First-principle Study Of Eu-doped α-SiAlON

In 1996, the Nichia Company developed white LED based on an InGaN chip, which can be used as lighting devices. White light LED has many advantages, such as, small size, low heat loss, long lifetime, and leading to a wide research all over the world.It is hard for one LED to emit white light due to the luminescent mechanism of semiconductor. Three fundamental ways to generat white light in white light LED are gererally used: (1) Combinate of three red, green, blue-LED chips, respectively, in accordance with the arrangement of the light emission intensity, but the technology are very complex to control. (2) Combination of a blue LED chip with a yellow phosphor. The emission from the blue LED was absorbed by the phosphor, which will reduce the ultraviolet radiation. (3) Combination of three color phosphors excited by one ultraviolet (UV) LED. This three-converter system can be improved in color rendering and directionality, compared to the white light LED composed of a yellow phosphor and blue emission LED. Recently, down-converting phosphors for application in white light LED attract much interest.To promote the development of white light LED, the phosopor is very important. In recent years, rare-earth doped nitridosilicate or oxonitridosilicate phosphors have attracted much attention, for example Ca-α-SiAlON: Eu²⁺, Sr2Si5N8:Eu²⁺. In these phosphors, a large splitting of 5d energy levels often appear due to the nephelauxetic effect and the relatively small electronegativity of N3- ion , making the redshift of the excitation and emission spectra, which make them as a potential candidate for the UV-LED or blue LED. The nitridosilicate or oxonitridosilicate phosphor also has the advantages of a kind of physicochemical properties.α-SiAlON is applied firstly as an oxonitridosilicate phosphor. It was found from the experimental results that in the M-α-SiAlON: Eu²⁺ (M= Li, Mg, Ca, Y), the optical property is much influenced by the M cations. This paper focuses on the reason of the phenomenon and tries to explain it from theoretical calculation. Considering that Ca and Mg have the same valence, to save calculation cost, we mainly consider the Eu-doped Ca-α-SiAlON and Mg-α-SiAlON.In the first chapter, the application of white LED and phosphors used for LED were introduced and the theoretical basis of first principles was also introduced. The methods of studying M-α-SiAlON: Eu²⁺, including X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Photoluminescence (PL) and the calculation software Materials Studio were introduced in the second chapter. X-ray diffraction (XRD) was used to analyze the crystallization quality of the synthesized materials. We can see the surface morphology and the size of particles by Scanning Electron Microscope (SEM). Photoluminescence (PL) spectra are used to study the optical properties of materials. The calculation software Materials Studio is used to calculate the constructed structure.In the third chapter, Eu-doped Ca-α-SiAlON and Mg-α-SiAlON were prepared experimentally, and their photoluminescence results were studied. Then we built reasonable structures and calculated the electronic density of states and band structure. By analysis and comparison of experimental results, we can explain the causes of this phenomenon from the perspective of the electronic structure.