Effects of tin on IR reflectivity, thermal emissivity, Hall mobility and plasma wavelength of sol–gel indium tin oxide films on glass

Sol–gel indium tin oxide films (ITO) of various composition (In:Sn atomic ratio=90:10 to 30:70) prepared from salt-based precursors were deposited on bare and SiO2 coated (∼200 nm thickness) sheet glass substrates. The films were cured in air and then annealed under N2/H2O at ∼500 °C to obtain ITO films of thickness 250–320 nm. Directional hemispherical reflectance (Rh) and trasmittance (Th) of the films were measured in the wavelength range 0.25–18 μm. The reflectance of the films at 10 μm (near the peak wavelength of black body radiation) was in the range 0.18–0.55. The thermal emissivity (ed) was evaluated from the relation: ed=1−Rh−Th (Th≈0, for sheet glass in the wavelength range 5–18 μm) and it was in the range 0.47–0.90. Reflectance (Rh) was also evaluated from measured sheet resistance values (R□), from the relation Rh=(1+2e0c0R□)−2, where e0 and c0 are the permittivity of electron in vacuum and velocity of light, respectively. Specific resistivity of the films, measured by van-der Pauw method at ambient temperature was in the range 1.7±0.3×10−3 to 6.6±1.2×10−3 Ω cm. The sheet resistance (R□) of the films was in the range 68–212 Ω/□. Free electron carrier concentration (N) and Hall mobility (μ) of the films were in the ranges (0.66±0.18)×1020 to (3.7±1.0)×1020 cm−3 and (4.4±1.5) to (20±5) cm2/V s, respectively. These values were utilized to evaluate plasma wavelength (λP) of the conducting films which were in the range 1.72±0.24–4.07±0.58 μm. Considerable variations of the above properties were observed with increasing Sn content but minimum thermal emissivity and maximum IR reflectivity were observed in the case of In:Sn=70:30.