Influence of Mn doping on structural and optical properties of ZnO nano thin films synthesized by sol-gel technique

Undoped and Mn-doped ZnO samples with different percentages of Mn content (1, 5 and 10 at%) were synthesized by a dip-coating sol-gel method. We have studied the structural, chemical and optical properties of the samples by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-visible spectroscopy. The XRD spectra show that all the samples are hexagonal wurtzite structures. We note that doping favors c-axis orientation along (002) planes. Up to 5 at% of Mn doping level, the c-axis lattice parameter shifts towards higher values with the increase of manganese content in the films. The expansion of the lattice constant of ZnO-Mn indicates that Mn is really doped into the ZnO. The SEM investigations of all samples revealed that the crystallites are of nanometer size. The surface quality of the ZnO-Mn film increases with Mn doping but no significant change of the grain size is observed from SEM images. The transmittance spectra show that the transparency of all the samples is greater than 85 %. We note, also, that a small doping (1 %) lowered the refractive index while the thickness of the layers and the gap increase. However, on raising the proportion of Mn beyond 5 %, practically the same values of index and gap as pure ZnO are found.