Deposição e caracterização de filmes finos de TiO2 dopados com Ag via o método sol-gel

Abstract

Titanium dioxide (TiO2) is a highly investigated semiconductor material renowned for its exceptional optical and catalytic properties, making it an ideal candidate for various applications, including sensors, antibacterial coatings, and photovoltaic cells. In this study, it was investigated the effect of silver (Ag) doping on the optical and electrical characteristics of thin film layers composed of TiO2 deposited on glass, indium tin oxide (ITO), as well as p-type and n-type crystalline silicon substrates. This doping process led to the formation of heterostructures exhibiting electrical behavior like the junction diodes, with the specific characteristics strongly dependent on the concentration of Ag used as a dopant. The deposition of these films was accomplished through the reverse micelle sol-gel method. For this purpose, a solution was prepared by combining titanium tetrabutoxide, xylene, and Triton X-100. To introduce Ag as a dopant, silver nitrate (AgNO3) was incorporated into the solution at concentrations of 0.1%, 0.5%, and 1%. The optical analysis of the TiO2 sol-gel solution through absorbance measurements in the ultraviolet (UV) region, it was observed that the inclusion of Ag resulted in a decrease in the material’s band gap. The band gap values ranged from (3.4 ± 0.2) to (1.88 ± 0.02) eV, indicating a significant reduction caused by the presence of Ag. Additionally, the inclusion of Ag facilitated the formation of silver nanoparticles within the films. These nanoparticles exhibited an effective diameter ranging from 30 to 34 nm, and their concentration varied from 0.44x1012 to 1.33x1012 particles/ml insofar the concentration of AgNO3 increased from 0% to 1.0%, respectively. Prior to the deposition of the films, thorough substrate preparation was conducted. The substrates underwent a cleaning process involving an aqueous solution of sulfuric acid, followed by a brief immersion in hydrofluoric acid. In the case of silicon substrates, an ammonium-based aqueous solution was employed to eliminate organic contaminants. Subsequently, the silicon substrates were subjected to thermal oxidation at 900 ∘C. The deposited films underwent a similar treatment, with sintering temperatures set at 450 ∘C and 550 ∘C. The optical characterization of these films, conducted through UV absorbance measurements, revealed variations in the optical band gap depending on the type of transition. For an indirect transition, the band gap ranged from 2.8 to 3.6 eV, while for a direct transition, it fell between 3.5 and 3.9 eV. Notably, the size of the nanoparticles displayed a similar trend to that observed in the sol-gel solution. Furthermore, the size variation of nanoparticles influenced the band gap in a manner consistent with the aforementioned observations. Chemical analysis utilizing techniques such as infrared spectrometry (FTIR), X-ray diffraction (XRD), and X-ray energy dispersion spectroscopy (EDS) was performed to examine the films following sintering. The results indicated that the films primarily consisted of the anatase phase. Morphological characterization further revealed the formation of films with an average thickness of 1 𝜇m, along with particles measuring up to 10 𝜇m in size, in addition to the nanometric-sized particles of the sol-gel solution of TiO2. On the other side, the electrical characterization results revealed interesting findings regarding the impact of silver doping on the properties of the TiO2 matrix. When silver is introduced as a dopant, it supplies electrons to the TiO2 matrix. In the case of n-type silicon substrate, it enhances conductivity and reduces electrical resistance. Conversely, an opposite behavior is observed for the ITO substrate. However, a distinct behavior is observed in the heterostructures formed by TiO2/p-type silicon. In this case, the introduction of silver causes the recombination of electrons provided by silver with holes in the p-type silicon, resulting in an opposite effect. As a result, the electrical resistance increases in the TiO2/p-type silicon heterostructures. The doping-induced changes are further elucidated by Mott-Schottky analysis. Consistent with our previous observations, the concentration of electrons in the TiO2/n-type silicon junction increases due to doping, while it decreases when the substrate is p-type silicon. This behavior is captured by the Mott-Schottky analysis, which provides valuable insights into the changes in electron concentration at the junction interfaces. Lastly, regardless of the substrate used for TiO2 deposition, the presence of silver as a dopant leads to an increase in the height of the Schottky barrier associated with the junctions of the device, indicating a significant influence on the overall electronic properties.