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Red-emitting cerium-based phosphor materials for solid-state lighting applications
Chemical Physics Letters 423 (2006) 352–356 www.elsevier.com/locate/cplett
Red-emitting cerium-based phosphor materials for solid-state lighting applications R. Le Toquin, A.K. Cheetham
*
Materials Research Laboratory, University of California, MRL MC-CAM Building, Room 3117C, Santa Barbara, CA 93106, USA Received 1 March 2006 Available online 3 April 2006
Abstract The discovery of phosphor materials that can be excited around 380 or 460 nm is essential for improving the efficiency and light quality of LED-based solid-state lighting devices. We have found, for the first time, a luminescent cerium compound emitting bright red light. CaSiN2:Ce3+ is a new phase in the calcium silicon nitride ternary system that crystallizes in a face-centered cubic unit cell with a lattice ˚ . This compound can be used for white light applications: (i) to enhance the light quality of the system parameter of a = 14.8822(5) A based on a blue LED with a yellow/green phosphor, (ii) as the red phosphor in the UV LED plus three RGB phosphors setup or (iii) directly as a yellow phosphor with a 460 nm LED due to the emission/excitation band shift with chemical substitution. 2006 Elsevier B.V. All rights reserved.
1. Introduction The development of very efficient UV/blue light emitting diodes (LEDs) based on wideband gap semiconductor such as GaN has led to considerable progress in the field of solid-state lighting as well as backlighting for display application [1,2]. The longevity of the LEDs and most importantly their low energy consumption are the driving forces for such a rapid development. The photons emitted through a p–n recombination process can be converted into lower energy radiation using the luminescence properties of phosphor materials. Therefore, blue LEDs emitting around 460 nm can be combined with a yellow phosphor [2] or even with a mix of green and orange phosphors [3] in order to obtain white light. In this situation, only a fraction of the blue photons emitted by the chip are absorbed by the phosphor, the rest being combined with the yellow photoluminescence to give white light. Alternatively, the use of the UV LEDs requires that all UV photons must be absorbed by the blue, green and red phosphors. The first generation of commercially available white LED was based on an InGaN chip emitting blue photons at around 460 nm *
Corresponding author. E-mail address: [email protected] (A.K. Cheetham).
0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.03.056
combined with a Y3Al5O12:Ce3+ (YAG) phosphor layer that converts blue into yellow photons [1,2]. As a result, a slightly bluish white light with a color rendering index (CRI) of 70 is produced that may, in principle, be improved by adding red photons from a complementary phosphor. The second generation of white LEDs is still based on blue InGaN LEDs but with a mix of green and orange phosphors that increases the CRI up to 90 and makes the phosphor deposition process potentially easier [3]. In fact, new red or orange phosphors are urgently needed for the development of efficient white LEDs, regardless of the type of LED used, and they need to be very bright in order to compensate for the low red sensitivity of the human eye [4]. Bright red phosphors are, however, very difficult to achieve due to the quantum yield drop with increasing Stokes shift. We focused our work on cerium-based materials for two reasons. First, among all the trivalent lanthanides, Ce3+ is the only one characterized by strong f ! d transitions. Trivalent cerium has a 4f15 d0 ground state configuration and its optical transitions in the visible region arise from 4f1 ! 5d1 excitations. As with Eu2+, cerium-based phosphor materials have broad excitation/emission bands that can be matched to the characteristics of the LED and can possibly mimic the solar spectrum. Second, the involvement
R. Le Toquin, A.K. Cheetham / Chemical Physics Letters 423 (2006) 352–356
of 5d levels implies that the emission wavelength can be tuned by changing the energies of the 5d levels with respect to the deeper 4f levels. For instance, by adjusting the crystal field splitting, it is possible to lower the lowest 5d level in order to decrease the 5d–5f energy band gap and shift the phosphor emission towards the red (Fig. 1). For Ce3+-doped materials, emission is normally observed in the near UV [5], but blue or even green/yellow as been reported for Ca2Al2SiO7 [6] and CaS [7], respectively. However, high crystal field splitting or strongly covalent cerium environments can decrease the energy of the down-converted photons. Yttrium aluminum garnet (YAG) doped with Ce3+ is the most important example, exhibiting a strong yellow emission (540 nm) upon blue excitation (460 nm) due to the very low energy of the lowest 5d level [8]. A small tetragonal distortion of the cubic crystal field at the cerium site is responsible for this unusual yellow emission [9]. It is also possible to observe green-yellow Ce3+ emission in oxynitride [10] or sulfides [7,11] compounds, replacing oxygen by more covalent anions such as nitrogen and sulfur. Further increase of the covalent character has lead to new Eu2+ doped Sialon [12] or silicon (oxy)nitride [3,13] based materials that have been reported to show very efficient orange luminescence. Eu2+ doped Sr2Si5N8 (orange) and CaSi2O2N2 (green) are among the most interesting for solid-state lighting [3]. The longer emission wavelength observed for nitride compounds is associated with a broader excitation band that covers part of the UV and visible spectral range leading to colorful compounds. Normally, the red shift is associated with a larger Stokes shift that leads to very low temperature quenching. Fortunately, some compounds maintain a sufficiently high quantum efficiency to be useful in solid-state lighting. In view of the lack of good red phosphors, we have aimed to synthesize cerium-based red phosphor materials using a host with a high crystal field splitting due to symmetry, coordination number and covalency factors. Several (a)
(b)
(c)
4f1
4f1
t2g 5d
eg
Eem
4f1
Fig. 1. Description of the energy splitting between the 4f and 5d levels in the case of a cubic crystal field (a), a cubic crystal field with nitrogen replacing oxygen atoms (b) and a distorted cubic crystal field (c). Due to screening effects, the 4f levels are not significantly affected by the crystal field.
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phases have been reported in the Ca–Si–N ternary system, among which one may have cubic symmetry, but no detailed structural characterization has been presented [14]. Recently, Gal et al. [15] reported the symmetry of CaSiN2 to be orthorhombic with space group Pnma and ˚ , b0 = 10.2074 A ˚ and cell parameters a0 = 5.1229 A ˚ c0 = 14.8233 A using single crystal X-ray diffraction. In the present work, we were able to design, for the first time, a red phosphor material based on CaSiN2 that is very promising for solid-state lighting applications. 2. Experimental Stoichiometric amounts of Ca3N2 and Si3N4 were weighed in a glove box ([O2] < 1 ppm and [H2O] < 1 ppm) in order to prevent oxidation or hydrolysis of the reactants. CeO2, Eu2O3, Sm2O3, Tb4O7, Pr2O3, Er2O3 were used as rear earth sources. For doping purposes, we used AlN, Mg2N3 and Sr2N. Following thorough mixing and grinding, the mixture was loaded into an alumina crucible. The heating was carried out in a horizontal tube furnace under flowing N2 (2–4 l/min) between 1300 C and 1500 C for several hours. The purity of all samples was checked using conventional X-ray diffraction. Luminescence properties were measured using a Perkin Elmer LS55 UV/Visible spectrophotometer. The external quantum efficiency (QE) was measured at 515 nm using an Argon laser. The setup comprises an integration sphere, two filters (to differentiate the laser excitation and the fluorescence) and a calibrated Si diode as detector. The setup and data process follow the work described by Greenham et al. [16]. By using this setup, the external quantum efficiency of the Ce3+:YAG phosphor standard has been measured to be around 70%. Synchrotron X-ray powder diffraction was performed at Brookhaven National Labo˚. ratory on beamline X7A with a wavelength of 0.7972 A 3. Results and discussion The high temperature synthesis of CaSiN2 doped with Ce3+ leads to a pinkish/red colored sample. This coloration was a very promising observation since it is characteristic of substantial absorption in the blue–green region (450– 550 nm), which is of primary interest for solid-state lighting application. In order to characterize the structure of this phase, we used X-ray powder diffraction. Using conventional X-ray diffraction methods, we were able to show that CaSiN2 doped with Ce3+ seems to crystallize in an F-centered cubic unit cell with a very large unit cell parameter ˚ . In order to assess the true symmetry of about 14.88 A and further probe the structure, we used the very high resolution of an X-ray synchrotron beam to measure the powder diffraction pattern. The cell parameters and possible space groups were determined using the FULLPROF SUITE of programs [17]. This study showed that the rare-earth doped CaSiN2 phase has indeed cubic symmetry and that five possible space groups, F23, Fm3, F432, F 43m and
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Fm 3m, can account for all the reflections (Fig. 2). After ˚ refinement, the cell parameter value is a = 14.8822(5) A 3 ˚ leading to a cell volume larger than 3200 A . However, ab initio structure solution is very challenging, especially due to the high symmetry, the lack of heavy elements, ˚ 3, leading and most of all the large unit cell volume (3200 A to at least 15 independent atoms in the unit cell). This structural study is still in progress and the results will be the subject of another publication. Previously, several phases have been reported in the Ca– Si–O–N system, among which one has possibly an F-centered cubic unit cell with a cell parameter of about ˚ [14]. More recently, Gal et al. have solved 14.864(12) A the structure of the ternary phases MSiN2 with M = Ca, Sr, Ba using single crystal X-ray diffraction [15]. Their study suggests that the Ca phase has an orthorhombic symmetry (space group Pbca) with unit cell parameter values of ˚ , b = 10.2074(6) A ˚ , c = 14.8233(9) A ˚. about a = 5.1229(3) A Their crystals were synthesized at 1000 C from a mix of the metals and sodium azide in a sealed tantalum tube. Despite the very different synthetic conditions, we found that the cell parameters of the orthorhombic phase are relatedpto our cubic p cell through the following relations: ac = 2 2a0, ac = 2a0 ac = c0, Vc = 4 · V0. Substitution of cerium into CaSiN2 appears to stabilize the cubic polymorph. The structure of the orthorhombic CaSiN2 has been described to be based on vertex sharing SiN4 tetrahedra comparable to the D1-type tilting distortion of the b-cristobalite phase. The phase is reported to be isostructural with KGaO2. The corner sharing SiN4 tetrahedra form six member rings with Ca atoms in very distorted octahedral coordination. The description of the structure plays a crucial role in the understanding of the optical properties and further tailoring of new phosphor materials, since the local symmetry of cerium atoms significantly
affects the emission wavelength of the phosphor. For the Ce3+ doped YAG compound, it has been shown that the distorted cubic symmetry of the Y site has a direct effect on the emission wavelength [9]. We can already anticipate, therefore, that the stabilization of the high temperature cubic phase of CaSiN2 may lead to a large crystal field splitting for the 5d level and to a large red shift for the emission wavelength. The optical properties of this material were measured using a photoluminescence spectrometer as described above. The results show, for the first time, a red emission for Ce3+ atoms in any host reported so far (Fig. 3). The maximum of the emission band is centered around 625 nm, which is at least 50 nm longer than the largest cerium emission wavelength reported to date. The emission peak extends from 550 to 700 nm with a full width at half maximum (FWHM) of about 80 nm. For comparison, we also examined one of the red standards, Eu3+-doped Y2O2S, to emphasize the fact that using a broad-band emitter as opposed to a line emitter there is at least a 6-fold increase in intensity (Fig. 3). For solid-state lighting based on InGaN blue LEDs, our red phosphor can be used as a color complement to any yellow phosphor in order to access white light with different color temperatures. As expected from the observation of the sample body color, most of the green part of the visible spectrum is absorbed as well as part of the blue (Fig. 3). The excitation band has a maximum around 535 nm and extends from 425 to 575 nm with a FWHM of about 75 nm. Besides the obvious application as a phosphor, this material looks ideal for use in plant growth enhancement since it absorbs green photons where the chlorophyll is ineffective and down-converts them into red photons. Coating this material on glass window with a complementary UV to blue phosphor
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0 350
Fig. 2. Synchrotron X-ray diffraction pattern of the phase CaSiN2 doped with Ce3+. Le Bail refinement in the space group F23 with unit cell ˚ . The red circles, black and blue curves parameter a = 14.8822(5) A represent the experimental data, the calculated pattern and the difference curve respectively. The green marks show the position of the peaks.
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450
500
550
600
650
700
Fig. 3. Emission/excitation spectrum of the compound CaSiN2 doped with 3% Ce3+. The excitation curve is shown in black and has been recorded at 630 nm. The emission curve (in red) has been measured for a constant excitation wavelength of 535 nm. For comparison, the emission of the red standard Eu3+ doped Y2O2S (excitation 396 nm) has been shown in gray.
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Table 1 Optical properties of Ce3+ doped CaSiN2 substituted with different cations on the Ca and Si sites
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0 350
400
450
500
550
600
650
700
Fig. 4. Emission/excitation spectrum of the compound Ca(Si,Al)N2 doped with 3% Ce3+. The excitation curve is shown in black and has been recorded at 560 nm. The emission curve (in orange) has been measured for a constant excitation wavelength of 475 nm. For comparison, the emission of the red standard Eu3+ doped Y2O2S (excitation 396 nm) has been shown in gray.
would allow the use of both UV and green photons for photosynthesis. The value of the Stokes shift based on the maximum of the excitation and emission peaks was estimated to be about 2700 cm1. This value is rather smaller than that of the YAG (3800 cm1), thereby corroborating the difficulty in observing red emission for cerium in YAG-based phosphors. This finding is also consistent with the relatively high value of the external quantum efficiency at room temperature, QE 40%, prior to any improvement of the chemical composition, particle size or morphology. It is also worth mentioning that the laser excitation (515 nm) for the QE measurement was not exactly optimal according to the excitation curve of the phosphor (Fig. 3). In order to explore the effect of the cation size on the emission wavelength, we substituted Ca by Sr and Mg as well as Si by Al. The effect of a larger cation on the Si site tends to decrease the emission wavelength by about 70 nm (Fig. 4). The excitation blue-shifts, too, from a peak maximum at 535 nm to 475 nm for 10% Al substitution (Fig. 4). With the Al substitution, therefore, this compound becomes interesting as a yellow phosphor for direct application with a blue LED emitting around 460 nm. On the other hand, Sr substitution on the Ca site leads to a red shift of the emission which was not expected according to
Ce3+ doped compound
Emission maximum
Excitation maximum
Body color
CaSiN2 Ca(Al,Si)N2 (Ca,Sr)SiN2 (Ca,Mg)SiN2
625 560 640 540
535 475 500 460
Pink/red Orange Orange Yellow
the compression at the cerium sites (Table 1). Mg substitution seems to follow the same trend since the emission is blue shifted by about 90 nm (Table 1). Despite the fact that it is very difficult to draw conclusions without a detailed structure, it appears that this behavior is similar to that of YAG, where substitution of larger atoms on the Y and Al sites leads to opposite shifts of the emission wavelength [9]. Due to the yellow emission and the 460 nm excitation, this phosphor can therefore be used as a yellow phosphor with a blue LED. Other rare earths atoms have also been tested in this new host. The samples exhibit the same cubic symmetry and a typical decrease of the unit cell parameter is observed along the rare-earth series (Table 2). The optical properties of these materials are also interesting in terms of phosphor applications (Table 2). 4. Conclusions We have synthesized a high temperature cubic phase in the Ca–Si–N system with the stoichiometry CaSiN2. This phase is stabilized by cerium substitution on the calcium site. CaSiN2:Ce3+ has several important properties that make it the first of its kind and very promising for solid-state lighting applications. First, it represents the only Ce3+ phosphor materials showing an intense red emission with broad emission/excitation bands. The use of a red broad-band emitter will increase the CRI to as much as 90 for the setup comprising a blue LED with yellow and red phosphors. In order to achieve this target, the QE of the red phosphor must be further increased because it will be excited by the green photons from the yellow phosphor. Second, CaSiN2:Ce3+ is one of the rare phosphor materials that can be excited in the yellow–green region. This property is of great interest for plant growth enhancement since green light is not harvested in the photosynthetic process. With CaSiN2:Ce3+, the green photons can be down-converted into useful red photons. Thirdly, the use of chemical substitution has led to new possibilities
Table 2 Optical properties and unit cell parameters (refinement in space group F23) for CaSiN2 doped with different rare-earth ions (RE) Ca1xRExSiN2
RE = Ce3+
RE = Pr3+
RE = Sm3+
RE = Eu2+
RE = Tb3+
RE = Er3+
Body color ˚) Cubic unit cell parameter (A k Emission maximum (nm) k Excitation maximum (nm)
Pink/red 14.8822 625 535
White 14.8720 425 330
White 14.8463 610 400
Orange 14.8419 605 400
White 14.8587 550 275
Grey 14.8517 420 325
The RE doping level is 3%. For line emitter rare earths, only the strongest peak is presented.
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for the CaSiN2:Ce3+ phosphor. For example, the tunable emission of this phosphor makes it very interesting as a yellow/orange phosphor to use directly in combination with a 460 nm blue LED. It is also worth mentioning that this red phosphor can be used directly in a white light setup comprising a UV LED with three red, green and blue phosphors. Acknowledgements We thank the Solid State Lighting and Display Center at the University of California Santa Barbara for funding. We also acknowledge Profs. S.P. DenBaars and S. Nakamura for helpful discussions.
[5] [6] [7]
[8] [9] [10] [11] [12]
References [1] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1997; S.P. Den Baars, in: A.H. Kitai (Ed.), Solid State Luminescence Theory, Materials and Devices, Chapman and Hall, London, 1993. [2] P. Schlotter, R. Schmidt, J. Schneider. Appl. Phys A64 (1997) 417. [3] R. Mueller-Mach, G. Mueller, M.R. Krames, H.A. Hoppe, F. Stadler, W. Schnick, T. Juestel, P. Schmidt, Phys. Stat. Sol. (a) 202 (No. 9) (2005) 1727; G.O. Mueller, R. Mueller-Mach, M.R. Krames, Proc. SPIE 4776 (2002) 122. [4] N. Sharma, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 387 (2004) 2;
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N. Sharma, N. Kijima, A.K. Cheetham, Solid State Commun. 131 (2004) 65; T. Justel, H. Nikol, C. Ronda, Angew. Chem. Int. Ed. 37 (1998) 3084. G. Blasse, B.C. Grabmeier, Luminescent Materials, Springer, Berlin, 1994. N. Kodama, Y. Tanii, M. Yamaga, J. Luminescence 87–89 (2000) 1076. G.C. Kim, H.L. Park, S.I. Yun, G. Moon, J. Mater. Sci. Lett. 5 (1986) 359; S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, 1998. G. Blasse, A. Brill, Appl. Phys. Lett. 11 (1967) 53. G. Blasse, A. Brill, Chem. Phys. 47 (1967) 5139; A.K. Cheetham et al., unpublished results. J.W.H. van Krevel, H.T. Hintzen, R. Metselaar, A. Meijerink, J. Alloys Compd. 268 (1998) 272. G. Gauthier, S. Jobic, M. Evain, H.J. Koo, M.H. Whangbo, C. Fouassier, R. Brec. Chem. Mater. 15 (2003) 828. J.W.H. van Krevel, J.W.T. van Rutten, H. Mandal, H.T. Hinzen, R. Metselaar, J. Solid State Chem. 165 (2002) 19. K. Uheda, N. Hirosaki, H. Yamamoto, H. Yamane, Y. Yamamoto, W. Inami, K. Tsuda. Proceedings of the 2004 Joint International Meeting of the Electrochemical Society Oct. 3–Oct. 8. E.A. Pugar, J.H. Kennedy, P.E.D. Morgan, J.H. Porter, J. Am. Ceram. Soc. 71 (1988) C-288. Z.A. Gal, P.M. Mallinson, H.J. Orchard, S.J. Clarke, Inorg. Chem. 43 (2004) 3998. N.C. Grennham, I.D.W. Samuel, G.R. Hayes, R.T. Philips, Y. Kessener, S.C. Moratti, A.B. Holmes, R.H. Friend, Chem. Phys. Lett. 241 (1995) 89. FULLPROF J. Rodriguez-Carvajal, T. Roisnel, FULLPROF98 and Winplotr, New windows 95/NT Application for diffraction. Commission for Powder Diffraction, International Union of Crystallography. Newsletter No. 20 (May–August) Summer 1998.
Red-emitting cerium-based phosphor materials for solid-state lighting applications R. Le Toquin, A.K. Cheetham
*
Materials Research Laboratory, University of California, MRL MC-CAM Building, Room 3117C, Santa Barbara, CA 93106, USA Received 1 March 2006 Available online 3 April 2006
Abstract The discovery of phosphor materials that can be excited around 380 or 460 nm is essential for improving the efficiency and light quality of LED-based solid-state lighting devices. We have found, for the first time, a luminescent cerium compound emitting bright red light. CaSiN2:Ce3+ is a new phase in the calcium silicon nitride ternary system that crystallizes in a face-centered cubic unit cell with a lattice ˚ . This compound can be used for white light applications: (i) to enhance the light quality of the system parameter of a = 14.8822(5) A based on a blue LED with a yellow/green phosphor, (ii) as the red phosphor in the UV LED plus three RGB phosphors setup or (iii) directly as a yellow phosphor with a 460 nm LED due to the emission/excitation band shift with chemical substitution. 2006 Elsevier B.V. All rights reserved.
1. Introduction The development of very efficient UV/blue light emitting diodes (LEDs) based on wideband gap semiconductor such as GaN has led to considerable progress in the field of solid-state lighting as well as backlighting for display application [1,2]. The longevity of the LEDs and most importantly their low energy consumption are the driving forces for such a rapid development. The photons emitted through a p–n recombination process can be converted into lower energy radiation using the luminescence properties of phosphor materials. Therefore, blue LEDs emitting around 460 nm can be combined with a yellow phosphor [2] or even with a mix of green and orange phosphors [3] in order to obtain white light. In this situation, only a fraction of the blue photons emitted by the chip are absorbed by the phosphor, the rest being combined with the yellow photoluminescence to give white light. Alternatively, the use of the UV LEDs requires that all UV photons must be absorbed by the blue, green and red phosphors. The first generation of commercially available white LED was based on an InGaN chip emitting blue photons at around 460 nm *
Corresponding author. E-mail address: [email protected] (A.K. Cheetham).
0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.03.056
combined with a Y3Al5O12:Ce3+ (YAG) phosphor layer that converts blue into yellow photons [1,2]. As a result, a slightly bluish white light with a color rendering index (CRI) of 70 is produced that may, in principle, be improved by adding red photons from a complementary phosphor. The second generation of white LEDs is still based on blue InGaN LEDs but with a mix of green and orange phosphors that increases the CRI up to 90 and makes the phosphor deposition process potentially easier [3]. In fact, new red or orange phosphors are urgently needed for the development of efficient white LEDs, regardless of the type of LED used, and they need to be very bright in order to compensate for the low red sensitivity of the human eye [4]. Bright red phosphors are, however, very difficult to achieve due to the quantum yield drop with increasing Stokes shift. We focused our work on cerium-based materials for two reasons. First, among all the trivalent lanthanides, Ce3+ is the only one characterized by strong f ! d transitions. Trivalent cerium has a 4f15 d0 ground state configuration and its optical transitions in the visible region arise from 4f1 ! 5d1 excitations. As with Eu2+, cerium-based phosphor materials have broad excitation/emission bands that can be matched to the characteristics of the LED and can possibly mimic the solar spectrum. Second, the involvement
R. Le Toquin, A.K. Cheetham / Chemical Physics Letters 423 (2006) 352–356
of 5d levels implies that the emission wavelength can be tuned by changing the energies of the 5d levels with respect to the deeper 4f levels. For instance, by adjusting the crystal field splitting, it is possible to lower the lowest 5d level in order to decrease the 5d–5f energy band gap and shift the phosphor emission towards the red (Fig. 1). For Ce3+-doped materials, emission is normally observed in the near UV [5], but blue or even green/yellow as been reported for Ca2Al2SiO7 [6] and CaS [7], respectively. However, high crystal field splitting or strongly covalent cerium environments can decrease the energy of the down-converted photons. Yttrium aluminum garnet (YAG) doped with Ce3+ is the most important example, exhibiting a strong yellow emission (540 nm) upon blue excitation (460 nm) due to the very low energy of the lowest 5d level [8]. A small tetragonal distortion of the cubic crystal field at the cerium site is responsible for this unusual yellow emission [9]. It is also possible to observe green-yellow Ce3+ emission in oxynitride [10] or sulfides [7,11] compounds, replacing oxygen by more covalent anions such as nitrogen and sulfur. Further increase of the covalent character has lead to new Eu2+ doped Sialon [12] or silicon (oxy)nitride [3,13] based materials that have been reported to show very efficient orange luminescence. Eu2+ doped Sr2Si5N8 (orange) and CaSi2O2N2 (green) are among the most interesting for solid-state lighting [3]. The longer emission wavelength observed for nitride compounds is associated with a broader excitation band that covers part of the UV and visible spectral range leading to colorful compounds. Normally, the red shift is associated with a larger Stokes shift that leads to very low temperature quenching. Fortunately, some compounds maintain a sufficiently high quantum efficiency to be useful in solid-state lighting. In view of the lack of good red phosphors, we have aimed to synthesize cerium-based red phosphor materials using a host with a high crystal field splitting due to symmetry, coordination number and covalency factors. Several (a)
(b)
(c)
4f1
4f1
t2g 5d
eg
Eem
4f1
Fig. 1. Description of the energy splitting between the 4f and 5d levels in the case of a cubic crystal field (a), a cubic crystal field with nitrogen replacing oxygen atoms (b) and a distorted cubic crystal field (c). Due to screening effects, the 4f levels are not significantly affected by the crystal field.
353
phases have been reported in the Ca–Si–N ternary system, among which one may have cubic symmetry, but no detailed structural characterization has been presented [14]. Recently, Gal et al. [15] reported the symmetry of CaSiN2 to be orthorhombic with space group Pnma and ˚ , b0 = 10.2074 A ˚ and cell parameters a0 = 5.1229 A ˚ c0 = 14.8233 A using single crystal X-ray diffraction. In the present work, we were able to design, for the first time, a red phosphor material based on CaSiN2 that is very promising for solid-state lighting applications. 2. Experimental Stoichiometric amounts of Ca3N2 and Si3N4 were weighed in a glove box ([O2] < 1 ppm and [H2O] < 1 ppm) in order to prevent oxidation or hydrolysis of the reactants. CeO2, Eu2O3, Sm2O3, Tb4O7, Pr2O3, Er2O3 were used as rear earth sources. For doping purposes, we used AlN, Mg2N3 and Sr2N. Following thorough mixing and grinding, the mixture was loaded into an alumina crucible. The heating was carried out in a horizontal tube furnace under flowing N2 (2–4 l/min) between 1300 C and 1500 C for several hours. The purity of all samples was checked using conventional X-ray diffraction. Luminescence properties were measured using a Perkin Elmer LS55 UV/Visible spectrophotometer. The external quantum efficiency (QE) was measured at 515 nm using an Argon laser. The setup comprises an integration sphere, two filters (to differentiate the laser excitation and the fluorescence) and a calibrated Si diode as detector. The setup and data process follow the work described by Greenham et al. [16]. By using this setup, the external quantum efficiency of the Ce3+:YAG phosphor standard has been measured to be around 70%. Synchrotron X-ray powder diffraction was performed at Brookhaven National Labo˚. ratory on beamline X7A with a wavelength of 0.7972 A 3. Results and discussion The high temperature synthesis of CaSiN2 doped with Ce3+ leads to a pinkish/red colored sample. This coloration was a very promising observation since it is characteristic of substantial absorption in the blue–green region (450– 550 nm), which is of primary interest for solid-state lighting application. In order to characterize the structure of this phase, we used X-ray powder diffraction. Using conventional X-ray diffraction methods, we were able to show that CaSiN2 doped with Ce3+ seems to crystallize in an F-centered cubic unit cell with a very large unit cell parameter ˚ . In order to assess the true symmetry of about 14.88 A and further probe the structure, we used the very high resolution of an X-ray synchrotron beam to measure the powder diffraction pattern. The cell parameters and possible space groups were determined using the FULLPROF SUITE of programs [17]. This study showed that the rare-earth doped CaSiN2 phase has indeed cubic symmetry and that five possible space groups, F23, Fm3, F432, F 43m and
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Fm 3m, can account for all the reflections (Fig. 2). After ˚ refinement, the cell parameter value is a = 14.8822(5) A 3 ˚ leading to a cell volume larger than 3200 A . However, ab initio structure solution is very challenging, especially due to the high symmetry, the lack of heavy elements, ˚ 3, leading and most of all the large unit cell volume (3200 A to at least 15 independent atoms in the unit cell). This structural study is still in progress and the results will be the subject of another publication. Previously, several phases have been reported in the Ca– Si–O–N system, among which one has possibly an F-centered cubic unit cell with a cell parameter of about ˚ [14]. More recently, Gal et al. have solved 14.864(12) A the structure of the ternary phases MSiN2 with M = Ca, Sr, Ba using single crystal X-ray diffraction [15]. Their study suggests that the Ca phase has an orthorhombic symmetry (space group Pbca) with unit cell parameter values of ˚ , b = 10.2074(6) A ˚ , c = 14.8233(9) A ˚. about a = 5.1229(3) A Their crystals were synthesized at 1000 C from a mix of the metals and sodium azide in a sealed tantalum tube. Despite the very different synthetic conditions, we found that the cell parameters of the orthorhombic phase are relatedpto our cubic p cell through the following relations: ac = 2 2a0, ac = 2a0 ac = c0, Vc = 4 · V0. Substitution of cerium into CaSiN2 appears to stabilize the cubic polymorph. The structure of the orthorhombic CaSiN2 has been described to be based on vertex sharing SiN4 tetrahedra comparable to the D1-type tilting distortion of the b-cristobalite phase. The phase is reported to be isostructural with KGaO2. The corner sharing SiN4 tetrahedra form six member rings with Ca atoms in very distorted octahedral coordination. The description of the structure plays a crucial role in the understanding of the optical properties and further tailoring of new phosphor materials, since the local symmetry of cerium atoms significantly
affects the emission wavelength of the phosphor. For the Ce3+ doped YAG compound, it has been shown that the distorted cubic symmetry of the Y site has a direct effect on the emission wavelength [9]. We can already anticipate, therefore, that the stabilization of the high temperature cubic phase of CaSiN2 may lead to a large crystal field splitting for the 5d level and to a large red shift for the emission wavelength. The optical properties of this material were measured using a photoluminescence spectrometer as described above. The results show, for the first time, a red emission for Ce3+ atoms in any host reported so far (Fig. 3). The maximum of the emission band is centered around 625 nm, which is at least 50 nm longer than the largest cerium emission wavelength reported to date. The emission peak extends from 550 to 700 nm with a full width at half maximum (FWHM) of about 80 nm. For comparison, we also examined one of the red standards, Eu3+-doped Y2O2S, to emphasize the fact that using a broad-band emitter as opposed to a line emitter there is at least a 6-fold increase in intensity (Fig. 3). For solid-state lighting based on InGaN blue LEDs, our red phosphor can be used as a color complement to any yellow phosphor in order to access white light with different color temperatures. As expected from the observation of the sample body color, most of the green part of the visible spectrum is absorbed as well as part of the blue (Fig. 3). The excitation band has a maximum around 535 nm and extends from 425 to 575 nm with a FWHM of about 75 nm. Besides the obvious application as a phosphor, this material looks ideal for use in plant growth enhancement since it absorbs green photons where the chlorophyll is ineffective and down-converts them into red photons. Coating this material on glass window with a complementary UV to blue phosphor
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Fig. 2. Synchrotron X-ray diffraction pattern of the phase CaSiN2 doped with Ce3+. Le Bail refinement in the space group F23 with unit cell ˚ . The red circles, black and blue curves parameter a = 14.8822(5) A represent the experimental data, the calculated pattern and the difference curve respectively. The green marks show the position of the peaks.
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Fig. 3. Emission/excitation spectrum of the compound CaSiN2 doped with 3% Ce3+. The excitation curve is shown in black and has been recorded at 630 nm. The emission curve (in red) has been measured for a constant excitation wavelength of 535 nm. For comparison, the emission of the red standard Eu3+ doped Y2O2S (excitation 396 nm) has been shown in gray.
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Table 1 Optical properties of Ce3+ doped CaSiN2 substituted with different cations on the Ca and Si sites
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Fig. 4. Emission/excitation spectrum of the compound Ca(Si,Al)N2 doped with 3% Ce3+. The excitation curve is shown in black and has been recorded at 560 nm. The emission curve (in orange) has been measured for a constant excitation wavelength of 475 nm. For comparison, the emission of the red standard Eu3+ doped Y2O2S (excitation 396 nm) has been shown in gray.
would allow the use of both UV and green photons for photosynthesis. The value of the Stokes shift based on the maximum of the excitation and emission peaks was estimated to be about 2700 cm1. This value is rather smaller than that of the YAG (3800 cm1), thereby corroborating the difficulty in observing red emission for cerium in YAG-based phosphors. This finding is also consistent with the relatively high value of the external quantum efficiency at room temperature, QE 40%, prior to any improvement of the chemical composition, particle size or morphology. It is also worth mentioning that the laser excitation (515 nm) for the QE measurement was not exactly optimal according to the excitation curve of the phosphor (Fig. 3). In order to explore the effect of the cation size on the emission wavelength, we substituted Ca by Sr and Mg as well as Si by Al. The effect of a larger cation on the Si site tends to decrease the emission wavelength by about 70 nm (Fig. 4). The excitation blue-shifts, too, from a peak maximum at 535 nm to 475 nm for 10% Al substitution (Fig. 4). With the Al substitution, therefore, this compound becomes interesting as a yellow phosphor for direct application with a blue LED emitting around 460 nm. On the other hand, Sr substitution on the Ca site leads to a red shift of the emission which was not expected according to
Ce3+ doped compound
Emission maximum
Excitation maximum
Body color
CaSiN2 Ca(Al,Si)N2 (Ca,Sr)SiN2 (Ca,Mg)SiN2
625 560 640 540
535 475 500 460
Pink/red Orange Orange Yellow
the compression at the cerium sites (Table 1). Mg substitution seems to follow the same trend since the emission is blue shifted by about 90 nm (Table 1). Despite the fact that it is very difficult to draw conclusions without a detailed structure, it appears that this behavior is similar to that of YAG, where substitution of larger atoms on the Y and Al sites leads to opposite shifts of the emission wavelength [9]. Due to the yellow emission and the 460 nm excitation, this phosphor can therefore be used as a yellow phosphor with a blue LED. Other rare earths atoms have also been tested in this new host. The samples exhibit the same cubic symmetry and a typical decrease of the unit cell parameter is observed along the rare-earth series (Table 2). The optical properties of these materials are also interesting in terms of phosphor applications (Table 2). 4. Conclusions We have synthesized a high temperature cubic phase in the Ca–Si–N system with the stoichiometry CaSiN2. This phase is stabilized by cerium substitution on the calcium site. CaSiN2:Ce3+ has several important properties that make it the first of its kind and very promising for solid-state lighting applications. First, it represents the only Ce3+ phosphor materials showing an intense red emission with broad emission/excitation bands. The use of a red broad-band emitter will increase the CRI to as much as 90 for the setup comprising a blue LED with yellow and red phosphors. In order to achieve this target, the QE of the red phosphor must be further increased because it will be excited by the green photons from the yellow phosphor. Second, CaSiN2:Ce3+ is one of the rare phosphor materials that can be excited in the yellow–green region. This property is of great interest for plant growth enhancement since green light is not harvested in the photosynthetic process. With CaSiN2:Ce3+, the green photons can be down-converted into useful red photons. Thirdly, the use of chemical substitution has led to new possibilities
Table 2 Optical properties and unit cell parameters (refinement in space group F23) for CaSiN2 doped with different rare-earth ions (RE) Ca1xRExSiN2
RE = Ce3+
RE = Pr3+
RE = Sm3+
RE = Eu2+
RE = Tb3+
RE = Er3+
Body color ˚) Cubic unit cell parameter (A k Emission maximum (nm) k Excitation maximum (nm)
Pink/red 14.8822 625 535
White 14.8720 425 330
White 14.8463 610 400
Orange 14.8419 605 400
White 14.8587 550 275
Grey 14.8517 420 325
The RE doping level is 3%. For line emitter rare earths, only the strongest peak is presented.
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for the CaSiN2:Ce3+ phosphor. For example, the tunable emission of this phosphor makes it very interesting as a yellow/orange phosphor to use directly in combination with a 460 nm blue LED. It is also worth mentioning that this red phosphor can be used directly in a white light setup comprising a UV LED with three red, green and blue phosphors. Acknowledgements We thank the Solid State Lighting and Display Center at the University of California Santa Barbara for funding. We also acknowledge Profs. S.P. DenBaars and S. Nakamura for helpful discussions.
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