Optical Investigation of the Small Polaron in β‐FeSi 2

The reflectivity of α‐FeSi 2 and also of β‐FeSi 2 doped with Co (n‐type) or Al (p‐type) has been investigated for wavelengths between 1 and 25 μm. The results for α‐FeSi 2 indicate metallic conduction in this material. The reflectivity of the undoped semiconducting phase β‐FeSi 2 can be explained by a fundamental absorption corresponding to a band gap E g = 0.8 eV and a reststrahlen band. The highest branch of the longitudinal optical phonons is found at 20.6 μm. The reflectivity of p‐FeSi 2 can be interpreted using Drude's theory of free carrier absorption. The optical properties of n‐FeSi 2 cannot be explained by free carrier absorption. A consistent interpretation, however, is possible in terms of the small polaron picture. Using the experimental data of the de conductivity and its activation energy the reflectivity of n‐FeSi 2 can be calculated. There is good agreement with the observed values. These results confirm an analysis of the transport properties of β‐FeSi 2 which was presented in a previous paper.