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The formation of VO2

  The OV defects anneal out at around 300$^\circ$C and a new line appears at 894 cm-1 (4 K) which then grows in intensity [141]. This band shifts to 889 cm-1 at room temperature. The original proposal [141] was that the absorption is due a VO2 defect, i.e. two O atoms sharing a vacancy, each bonded between two of the Si vacancy neighbours (Figure 5.2). The symmetry is then D2d. There are three processes which contribute to the demise of VO.

Initially a fast process in which the [Oi] rapidly increases. This may be related to the break up of aggregates of Sii introduced during the irradiation. This then leads to the reaction: VO + Sii $ \rightarrow $ Oi [81,82,79].
A second slower process. This is assumed to be: VO + Oi $ \rightarrow $ VO2 and caused by VO diffusion to the immobile (at 350$^\circ$C) Oi. This assignment is based on the 1.86 eV activation energy [80], the migration barrier for VO diffusion.

Not all VO is lost by this second process and a third involves VO being captured by some (unknown) defect and producing Oi. Roughly half of VO can meet their fate in this third process.

Support for the VO2 assignment of the 889 cm-1 defect comes from its IR-absorption intensity which is approximately proportional to [Oi]2 when VO has disappeared. However, there is no loss of Oi, as measured by IR-absorption, during the slow stage when the 889 cm-1 defect is being formed [81]. This initially provided the necessity for the third formation mechanism given above, however recent studies by Londos et al [79] show two distinct activation energies in the slow process, with a switch in dominance at about 360$^\circ$C. In addition they showed that above 380$^\circ$C VO2 absorption continues to increase, even though VO levels have almost stabilised.

Further, early uniaxial stress studies on the 889 cm-1 defect [77] concluded that its symmetry was lower than D2d; unless the symmetry was D2d, two O-related LVMs would be expected. However, prompted by our theoretical investigations, Bech Neilsen et al [83] re-examined the defect with uniaxial stress and showed it to possess either C2v or D2d symmetry. Although it is difficult to distinguish between the two, they have recently performed a range of stress-induced dichroism experiments that were able to unambiguously show D2d symmetry [83] (see below). Finally, for mixed 16O-18O samples, only two LVMs were detected and not three [84,85], with isotopic shifts indicative of a single oxygen atom. This shows that the two O-modes in this model for the 889 cm-1 centre are decoupled even though their separation can only be a few Å. This objection is not fatal as earlier modelling studies [86] found that the O atoms were decoupled in the LVMs.

The difficulties [87] described here have led to an alternative assignment of the 889 cm-1 LVM to V3O [78]. This defect would possess only one LVM and possess C1h symmetry. Presumably, in this model vacancies are thermally released from VO or Vn complexes and form stable divacancies which subsequently trap VO. EPR studies have suggested [88] that complexes such as V2O and V3O, with S= 1, form and anneal out around 300$^\circ$C although Davies et al [89] suggest that V2O may be responsible for an LVM at 1005 cm-1 (10 K). This defect anneals out around 450$^\circ$C.

next up previous contents
Next: Higher order VOn defects Up: Introduction Previous: The VO Centre
Chris Ewels