Thickness-Dependent Structural, Morphological, Optical, and Electrical Properties of Sol-Gel Derived Aluminum-Doped Zinc Oxide Thin Films

Aluminum-doped zinc oxide (AZO) thin films were successfully deposited onto glass substrates via the sol-gel dip-coating technique. The impact of film thickness on their structural, morphological, optical, and electrical properties was systematically examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), Hall-effect measurements, UV–Visible spectroscopy, and photoluminescence (PL) analysis. XRD results confirmed that all films crystallized in the hexagonal wurtzite phase, exhibiting a strong preferential orientation along the c-axis. An increase in film thickness led to enhanced crystallinity, as evidenced by the rising intensity of the (002) diffraction peak and the growth in crystallite size. SEM and AFM analyses revealed that both grain size and surface roughness increased with film thickness, reflecting the evolution of surface morphology. Electrical characterization showed that the thinnest film exhibited the lowest resistivity and highest carrier concentration and mobility, while thicker films displayed a decline in electrical performance. Optical measurements indicated high average transmittance (76.9–81.1%) across the visible spectrum, which gradually decreased with increasing thickness. Moreover, a slight redshift in the absorption edge was observed, corresponding to a reduction in the optical band gap. PL spectra revealed two main emission bands in the UV-blue and blue-green regions (375–550 nm). With increasing thickness, a noticeable decrease in PL intensity and a redshift of the UV emission band were observed, consistent with the narrowing of the optical band gap. These findings highlight the significant influence of thickness on the structural and optoelectronic performance of AZO thin films.