Later Thermal Donors - TD3 and beyond

The problem of later thermal donors is a very different one. Here, experiment suggests a common core that undergoes small perturbation with increasing oxygen content. The standard model is one of oxygen aggregation along the $\langle$110$\rangle$ plane, resulting in a defect with a fixed core and two tails of O_i stretching out along $\langle$110$\rangle$ .

There have been a total of 16 different thermal donors observed[241]. In this discussion I will assume TD3 is the di-y-lid and hence has four oxygen atoms in it. Additionally I will assume that this lies at the core of all following thermal donors; these assumptions do not drastically alter any of the models should they later be revised. Analysis of oxygen loss from solution by Newman et al [143] suggests an average loss of ten oxygen atoms from solution per thermal donor, at 500$^\circ$C.

There are a number of possible formation models, but these can be roughly divided depending on whether they assume a serial development of thermal donors or allow isomers. The argument also varies depending on whether the TDs grow through O_i addition or dimer addition, which is currently not clear [272]. However standard O_i migration alone cannot account for the quantity of higher order TDs formed [272], and TD formation from TD2 to TD6 has been shown to have a formation barrier of 1.7 eV [273], suggesting that dimers are the dominant active species rather than O_i.