Optical properties of SimGen superlattices: A CPA treatment of the interface diffusion

The use of the coherent potential approximation (CPA) is proposed for the description of the interdiffusion across the Si-Ge interfaces of the SimGen superlattices (SL's). A generalization of the Kubo-Greenwood formula for the optical conductivity within the CPA is used to describe quantitatively the effect of the interface diffusion on the optoelectronic properties of the SL's, the latter described within the tight-binding approximation using an orthogonal basis set including interactions up to third-nearest neighbors. Our results being in very good agreement with other theoretical approaches and recent experimental measurements of the optical-absorption coefficient and the optical transitions in the Si4Ge4 and SimGen (m + n = 10) SL's indicate that our approach is not only computationally efficient but accurate as well.