Surface and interface structures of epitaxial silicon nitride on Si(111)

Surface and interface structures of an ultrathin silicon nitride film grown epitaxially on the Si(111) surface are investigated by core-level and valence-band photoelectron spectroscopy using synchrotron radiation. The Si $2p$ photoelectron spectra reveal a characteristic series of components for the Si species not only in stoichiometric ${\mathrm{Si}}_3{\mathrm{N}}_4$ $\left({\mathrm{Si}}^{4+}\right)$ but also in the intermediate nitridation states with one $\left({\mathrm{Si}}^{+}\right)$ or three $\left({\mathrm{Si}}^{3+}\right)$ nitrogen nearest neighbors. The Si $2p$ core-level shifts for the ${\mathrm{Si}}^{+},$ ${\mathrm{Si}}^{3+},$ and ${\mathrm{Si}}^{4+}$ components are determined to be 0.64, 2.21, and 2.74 eV, respectively. In sharp contrast to the well-known ${\mathrm{SiO}}_2/\mathrm{Si}\mathrm{}\left(111\right)\mathrm{}$ case, no trace of the ${\mathrm{Si}}^{2+}$ species is observed, indicating an atomically abrupt and defect-free interface in accordance with a recent interface model of $\beta -{\mathrm{Si}}_3{\mathrm{N}}_4/\mathrm{Si}\mathrm{}\left(111\right).$ In addition, the origin of the characteristic N $1s$ spectra and a strong surface-state emission observed in the valence-band spectra are discussed.