Effect of thickness on properties of La-doped Zinc Blende/Wurtzite ZnO thin films prepared by spray pyrolysis

Thin films of ZnO doped with 3 wt% Lanthanum were deposited on glass substrates using spray pyrolysis for varying thicknesses. The impact of thickness on the physical properties of the films was investigated using various techniques. XRD results revealed the formation of a mixture of two phases, cubic Zinc Blende (ZB) and hexagonal Wurtzite (WU), with their ratios changing based on film thickness. Crystallite size measurements indicated that the cubic phase granules remained stable at 14.6 nm up to a thickness of 719 nm, decreasing to 14 nm at 846 nm, while the crystallite size of the hexagonal phase increased to 18 nm at 846 nm. FE-SEM images showed changes in surface morphology with increasing thickness, and EDX analysis demonstrated a consistent La/Zn ratio across all film thicknesses. The refractive index exhibited a shift from normal to abnormal dispersion when the thickness exceeded 719 nm, and the high-frequency dielectric constant decreased from 3.2 to 2.91 as the thickness increased. Additionally, optical band gaps ranged from a minimum of 3.19 eV at 719 nm thickness to a maximum of 3.23 eV at 846 nm thickness. The study examined the effectiveness of the deposited films in blocking UV light with a wavelength of 365 nm. It was discovered that the decay time significantly decreased to 143 s at a thickness of 661 nm, compared to longer times observed at other thicknesses.