Optical critical points of thin-film Ge1-y Sny alloys: A comparative Ge1-y Sny Ge1-x six study

The E0, E0 + Δ0, E1, E1 + Δ1, E0′, and E2 optical transitions have been measured in Ge1-y Sny alloys (y<0.2) using spectroscopic ellipsometry and photoreflectance. The results indicate a strong nonlinearity (bowing) in the compositional dependence of these quantities. Such behavior is not predicted by electronic structure calculations within the virtual crystal approximation. The bowing parameters for Ge1-y Sny alloys show an intriguing correlation with the corresponding bowing parameters in the Ge1-x Six system, suggesting a scaling behavior for the electronic properties that is the analog of the scaling behavior found earlier for the vibrational properties. A direct consequence of this scaling behavior is a significant reduction (relative to prior theoretical estimates within the virtual crystal approximation) of the concentration yc for a crossover from an indirect- to a direct-gap system.