The Rare Earth Doped LiYF₄Single Crystal For White Light-emitting-diodes Research

Recently, there has been a significant increase of interest in the white light emittingdiode (LED) because of their remarkable properties such as high brightness, ruggedstructure, lower power consumption, a long lifetime span and environmental benefit incontrast with conventional incandescent and fluorescence lamps[1-3]. The common way togain white light emitting diode is to use a blue-emitting GaN chip to pump a YAG: Ce3+phosphor or an ultraviolet InGaN to excite blue, green, and red-emitting multiphase.However, there are many drawbacks in this type of white LEDs [4,5]. For instance, theheat resistance of the epoxy resins is relatively poor, and the refractive index mismatchbetween the phosphors and the epoxy results in light scattering [6].The white light colortemperature on the high side, color rendering index is low; In order to solve these problems,single-host full-color emitting glasses [7,8], glass ceramics [9] and transparent ceramics forachieving white light emission had been tried and reported such as oxyfluoride glasses co-doped with Ho3+/Tm3+/Yb3+[10], bismuth tellurite glasses triply-doped Tm3+/Ho3+/Yb3+[11]and Tm3+/Er3+/Yb3+triply-doped aluminum Y2O3transparent ceramics [12]. The successfulwhite light emitting of above materials has motivated the search for new materials forwhite LED.In previous studies, the glass and transparent ceramics were mainly chosen as hosts forwhite LED. The low optical transmittance of transparent ceramics and the low luminescentefficiency of rare earth ions in glasses, the poor mechanical, thermal, physical andchemical properties of glasses made them difficult for any practical applications. In fact,many single crystals exhibit better properties for white LED. The LiYF4crystal has beenproven to be a suitable laser material because of its many advantages such as lowmaximum phonon energy, long fluorescence lifetime, low thermal lens effect and laserthreshold, and high incorporation of trivalent rare-earth ions. These properties make LiYF4crystal a potential phosphor for solid-state lighting, in particular as a phosphor material forhigh brightness white LED.The modified Bridgman method have been used to grow Dy3+、Dy3+/Tm3+、Dy3+/Eu3+co-doped LiYF4crystal [17-19] successfully in our laboratory. Through the optical testing,we can concluded that the LiYF4single crystal doped with rare earth ions is a potentialphosphor for white LED.