Studies of optical properties of ZnO:MgO thin films fabricated by sputtering from home-made stable oversize targets

Abstract

In this work, ZnO:MgO targets with different MgO contents were prepared by solid state-method. The results show that the target with 2 wt% MgO content presents the best quality. This target presents the highest crystallite size and lowest lattice strain values in comparison with the other concentrations. Its highest density proves that it is the most appropriate target for making Mg doped ZnO (MZO) thin films with good quality. The effects of working pressure on structural, morphological and optical properties of the sputtered thin films were studied. XRD analysis of MZO thin films showed wurtzite structure with (002) reflection as preferred orientation. In addition, the samples exhibited a surface morphology dominated by spherical grains, RMS roughness ranged from 1.9 to 5.1 nm, and an electron-LO phonon coupling from Resonant Raman spectra, much stronger with the working pressure. The average transmittance values were close to 92 % in the visible range and the optical band gap decreased from 3.42 to 3.32 eV with the working pressure. Room temperature photoluminescence (PL) spectra showed a sharp ultraviolet emission centered at 3.17 eV and a broad green emission band at 2.25 eV. All these enhanced properties of MZO thin films were linked to the good crystalline quality, high density and compactness of the prepared ZnO:MgO (2 wt%) target. These transparent conductive films presented good performances for optoelectronic device applications.