National Research Council of Canada. Measurement Science and Standards
The optical absorption coefficient of pure Ge has been determined from high-accuracy, high-precision optical measurements at photon energies covering the spectral range between the indirect and direct gaps. The results are compared with a theoretical model that fully accounts for the resonant nature of the energy denominators that appear in perturbation-theory expansions of the absorption coefficient. The model generalizes the classic Elliott approach to indirect excitons, and leads to a predicted optical absorption that is in excellent agreement with the experimental values using just a single adjustable parameter: the average deformation potential DΓL coupling electrons at the bottom of the direct and indirect valleys in the conduction band. Remarkably, the fitted value, DΓL=4.3×108eV/cm, is in nearly perfect agreement with independent measurements and ab initio predictions of this parameter, confirming the validity of the proposed theory, which has general applicability.