Nitrogen is commonly used as an inert atmosphere for processing Si, but it has important effects on the material. For example, N impurities can pin dislocations [161,162] and form electrically active defects such as the substitutional deep donor [163,164]. There have been suggestions that nitrogen-oxygen complexes can form shallow donors [164,165,166], although whether N forms an essential component of these defects is controversial [167,168], and this is discussed further in Chapter 8.
Nitrogen can be introduced into Si by adding Si3N4 to the melt, annealing in an atmosphere of N2, or by implantation. N is known to enhance O precipitation and inhibit C enhanced O precipitation at 750C . Photoluminescence (PL) experiments showed a decrease in oxygen precipitate strain-induced defects in samples containing a higher N content.
The principal N defect in O-free silicon is a nitrogen pair consisting of two adjacent  oriented nitrogen interstitials, Ni, with C2h symmetry (see Figure 7.1) [145,170,145]. It is stable until about 800C . Isolated Ni defects have also been observed (see Section 7.3), and although substitutional Ns is a rare defect it has been detected by EPR  and local vibrational mode spectroscopy .
In N and O implanted FZ-Si , or in Cz-Si grown in a N2 atmosphere [174,175], N-O complexes are formed by annealing between 400 and 700C. Under these conditions, the intensity of the localised vibrational modes (LVMs) due to the pair are progressively reduced and three other IR-absorption lines at 1026, 996 and 801 cm-1 (room temperature) become increasingly prominent. The intensities of these lines are correlated with each other  and they are believed to be due to NNO complexes. This is because Oi atoms become mobile around 400C and become trapped by N-pair defects which are stable at this temperature. Beyond C, these complexes break up with the emission of Oi leaving the N-pair. Cooling allows the NNO complexes to reform. This dissociation and reformation is reversible.