Investigation On Optical Properties And Defects Of V-doped ZnO Thin Films Prepared By RF Magnetron Sputtering

ZnO is a kind of II-VI semiconducting materials with wide band gap?3.37 eV?and high exciton binding energy?60 meV?,and has many excellent characteristics,such as transparent conductive,piezoelectric,dilute magnetic and voltage-sensitive characteristics after properly doping.Consequently,it is one of the most important candidate materials for flat panel displays,sensors,spintronics and short-wave luminescent devices,and then it becomes the hot pot of current research.Using V₂O₅ doped Zn O ceramics as target,V-doped ZnO?ZnO:V?films were deposited on glass substrates by RF magnetron sputtering,and the influence of substrate temperatures,O₂/?O₂+Ar?ratio and vanadium contents on the microstructures,defects and optical properties of ZnO:V films was systematically studied.The results are as follows:The Zn O:V films are nanometer granular films with wurtzite lattice structure and preferred[002]orientation vertical to the surface of substrate.The root-mean-square?RMS?surface roughness of all films is less than 10 nm and no vanadium compound is detected by XRD.With increasing substrate temperature,the crystallinity of Zn O:V films has been gradually improved,but some abnormal changes happen due to the components seriously deviated from the stoichiometry at some particular temperatures?150°C and 300°C?.The crystallinity of Zn O:V films is deteriorated with increasing the content of vanadium while those deposited at the O₂/?O₂+Ar?ratio of 0%and 50%have relatively good crystallinity.Zinc exists in Zn O:V films with the standard state of Zn²⁺,while V⁴⁺and V⁵⁺ions coexisted in the ZnO:V films.O exists in Zn O:V films with two states,that is,normal state and that in oxygen-deficient regions.ZnO:V films exhibit an average transmittance over 84%in the visible range?400-800 nm?.The band gap of ZnO:V films was lower than that of pure ZnO due to the V₂O₅doping,while the band gap of ZnO-1mol%V₂O₅ films with higher V content was higher than that of ZnO-0.25mol%V₂O₅ films.The band gap decreases first and then increases with increasing substrate temperature.As the O₂/?O₂+Ar?ratio increases,the band gap increases first and then decreases.The PL spectra indicate that several emissions with various wavelengths are obtained in ZnO:V films,corresponding to different luminescence mechanisms,that is,the UV emission?382-387 nm?associated with free excitons recombination,the violet emission?395-414 nm?attributed to the interface traps in grain boundaries,the blue emission?451 nm?derived from VO and Zni,the yellow emission?561-571 nm?and the NIR emission?765-779 nm?which both attributed to V_o⁺⁺defects.The defects in ZnO:V films are the complex of V_O and Zn_i.The contents of V_O and Zn_i change with the O₂/?O₂+Ar?ratios and substrate temperatures.However,a single defect,such as V_O or Zn_i,and O_i does not form in ZnO:V films.More defects exit in ZnO:V films deposited under the condition of oxygen-deficient?O₂/?O₂+Ar?=0%?and seriously oxygen-rich?O₂/?O₂+Ar?=75%,87.5%?ambience.Under proper O₂/?O₂+Ar?ratio?O₂/?O₂+Ar?=12.5%-62.5%?,defects content gradually decrease with increasing O₂/?O₂+Ar?ratio.As the substrate temperature increases to 300°C and above,defects decrease markedly.In our work,the Optimum deposition conditions for Zn O:V film are doping vanadium of 0.25mol%,substrate temperature of 350°C and O₂/?O₂+Ar?=50%,respectively.