A method for growing an oxynitride film on a substrate includes positioning the substrate in a process chamber, heating the process chamber, flowing a first wet process gas comprising water vapor into the process chamber, and reacting the substrate with the first wet process gas to grow an oxide film on the substrate. The method further includes flowing a second wet process gas comprising water vapor and a nitriding gas comprising nitric oxide into the process chamber, and reacting the oxide film and the substrate with the second wet process gas to grow an oxynitride film. In another embodiment, the method further comprises annealing the substrate containing the oxynitride film in an annealing gas. According to one embodiment of the method where the substrate is silicon, a silicon oxynitride film can be formed that exhibits a nitrogen peak concentration of approximately 3 atomic % or greater.