Silicon nitride (Si_3N_4) is an important material as insulating film in microelectronics devices as well as a high temperature structural ceramic. The electronic structure of the bulk Si_3N_4 and the defect in Si_3N_4 are of great importance. Recently, It is possible to predict the atomic and electronic structure and properties of materials quantitatively due to the development of the pseudopotential method based on the density functional theory (DFT). It should be noted that the atomic configuration of the bulk Si_3N_4 crystal is very complex with large number of atoms in the unit cell, and the precise configuration has not been settled. This problem is of great importance in order to study the amorphous Si_3N_4 used in the devices, because the local configurations around each Si and N atoms in Si_3N_4 crystal and amorphous form of SI_3N_4 are rather similar to each other, which determine the main feature of the electronic structure.
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