Determination of the free carrier concentration in atomic-layer doped germanium thin films by infrared spectroscopy

Novel silicon photonics applications requiring heavy n-type doping have recently driven a great deal of interest towards the phosphorous doping of germanium. In this work we report on infrared reflectance spectroscopy measurements of the electron density in heavily n-type doped germanium layers obtained by stacking multiple phosphorous δ- layers. Here, we demonstrate that the conventional Drude model of the electrodynamic response of free carriers in metals can be adapted to describe heavily doped epitaxial Germanium