Effect of Na doping on microstructures, optical and electrical properties of ZnO thin films grown by sol-gel method

Abstract

Undoped ZnO film and Na doped ZnO films with different Na/Zn molar ratios were deposited on glass substrates by the sol-gel spin coating method. The X-ray diffraction (XRD) pattern showed that all the films exhibited a high c-axis orientation with a hexagonal wurtzite structure. The diffraction angle shifted significantly to the lower angle with sodium doping. X-ray photoelectron spectroscopy (XPS) analysis confirmed that Na was incorporated into the ZnO matrix by occupying Zn sites. Field emission scanning electron microscope (FESEM) and Atomic force microscope (AFM) analysis showed that the surfaces of the films consisted of homogeneously distributed dense grains. FESEM images also exhibited hexagonal-like shape nanoparticles. The UV–Vis–NIR spectroscopy exhibited very sharp near UV absorption edge. Hall effect measurement results indicated that the undoped ZnO film exhibited n-type conductivity, whereas the Na-doped ZnO films generally exhibited p-type conductivity and all of the films had low carrier concentration. Room temperature photoluminescence (PL) spectra showed an ultraviolet emission (380–384 nm) and a broad visible emission (450–700 nm).