Preparation And X-Ray Photoelectron Spectroscopy Studies Of ZnGa₂O₄ Powders And Films

ZnGa₂O₄ is a kind of AB₂O₄ spinel oxide luminescent material.ZnGa₂O₄ has shown broad application prospects in vacuum fluorescent display,field emission display and cathodoluminescence because of its good stability,low excitation voltage and high luminous purity,and it has attracted much attention in recent years.In the structure of AB₂O₄ spinel,oxygen ions arrange in close packed cubic structure,forming tetrahedral and octahedral vacancies.A and B cations occupy these two vacancies.The distribution of cations in the tetrahedral and octahedral sites of spinel structure is influenced by many factors,such as material composition,preparation method,grain size,cation radius,crystal field stabilization energies?CFSE?and so on.The change of cation distribution in the microstructure of the material will lead to lattice distortion,thus affecting the luminescence properties of the material.In the thesis,ZnGa₂O₄ luminescent powder and thin film materials were prepared,the distribution of cations in the structure of ZnGa₂O₄,and the influence of material composition and heat treatment temperature on the cation distribution have been studied using X-ray photoelectron spectroscopy?XPS?.The relationship between cation distribution and luminescence properties was discussed.In addition,the composition of ZnGa₂O₄ film and its change with depth were studied by XPS combined with Ar+ ion etching technique,and the interface structure between the film and the substrate was analyzed and discussed.Main contents and results are as follows:?1?Zn,Ga₂O₃+x powderss with different Zn/Ga ratio were prepared by sol-gel method,and the particle size was about 10?30 nm.The samples were characterized by powder X-ray diffraction?XRD?,transmission electron microscopy?TEM?and XPS.The results showed that the formation of solid solution of ZnGa₂O₄ and ?-Ga₂O₃ when x<1.0,and the grain size and lattice constant increased with the increase of x.Zn2+and Ga3+ cations occupied tetrahedral and octahedral sites in all the samples and the inversion parameters?2 times the fraction of Ga3+ ions in tetrahedral sites?of samples increased with the increase of x.The luminescence spectra indicated that the samples exhibited a strong emission peak at-700 nm,which was attributed to the oxygen vacancies,and electron paramagnetic resonance?EPR?spectra confirmed the existence of oxygen vacancies in the samples.?2?Stoichiometric ZnGa₂O₄ spinel powders were prepared by two different methods:sol-gel?SG?method and solid state reaction?SSR?method.The effects of synethesis method and annealing temperature on morphology,cation distribution and luminescence properties were studied,and the relationship between luminescence properties and cation distribution were discussed.The samples prepared by SG method were nanoparticles with irregular shape,and the particle size was in the range of 20?60 nm.The samples prepared by SSR method were rodlike structure with diameter of 600?700 nm and length of 1.7?3.2 pm.Zn2+ and Ga3+ cations occupied the tetrahedral and octahedral sites for all the samples.For luminescent powders prepared by SG,the inversion parameter was 0.30?0.23 and the value decreased with the increase of annealing temperature.For the powders prepared by SSR,the inversion parameters was relatively small,about 0.18-0.13.The luminescence spectra of the samples prepared by SSR were determined.The samples exhibited a broad emission band at 350?600 nm when annealed at 900?.The emission at 350-450 nm became very weak and an obvious emission peak at about 500 nm appeared when the annealing temperature increased to 1100?.We think that the change of luminescence properties with temperature is related to the change of coordination environment of Ga3+ ions.The broad emission band at 350?600 nm can be attributed to the overlap of[GaO4]and[GaO6]self-activation centers and the emission peak at 500 nm is derived from the energy level transition of Ga3+ ions in the octahedral position.When the annealing temperature was 1100?,the Ga3+ ions in the tetrahedral position relatively less?about 6.5%?,so only the emission peak at 500 nm was observed.?3?ZnGa₂O₄ spinel films were prepared by two different methods:sol-gel spin coating method and pulsed laser deposition?PLD?,the surface morphology,thickness and luminescence properties of ZnGa₂O₄ films were characterized.The depth profile analysis of the films was carried out using XPS combined with Ar+ ion etching technology.We found that the surface grain size,compactness and luminescence of films were related to the molar ratio of PVP and cation for the films prepared by sol-gel method.When the ratio was about 1.0,the films have smaller grain size,better compactness and stronger emission.In addition,the thickness of films increased with the increase of PVP concentration.For the films prepared by PLD,the thickness of films increased from 250 nm to 800 nm when the growth time increased from 1 h to 3 h.The content of component elements?Zn,Ga,O?was very low before etching,and the surface was covered by contaminated carbon.The content of Zn,Ga,O elements increased first and then kept their stoichiometry unchanged with the increase of etching time.We think this change was related to the surface contamination layer.In addition,we found that with the increase of etching time,the molar ratio of Ga/Zn gradually decreased from 2.0 to?1.5,while the O/Si ratio has remained at around 1.5 at the interface.We analyzed the possible structure of the interface between ZnGa₂O₄ film and substrate,and speculated that the diffusion of Si occurred at the interface and Zn2SiO4 phase formed.The interface consisted of a mixture of ZnGa₂O₄ and Zn2SiO4.