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

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 cm^-1 region are due to vibrations of interstitial oxygen atoms perturbed by lithium ions, and substitutional boron ions, also perturbed by lithium ions. The 517-cm^-1 band of interstitial oxygen is displaced to 525 cm^-1 by Li^7 and to 537 cm^-1 by Li^6, and the 1106-cm^-1 band is displaced to 1006 cm^-1 by both lithium isotopes. (Frequencies are for 78^ except for the 1106- and 1006-cm^-1 bands which are for 300^. ) No counterpart of the 1203-cm^-1 band of interstitial oxygen is detected for the lithium-oxygen complex, and on this basis the 1203-cm^-1 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 cm^-1 for the frequency of the vibrational mode associated with the libration. The 623-cm^-1 triply degenerate vibration of isolated substitutional boron ions is split into two bands at 567 and 656 cm^-1 by interaction with lithium, indicating axial symmetry for the lithium-boron complex. (These frequencies are for the 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 (Li^+).