Conclusions

In this chapter we have shown that there is a binding energy between two O_i atoms to form an assymetric oxygen dimer, O_2i. Our calculations predict a binding energy in the range 0.7 $\rightarrow$1.7 eV. This assymetric dimer is 0.26 eV more stable than a symmetric dimer pair, previously predicted using AIMPRO and other methods. We have shown that the dimer can migrate with a barrier substantially lower than that of isolated O_i.

In addition we have shown that it has vibrational modes in excellent agreement with the experimentally observed 1012 cm^-1 and associated modes. An additional mode at 1105 cm^-1 has been tentatively correlated with the symmetric dimer. We have attempted to show how the dimer is crucial in the formation processes of many oxygen related defects in Cz-Si, that form in the temperature range $\sim$300-450$^\circ$C in which the dimer is stable.

Fast diffusing dimers will rapidly encounter relatively slow moving single O_i to produce an oxygen trimer, O_3i. We have performed some preliminary investigations of possible trimer structures, which show three bond centred O_i atoms around a shared Si atom to have the lowest energy by 0.248 eV, but a linear structure to have vibrational modes closer to experiment. The role of dimers and trimers in thermal donor formation are discussed further in Chapter 9.