Theory of infrared absorption in silicon

A phenomenological theory of infrared (ir) absorption based on dipole moments derived from local charges produced by bond-length and bond-angle distortions is applied to calculate the ir absorption in crystalline (c-Si) and amorphous (a-Si) silicon. In a-Si, the first-order ir absorption is dominated by effective charges due to phonon-induced bond-angle distortions. In c-Si, the second-order ir absorption is best described by a phonon-modulated version of the same mechanism. A single value for the coupling constant in the dipole moments ($e_0^{\mathrm{*}}$=0.35e) provides reasonably good agreement with the absolute experimental ir absorption spectrum of both c-Si and a-Si, demonstrating that the ir absorption in both cases can be well described by the same local charge-transfer mechanisms.