A kinetic study by Suezawa et. al.  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 Nn-Om 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 Oi is electrically inactive [103,177].
It was proposed that the STD could consist of a core containing Oi, surrounded by N2i pairs in the 110 direction . However in order to maintain C2v symmetry with only one Oi, the Oi must lie along the central C2 axis. This structure will therefore be of a  split interstitial type, similar to Ni. 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 C2v symmetry. No shallow donor level was found and we therefore conclude that this cannot be a valid model for the STD. However, N2i pairs could still co-exist with the NiO2i shallow thermal donor core proposed above. Although Oi atoms will continue to agglomerate at the defect core due to the polar bond formation, the outer Oi 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 N2i pairs to also collect along the same 110 plane. This formation mechanism is identical to that of the NNO defect [103,177] but requires N2i to be mobile.