Abstract Oxidation of nitrogen implanted substrates results in so called silicon-oxinitride layers (Si x O y N z layers) which are dependent on implantation dose and energy always thinner than pure silicon-oxides (SiO 2 ) produced under the same oxidation conditions. Elastic recoil detection profiles indicate that the implanted nitrogen diffuses out of the substrate into the silicon-oxide layer what improves the electrical quality of these insulators. The Si x O y N z layers show lower Fowler–Nordheim tunnelling currents as well as lower interface state densities ( D it ) than the corresponding SiO 2 layers or N 2 O–silicon-oxinitride insulators. NH 3 –Si x O y N z layers show the lowest D it values because of H 2 -annealing effects but contain fixed charges.