It was pointed out that the optical absorption coefficient of SinGem superlattices depended strongly upon the quality of the Si/Ge interfaces. Interdiffusion across these interfaces was reflected by variations in the optical absorption coefficient as a function of energy, for energies that were near to the energy gap. By using the coherent potential approximation to simulate interdiffusion across the Si/Ge interfaces, and by using the Kubo-Greenwood formula for optical absorption, it was shown that (for Si5Ge5) the optical absorption coefficient was proportional to a power of the deviation from the energy gap; where the exponent was between 0.5 and 2.0 (depending upon the degree of interdiffusion across the Si/Ge interfaces). An exponent of 0.5 reflected the existence of abrupt interfaces, while an exponent of 2 corresponded to highly diffused interfaces. The latter case agreed well with available experimental data.

I.A.Papadogonas, A.N.Andriotis, E.N.Economou: Physical Review B, 1997, 55[8], R4887-90