The use of high-k materials as gate dielectric still meets a lot of unsolved problems such as thermal instability during post deposition anneals resulting in the formation of interfacial oxide layers or bad process compatibility. As long as these requirements are not accomplished alternative gate dielectrics have to be formed by oxynitrides or gate stacks built of oxynitrides and some high-k material. In order to achieve a low equivalent oxide thickness (EOT) it is necessary to grow homogeneously thin oxynitrides which are nitrogen-rich and which have a high interface quality. Therefore we have studied the growth of thin nitrides and oxynitrides (EOT = 1 – 2nm) formed by rapid thermal nitridation in NH3 and wet reoxidation. By varying the partial pressure of NH3 in the process gas ambient NH3/Ar the nitride quality could be optimized: it was found that an optimized ratio of NH3 and Ar during nitridation improves the electrical properties of the nitrides and oxynitrides significantly. Interface state densities as low as those of dry thermal oxides and leakage current densities reduced by four orders of magnitude compared to SiO2 of the same EOT have been obtained. Due to the high incorporation of nitrogen into the oxynitride by rapid thermal nitridation and following oxidation the leakage current densities are also lower than those of most oxynitrides reported in literature. In addition we present data concerning the suppression of boron diffusion from p+ poly-Si electrodes. In summary the developed oxynitrides are suitable to bridge the gap between common SiO2 and new alternative gate dielectrics or to form gate stacks in combination with high-k materials.