Effect of iron oxidation state and annealing atmosphere on functional properties of zinc oxide thin films
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The ability to produce high-quality single-phase diluted magnetic semiconductors (DMS) is the driving factor to study DMS for spintronic applications. ZnO is a wide band gap semiconductor of the II-VI semiconductor group and exhibits several favorable properties. The effective incorporation of dopant species into ZnO host structure should induce changes in its physical and chemical properties enabling the establishment of novel functional properties. In the case of doping with transition metal ions, such as Fe ions, the subsequent exchange interaction from magnetic spins should induce a ferromagnetic behavior, but we found a paramagnetic response in our Fe doped ZnO thin films. The present research addresses the study of the effect of the oxidation state of Fe species and the influence of the annealing atmosphere on the structural and functional properties of nanocrystalline ZnO thin films. We report structural, optical and magnetic properties of annealing temperature of paramagnetic Fe doped ZnO thin films, in a range of 500°C to 700°C. Fe-doped ZnO films samples were synthetized via a sol-gel approach. The produced Zn (1−x) FexO nanoparticles at [x = 0, 0.05, and 0.07], films were grown on silicon wafer substrates. The structural characterization was performed by X-Ray Diffraction (XRD) examination, Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) analysis to verify the formation of the ZnO host structure after annealing of the precursor phases. Incorporating Fe ions into ZnO did not affect the crystal structure of the wurtzite host. The variation of the average crystallite size of Fe [0-7 at. %] ZnO films annealed in nitrogen and air, in a range of 500°C to 700°C, were negligible and averaged between ZnO thin films prepared, have a hexagonal wurtzite structure based on XRD evidence, with crystals preferentially oriented along the c-axis. After 5 at% Fe is doped, the crystalline quality and the preferential orientation of ZnO thin film was improved. The results show that Fe atoms, were substituting Zn ions successfully due to the small ionic radius of Fe ions, compared to that of a Zn ion, the crystal size decreased with an increasing dopant concentration. There is no evidence of a secondary phase (Fe, FeO, Fe2O3 or ZnFe2O4). Hence, XRD and Photoluminescence spectroscopy (PL) measurements confirmed an enhanced crystallinity of the ZnO host, with an average crystallite size of 15 nm, for films annealed over 500°C in air and under a nitrogen atmosphere. The optical properties of thin films were determined by Infrared, Ultraviolet–visible spectroscopy (UV-vis) and (PL) techniques. The results confirmed that the crystallinity of the ZnO is deteriorated due to the Fe-doping. Photoluminescence (PL) measurements corroborated the formation of high-quality ZnO host structure, in films annealed in air and controlled nitrogen atmospheres. Fe-incorporation hardly influences the transmittance in the visible range, however the optical band gaps of ZnO thin films gradually increase with the improved Fe-doping concentration. The PL spectrum displays that all of the samples have an ultraviolet emission peak centered at 388 nm. At, x = 0 at% thin film has the strongest ultraviolet emission peak, which decreased with increasing dopant percentage. As a result of the quenching by concentration effect attributed to the formation of trapping states by the dopant species. First principle calculations indicate that the oxidation states of iron are Iron (II) and Iron (III) with a zinc vacancy or an interstitial oxygen anion, respectively. The calculations predict that the exchange interaction between transition metal (TM) ions can switch from the antiferromagnetic (AFM) coupling into its quasi-degenerate ferromagnetic coupling by external perturbations. This is further supported and explained the observed paramagnetic behavior at magnetic measurements. Magnetic measurements revealed a paramagnetic signal comes from Fe ions doped ZnO crystals. Furthermore, introducing Fe into ZnO induces a magnetic moment without any distortion in the geometrical symmetry. M-H measurements evidenced a paramagnetic behavior at room temperature in films that were dependent on the type and amount of the dopant species.