Effect of annealing temperature on the structure and dielectric characterization of ITO thin films on a boro-float substrate prepared by radio frequency sputtering

Abstract

The effect of annealing temperature (T_a= 200, 250, and 300 °C) on the structural properties, ac conductivity, and complex dielectric constants (${\epsilon }^{{\prime }}$ and ${\epsilon }^{{\prime }{\prime }}$) of indium-doped tin oxide (ITO) thin films (~ 90 nm thick)/0.5 mm boro-float substrates (BFS) synthesized by radio frequency (RF) sputtering is investigated. The X-ray diffraction (XRD) examination demonstrated that indium was successfully substituted with tin atoms to form ITO films and the crystallite size for the cubic phase, as well as particle size, were impacted by T_a. The real part of complex dielectric constants (${\epsilon }^{{\prime }}$) was significantly reduced for all ITO/BFS from the range of 2.7 × 10⁴–5.1 × 10⁴ to 5.3–19 as the frequency (f) was increased to 0.25 Hz, while it remained constant for further increases in f. The value of ${\epsilon }^{{\prime }}$ for the as-prepared ITO/BFS was increased as T_a increased up to 250 °C, then was decreased at T_a=300 °C. A similar finding was detected for the loss factor with no observation of any relaxation peaks. The Q-factor was increased for all ITO/BFS as f increased to 100 Hz and then was reduced with increasing f up to 20 MHz, while steadily increasing with T_a. The deduced frequency exponent is greater than 0.5 for the ITO/BFS, indicating their electronic conduction nature. The density of the localized states and hopping frequency of the ITO/BFS were increased by annealing at 200 °C, meanwhile was decreased for T_a = 300 °C. The binding energy was decreased from 0.647 eV for the as-prepared ITO/BFS to 0.518 eV by annealing at 200 °C, meanwhile was increased to 0.74 and 0.863 eV for T_a equals 250, and 300 °C, respectively. The Cole-Cole plots revealed a single semicircular arc for all films, and their corresponding equivalent circuit was analyzed. The equivalent bulk resistance was gradually decreased by annealing in the range of 200–300 °C, whereas the equivalent capacitance was increased. The resistance of grains and resistance of grain boundaries of the as-prepared ITO/BFS was gradually decreased by increasing T_a to 250 °C, while it was increased for T_a = 300 °C. These outcomes recommended the RF sputtered ITO/BFS for high-frequency devices, integrated circuits, and supercapacitors.