In a recent publication, Shirasawa [Phys. Rev. Lett. 98, 136105 (2007)] have experimentally shown that incorporation of nitrogen at the interface of a silicate adlayer on 6H-SiC (0001) leads to the formation of a well ordered, highly stable epitaxial silicon oxynitride (SiON) layer without dangling bond states. We investigate the structural and electronic properties of this intriguing system by employing density functional theory with self-interaction-corrected pseudopotentials. Our results corroborate the structural model inferred from low-energy electron diffraction. In addition, our calculated filled- and empty-state scanning tunneling microscopy images are in excellent agreement with the experimental data, clearly revealing that O and Si surface states of the silicate double layer on top of the system, respectively, give rise to the observed images. The calculated surface band structure exhibits a surface band gap of 9 eV in very good agreement with the scanning tunneling spectroscopy data. The physical origin of this amazingly large gap is clarified.