  The layered oxychalcogenide semiconductor Bi$_2$O$_2$Se (BOS) hosts a
multitude of unusual properties including high electron mobility. Owing to
similar crystal symmetry and lattice constants, the perovskite oxide SrTiO$_3$
(STO) has been demonstrated to be an excellent substrate for wafer-scale growth
of atomically thin BOS films. However, the structural and electronic properties
of the BOS/STO interface remain poorly understood. Here, through
first-principles study, we reveal that polar discontinuities and interfacial
contact configurations have a strong impact on the electronic properties of
ideal BOS/STO interfaces. The lowest-energy [Bi-TiO$_2$] contact type, which
features the contact between a Bi$_2$O$_2$ layer of BOS with the
TiO$_2$-terminated surface of STO, incurs significant interfacial charge
transfer from BOS to STO, producing a BOS/STO-mixed, $n$-type metallic state at
the interface. By contrast, the [Se-SrO] contact type, which is the most stable
contact configuration between BOS and SrO-terminated STO substrate, has a much
smaller interfacial charge transfer from STO to BOS and exhibits $p$-type
electronic structure with much weaker interfacial hybridization between BOS and
STO. These results indicate that BOS grown on TiO$_2$-terminated STO substrates
could be a fruitful system for exploring emergent phenomena at the interface
between an oxychalcogenide and an oxide, whereas BOS grown on SrO-terminated
substrates may be more advantageous for preserving the excellent intrinsic
transport properties of BOS.
