  We apply hard X-ray photoelectron spectroscopy (HAXPES) to investigate the
electronic structures in ~50-nm thick epitaxial GaP layers grown on Si(001)
under different conditions. Depth profiles of the local binding energies for
the core levels are obtained by measuring the photoemission spectra at
different incident photon energies between 3 and 7 keV and analyzing them with
simple numerical models. The obtained depth profiles are in quantitative
agreement with the band bending determinations for the same samples in a
previous coherent phonon spectroscopic study. Our results demonstrate the
applicability of the HAXPES with varying incident photon energy to characterize
the electric potential profiles at buried semiconductor heterointerfaces.
