Femtosecond ellipsometric study of nonequilibrium carrier dynamics in Ge and epitaxial Si1−xGex

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Abstract
The time‐resolved, above‐gap optical response of optically thick Si1−xGex alloys to carrier injection by a femtosecond pump pulse is measured across the entire compositional range (0≤x≤1) using a novel femtosecond ellipsometric technique which clearly distinguishes the real and imaginary parts of the time‐varying dielectric function ε1(t)+iε2(t). The results are modeled microscopically in terms of the Drude contribution from a diffusing hot electron‐hole plasma, augmented by transient‐induced absorption from hot‐carrier‐induced band renormalization. Further corrections from thermal band‐gap shrinkage, intervalley scattering, and transient interband absorption saturation are also discussed.