Infrared absorption spectrum of B-doped Si

It is shown that the presence of a sharp resonance (quasilocal mode) at 227 ${\mathrm{cm}}^{-1\mathrm{}}$ in the one-phonon absorption spectrum associated with isolated boron substitutional impurities in silicon implies substantial stiffening of the bond-bending force constant at the impurity atom. Previous interpretations, in terms of the mass defect only, rely upon artificial assumptions concerning the host-lattice density of modes in the TA region. Changes in bond-bending and bond-stretching force constants are found by fitting to the quasilocal- and local-mode frequencies. Detailed calculations of the band absorption which include for the first time elements of the apparent charge tensor associated with first and second neighbors of the impurity (as well as the ${\mathrm{B}}^{-}$ impurity itself) are presented. These calculations illustrate the important effects of near-neighbor apparent charges on the strengths of the various features in the absorption spectrum. Asymptotic expressions are invoked to assign second-neighbor apparent-charge tensor elements, and the number of independent first-neighbor elements is reduced from three to two by application of a sum rule. The remaining two elements are fitted to the strengths of the 227-${\mathrm{cm}}^{-1\mathrm{}}$ quasilocal-mode absorption and the local-mode absorption. These apparent charges are then used in the calculations for the entire band. Parallels are drawn between the absorption peak around 330 ${\mathrm{cm}}^{-1\mathrm{}}$ and the 227-${\mathrm{cm}}^{-1\mathrm{}}$ resonance. In an appendix it is shown that the contribution to the absorption arising from apparent charges on more distant neighbors (which are neglected in our calculations) is concentrated near the Raman frequency. In the band the integrated absorption from apparent charges on third neighbors and beyond is comparable with that from the apparent charges on the impurity and its first and second neighbors.