  Bismuth oxyselenide (Bi$_2$O$_2$Se), a novel quasi-2D charge-carrying
semiconductor, is hailed as one of the best emerging platforms for the next
generation semiconductor devices. Recent efforts on developing diverse
Bi$_2$O$_2$Se heterojunctions have produced extensive potential applications in
electronics and optoelectronics. In-depth understanding of the band alignment
and especially interface dynamics is, however, still challenging. In this work,
a comprehensive experimental investigation on the band alignment is performed
by a high-resolution X-ray photoelectron spectrometer (HRXPS), and the
properties of interface states are also fully discussed. The results show that
the ultrathin film Bi$_2$O$_2$Se grown on SrTiO$_3$ (TiO$_2$ (001) termination)
exhibits Type-I (straddling gap) band alignment with a valence band offset
(VBO) of about 1.77\pm0.04 eV and conduction band offset (CBO) of about
0.68\pm0.04 eV. However, further considering the contribution of the interface
states, the bands on the interface present a herringbone configuration due to
sizable build-in electric fields, which is significantly different from the
conventional band alignment. In this sense, our results provide an insightful
guidance to the development of high-efficiency electronic and optoelectronic
devices, specifically of the devices where the charge transfer is highly
sensitive to interface states.
