Extracting the Complex Refractive Index of an Ultrathin Layer at Terahertz Frequencies With no Prior Knowledge of Substrate Absorption Loss

A method based on complementary transmission and reflection measurements is developed to determine the complex refractive index of an ultrathin layer, despite no knowledge of the substrate's absorption loss. Such a technique avoids error introduced by conventional methods in which the extinction coefficient of the substrate is extracted from a different region of the wafer, or an entirely different reference wafer. This method is implemented to determine the terahertz (THz) complex refractive index of highly doped (i.e., >10
¹⁸
cm
⁻³
) thin films from the emerging group IV semiconductor of GeSn, as well as the THz extinction coefficient of the underlying substrate region.