k.p perturbation theory for light absorption in semiconductors doped by 3d metals

A theory of light absorption in semiconductors doped by transition metal impurities is constructed. The absorption cross section near the edge of the optical band is calculated by means of k.p perturbation theory within the framework of the Kane model supplemented by the d impurity levels. Special attention is given to the problem of gauge invariance in k.p theory. It is shown that in the case of an impurity state with definite parity the Lucovsky formula is valid when modified by the more detailed account of the central cell corrections. In the substitutional positions without the inversion centre the impurity states possess no definite parity which allows the dipole transitions forbidden by the conventional crystal field theory and causes the linear Stark effect for the t₂ levels.