Temperature-Dependent Dielectric Response, Index of Refraction, and Absorption Coefficient of GeSn Films up to 8.4% Sn

Three Ge<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(1-x)</sub>Sn<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> films were measured by spectroscopic ellipsometry to extract their optical properties. The Sn contents of the films were 3.6%, 6.5%, and 8.4%, and all were fully strained to a Ge (001) substrate. Optical constants were collected from 0.39–4.116 eV, at temperatures between 78 K and 475 K. Critical point energies in the band structure were red-shifted with increasing Sn content and increasing temperatures. An extra critical point appears between E<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>+Δ and E<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> transitions in GeSn samples that does not appear in Ge.