(N_2i)_n-O_i - Suezawa's model

A kinetic study by Suezawa et. al. [165] linking the rate of loss of N-pairs to the increase in STDs (using IR electronic absorption intensities), suggested that STDs consisted of an unknown O-containing core, to which N-pairs attached themselves. However this modelling neglected dissociation of N_n-O_m complexes, which will be crucial in the kinetics of these defects. In addition it is known that the NNO defect consisting of an N-pair neighbouring a single O_i is electrically inactive [103,177].

It was proposed that the STD could consist of a core containing O_i, surrounded by N_2i pairs in the 110 direction [165]. However in order to maintain C_2v symmetry with only one O_i, the O_i must lie along the central C₂ axis. This structure will therefore be of a [100] split interstitial type, similar to N_i. Since it contains an even number of N atoms it is EPR inactive in the neutral state.

We investigated the cluster Si₅₇H₅₆N₄O, containing this defect and constrained it to possess C_2v symmetry. No shallow donor level was found and we therefore conclude that this cannot be a valid model for the STD. However, N_2i pairs could still co-exist with the N_iO_2i shallow thermal donor core proposed above. Although O_i atoms will continue to agglomerate at the defect core due to the polar bond formation, the outer O_i atoms will not benefit from the tensile strain field of the defect core. They will themselves still exert tensile stress on the surrounding lattice, and therefore there will be a driving force for N_2i pairs to also collect along the same 110 plane. This formation mechanism is identical to that of the NNO defect [103,177] but requires N_2i to be mobile.