Characterization of Fluoride Phosphors for Diode Pumped Solid-State Lighting Applications

Phosphor coated diodes are critical to the lighting industry. Rare-earth doped fluoride phosphors are characterized in this work. Spectroscopy and lifetime measurements of CaF2: Dy, LaF3: Eu and Tb-doped LaF3 and CaF2 crystals were performed by irradiating the samples with 405 and 375 nm diode lasers. Chromaticity diagrams are developed from spectral measurements. Dy-doped CaF2 revealed bright white light emission having peaks at 492, 570 and 654 nm. The measured color coordinates are x = 0.322 and y = 0.340 for Dy-doped CaF2 and these values are close to those of sunlight and the coordinated color temperature is 6147 K, under 405 m laser excitation. Eu-doped LaF3 revealed twenty emission peaks in the visible wavelength region. The color coordinates measured for LaF3: Eu are x = 0.283 and y = 0.293 under 405 nm diode laser excitation and the color temperature is 9557 K. Eu-doped LaF3 provides cool white-light, under 405 nm diode laser excitation. Tb-doped fluoride crystals revealed bright white light under low power diode laser excitation. The measured color coordinates for Tb-doped CaF2 are x = 0.329 and y = 0.558 and the color temperature is 7713 K. The color coordinates for LaF3: Tb are x = 0.342 and y = 0.365 and the coordinated color temperature is 5370 K. These values are close to those of sunlight.


Introduction
the current research areas is the design of all solid-state white-light emitters which are energy efficient, robust and free from pollution [1].There is an ongoing global effort to design efficient solid-state white-light sources.For this purpose, both organic and inorganic materials are being investigated which emit white-light under electrical pumping [2] [3] [4].In the category of inorganic materials, semiconductor diodes, quantum dots and diode pumped phosphor materials are under consideration.One class of materials that emit white light is diode pumped phosphors which are embedded with inorganic ions [5]- [11].
Light emitting diodes (LED) consume less energy, do not pollute the environment and last much longer than their counterparts.Some of the rare-earth ions possess numerous energy levels and emit in the visible and infrared wavelength regions.Depending on the energy level structure, a rare-earth ion may absorb certain discrete lasers wavelengths.Because of these characteristic spectral fea- However, Eu 3+ -doped materials were extensively investigated because their possible applications in optical hole burning memory devices [16].Published literature on Dy 3+ -doped materials is far less than that for Er 3+ -doped materials because the latter is used in the laser development.Moreover, at higher doping levels concentration quenching dominates in Dy 3+ -doped materials.Lately, rare-earth doped materials are being investigated for possible applications in solid-state lighting [19]- [24].Rare-earth doped LaF 3 and CaF 2 are relatively stable when compared to other materials, because of their high melting points.
Crystal growth of CaF 2 is a mature technology and large windows are grown for infrared applications.In the current study the suitability of these materials for solid-state lighting applications is discussed.In a solid, excited rare-earth ions relax radiatively and non-radiatively [25].The former depends on the transition probabilities which are calculated using Judd-Ofelt intensity parameters which, in turn, are derived from the absorption spectrum.The radiative probabilities are independent of temperature.The non-radiative relaxation is temperature dependent and its value depends on the energy gap between the levels and the cut-off phonon frequency [26].The cut-off phonon frequency depends on the reduced mass of the molecule and force constants.The phonon structure is revealed by Raman and infrared spectral measurements.The cut-off phonon frequency of LaF 3 and CaF 2 crystals are 360 and 380 cm −1 respectively.

Experimental Details
The crystals used for this investigation were obtained from Optovac, Inc.The crystals are of the size 8x6x4 mm 3    Excited ions relax non-radiatively in cascade to 4 F 9/2 .The energy gap between 4 F 9/2 and its lower level is very large and hence 4 F 9/2 relaxes radiatively to the lower levels with 100% quantum efficiency, in the absence of concentration quenching effects.The fluorescence spectrum observed under a 375 nm diode laser is shown in Figure 3 and the sample color is shown in the inset.Figure 3 reveals emission peaks at 492 ( 4 F 9/2  6 H 15/2 ), 570 ( 4 F 9/2  6 H 13/2 ) and 654 nm ( 4 F 9/2  6 H 11/2 ); the spectral identifications are given in parentheses.The fluorescence contains blue, green and red colors and hence the sample color is white along the beam path.The emission spectrum is identical even for 405 nm excitation.The measured lifetime of 4 F 9/2 level is 1.3 ± 0.1 ms.Each emission peak is a

LaF3: Eu 3+ (0.3%)
A partial energy level diagram of Eu 3+ is shown in Figure 5. Eu 3+ ions occupy a single type of site in this material.405 and 375 nm lasers resonantly excite 5 D 3 and 5 L 7 levels, respectively.Excited ions relax radiatively and non-radiatively in cascade to the lower levels.Transition probabilities predict radiative transitions  The lifetimes of 5 D J manifolds (J = 0, 1, 2 and 3) are in the millisecond range.

Tb 3+ -Doped CaF2 and LaF3
The absorption spectrum of CaF 2 : Tb 3+ (0.1%) is shown in Figure 8.It revealed peaks at 257 ( 5 K 9 ), 274 ( 5 F 4 ), 286 ( 5 F 5 ), 295 ( 5 H 5 ), 303 ( 5 H 6 ), 320( 5 D 0 ), 329 ( 5 D 1 ), 343 ( 5 G 3 ), 352 ( 5 G 4 ), 367 ( 5 L 10 ), 380 ( 5 D 3 ), and 488 nm ( 5 D 4 ).The symbols in the parentheses represent the corresponding spectral terms of the excited energy states.The absorption spectrum does not reveal any peak in the vicinity of 405 nm.Accordingly, the material did not reveal any fluorescence from 5 D J (J = 3, 4) levels under 405 nm excitation.However, the sample emitted bright white light under 375 nm laser excitation (Figure 9).When the fluorescence was analyzed with a spectrometer it revealed a weaker peak at 426 nm and more intense peaks in the blue, green, orange and red regions, respectively at 492 ( 5 D 4  7 F 6 ), 549 About 20 percent of the electricity supplied to homes is used in lighting.One of B. R. Reddy et al.DOI: 10.4236/opj.2018.88024289 Optics and Photonics Journal photometer.The fluorescence spectroscopy measurements were performed using a Stellarnet spectrometer.Luminescence was generated from the samples by exposing them to Coherent OBIS 375 or 405 nm diode lasers.For lifetime measurements the laser beams were chopped at 100 Hz by a mechanical chopper to generate laser pulses.The luminescence was acquired by a fiber bundle whose other end was connected to an Acton Research Corporation, Spectra Pro 500 spectrometer (Figure1).The spectrometer output was detected by a Hamamatsu R928 photomultiplier tube (PMT).The PMT output was amplified and then acquired by a Stanford Research Corporation, SRC 430 multichannel scaler.The resulting decay signals were fitted to single exponentials to derive the lifetimes of the excited levels.For the measurement of color coordinates and CCT the spectra were acquired by a Stellar Net spectrometer.

3. Results and Discussion 3 . 1 .Figure 1 .
Figure 1.A block diagram of the experimental setup used for lifetime measurement.

Figure 2 .
Figure 2. Partial energy level diagram of Dy 3+ ion depicting emission transitions, under 375 and 405 nm diode laser excitations.

Figure 3 .
Figure 3. Emission spectrum of CaF 2 : Dy 3+ observed under 375 nm laser excitation.Inset in the figure is the sample photograph depicting the emission color.

Figure 4 .
Figure 4. Chromaticity diagram of CaF 2 : Dy 3+ measured with 375 nm diode laser excitation.The color coordinates are x = 0.338 and y = 0.36 and CCT is 5474 K.

Figure 5 .
Figure 5. Partial energy level diagram of LaF 3 : Eu 3+ depicting absorption and emission transitions under diode laser excitation.

Figure 6 .
Figure 6.Fluorescenece spectrum of LaF 3 : Eu 3+ observed with 405 nm diode laser excitation.The inset in the figure is a photograpgh of the sample, which reveals white emission color.

Figure 7 .
Figure 7. Chromaticity diagram of LaF 3 : Eu 3+ measured with 405 nm diode laser excitation.Color coordinates are x = 0.273 and y = 0.282 and CCT is 9557 K.

Figure 9 .
Figure 9. Fluorescence spectrum of CaF 2 : Tb 3+ generated with 375 nm laser excitation.Inset is the sample photograph under laser illumination.