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The VO3 Defect

  VO3 forms at 450$^\circ$C when VO2 traps an additional Oi. This defect (shown in Figure 5.4) has an additional O atom, say Oc, next to VO2. The bonds of the Oc atom are 1.632 Å to the Si atom bordering the vacancy and 1.686 Å to the further Si. These are much shorter bonds and naturally lead to higher O-related local modes. Oc is bonded to a Si atom which is also bonded to another O-atom, say Oa, with length 1.643 Å. Oa is also 1.696 Å away from the other Si atom bordering the vacancy. The third O atom, Ob, also lies within the vacancy and possesses two bonds of length 1.696 Å. Thus the presence of the additional O atom has only slightly perturbed the O atoms within the vacancy. Their separation is 2.646 Å.

The LVMs are given in the table. The 998 and 902 cm-1 modes are localised on Oc and Oa. The 813 cm-1 is localised on Ob. The 665 cm-1 and other modes are extended over all the O atoms. Thus the effect of an additional O is to displace upwards the mode due to VO2 by about 100 cm-1 and introduce an additional mode $\sim$1000 cm-1. This top mode is in good agreement with a mode found at 1005 cm-1, although the other modes are about 70-100 cm-1 too low. A similar error was found for VO2, and this error lies within the uncertainties of the method, and again, the upward mode shift from VO2 $ \rightarrow $ VO3 is correctly predicted. The prediction and observation of three O-related modes supports the assignment of the modes at 1005, 976, 910 cm-1 to VO3 and therefore indirectly the 889 cm-1 mode to VO2.

Figure 5.4: The O3V centre in Si. The box indicates the [100] directions.
\psfig {figure=oxygen/vacancy/diags/o3v.eps,width=13cm}


Just as we had trouble finding the lowest energy structure for VO2, we also found a low energy structure for VO3 consisting of an OV centre with two Oi atoms in the dilated bonds neighbouring the Si-O-Si of the OV. This is similar to the alternative VO2 model described above but with an additional Os sitting in the vacancy. This proved to be lower energy than the VO3 model described above, and also had good agreement with experimental LVMs. However since the energies of VO2 have to be corrected in order to find the actual lowest energy structure we have assumed this also applies to VO3, and so exclude this model. Experimental stress induced alignment experiments should be able to differentiate between these two centres, since the VO3 presented above has C1h symmetry, whereas this low energy VO3 has C2v symmetry. Work is currently underway on the higher order VOn complexes by Lindström and Hallberg, and hopefully they should be able to check this.

next up previous contents
Next: The OV-Oi pre-cursor to VO Up: Method and Results Previous: Alternatives to VO - VO
Chris Ewels