Nitrogen and silicon atoms have the smallest mismatch of bonding in the Si surface (111), and different percentages of nitrogen atoms are adopted to passivate silicon surface dangling bonds in the Si (111) orientation. The first-principle calculations showed that the band gap is broadened and the localized trap states are generated when the nitrogen atom content is 75%—100% in the Si surface (111). Then the corresponding local electronic-state model is proposed, and the physical mechanism of the previous experimental results of the visible PL emission on SiN films is clarified.