Display Applications of Rare-Earth-Doped Materials
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MRS BULLETIN/SEPTEMBER 1999
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Wavelength (nm) Figure 2. Cathodoluminescence emission spectrum for Y203:Eu
cathodoexcitation. The 4/ —* 5d absorp tion for Ce 3 + is at a lower energy than that for Tb 3 + and, therefore, may be used to transfer energy to the Tb 3 + for emis sion in the green. For greater specifics on the energy-transfer dynamics and interesting influences of host crystallography on Ce 3 + and Ce 3 + /Gd 3 + energy transfer to T b 3 + , the reader is referred to the literature.5'16'17 Green emissions are also observed from the 4S3/2 —» 4 Iis/ 2 transition of triva lent erbium (Er 3+ ). The transitions from this excited State are all spin-allowed, so Er 3+ possesses a large absorption cross section and a short luminescent lifetime. Emission is typically centered near 545 nm, making it well suited for a saturated green phosphor, provided that a host with suitably low phonon energy is used to lessen the nonradiative relaxation to the 4 F 9/2 State (-3000 cm" 1 below the emitting level). Since Tb 3 + emits well in oxide Systems, Er 3+ has not been developed very actively for visible-display technologies. Cerium is used as a luminescent Cen ter in blue phosphors. The broad-band emission spectrum is shown in Figure 4, along with a spectrum calculated using the semiempirical molecular orbital theory as described by O'Brien et al.18 The two emission bands arise from a Split ting of the ground State due to spin-orbit coupling. In both the divalent (Ce 2+ ) and trivalent (Ce 3+ ) State, a blue emission occurs through a 5d —* 4/transition. There fore, the host plays a very large role in the color of this emission, such that it may ränge from UV to red. Since the transi tions are allowed, the absorption cross sections are very large, and the resulting
emission is strong and highly efficient, exhibiting a luminescent decay time on the order of tens of nanoseconds.
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730
780
Figure 3. Room-temperature electroluminescence emission spectrum forZnS.TbOF.
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Wavelength (nm)
Figure 4. Measured and calculated photoluminescence emission spectra for SrS:Ce.
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Display Applications of Rare-Earth-Doped Materials
Line emission at 450 n m from the G 4 -» 3 H 6 (or 'D 2 -> 3F4) transition of trivalent thulium (Tm 3+ ) is a saturated, primary blue. A limitation for the use of Tm3+ in EL-driven displays is the large IR component of the emission. This is due to both "field Stripping" and the existence of a multitude of lower-lying states that can be excited by lower-energy electrons (since for CL and EL, there is little control over the excitation energy). This leads to a "cooling" of the electrons, such that they no longer have enough energy to excite the levels needed to generate a blue emission. This would not be a limi tation in PL, since the hosts typically have a sufficientl
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