Using a density functional scheme, the authors investigate the electronic properties of an epitaxial silicon oxynitride layer on a 6H-SiC(0001) surface, as recently realized experimentally. Simulated scanning-tunneling-microscopy images of filled and empty states agree well with the experiment, lending support to the proposed atomic structure. In accord with the experiment, the local density of states indicates that the electronic band gap in the thin silicate layer at the surface is close to that of bulk SiO2. The authors show that this effect results from the surface of the epitaxial adlayer acting as a high-barrier potential for the SiC states induced in the oxide band gap.