Piezoabsorption of Germanium Thin Films above the Fundamental Energy Gap

Reported here are measurements at 300 and 80^∘{}K of the stress-modulated absorption between 2 and 5 eV of thin polycrystalline films of Ge deposited directly on quartz transducers. Changes in the index of refraction and extinction coefficient as well as changes in the real and imaginary part of the dielectric constant due to the applied periodic strain are calculated. Strong polarization effects are observed for polarizations parallel and perpendicular to the strain axis. These effects are explained in terms of different contributions of the piezo-optic tensor components to the change in the real and imaginary parts of the dielectric-constant tensor. Temperature coefficients of the Λ spin-orbit-split transitions and the Σ-X transitions are reported. A transition at 3.1 eV which has a temperature coefficient of (-2.3±{}0.4)×{}10^-4 eV/^∘{}C is also discussed.