Growth and characterization of Mn-doped ZnO thin films

Zinc Oxide (ZnO) is a very interesting material due to its wide bandgap (3.37eV) suitable to transparent optoelectronic devices. Manganese (Mn) incorporation in wide bandgap semiconductors is a current topic due to new magnetic and optical properties. Many methods can be applied to obtain Mn-doped ZnO. This work shows an investigation of ZnO thin films doped with Mn at different concentrations (up to 10%) as well two reference-samples: Mn3O4 and ZnO. The samples were grown by Spray Pyrolysis on top of glass substrates using as precursors zinc acetate dihydrate and manganese acetate tetrahydrate in aqueous solution with 10-2 molarity. The system was characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Optical Transmittance and Absorbance. ZnO and Mn3O4 were identified by XRD as wurtzite phase (grown preferentially along the (002) direction) and hausmannite phase (grown preferentially along the (121) direction), respectively. The crystallite size in the sample of ZnO was approximately 13 nm. This size decreased a quarter from 3% Mn-doping with no evidence of secondary Mn-related phases. There was no significant change in the optical gap value up to 7% Mn. Half of the samples was subjected to an annealing at 500°C during one hour. XRD analysis indicated a phase transition from Mn3O4 to α-Mn2O3 in the bixbiite phase without a preferential growth direction, and a crystallite size around 25 nm. The images obtained by SEM for as-grown and annealed samples showed Mn segregation after thermal treatment. The bandgap decreased above 1% Mn inserted in the lattice, which corroborates the Mn segregation for the annealed samples. Our results indicate that the Spray Pyrolysis method is a convenient route to obtain well diluted ZnMnO thin films.