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\bf {\it AB INITIO} INVESTIGATION OF OXYGEN DEFECTS IN Si

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\bf \underbar{C.~Ewels}$^{\bf a}$, R.~Jones$^{\bf a}$ and 
S.~\"Oberg$^{\bf b}$

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$^{\bf a}$	Department of Physics, University of Exeter, \\
		 Exeter, EX4~4QL, UK\\

$^{\bf b}$	Department of Mathematics, University of Lule\aa,\\
		Lule\aa, S95187, SWEDEN

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Key words: oxygen, silicon, {\it ab initio} theory, vacancy.

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{\it Ab initio} techniques are  used to study vacancy oxygen complexes
in Si. Substitutional  oxygen  is found to be  an  off-site  defect in
agreement with  experiment. It possesses  one high frequency O-related
LVM  at 788~cm$^{-1}$  in  good  agreement with  an  assigned  mode at
831~cm$^{-1}$. The VO$_2$ defect is found to possess $D_{2d}$ symmetry
and  only   one  O-related    high  frequency  IR    active  mode   at
807~cm$^{-1}$. The  VO$_3$ defect has  three high frequency  IR active
modes  lying above the LVM for  VO$_2$. The calculated frequencies are
in fair  agreement with those  tentatively assigned to VO$_3$ at 1005,
976 and 910~cm$^{-1}$.  On the other hand, the  V$_2$O defect has  one
such LVM  at 790~cm$^{-1}$.  These   results provide support for   the
assignment  of the  889~cm$^{-1}$  (300~K)  local vibrational mode  to
VO$_2$ rather than to V$_2$O.

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