A low-temperature, 200–300°C, plasma-assisted oxidation-deposition process sequence has been developed for formation of SiO2/Si heterostructures. Adding a nitride layer to form an ON, or ONO composite dielectric, increases the density of trapping states, Dit, at the SiO2/Si interface, unless the entire structure is subjected to a high-temperature rapid thermal anneal, e.g., 30 s at 900°C. By interrupting the nitride deposition, and using on-line Auger electron spectroscopy, AES, the increase in Dit correlates with a migration of N-atoms to the SiO2/Si interface during the nitride deposition. There is no evidence for N-atom incorporation into the oxide layer itself. In contrast, for remote PECVD deposition of oxides onto nitrides, O-atoms react with the nitride, and form an oxy-nitride alloy interfacial region layer.