Structural And Luminescent Properties Of Rare Earths Doped Zno Thin Films

Zinc oxide?ZnO?is a?-?semiconductor with a direct and wide band gap.The band gap is about 3.37 eV,the exciton binding energy is 60 meV,and the thermal ionization energy about 26 meV at room temperature.ZnO nanostructures have been widely applied in the fields of photo-catalysis,solar cells,sensors and photoelectric detectors.Since 1997 people found that the ultraviolet stimulated emissions of ZnO microcrystalline films at room temperature,the near band edge emission from ZnO nanostructures has attracted extensive attentions.In recent years,people discovered the luminescence properties of ZnO thin films can be improved remarkably through doping appropriate concentration of rare earth elements.The main work of this paper focuses on the structural and luminescent properties of Ce and Yb singly doped or Ce/Yb co-doped ZnO microcrystalline films,especially the immensely enhancement of the near band emission of the nano ZnO thin films.We have prepared the sample films by using magnetron sputtering.Then these films were thermally treated in a high-temperature tubular furnace.The films compositions and the structural evolution after the annealing were characterized by using Rutherford backscattering spectrometry?RBS?,X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,transmission electron microscopy?TEM?and scanning electron microscopy.The photoluminescence?PL?spectra,PL excitation?PLE?spectra and time-resolved PL spectra?PL decay?,were used to investigate the optical properties of the films.For Ce singly doped ZnO films?ZnO:Ce with Ce concentration of about 0.1 at.%?,after 1100^oC annealing,the films are highly c-axis oriented and the intensity of the near band edge emission?with peak position at 3.34 eV and width of¹13 meV?is about two orders of magnitude stronger than that of the un-doped films.The strong UV emission could be due to the improvement of the film quality and the decrease of non-radiative recombination centers.Besides the effects of high temperature annealing,the doping of Ce can also be beneficial for the self-texturing of the films.For Yb singly doped ZnO films?ZnO:Yb with Yb concentration about 5.0 at.%?,after the identical 1100?annealing the XRD patterns show that ZnO:Yb films have polycrystalline structure.SEM shows that Yb₂O₃ have precipitated on the surface of the films and TEM observation confirms the formation of Zn and Yb silicates within the films.And the UV emissions are also hundreds times stronger than that of the un-doped films,but the near-infrared emission of Yb³⁺is very weak.The strong UV enhancement could be due to the interfaces of the ZnO nanoparticles been passivated,where many of the recombination centers that are normally on a ZnO surface may have being effectively removed by the silicate reaction.The Yb doping has also effect the film crystal quality.For?Ce,Yb?co-doped ZnO films?ZnO:Ce/Yb?,we have prepared samples with different Ce and Yb concentrations.The concentration of Ce in the films is 0.09,0.14and 0.31at.%respectively,while the concentration of Yb is 0.24,0.55 and 0.88 at.%.Our initial aim of this work was to observe Ce/Yb down-conversion emission in ZnO film matrix.But we found that the near-infrared emission of Yb³⁺of the samples were very weak even invisible when the annealing temperature exceed 1000^oC.The energy transfer efficiency from ZnO or Ce³⁺to Yb³⁺is very low.However,the near band edge emissions are hundreds times stronger than that of the un-doped films after the high temperature annealing.XRD analysis indicated that Zn and Yb silicate have also formed when the anneal temperatures was 1000^oC.SEM results also indicated that Ce/Yb co-doping and annealing have significant effect on the surface morphology and crystallinity of the films.In conclusion,the structural and PL properties of Ce/Yb singly doped or co-doped ZnO films have been studied.The UV emissions from the doped sample films are two orders of magnitude higher than the undoped sample films,and initiatory assumption is given.This observation may have potential application in the field of short-wave semiconductor photoelectric devices.