Abstract
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 terahertz time domain spectroscopy (THz-TDS) in the frequency range from 0.5 to 18 THz using air plasma techniques, supplemented by the photoconductive antenna (PCA) method. The complex conductivities were accurately calculated using a thin film extraction algorithm and analyzed in terms of the Drude conductivity model. All the measured TCOs have a scattering time below 15 fs. 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 in these materials, which is strongly influenced by the zinc oxide phonon resonance tail even in the low frequency part of the spectrum. The conductivity of AZO is found to be more thickness dependent than GZO and ITO, indicating high importance of the surface states for electron dynamics in AZO. Finally, we measure the transmittance of the TCO films from 10 to 200 THz with Fourier transform infrared spectroscopy (FTIR) measurements, thus closing the gap between THz-TDS measurements (0.5-18 THz) and ellipsometry measurements (200-1000 THz).
Highlights
Transparent conducting oxides (TCO) have attracted significant attention as alternative plasmonic materials to conventional metals [1,2,3]
We find that the ITO conductivities are well described by the simple Drude model, while the conductivity of gallium-doped zinc oxide (GZO) and aluminum-doped zinc oxide (AZO) must be described by a Lorentz oscillator model in addition to the Drude response in order to take the phonon response of zinc oxide (ZnO) into account [5]
The conductivities were extracted with an iterative thin film extraction algorithm and analyzed by fitting with the Drude model for ITO and Drude-Lorentz oscillator model for GZO and AZO
Summary
Transparent conducting oxides (TCO) have attracted significant attention as alternative plasmonic materials to conventional metals [1,2,3]. With the emerging of terahertz time domain spectroscopy (THz-TDS), the frequency resolved conductive properties of a range of conductors, including thin gold films [9], highly doped silicon [10,11] and ITO [8] have been investigated between 0.2 and 2.7 THz. no broadband characterization of the complex conductivities of these TCO films has been performed in the THz range far. Due to the development of two-color femtosecond air plasma THz-TDS [12,13,14], it has become possible to extend THz-TDS measurements on chalcogenide glasses [15] and water [16] up to 18 THz. Here we report the complex conductivities of ITO, AZO and GZO up to >18 THz covering the important crossover point where the scattering rate equals the frequency, ωτ = 1 , allowing precise determination of the scattering time τ. By performing a Fourier transform infrared spectroscopy (FTIR) and ellipsometric measurements, we get the transmittance of the TCOs over an exceedingly broad frequency range from 0.5 THz to 1000 THz
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