Optical And Gas Sensitive Properties Of Rare Earth Elements(La、Ce、Nd) Doped ZnO Thin Films

As the third generation multifunctional wide band gap semiconductor, ZnO hasdrawn much attention in the past decades. In this paper, the rare earth elements （La,Ce, Nd）-doped ZnO thin films were successfully prepared by RF magnetronsputtering. X-ray diffraction （XRD）, X-ray photoelectron spectroscopy （XPS）,atomic force microscope （AFM）, UV-visible absorption spectrophotometer,fluorescence spectrometer and gas sensing tester were used to analyze the influenceof doping content, nuclear charge number and oblique angle on the phase structure,surface morphology and properties （optical properties, gas sensitive properties andphotocatalytic properties）.It was found that with La doping content increased, the film surface roughnessincreased gradually. The band gap of the films enhanced because when La centred atthe substitutional sites of Zn, the interacation between p and d orbital electronsdecreased which induce the valence band maximum moved to lower energy. Therewas no difference between the undoped ZnO thin film and1%La、2%La doped ZnOthin films. However, the UV emission peak blueshifted when the doping content ofLa was3%. With the increase of La doping, the sensitivity to HCHO is graduallyimproved and the photocatalytic activity first increased and then decreased.The properties of ZnO thin films varied when doping with different rare earthelements. The gas sensing performance and photocatalytic activity of Nd dopedZnO thin film was best at low temperature （T<320°C） than the other two which wasattributed to Nd doped ZnO thin film surface roughness was greater than that of Laor Ce-doped ZnO thin films. When at high temperature（T>320°C）, the surface ofZnO thin films tended to be even, the gas sensitive property of Ce-doped ZnO thinfilm was better that La、Ce-doped ZnO thin films as Ce⁴⁺could provide moreelectrons. Under UV irradiation, Ce⁴⁺could easily become Ce3+by trapping anelectron, the photocatalytic activity was improved by decrease the recombination ofelectron-hole pair.The properties of the ZnO thin films varied with different oblique angles. The UVemission intensity of both ZnO and La doped ZnO thin films was improved byapproximately5times than the films deposited with other oblique angles becausethe best crystallinity and quality when the substrate oblique angle was90°. Thegas sensing property and photocatalytic activity of ZnO thin films were best whenthe oblique angle was60°, which was attributed to the shadow effect caused by the oblique inslands at the surface of ZnO thin films. However, the best gassensitive and photocatalytic properties appeared when the substrate parallelled tothe target in La-doped thin films. This is due to the quality of the La atom waslarger than both Zn atom and O atom, which made it easy to form secondary-defecton the substrate hence increased the number of surface island as well as the surfaceroughness of thin films.