A comparative study of structural, surface morphology and optical properties of Na and Mg codoped ZnO nanocrystalline thin films prepared using sol–gel spin coating technique

• This codoped film is new material for Raman analysis.
• Wrinkle structures identified from SEM & AFM images.
• Intrinsic interband Frohlich contribution is responsible for broadening of LO phonons.
• Sodium plays an important role in controlling the NBE shift in the codoped ZnO films.
• The optical conductivity is high in the UV region when compared to visible region.

Nanocrystals of Na-doped and Na, Mg codoped ZnO thin films were prepared by sol–gel spin coating method on glass substrate at different annealing temperatures. The deposited films were characterized by X-ray diffraction (XRD), Micro Raman, photoluminescence (PL) and UV–Visible measurements. The codoped films had hexagonal wurtzite structure and there were no peaks belonging to Na and Mg or its oxides observed from XRD. Surface morphology of the films was scrutinized by scanning electron microscope (SEM) and atomic force microscope (AFM) showing wrinkle structures that appeared on the film surface. Elemental compositions of the films were analysed using energy dispersive X-ray (EDX) spectra. The LO phonon modes were observed up to the fifth order and their enhancement was observed by the presence of Frohlich contribution. From μ-PL, the strong near band edge emission observed at 3.24 eV was blue shifted due to the band filling of free carriers. Fundamental absorption edge was observed at 370 nm and the optical band gap was decreased with respect to ZnO. Reduction of Urbach energy among codopants is small indicating less disorder.