Vibrational Spectra of Lithium-Oxygen and Lithium-Boron Complexes in Silicon

Abstract

Vibrational spectra of lithium-oxygen and lithium-boron complexes in silicon have been studied using enriched boron and lithium isotopes. The absorption bands in the 300-4000 ${\mathrm{cm}}^{-1\mathrm{}}$ region are due to vibrations of interstitial oxygen atoms perturbed by lithium ions, and substitutional boron ions, also perturbed by lithium ions. The 517-${\mathrm{cm}}^{-1\mathrm{}}$ band of interstitial oxygen is displaced to 525 ${\mathrm{cm}}^{-1\mathrm{}}$ by ${\mathrm{Li}}^7$ and to 537 ${\mathrm{cm}}^{-1\mathrm{}}$ by ${\mathrm{Li}}^6$, and the 1106-${\mathrm{cm}}^{-1\mathrm{}}$ band is displaced to 1006 ${\mathrm{cm}}^{-1\mathrm{}}$ by both lithium isotopes. (Frequencies are for 78°K except for the 1106- and 1006-${\mathrm{cm}}^{-1\mathrm{}}$ bands which are for 300°K.) No counterpart of the 1203-${\mathrm{cm}}^{-1\mathrm{}}$ band of interstitial oxygen is detected for the lithium-oxygen complex, and on this basis the 1203-${\mathrm{cm}}^{-1\mathrm{}}$ band is reassigned to a combination of the asymmetric SiO stretching vibration with the librational motion of oxygen around the $〈111〉$ axis rather than to the symmetric SiO stretching fundamental. This new assignment provides a value of 67 ${\mathrm{cm}}^{-1\mathrm{}}$ for the frequency of the vibrational mode associated with the libration. The 623-${\mathrm{cm}}^{-1\mathrm{}}$ triply degenerate vibration of isolated substitutional boron ions is split into two bands at 567 and 656 ${\mathrm{cm}}^{-1\mathrm{}}$ by interaction with lithium, indicating axial symmetry for the lithium-boron complex. (These frequencies are for the ${\mathrm{B}}^{11}$ isotope.) The precipitation of lithium from the lithium-oxygen complex is inhibited in the presence of boron, probably because in the absence of free electrons the precipitation nuclei develop a positive charge which repels the diffusing lithium species (${\mathrm{Li}}^{+}$).