Microstructural and optical properties of high-quality Mg–Zn oxide thin films

Abstract

Abstract The microstructural and optical properties of Mg–Zn oxide thin films deposited by aerosol assisted chemical vapor deposition onto fused silica substrates were studied as a function of the Mg concentration in the precursor solution, which was varied from 0 to 100\xa0mol %. The thickness of all the samples was around 150\xa0±\xa020\xa0nm. Scanning electron microscopy and grazing incidence x-ray diffraction were used for the analysis of the morphology, composition, and crystalline structure. Outcomes showed a gradual evolution of the microstructural and optical properties of the films as the Mg content increased. The systematic increase of the lattice parameter a, and the decrease of parameter c were observed with the increase of Mg concentration, nevertheless, the unit cell volume remained nearly constant, around 0.0471\xa0±\xa00.0007 nm3. Energy dispersive x-ray microanalysis results indicated that Mg /(Zn\xa0+\xa0Mg) atomic ratio (x) up to 0.59 was found for a single-phase hexagonal MgxZn1-xO film. This value is higher than most of the reported Mg content in wurtzite ZnO. At higher Mg loadings (>70\xa0mol % in solution), the segregation of periclase MgO phases occurred. X-ray photoelectron spectroscopy confirmed the composition of selected Mg doped ZnO films and the substitution of Zn+2 by Mg+2 atoms. The Williamson-Hall analysis of diffracted peaks was employed to determine the micro-strain in the films, it was found ∈ \xa0=\xa00.2\xa0±\xa00.1 % with a slight tendency to decrease ∈ as x increases. For single-phase hexagonal MgxZn1-xO films, the computation of the dispersion relations of the optical constants was achieved using direct transmittance and near-normal absolute reflectance. For this objective, TFcalc software was used to fix the Sellmeier dispersion relation parameters by fitting the modeled transmittance and reflectance spectra of the stack film-substrate with corresponding experimental spectra. A detailed analysis of the optical band gap energy and its correlation with the microstructural characteristics of the films was performed, in particular with lattice parameters. The modification of optical band gap energy and lattice parameters with the inclusion of Mg in the wurtzite structure can be explained because the MgO4 unit has a compressed tetrahedral geometry with a much shorter height than those of ZnO4 tetrahedra, and due to the change in electronegativities and ionic radius between Mg and Zn, the bonding character becomes more ionic, and the increased localization of Zn 3d and O 2p orbitals. The optical band gap energy was tuned by changing the Mg content, from about 3.3\xa0eV of undoped ZnO up to 4.3\xa0eV of hexagonal MgxZn1−xO (x\xa0~\xa00.59). Moss relation was used to analyze the relationship of optical band gap energy and refractive index.