The application of an insulating stack composed of a few-nm thick thermally grown oxynitride and a thicker (15-20 nm) deposited high-permittivity metal oxide (HfO2, epsilon approximate to20) significantly improves the electrical properties of 4H-SiC(0001) metal-oxide semiconductor (MOS) structures. This is achieved through the beneficial combination of two features: First, the use of a thin native oxynitride allows minimization of the carbon supply from the consumed SiC and, as a consequence, to reduce the density of C-clusters at the SiC/oxide interface. In this way, the density of donor-type interface states near the top of the SiC valence band is reduced to below 10(12) cm(-2)eV(-1). Second, the thin oxynitride allows to reduce the density of acceptor-type interface states near the conduction band edge of SiC to values in the low 10(12) cm(-2) eV(-1) range, as compared to the high 10(12) cm(-2) eV(-1) range observed in thicker oxynitrides or dry oxides on 4H-SiC. The resulting total interface state density in the energy interval of similar to2.7 eV between the Fermi levels in n- and p-type SiC appears to be about 7x10(11) cm(-2). The attained low interface defect density together with the good insulating properties of the SiON/HfO2 stack suggests that the deposition process does not degrade the underlying oxynitride. This offers the possibility of further optimisation of the stacked insulator for 4H-SiC MOS transistor fabrication.