Abstract
Indium tin oxide (ITO) exhibits two outstanding properties which are the high optical transparency and electrical conductivity. It has been widely used for various optoelectronic devices, such as solar cells, liquid crystal displays, and light emitting diodes (LED). Recently, because of the broadband and omnidirectional antireflection (AR) characteristics, ITO nanostructures have been successfully employed to enhance efficiency of photovoltaics and LED. In this thesis, we aimed to study the frequency-dependent complex refractive indices and conductivities of ITO thin films, nanorods, and nanowhiskers by THz time-domain spectroscopy (THz-TDS) based on photoconductive (PC) antennas and laser-induced gas plasma, respectively. ITO thin films were grown on the high resistivity silicon substrate by DC reactive magnetron sputtering. On the other hand, ITO nanorods and nanowhiskers were deposited on the high resistivity silicon substrate using glancing-angle electron-beam evaporation. In order to obtain the complete optical and electrical information, we combined the experimental results of complex refractive indices measured from the PC antenna (0.15-1.4THz) and laser-induced gas plasma (0.5-4THz) THz-TDS system. Because the complex conductivities can be extracted from the refractive indices, the important electrical parameters for optoelectronic materials, such as DC mobilities and carrier densities fit complex conductivities of the ITO thin films, nanorods, and nanowhiskers by Drude-Smith model, will be much more accurate. We have determined that the plasma frequencies of the ITO films are in the range of 1547-3170 rad•THz, while the corresponding scattering times are 4.32-9.19 fs. For nanowhiskers and nanorods, the plasma frequencies are 751-853 versus 561-1006 rad⋅THz, and carrier scattering time are 13.2-39.6 versus 13.5-31.7 fs, respectively. The mobility, electron density of the thin films were determined to be 2.14-16.2 cm2 V−1 s−1 and 2.26-9.49 × 1020 cm−3, while 20.26-92 cm2 V−1 s−1 and 5.33-6.86 × 1019 cm−3 in nanowhiskers, 9.13-53.6 cm2 V−1 s−1 and 2.97-9.56 × 1019 cm−3 in nanorods. Our results showed that the ITO nanowhiskers and nanorods exhibit longer carrier scattering times than ITO thin films. This denotes that the two kinds of nanostructures have an excellent crystallinity with large grain size. In addition, we also discussed the backscattering and localization effect in the different nanostructures, while it caused the nanowhiskers to show the more remarkable conductivity than nanorods.