard X-ray photoelectron spectroscopy enables band bending across the
SrNbxTi1-xO3-{\delta} / Si heterojunction to be spatially mapped. Band
alignment at the interface and surface depletion in SrNbxTi1-xO3-{\delta} are
implicated in the formation of the hole gas and built-in fields. Control of
charge transfer and built-in electric fields across semiconductor-crystalline
oxide interfaces opens a pathway to novel functional heterojunctions.
