Background

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 750$^\circ$C [169]. 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 [100] oriented nitrogen interstitials, N_i, with C_2h symmetry (see Figure 7.1) [145,170,145]. It is stable until about 800$^\circ$C [170]. Isolated N_i defects have also been observed (see Section 7.3), and although substitutional N_s is a rare defect it has been detected by EPR [171] and local vibrational mode spectroscopy [172].

  

Figure 7.1: The N-pair defect in Si. Vertical direction is $\langle$001$\rangle$ , horizontal is $\langle$110$\rangle$ 

In N and O implanted FZ-Si [173], or in Cz-Si grown in a N₂ atmosphere [174,175], N-O complexes are formed by annealing between 400 and 700$^\circ$C. 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 [175] and they are believed to be due to NNO complexes. This is because O_i atoms become mobile around 400$^\circ$C and become trapped by N-pair defects which are stable at this temperature. Beyond $\approx 700^\circ$C, these complexes break up with the emission of O_i leaving the N-pair. Cooling allows the NNO complexes to reform. This dissociation and reformation is reversible.