Absorption by the vibrations of uncharged atoms

Abstract

A charged impurity atom in an unpolar material (e. g. B - or P + in silicon) polarizes the elec­trons in its surroundings, but the equilibrium configuration is symmetrical hence the total dipole moment vanishes. But it is known from the Raman scattering and the elasto-optical effects that a lattice displacement changes the electronic polarizability. If a displacement is such that the polarizability changes by different amounts on opposite sides of the impurity then the polarization induced by the impurity is no longer symmetrical and a net electronic dipole moment results. Vibrations which have this property are optically active, even if they involve no displacement of the impurity itself. One can attribute 'apparent charges’ to the atoms near the impurity, and a sum rule for the total absorption is derived in terms of these apparent charges. Approximate expressions obtained for the apparent charges from macroscopic relations indicate that the absorption by the vibrations of the neighbours of the impurity is of similar magnitude to or larger than the absorption by the vibration of the impurity itself. The absorption by the non-localized vibrations is therefore much larger than it would otherwise be. The experimental spectra agree with this conclusion.