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Description
Transparent conductive oxides (TCOs) coatings play an important role in several optoelectronic applications, such as transparent electrodes in liquid-crystal displays (LCDs), organic light-emitting diodes (OLEDs), solar cells, and light-emitting diodes [1-3]. One of the most prominent TCOs for decades has been tin-doped indium oxide, also known as indium tin oxide or simply ITO [4]. This material has a high optical transmittance in the visible spectral range and an electrical resistivity in the order of $10^{-4}\Omega$cm. This is owing to its wide optical bandgap and the degenerated states of the semiconductor allowing a relatively high charge density whilst keeping the optical transparency. These material properties can be tailored by modifying the charged defects density, typically oxygen vacancies. In particular, the tailoring can be performed by post-deposition thermal treatments. In this work, a systematic study regarding the influence of post-deposition annealing treatments on the structural, electrical and optical properties of sputtered ITO thin films is presented. The films were deposited by radio frequency magnetron sputtering on fused silica substrates using a high-density ITO target (90 wt% In$_2$O$_3$ and 10 wt% SnO$_2$). Annealing treatments were carried out in argon or air atmospheres in order to induce the variation of the optoelectronic properties. Optical transmittance, X-ray diffraction, elemental composition and electrical resistivity measurements were performed after each annealing step. Annealing in both argon and air increased the grain size with increasing temperature. After the annealing up 400°C, the electrical resistivity decreases in both atmospheres. For higher temperatures, annealing in air increases the resistivity while annealing in argon decreases the resistivity of the material. In this way, the impact of the atmosphere and annealing temperature on the electrical resistivity was assessed. The lowest resistivity obtained in this work was 2.5$\times10^{-4}\Omega$cm after heating in argon at 500°C. The optical transmittance of the film in the visible range did not show any noticeable differences in both argon and air annealing. Optical parameters, such as refractive index, absorption coefficient, band gap and Urbach energy were evaluated accordingly at each annealing temperature.
