The Shallow Thermal Donors in Silicon

Families of STDs involving H, A/ and another ingredient X, with very close ionization energies, have been labelled STD-iV(H), STD-A A/), and STD-N(X), respectively [216]. There has, however, been no evidence of the presence of N in the atomic core of these STDs. Nevertheless, on the basis of recent correlations between the intensities of the electronic lines of some of them and the N concentration, it has been concluded that at least four of them each contain one N atom Alt et al. [6]. 

The positions of the first lines of the N-induced STDs spectra and of the SDs reported by Yu [275] are listed in Table 6.30. [6] reported a new STD spectrum (N-O-8), close to N-O-4, detected because of the smaller FWHMs in these samples. 

No transitions which could be associated with a second donor electron have been observed for the N-related STDs. Therefore, it is concluded that they are single donors [80] and this is probably also true for the other STDs [216], This single-donor behaviour of the STDs is associated with an ESR signature known as Si-NLIO [79] and the ESR results indicate a C2V symmetry for these centres, the same as that of the TDDs. 

Possible atomic compositions of (N,0) STDs have been proposed by Alt et al. [6] from the measurements of the intensities of their spectra as a function of [Oj], in relation with the theoretical calculations of [69] on the (N,0) complexes in silicon. In this study, N-O-5 and N-O-3 are attributed to NO and ONO structures, respectively. The NO structure is similar to the split nitrogen pair of Fig. 2.6, where one O atom replaces one N atom, and the tricoordinated O atom accounts for the donor behaviour of this pair [69]. The N-O-6 spectrum is elusive and it is only observed in the CZ samples with the highest values of [Oi]. A recent IR study of the (N,0) STDs, focused on the N-O-3, N-O-5, and N-O-6 spectra, has been reported by Ono [193]. It is shown that N-O-6 anneals at a temperature ( 600°C) lower than that for the two other STDs, and that the thermal regeneration of the STDs after 

The observation of STDs in annealed N-free CZ silicon nominally suggests that other families of STDs may exist. This is indeed the case in hydrogenated CZ silicon and CZ AZ-doped silicon [158,216]. In these materials, STD spectra with lines slightly shifted in position from those in silicon not containing these impurities are observed, as shown in Fig. 6.33. 

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