Ultrabroadband terahertz characterization of highly doped ZnO and ITO

The broadband complex conductivities of transparent conducting oxides (TCO), namely, aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO) and tin-doped indium oxide (ITO), were investigated by using THz-TDS from 0.5 to 18 THz. The complex conductivities were accurately calculated using a thin film extraction algorithm and analyzed in terms of the Drude conductivity model. We find that a phonon response must be included in the description of the broadband properties of AZO and GZO for an accurate extraction of the scattering time, which is strongly influenced by the zinc oxide phonon resonance tail even in the low frequency part of the spectrum.