Atomic-order nitridation of Si(100) by an ultraclean electron-cyclotron-resonance nitrogen plasma and contribution of radicals and ions to the nitridation have been investigated. The N atom concentration on Si(100) increases with the plasma exposure time and tends to saturate to a value corresponding to a few atomic layers. In the initial stage, the N atom concentration is normalized by the product of the relative radical density with the nitrogen plasma exposure time and the number of the incident ions is much smaller than the nitridation amount, which means the radical reaction is dominant. Assuming Langmuir-type kinetics neglecting desorption, an excellent agreement is observed by fitting the experimental data. In the saturation region, the N atom concentration is normalized by the number of incident ions and becomes higher than that corresponding to the double atomic layers. Therefore, it is suggested that nitridation of the deeper atoms below the surface is induced by the ion incidence.
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