Ni is a  split interstitial, structurally similar to interstitial carbon, Ci. N is much more electronegative than Si, and hence draws charge from its neighbouring atoms. However, unlike Ci, its lone pair is already filled, and so it cannot remove the excess electron from the neighbouring undercoordinated silicon radical. The wavefunction of this state is small in the vicinity of the N atom and lies deep in the gap (see Figure 8.2). Like C, the small size of the N atom suggests that the Si-Si bonds along  are dilated, and are therefore sites where O atoms would preferentially precipitate.
By pairing up as (Ni)2 this leaves all the Si and N atoms fully coordinated and removes any associated electrical activity. However the localised lattice polarisation due to the strong electronegativity of N remains, as does the tensile lattice strain.
A single interstitial N atom was inserted into a 148 atom cluster, Si79H68N and all atoms were allowed to relax. The Kohn-Sham eigenvalues are given in Figure 8.2, and it can be seen that the single donor state lies quite low in the gap. The top N-Si bonds oriented along  are 1.83 Å and the  N-Si bond is 1.75 Å. These Si atoms are pulled towards the core leading to a dilation of the Si-Si backbonds (the two 110 back-bonds are dilated to 2.397 Å, and the 110 pair above the defect are compressed to 2.293 Å; these compare to ideal Si-Si bond lengths of 2.349 Å). This makes these bonds attractive sites for oxygen precipitation. Vibrational modes of the defect have been given earlier  and are in good agreement with an observed 691 cm-1 line (see Table 7.2).