Alternatives to VO2 - the V2O Defect

This possesses Si-O lengths of 1.73 and 1.72 Å with a Si-O-Si angle of 143 $^\circ$ . The Si-O bonds are longer than in VO₂ and close to those in the VO centre. There is very little reconstruction of the Si-Si bonds in the divacancy and consequently there is an empty state very close to a filled mid-gap state. The LVMs are given in Table 5.1, and the structure is shown in Figure 5.3. There is a single mode at 791 cm^-1: almost the same frequency as in the VO defect. Thus the calculations give no support for an assignment of the 889 cm^-1 defect to a V₂O complex [89]. It is then unlikely that V₃O complexes have high frequency modes and this casts doubt on the assignment [78] of the 889 cm^-1 line to this defect. In addition the V₂O and V₃O defects do not have the required D_2d symmetry [83]. The table also shows the effect of replacing a ²⁸Si neighbour of O by ²⁹Si.

Divacancy complexes are a common vacancy impurity, which combine with O_i to form V₂O. These centres are more common in heavily damaged samples. V₂O and V₃O can, in addition to our studies, be ruled out as candidates for the 889 cm^-1 line on the basis of various annealing studies. Lee et al showed V₂O anneals out at the same temperatures as VO [88]. Other studies show it to anneal out at around 450 $^\circ$ C to form V₃O and V₂O₂ [89], with V₂O₂ going on to form either V₃O₂ or V₃O₃ [96]. Although these results are not consistent with each other, neither fits the annealing properties of the 889 cm^-1 mode. For these reasons V₃O can also be ruled out as a candidate for the 889 cm^-1 line.

**Figure 5.3:** The OV₂ centre in Si. The box indicates the [100] directions. The two central dots mark the vacant lattice Si sites.
$\begin{figure} \begin{center} \vspace{-4.0cm} \ \psfig {figure=oxygen/vacancy/diags/ov2.eps,width=13cm} \vspace{-3.5cm}\end{center}\end{figure}$

**Table 5.1:** Calculated and Experimental Frequencies, cm^-1, of LVMs of VO_n defects - isotope values show downwards shift of the modes with shifting isotope. a,b,c for VO₃ refer to Figure 5.4.

Symm.	²⁸Si¹⁶O	²⁸Si¹⁸O	²⁸Si¹⁶O¹⁸O	²⁹Si¹⁶O

VO
B₁	787.4	38.1	-	0.9
Expt. [75,85]	831	33

VO₂
E	806.5	38.8	0.0, 38.8	0.0, 1.0
A₁	656.4	18.5	8.8	1.3
E	574.2	3.5	0.1, 3.5	0.0, 2.7
B₂	563.8	8.6	4.7	1.4
Expt. [75,84]	894 (4 K)	49 (300 K)

V₂O
A	790.7	38.2	-	1.1, 0.9

VO₃	¹⁶O	¹⁸O_c	¹⁸O_b	¹⁸O_a
A	997.8	22.7	0.1	12.8
A	902.2	23.5	0.1	30.5
B	813.5	0.3	39.2	0.1
A	669.1	4.1	5.6	6.6
A	634.3	6.5	8.2	2.1
B	611.6	4.5	0.0	1.9
A	597.5	1.1	2.4	1.8
Expt. [141]	1005, 976, 910