Thickness-Driven Modulation of Structural, Optical, and UV Photoconductive Properties of ZnO Thin Films for Optoelectronic Applications

Numerous researchers agree that zinc oxide (ZnO) is a highly prospective metal oxide semiconductor for use in gas sensing and optoelectronics. In this work, RF magnetron sputtering was used to deposit ZnO thin films with thicknesses ranging from 40 nm to 200 nm on soda-lime glass substrates. Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), photoluminescence (PL), optical spectroscopy, and current–voltage (I–V) measurements under both dark and ultraviolet (UV) illumination conditions were used to systematically examine the impact of film thickness on the structural, morphological, optical, and electrical characteristics. All films had a highly preferred (002) orientation, which indicates c-axis-oriented growth, according to XRD analysis. As thickness increased, crystallite size increased from about 15 nm to about 35 nm. In addition to the beginning of morphological coarsening at 200 nm, FESEM observations revealed a gradual evolution from fine, densely packed grains at lower thickness to larger, coalesced grains at higher thickness. High transparency (> 80%) in the visible region was found in optical studies, and as thickness increased, transmittance gradually decreased, and the absorption edge shifted to the red. Microstructural evolution and potential strain relaxation effects are responsible for the optical band gap’s decrease from 3.41 eV to 3.29 eV. The photoluminescence spectra showed a broad deep-level emission in the visible region and near-band-edge emission in the UV region. While thinner and intermediate thicknesses showed strong near-band-edge emission, thicker films (> 120 nm) showed broadening and decreased intensity of this peak along with increased deep-level emission, indicating a greater contribution from recombination processes related to defects. Ohmic contact behavior and the existence of photoconductive response were indicated by electrical measurements that showed linear and symmetric I-V characteristics under both dark and UV illumination. Up to an intermediate range, the photocurrent increased with thickness; at higher thicknesses, the contrast between illuminated and dark currents decreased. Overall, the results show how ZnO thin-film structural and optoelectronic properties are influenced by film thickness, with intermediate thicknesses providing a balance between crystallinity, optical transparency, and photoconductive response.