Dominant Second-Order Dipole-Moment Contribution in the Infrared Absorption of III-V Compounds

It is shown that a relationship exists between the coefficient describing the infrared lattice absorption and the ionic contribution to the second-order electric susceptibility in a polar noncentrosymmetric crystal. By analyzing the available data of the latter it is concluded that two-phonon sidebands in the infrared absorption are due to the second-order dipole moment in all the III-V compounds and to the third-order potential in ZnO and CdS. Further, the magnitude of the integrated infrared absorption coefficient in a cubic homopolar IV-IV semiconductor is discussed.