In this chapter we have shown that there is a binding energy between two Oi atoms to form an assymetric oxygen dimer, O2i. Our calculations predict a binding energy in the range 0.7 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 Oi.
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 300-450C in which the dimer is stable.
Fast diffusing dimers will rapidly encounter relatively slow moving single Oi to produce an oxygen trimer, O3i. We have performed some preliminary investigations of possible trimer structures, which show three bond centred Oi 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.