A method for growing an oxynitride film on a substrate includes positioning the substrate in a process chamber, heating the process chamber, flowing a wet process gas comprising water vapor and a nitriding gas comprising nitric oxide into the process chamber. The wet process gas and the nitriding gas form a processing ambient that reacts with the substrate such that an oxynitride film grows on the substrate. In yet another embodiment, the method further comprises flowing a diluting gas into the process chamber while flowing the wet process gas to control a growth rate of the oxynitride film. In another embodiment, the method further comprises annealing the substrate and the oxynitride film in an annealing gas. According to embodiments of the method where the substrate is silicon, a silicon oxynitride film forms that exhibits a nitrogen peak concentration of at least approximately 6 atomic % and an interface state density of less than approximately 1.5 ×10 12 per cc.