Stuidies On Growth And Luminescence Properties Of A Novel Ce:YAG Fluorescent Crystal For White LED

The leading commercial available white LED can be fabricated by coating a blue emitting In Ga N die with Ce:YAG phosphor dispersed in an organic resin or silicone.Due to the bad thermal conductivity and chemical stability of organic resin or silicone,phosphor-converted LED always undergoes degradation of yellowing and aging under condition of high irradiation and high temperatures,which will cause the chromaticity shift and seriously deteriorate the lifetime of LED device.In addition,due to the lacking of a red component,this LED device often exhibits high correlated color temperature and a poor color rendering index.It is reported that Ce:YAG fluorescent crystal exhibits more excellent luminescence properties and higher thermal stability,compared with the commercial phosphor.So,in this paper,high quality of Ce:YAG single crystal is proposed to replace the traditionally commercial phosphor,and Tb,Mn and Eu ions are doped into the YAG single crystal to add the red component,which will improve the luminescence properties of white LED.High quality of series of Ce:YAG single crystal including Ce,Tb:YAG,Ce,Mn:YAG,Ce,Mn,Tb:YAG and Ce,Eu:YAG are grown by Czochralski method.XRD,SEM/STEM,absorption spectra,fluorescence spectra,variable temperature spectra,decay curves,EXAFS,EPR and photoelectric performance are used to investigate the optical properties in detail.Main research contents and conclusions are listed as the following:1.The results of XRD indicate that the doping of rare earth ionssuch as Tb,Eu and transition metal ions such as Mn does not induce significant changes in YAG host structure.2.The absorption spectra indicate that all series of Ce:YAG single crystals have an intensive absorption peaking at 460 nm,which can be attributed to the 4f?5d absorption transition of Ce³⁺,except for Ce³⁺,the characteristic peaks of Tb,Mn and Eu ions are also observed at the same time.Under optical excited at 460 nm,a broad emission peaking at530nm(5d^{1 2}E_g?_8g?4f^{1 2}F_7/2?_8u)in range of 500nm-650 nm has been the components of the emission spectra for all single crystals,and a broad emission peaking at 567nm(⁴T₁?⁶A₁ absorption transition of Mn²⁺),which partial compensates the lacking of red component.3.The temperature dependent emission spectra for Ce,Tb,Mn:YAG single crystal in range of 303K-563 K are measured,compared with the commercial phosphor,increasing the temperature,Ce,Tb,Mn:YAG single crystal exhibits higher thermal stability and has lower light loss.4.The results of EXAFS and EPR spectra of Mn ions in Ce,Mn:YAG single crystal indicate that the valence state of doped manganese is mainly +2.5.The optical parameters of LED devices based on Ce:YAG,Ce,Tb:YAG,Ce,Mn:YAG,Ce,Mn,Tb:YAG and Ce,Eu:YAG crystal wafers are measured,and the influences of doped ions,single crystal wafers thickness,LED chips,silicone and driven current on the luminescence properties of crystal wafers are discussed in detail.The related results indicate that the introduction of Tb partially improve the luminescence efficiency and lower the correlated color temperature?CCT?,but have no significant influences on the color rendering index?CRI?,and the introduction of Mn can partially improve the color rendering index of white LED,but lower the luminescence efficiency,however,the introduction of Eu lower the correlated color temperature,but leads to the luminescence quenching of Ce³⁺.6.The annealing process of Ce,Mn:YAG single crystal under H₂ is investigated in detail,the corresponding results indicate that annealing process under suitable temperature can eliminate the detects and stress during the growth and technical process of single crystal,which partiallyimproves the luminescence properties of crystal wafers.