The initial processes of nitridation of silicon thin film have been examined by means of the first-principle calculations. The nitrogen atom on the bare silicon(1 0 0) surface stays around the surface due to the dangling bond of the first-layer silicon, and the large stabilization on the bare silicon surface raises the energy barrier of the penetration of the nitrogen atom into the surface. The nitrogen atom on the H-terminated silicon surface can penetrate into the surface by crossing over the small energy barrier. In the nitridation by using nitrous oxide (N2O) molecule, N2O can be adsorbed only with the large N-N bond length, and then an exothermic elimination of nitric oxide (NO) molecule from the adsorbed nitrogen atom on the bare silicon surface takes place. The electronic processes have been discussed in terms of the quantum mechanical energy densities, which give new images of microscopic electronic stresses in the interaction system. (C) 2003 Elsevier Science B.V. All rights reserved.