Preparation And Physical Properties Of ZnO Thin Films Co-doped With Co And Cu

As a wide bandgap (3.4 eV) II-VI compound semiconductor, zinc oxide (ZnO) possesses a stable wurtzite structure with lattice spacing a = 0.3249 nm and c = 0.5206 nm. So far, ZnO has attracted intensive research effort because of its unique properties and versatile applications in the fields such as transparent electronics, ultraviolet (UV) light emitters, chemical sensors, piezoelectric devices, surface acoustic wave devices, solar cells and spin electronics. As an important candidate of diluted magnetic materials, TM (transition metal)-doped ZnO have been predicted to possess high Curie temperature up to room temperature, which provides a promising future for the development of spintronic devices. So, the investigation on the optical and magnetic properties of ZnO becomes indispensable at the present.In this thesis, we report on the systematic studies of Co-, Cu-doped and (Co, Cu) co-doped ZnO-based diluted magnetic semiconductor thin films by means of Sol-Gel method. Based on a large number of experiments and theoretical calculations, we analyzed the optical and magnetic properties of all the samples. The main work is as follows:Firstly, according to stoichiometric ratio of Zn,Co,Cu, we mixed the appropriate amount of zinc nitrate Zn (NO₃)₂·6H₂O, cupric acetate Cu (CH₃COO)₂·H₂O, cobalt acetate tetra hydrate Co (CH₃COO)₂·4H₂O in a mixture of 40mL anhydrous alcohol. Co, Cu single doped and co-doped ZnO thin films were fabricated on the glass substrate. The influence of Co and Cu doping on surface morphologies of ZnO films have been investigated. X-ray diffraction (XRD) reveals that all the ZnO thin film samples were well oriented and the grain size of Cu-doped ZnO film is bigger than others. Strong blue double emission and weak green emission were observed in photoluminescent spectrum of all samples at room temperature and both long wavelength blue peak and green peak could be modulated by doping. The blue double peaks are caused by transition of electrons from the bottom of the conduction band to zinc vacancy or zinc interstitial to the top of valence band, however, the green peak is highly relevant to oxygen slip formed by doping. The ferromagnetic properties of (Co, Cu) doped ZnO thin films were detected. In combination with the above structural and luminescent results, the ferromagnetism of our (Co, Cu) doped ZnO thin films are likely to originate from the oxygen-related defects.Furthermore, the effect of Co-doping on the Zn_0.98Cu_0.02O film was investigated. Although uniform surface morphology was observed, the amount and particle size decrease with the increase of Co content. The sample exhibits best crystal quality when the Co concentration is 6%. The effects of the Co content on the photoluminescence of TM-doped ZnO and defects such as O vacant, Zn vacant, and Zn interstitial position in the TM-doped ZnO films were discussed. Our results indicate that the Co doping can be used an effective method to control the blue and green emission in the Cu-doped ZnO film.