**1. Introduction**

Transparent conductive oxide (TCO) thin films possess excellent conductivity and optical transmittance in the visible and near-infrared regions, and are thus applied in many photoelectric components nowadays, including solar cells [1,2], organic light-emitting diodes [3,4], thin-film transistors [5,6], photovoltaic batteries [7–9], electrochromic devices [10–12], and tablet displays [13–16]. Metallic films are generally opaque in the visible light range. However, for film thicknesses of less than 100 Å, visible light is transmitted through the film, while infrared (IR) light is reflected. Moreover, for metals such as In2O3, ZnO, SnO2, TiO2, and CdO with energy gaps of 3 eV or more, the film also has excellent semiconducting properties [17].

The literature contains many studies on the optoelectronic properties of metallic films [18–20]. In addition, various authors have investigated the properties of three-layer TCO films with oxide/metal/oxide or metallic oxide/metal/metallic oxide structures [21]. The results have shown that such films not only suppress the reflection from the metallic layer in the visible light range but also produce a transmittance effect [22,23]. Consequently, the TCO thin films are used in solar cells, gas sensors, LCD displays, etc.

Among the various metal oxides in common use nowadays, SnO2 has poorer electrical properties than ITO, but a superior photoelectric performance in the IR region. Furthermore, SnO2 has good chemical and thermal stability and is also amenable to surface modification in order to expand its working wavelength range. As a result, SnO2 conductive films are widely used for such applications as gas sensors, solar energy battery electrodes, low-radiation glasses, etc. [24,25]. However, SnO2 films are less easily used in tablet display applications due to their high electrical resistance and poor etching effect.

Accordingly, the present study explores the feasibility of improving the optical and electrical properties of SnO2 thin films by doping the films with Ti. Note that Ti is deliberately selected as the dopant material here, since it has a maximum chemical valence of +4 [26], where the radius of Ti4<sup>+</sup> is 0.0605 nm, while that of Sn4<sup>+</sup> is 0.069 nm. Due to the similarity of the ion radii, the Ti4<sup>+</sup> ions readily replace the Sn4<sup>+</sup> ions in the crystal lattice of the SnO2 and hence modify its electrical and optical behavior. The Ti:SnO2 films are deposited on glass substrates using a radio frequency (RF) magnetron sputtering system and then are annealed at various temperatures in the range of 200–500 ◦C to prompt the diffusion of the Ti atoms into the SnO2 layer. The optoelectronic properties of the films are then systematically explored in order to determine the annealing temperature, which results in the optimal tradeoff between the electrical and optical properties of the film, respectively.
