Exploring the structural and optical properties of Ir-doped ZnO thin films

Abstract

In this investigation, ZnO and Ir doped ZnO thin films were deposited on glass substrates by sol–gel spin coating technique. X-Ray diffraction study revealed that the obtained thin films possessed polycrystalline structure. The average crystallite size of ZIRO0, ZIRO2, and ZIRO6 thin films was 38 nm, 51 nm, and 43 nm, respectively. Structural parameters (lattice parameters, the volume of unit cell, atomic packing fraction, bond length, and dislocation density) of ZnO and Ir substituted ZnO thin films were determined. The unit cell's volume increased from 47.52 Å3 to 54.50 Å3 when the Ir dopant level became 6%. Moreover, the atomic packing fraction value also raised from 75.3% to 80.5% by increasing Ir doping. The morphological characteristics of the samples were studied by field emission scanning electron microscopy (FESEM). It is seen from those images that the diameters of the grains on the surface of the undoped ZnO thin film range roughly from 30 to 45 nm. On the other hand, the diameters of the grain sizes of the 2% Ir and 6% Ir doped ZnO thin films vary between 45 nm and 75 nm. The thickness of ZnO and Ir doped ZnO thin films was obtained from a cross-section of the FESEM image. The thickness value of undoped, 2% Ir doped, and 6% Ir doped ZnO thin films was found to be ∼512 nm, 350 nm, and 393.4 nm, respectively. X-ray photoelectron spectroscopy studies revealed the valence states of Zn and Ir as 2+ and 4+, respectively. The optical band gap of the thin films was determined by diffuse reflection spectroscopy. The optical band gap value of the studied thin films was determined using both Tauc's plots and Kubelka-Munk plots. While Tauc's plots showed that the optical bandgap of ZnO decreased from 3.28 eV to 3.21 eV with Ir substitution, Kubelka-Munk plots demonstrated that the optical bandgap of ZnO declined from 3.26 eV to 3.11 eV. In addition, diffuse reflectance spectroscopy was used to investigate various optical characteristics, including transmittance, reflectance, refractive index, extinction coefficient, and the real and imaginary components of the dielectric constant, as well as conductivity. All the films exhibit high transmittance in the visible region. The ZnO sample has the highest transmittance value (∼90%) compared to the Ir doped ZnO thin films. It was found that optical dielectric and conductivity values increased with Ir doping. Urbach energies of the fabricated thin films were deduced, and it was seen that Ir incorporation boosted Urbach energies from 106 meV to 689 meV.