Ce 3+ :CaSc 2 O 4 Crystal Fibers for Green Light Emission: Growth Issues and Characterization

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Ce3+:CaSc2O4 Crystal Fibers for Green Light Emission: Growth Issues and Characterization Detlef Klimm, Jan Philippen, Toni Markurt, and Albert Kwasniewski Leibniz Institute for Crystal Growth, Max-Born-Str. 2, 12489 Berlin, Germany ABSTRACT Ce3+ is known to show broad optical emission peaking in the green spectral range. For the stabilization of 3-valent cerium in ceramic phosphors such as calcium scandate CaSc2O4, often co-doping with sodium for charge compensation is performed (Na+, Ce3+ ↔ 2 Ca2+). At the melting point of CaSc2O4 (≈2110°C), however, alkaline oxides evaporate completely and codoping is thus no option for crystal growth from the melt. It is shown that even without codoping Ce3+:CaSc2O4 crystal fibers can be grown from the melt by laser-heated pedestal growth (LHPG) in a suitable reactive atmosphere. Reactive means here that the oxygen partial pressure is a function of temperature and pO2(T) rises for this atmosphere in such a way that Ce3+ is kept stable for all T. Crystal fibers with ≈1 mm diameter and ≤50 mm length were grown and characterized. Differential thermal analysis (DTA) was performed in the pseudo-binary system CaO–Sc2O3, and the specific heat capacity cp(T) of CaSc2O4 was measured up to 1240 K by differential scanning calorimetry (DSC). Near and beyond the melting point of calcium scandate significant evaporation of calcium tends to shift the melt composition towards the Sc2O3 side. Measurements and thermodynamic calculations reveal quantitative data on the fugacities of evaporating species. INTRODUCTION The direct conversion of electric energy to visible light is one of the big research challenges nowadays. From the 1960s on, Ga(As,P) based light emitting diodes (LED’s) became available which were used mainly as indicators with red emission. Not before 1994, (Ga,In)N based LED’s were developed for the blue and even ultraviolet spectral range [1]. Unfortunately, just the green spectral range where human eyes have maximum sensitivity is hard to reach by direct emission, because material systems mentioned above suffer from reduced electro-optical conversion efficiency in the center of the visible spectral range. Irrespective of some recent progress with (In,Ga)N layers on GaN substrates [2], still frequency conversion from UV to the green spectral region by photoluminescence is the most important process for the generation of green light, especially in white LED’s for general illumination. Ce3+ ions in oxide environment can convert UV light efficiently to the green, with maximum intensity around 515 nm [3], and calcium scandate CaSc2O4 can be doped with several trivalent rare earth ions, among them Ce3+ [4,5]. The octahedral radii (Shannon) of Ce3+ (115 pm) and Ca2+ (114 pm) are almost identical, and it can be assumed that Ce3+ substitutes for Ca2+ rather than for the much smaller (88.5 pm) Sc3+. For the production of white LED’s, ceramic phosphors are used. The incorporation of Ce3+ into the CaSc2O4 matrix is then enabled by co-doping with the similarly sized Na+ (octahedral radius116 pm