Effects of Annealing Ambient on the Structural, Optical and Electrical Properties of TiO2:Ge Thin Films

Abstract

TiO_2, a semiconducting material with a wide energy band gap, has been researched intensively due to its interesting properties and many potential applications. The films properties can as well be modified by preparing the films under diverse ambient to suit diverse applications. In this work, the effects of the annealing atmosphere on the structural, optical and electrical properties of pure TiO₂ and TiO₂:Ge composite thin films deposited by sputtering from commercially available pure TiO₂ and TiO₂:Ge (85:15) targets respectively were studied. The films were deposited on fluorine doped tin oxide glass substrates by radio frequency magnetron sputtering technique at room temperature 23–25 ℃ and then annealed at 450 ℃ for one hour in three different atmospheres: air, argon and nitrogen. The XRD results revealed that, both pure TiO₂ and TiO₂:Ge thin films were crystalline and had a mixed phase of anatase and rutile regardless of the annealing ambient. The calculated crystallite sizes ranged between 19–21 nm with the particle sizes for TiO₂:Ge films being larger compared to pure TiO₂ as calculated from ImageJ software. Electrical measurements showed that the resistivity of as deposited pure TiO₂ films (about 13.03 × 10^–2 Ω-cm) was greater than the TiO₂: Ge composite (5.91 × 10^–2 Ω-cm). Furthermore, on annealing, pure TiO₂ had a resistivity of 9.47 × 10^–2 Ω-cm–10.4 × 10^–2 Ω-cm, while the TiO₂:Ge composite had a resistivity of 2.24 × 10^–2 Ω-cm–3.61 × 10^–2 Ω-cm depending on the annealing atmosphere. Variation of annealing atmosphere had minimal influence on the electrical resistivity of the films. The films annealed in nitrogen recorded the least resistivity values averaging about 5.86 × 10^–2 Ω-cm, while those annealed in argon and air recorded a resistivity of 6.94 × 10^–2 Ω-cm and 6.65 × 10^–2 Ω-cm, respectively. The transmittance spectra showed that, all the annealed films were transparent in the visible region (400–700 nm) with a mean bandgap of 3.60 eV for each annealing atmosphere. It is recommended that films annealed in nitrogen atmosphere could be considered for potential applications in photovoltaics.

Keywords: Titanium dioxide; Germanium; Ambient; Structural; Optical, Electrical