Absorption of oxygen in silicon in the near and the far infrared

Previous work on oxygen in silicon has shown that oxygen dissolves interstitially in silicon forming a complex which may be approximately described as Si 2 O. Absorption bands of Si : O occur at 517, 1136 and 1203 cm -1 and these have been assigned by earlier authors to the v 2 (symmetric bending), v 3 (antisymmetric stretch) and v 1 (symmetric stretch) normal modes of vibration of Si 2 O. The present investigation confirms the v 3 origin of the 1136 cm -1 band (the well-known 9 μ m band) but we disagree with the earlier assignments of the 517 and 1136 cm -1 bands. The results reported here are relevant to organic siloxanes. We have extended the investigation of Si : O into the far infrared and we find sharp absorption lines at 29.3, 37.8, 43.3 and 49.0 cm -1 which we have assigned to the v 2 mode of Si 2 O. The isotope shift due to l8 O has been observed in the far infrared spectrum. Effects of uniaxial stress on the 29.3 cm -1 line have been investigated and are found to be consistent with the assignment to the v 2 mode. The main features of the far infrared spectrum are accounted for with a simple anharmonic potential which ignores coupling of the Si 2 O to the crystal lattice. We have investigated effects of uniaxial stress on the 517, 1136 and 1203 cm -1 bands of Si 2 O. Our stress results for the 1136 cm -1 band are consistent with the earlier v 3 assignment. Using our normal mode description, we conclude that the 1203 cm -1 band is a combination band involving v 3 and v 2 excitations. We have not been able to give a clear cut assignment to the 517 cm -1 band, but we suggest that v 1 type excitation may be involved. The appendix describes the stress splitting of the 836 cm -1 band of the silicon A centre in electron irradiated Si : O and our results confirm an earlier model for this centre. In all cases investigated here, the stress splittings arise from raising the orientational degeneracy of the oxygen complex.