Infrared study of localized vibrations in silicon due to boron and lithium

Localized modes in silicon associated with compensated boron and lithium impurities, observed previously⁽¹⁾ have been investigated in more detail. In particular the isotopic shift of absorption peaks due to change in the lithium and boron isotopes and temperature dependence of the position, width and intensity of peaks have been measured. On the basis of these measurements the impurity and isotopic origin are given for all observed peaks. The symmetry at the impurity sites has been also deduced.Thus, two peaks at 620 and 644 cm⁻¹ are assigned to localized vibrations of isolated boron atoms, B¹¹ and B¹⁰ respectively, at the substitutional site with the local symmetry T_d. The peaks at 681 and 584 cm⁻¹ are attributed to B¹⁰, and the peaks at 653 and 564 cm⁻¹ to B¹¹ which are located at a substitutional site deformed axially by the lithium atom in the nearest vicinity (pairing effect). The lithium reduces the local symmetry at the boron site, from T_d to C_3v, and partly removes the degeneracy, so that the threefold degenerate line is split into two components, a singlet and a doublet.The peaks at 522 and 534 cm⁻¹ are associated with the localized vibration of lithium isotopes, Li⁷ and Li⁶ respectively, in an interstitial position, interacting with a boron or an oxygen atom.