The N_i defect

  Isolated N_i defects also exist in implanted material, but lie below the detection limit in doped as-grown material. They possess a local vibrational mode (LVM) around 691 cm^-1 [170], which is in close agreement with the calculated N stretch mode of the [100] oriented nitrogen-silicon split interstitial at 700 cm^-1 [145] (see Table 7.2). In this defect N and Si share a lattice site. Berg Rasmussen et. al. [176] observed N_i in N-implanted Si singly annealed at 600$^\circ$C. They concluded that N_i was mobile during annealing, and rapid cooling was necessary if N was to be trapped as N_i rather than forming other defect complexes.

N_i is a [100] split interstitial, structurally similar to interstitial carbon, C_i. N is much more electronegative than Si, and hence draws charge from its neighbouring atoms. However, unlike C_i, 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 [011] are dilated, and are therefore sites where O atoms would preferentially precipitate.

By pairing up as (N_i)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 [011] are 1.83 Å and the [100] 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 $\langle$110$\rangle$ back-bonds are dilated to 2.397 Å, and the $\langle$110$\rangle$ 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 [145] and are in good agreement with an observed 691 cm^-1 line (see Table 7.2).