Exploring thickness-dependent physical properties in Cs-doped ZnO thin films

In this study, we investigate the influence of film thickness on the properties of cesium (Cs)-doped zinc oxide (ZnO) thin films fabricated via spray pyrolysis. By varying the thickness from 305 nanometers (nm) to 677 nm, we consistently formed a hexagonal ZnO phase across all samples. The crystallite size varied from 51 nm to 58.13 nm, indicating a nuanced relationship between thickness and structural properties. Field emission scanning electron microscopy (FE-SEM) revealed diverse grain size distributions, which are critical for optimizing film performance. From optical measurements, the band gap values were all around 3.26 ± 0.01 electronvolts (eV), while the refractive index decreased significantly from 2.3 to 1.7 with increasing thickness and fixed Cs nominal content at 5 wt percent (wt%). Employing the Spitzer-Fan model, we observed a consistent reduction in high-frequency dielectric constant (ε_∞) from 3.3 to 2.89 as thickness increased, highlighting changes in electronic properties. Importantly, electrical conductivity (σ) exhibited a remarkable increase from 0.0053 ohm⁻¹ meter⁻¹ to 0.044 ohm⁻¹ meter⁻¹ with increasing thickness, suggesting enhanced charge transport properties crucial for optoelectronic applications. Correspondingly, activation energy (E_a) was found to vary between 0.25 eV and 0.415 eV, indicating significant thermal sensitivity across the studied thickness range. Furthermore, ultraviolet (UV) performance under 365 nm illumination showed substantial variations in rise and decay times correlated with film thickness; minimum rise times were recorded at 5.08 s for the 305 nm film, while minimum decay times reached 68.86 s for the 400 nm film. These findings underscore the potential for tailoring ZnO thin films to optimize performance in optoelectronic applications, demonstrating a strong initial impact through quantifiable enhancements in material properties.