Time-domain THz characterization of semiconductors across a wide doping range by combined transmission spectroscopy and ellipsometry

Abstract

Terahertz (THz) time-domain techniques provide a powerful, contactless tool for probing the electrodynamic response of semiconductors. However, conventional THz transmission spectroscopy becomes increasingly ineffective for highly doped or conductive samples due to strong absorption and reduced penetration depth, whereas reflection methods for opaque samples are extremely sensitive to even small errors in phase. In this work, we present a single, unified THz time-domain platform, operated either in transmission or in reflection ellipsometric configuration, which enables the characterization of bulk semiconductor wafers over a broad range of doping levels, spanning more than four orders of magnitude in carrier concentration (between 1014 and 1018 cm−3). The technique leverages the unique sensitivity of THz radiation to free-carrier dynamics in semiconductors, ensuring continuous access to the complex dielectric function across markedly different conductivity regimes and enabling reliable extraction of carrier-related parameters. The measured response is parameterized using an effective generalized Drude description that accounts for frequency-dependent scattering and naturally converges to the classical Drude limit at high carrier densities. The results establish the combined transmission and ellipsometric time-domain approach as a robust contactless method for extracting transport-related parameters over a broad conductivity range in semiconductors and other materials of interest in the THz region.