Structural, electrical, optical and morphological properties of aluminum-doped TiO₂ thin films deposited by spray pyrolysis method

In this research, highly transparent thin films of pure and doped titanium dioxide (TiO₂) with various amount of aluminum (0–5 mol%) were prepared by sol–gel method and were coated on a FTO substrate at 500 °C by spray pyrolysis. Thermogravimetry and differential thermal analysis were employed to study the thermal transformation behavior of the xerogel. The effects of aluminum doping on structural, morphological, optical, and electrical properties of TiO₂ samples were evaluated using X-ray diffraction (XRD), field emission scanning electron microscopy, atomic force microscopy, UV–Vis absorption spectroscopy, ellipsometry and four point probe techniques. XRD results reveal that all thin films have anatase crystal structure and it can be seen that the preferred growth orientation of the TiO₂ thin films depends on the Al doping levels. The results show that by increasing the amount of aluminum dopant (0 to 5 mol%), the grain size, crystalline plane distance and roughness are decreased, as well the resistivity of the TiO₂ layers decreases until it reaches to a minimum value at 3 mol% of aluminum. On the other hand, the band gap rises from 3.28 to 3.39 eV. The ellipsometry measurements demonstrate that the refractive index of TiO₂ thin films decrease from 1.96 to 1.89 at 632.8 nm and the extinction coefficient increase with Al doping content.