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

Abstract

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 $\upsilon _{2}$ (symmetric bending), $\upsilon _{3}$ (antisymmetric stretch) and $\upsilon _{1}$ (symmetric stretch) normal modes of vibration of Si$_{2}$O. The present investigation confirms the $\upsilon _{3}$ origin of the 1136 cm$^{-1}$ band (the well known 9$\mu $m band) but we disagree with the earlier assignments of the 517 and 1136 cm$^{-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 $\upsilon _{2}$ mode of Si$_{2}$O. The isotope shift due to $^{18}$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 $\upsilon _{2}$ 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 $\upsilon _{3}$ assignment. Using our normal mode description, we conclude that the 1203 cm$^{-1}$ band is a combination band involving $\upsilon _{3}$ and $\upsilon _{2}$ excitations. We have not been able to give a clear cut assignment to the 517 cm$^{-1}$ band, but we suggest that $\upsilon _{1}$ 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.