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1D thermal donors

The first model is the linear tail model described above, the currently accepted `standard model'. In this case, Oi atoms aggregate in the same $\langle$110$\rangle$ plane as the defect core, in neighbouring BC sites. To form TD16 we require 14 new thermal donors after TD3. If each of these grows through a dimer adding to the previous defect that implies 13$\times$2+4=30 oxygen atoms in TD16. If the scheme is slightly modified and the TDs grow through Oi addition then this number can be reduced to 17. Thus there is a chain of either 7 or 13 Oi atoms on each side of the defect core.

This model seems very unphysical. Oi strains its surrounding lattice, causing 30% Si-Si bond dilation. When paired to form O2i, there is once again a large strain field produced both above (001) and in front (110) of the defect. Two dimer structures together, the structure of the di-y-lid, seem sufficiently strained to force the core oxygen atoms into tri-valent positions. A model where Oi atoms collect along $\langle$110$\rangle$ next to the defect core is reasonable for the first few Oi atoms, since TD3 is highly tensile along this direction. But it would be unable to equalise the strain field of 7 to 13 Oi atoms on each side. Although the strain field in such a chain might be high enough to force more of these chain Oi atoms into y-lid sites, these would be electrically active, and the Coulombic repulsion with the TD core would be too high.

One way in which a chain model can achieve these 13 different structures is if the chains on either side of the core can be of different lengths, and thus isomers are allowed. In this case, structures such as Oi-TD3-O3i would be allowed. Table 9.8 shows that a minimum of 6 total `tail' Oi atoms are required for this to explain all the TDs, i.e. a maximum tail length of 5Oi on each side.

The higher order annealing studies do not allow differentiation between an isomer and a serial development model. TDs 12 to 16 were seen simultaneously in a sample annealed for 3 hours at 470$^\circ$C [241], and so there is no information on their relative development; they are named in sequence of decreasing ground state binding energy.

Finally, extremely long tail models may not be consistent with reorientation data[236]. For TD3 $\sim$5 Oi hops are required for reorientation; this would be possible with the di-y-lid model. For higher order TDs there is a small increase in the reorientation time prefactor, corresponding to more hops. However for a long tail thermal donor to reorient through each atom hopping individually, the number of reorientation steps would rapidly increase, as the line has to sweep out a circle to realign itself. One possibly way around this is if the atoms hop towards the core, through the core, and back out along the new defect line; in this case the number of hops required would equal the number of Oi atoms in each `arm' of the defect plus some constant for the core to reorient (presumably, from the TD3 result, five hops for the core).

Table 9.8: Number of thermal donor structures possible with a fixed common core and Oi adding in two linear tails, one either side of the core. The defect is assumed symmetric. The table lists number of isomeric combinations possible for a given tail length, and cumulative total. There are 13 experimentally observed TDs after TD3.
Number of Number of Cumulative
`tail' Oi TD isomers number of TDs
1 1 1
2 2 3
3 2 5
4 3 8
5 3 11
6 4 15

next up previous contents
Next: 2/3D thermal donors Up: Later Thermal Donors - Previous: Later Thermal Donors -
Chris Ewels