Using hybrid density functionals, we investigate the origin of the large band gap experimentally observed at the 6H-SiC(0001) surface when passivated by an epitaxial silicon oxynitride layer. In order to distinguish the effects resulting from the interfacial nitrogen layer and from the thinness of the epitaxial oxide layer, we use various models of the 6H-SiC(0001)/SiO2 interface and perform a comparative study through the evolutions of their planar-averaged local density of states across the interface. Our study attributes the large band gap opening to a combined effect. The presence of the nitrogen layer causes the band gap to open already in the last planes of the substrate. The thinness of the epitaxial layer contributes to a further increase of the band gap in the close vicinity of the outer surface and its effect is enhanced by the presence of the nitrogen layer. (C) 2008 Elsevier B.V. All rights reserved.