Non-Drude Behavior in Indium-Tin-Oxide Nanowhiskers and Thin Films Investigated by Transmission and Reflection THz Time-Domain Spectroscopy

A comparative study of indium-tin-oxide (ITO) nanowhiskers (NWhs) and thin films as transparent conductors in the terahertz frequency range are conducted. We employ both transmission-type and reflection-type terahertz time-domain spectroscopies (THz-TDTS and THz-TDRS) to explore the far-infrared optical properties of these samples. Their electrical properties, such as plasma frequencies and carrier scattering times, are analyzed and found to be fitted well by the Drude-Smith model over 0.1–1.4 THz. Further, structural and crystalline properties of samples are examined by scanning electron microscopy and X-ray diffraction, respectively. Non-Drude behavior of complex conductivities in ITO NWhs is attributed to carrier scattering from grain boundaries and impurity ions. In ITO thin films, however, the observed non-Drude behavior is ascribed to scattering by impurity ions only. Considering NWhs and thin films with the same height, mobility of the former is
${\sim}{\rm 125}~{\rm cm}^{2}{\rm V}^{{-1}}{\rm s}^{-1}$
, much larger than those of the ITO thin films,
${\sim}{\rm 27}~{\rm cm}^{{2}}~{\rm V}^{{-1}}~{\rm s}^{{-1}}$
. This is attributed to the longer carrier scattering time of the NWhs. The dc conductivities
$({\sim}{\rm 250}~\Omega^{{-1}}~{\rm cm}^{{-1}})$
or real conductivities in the THz frequency region of ITO NWhs is, however, lower than those of the ITO thin films
$({\sim}{\rm 800}~\Omega^{{-1}}~{\rm cm}^{{-1}})$
but adequate for use as electrodes. Partly, this is a reflection of the much higher plasma frequencies of thin films. Significantly, the transmittance of ITO NWhs (
$\cong$
60%–70%) is much higher (
$\cong$
13 times) than those of ITO thin films in the THz frequency range. The underneath basic physics is that the THz radiation can easily propagate through the air-space among NWhs. The superb transmittance and adequate electrical properties of ITO NWhs suggest their potential applications as transparent conducting electrodes in THz devices.