Characterization of Indium Tin Oxide (ITO) Thin Films Towards Terahertz (THz) Functional Device Applications

Abstract

In this study, we explored terahertz (THz) optical properties of the as-deposited and rapid thermal annealing (RTA) at 400°C, 600°C and 800°C of indium-tin-oxide (ITO) thin-films sputtered on high-resistivity silicon (HR-Si) substrate. We further investigated THz frequency dependence complex refractive indices and conductivity of those as-prepared and annealed ITO thin films. We note the transmittance enhanced from 27 % to 39 % for ITO thin film treated with RTA at 800°C while the real part of conductivity improved from 1019 Ω-1∙cm-1 to 4881 Ω-1∙cm-1 for ITO thin film treated with RTA at 600°C in the frequency range of 0.2 to 1 THz. The mobility, carrier concentration, plasma frequency and relaxation time of those sample were also deduced by fitting the experimental results with the Drude-Smith model. For example, the mobility of 29 cm2/V∙s, 110 cm2/V∙s, 226 cm2/V∙s and 70 cm2/V∙s, respectively is deduced for as-deposited, RTA at 400°C, RTA at 600°C and 800°C of ITO thin-films. In addition, the optical bandgap of those ITO thin films were evaluated with a typical Tauc plot showing extrapolation of photon energy spectrum. The transmittance, absorption coefficient and complex refractive indices of those ITO thin films were extracted by Swanepoel method using experimental transmittance and reflectance spectrum throughout the wavelength range from 400 to 1000 nm. Moreover, as expected, the transmittance of the ITO thin film treated with RTA at 800°C is improved at the wavelength of the visible light spectrum. We also displayed the surface morphology, microscopic structural properties and typical chemical composition of as-deposited and RTA at 400°C, RTA at 600°C and 800°C of ITO thin-films. We strongly believed the present study will lead to further enhancement for THz optoelectronics applications.