Thin (11.4 nm) gate quality silicon dioxide films were subjected to high‐temperature (850–1150 °C) rapid thermal nitridation cycles in ultrapure ammonia. Secondary‐ion mass spectroscopy and neutron depth profiling results indicate a significant level of nitrogen diffusion into the silicon substrate. After stripping the dielectric layers, Schottky diode studies were performed using an electrolyte‐based technique. We found, for the first time, the formation of nitridation‐induced ultrathin (less than 60 nm)n‐type layers at the top surface of nominallyp‐type silicon substrates used in the study. A nitrogen‐oxygen donor complex formation mechanism is invoked to explain the presence of the ultrathinn‐type layers.