Red-emitting cerium-based phosphor materials for solid-state lighting applications

Red-emitting cerium-based phosphor materials for solid-statelighting applicationsR.Le Toquin,A.K.Cheetham*Materials Research Laboratory,University of California,MRL MC-CAM Building,Room 3117C,Santa Barbara,CA 93106,USAReceived 1March 2006Available online 3April 2006AbstractThe discovery of phosphor materials that can be excited around 380or 460nm is essential for improving the efficiency and light qual-ity of LED-based solid-state lighting devices.We have found,for the first time,a luminescent cerium compound emitting bright red light.CaSiN 2:Ce 3+is a new phase in the calcium silicon nitride ternary system that crystallizes in a face-centered cubic unit cell with a latticeparameter of a =14.8822(5)A˚.This compound can be used for white light applications:(i)to enhance the light quality of the system 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 460nm LED due to the emission/excitation band shift with chemical substitution.Ó2006Elsevier B.V.All rights reserved.1.IntroductionThe 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 appli-cation [1,2].The longevity of the LEDs and most impor-tantly 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 460nm 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 photo-luminescence 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 460nmcombined with a Y 3Al 5O 12:Ce 3+(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 $70is produced that may,in principle,be improved by adding red photons from a complementary phosphor.The second generation of white LEDs is still based on blue InGaN LEDs but with a mix of green and orange phosphors that increases the CRI up to 90and makes the phosphor deposition process potentially easier [3].In fact,new red or orange phosphors are urgently needed for the development of efficient white LEDs,regardless of the type of LED used,and they need to be very bright in order to compensate for the low red sensitiv-ity of the human eye [4].Bright red phosphors are,how-ever,very difficult to achieve due to the quantum yield drop with increasing Stokes shift.We focused our work on cerium-based materials for two reasons.First,among all the trivalent lanthanides,Ce 3+is the only one characterized by strong f !d transitions.Tri-valent cerium has a 4f 15d 0ground state configuration and its optical transitions in the visible region arise from 4f 1!5d 1excitations.As with Eu 2+,cerium-based phos-phor materials have broad excitation/emission bands that can be matched to the characteristics of the LED and can possibly mimic the solar spectrum.Second,the involvement0009-2614/$-see front matter Ó2006Elsevier B.V.All rights reserved.doi:10.1016/j.cplett.2006.03.056*Corresponding author.E-mail address:cheetham@ (A.K.Cheetham)./locate/cplettChemical Physics Letters 423(2006)352–356of5d levels implies that the emission wavelength can be tuned by changing the energies of the5d levels with respect to the deeper4f levels.For instance,by adjusting the crystal field splitting,it is possible to lower the lowest5d level in order to decrease the5d–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],respec-tively.However,high crystalfield 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(540nm)upon blue excitation(460nm)due to the very low energy of the lowest 5d level[8].A small tetragonal distortion of the cubic crys-talfield at the cerium site is responsible for this unusual yel-low emission[9].It is also possible to observe green-yellow Ce3+emission in oxynitride[10]or sulfides[7,11]com-pounds,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 suffi-ciently 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 crystalfield splitting due to sym-metry,coordination number and covalency factors.Several 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 CaSiN2to be orthorhombic with space group Pnma and cell parameters a0=5.1229A˚,b0=10.2074A˚and c0=14.8233A˚using single crystal X-ray diffraction.In the present work,we were able to design,for thefirst time, a red phosphor material based on CaSiN2that is very promising for solid-state lighting applications.2.ExperimentalStoichiometric amounts of Ca3N2and Si3N4were weighed in a glove box([O2]<1ppm and[H2O]<1ppm) in order to prevent oxidation or hydrolysis of the reactants. CeO2,Eu2O3,Sm2O3,Tb4O7,Pr2O3,Er2O3were used as rear earth sources.For doping purposes,we used AlN, Mg2N3and Sr2N.Following thorough mixing and grind-ing,the mixture was loaded into an alumina crucible. The heating was carried out in a horizontal tube furnace underflowing N2(2–4l/min)between1300°C and 1500°C for several hours.The purity of all samples was checked using conventional X-ray diffraction.Lumines-cence properties were measured using a Perkin Elmer LS55UV/Visible spectrophotometer.The external quan-tum efficiency(QE)was measured at515nm using an Argon laser.The setup comprises an integration sphere, twofilters(to differentiate the laser excitation and thefluo-rescence)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 effi-ciency of the Ce3+:YAG phosphor standard has been mea-sured to be around70%.Synchrotron X-ray powder diffraction was performed at Brookhaven National Labo-ratory on beamline X7A with a wavelength of0.7972A˚.3.Results and discussionThe high temperature synthesis of CaSiN2doped 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($450–550nm),which is of primary interest for solid-state lighting application.In order to characterize the structure of this phase,we used X-ray powder diffing conven-tional X-ray diffraction methods,we were able to show that CaSiN2doped with Ce3+seems to crystallize in an F-cen-tered cubic unit cell with a very large unit cell parameter of about14.88A˚.In order to assess the true symmetry and further probe the structure,we used the very high res-olution of an X-ray synchrotron beam to measure the pow-der diffraction pattern.The cell parameters and possible space groups were determined using the F ULLPROF S UITE of programs[17].This study showed that the rare-earth doped CaSiN2phase has indeed cubic symmetry and that five possible space groups,F23,Fm 3,F432,F 43m andR.Le Toquin,A.K.Cheetham/Chemical Physics Letters423(2006)352–356353Fm 3m,can account for all the reflections (Fig.2).Afterrefinement,the cell parameter value is a =14.8822(5)A˚leading to a cell volume larger than 3200A˚3.However,ab initio structure solution is very challenging,especially due to the high symmetry,the lack of heavy elements,and most of all the large unit cell volume (3200A˚3,leading to at least 15independent 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-cen-tered cubic unit cell with a cell parameter of about14.864(12)A˚[14].More recently,Gal et al.have solved the structure of the ternary phases MSiN 2with M =Ca,Sr,Ba using single crystal X-ray diffraction [15].Their study suggests that the Ca phase has an orthorhombic sym-metry (space group Pbca)with unit cell parameter values ofabout a =5.1229(3)A˚,b =10.2074(6)A ˚,c =14.8233(9)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 related to our cubic cell through the following relations:a c =2p 2a 0,a c =p 2a 0a c =c 0,V c =4·V 0.Substitution of cerium into CaSiN 2appears to stabilize the cubic poly-morph.The structure of the orthorhombic CaSiN 2has been described to be based on vertex sharing SiN 4tetrahe-dra comparable to the D1-type tilting distortion of the b -cristobalite phase.The phase is reported to be isostruc-tural with KGaO 2.The corner sharing SiN 4tetrahedra 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 significantlyaffects the emission wavelength of the phosphor.For the Ce 3+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 CaSiN 2may 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 Ce 3+atoms in any host reported so far (Fig.3).The maximum of the emission band is centered around 625nm,which is at least 50nm longer than the largest cer-ium emission wavelength reported to date.The emission peak extends from 550to 700nm with a full width at half maximum (FWHM)of about 80nm.For comparison,we also examined one of the red standards,Eu 3+-doped Y 2O 2S,to emphasize the fact that using a broad-band emit-ter 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 535nm and extends from 425to 575nm with a FWHM of about 75nm.Besides the obvi-ous 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-con-verts them into red photons.Coating this material on glass window with a complementary UV to bluephosphorFig.2.Synchrotron X-ray diffraction pattern of the phase CaSiN 2doped with Ce 3+.Le Bail refinement in the space group F23with unit cellparameter a =14.8822(5)A˚.The red circles,black and blue curves represent the experimental data,the calculated pattern and the difference curve respectively.The green marks show the position of the peaks.354R.Le Toquin,A.K.Cheetham /Chemical Physics Letters 423(2006)352–356would 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 about2700cmÀ1.This value is rather smaller than that of the YAG(3800cmÀ1),thereby corroborating the diffi-culty in observing red emission for cerium in YAG-based phosphors.Thisfinding is also consistent with the rela-tively 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(515nm) 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 about70nm (Fig.4).The excitation blue-shifts,too,from a peak max-imum at535nm to475nm for10%Al substitution(Fig.4). With the Al substitution,therefore,this compound becomes interesting as a yellow phosphor for direct appli-cation with a blue LED emitting around460nm.On the other hand,Sr substitution on the Ca site leads to a red shift of the emission which was not expected according to the compression at the cerium sites(Table1).Mg substitu-tion seems to follow the same trend since the emission is blue shifted by about90nm(Table1).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 wave-length[9].Due to the yellow emission and the460nm exci-tation,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(Table2). The optical properties of these materials are also interest-ing in terms of phosphor applications(Table2).4.ConclusionsWe 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 thefirst 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 as90for the setup comprising a blue LED with yel-low 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 phos-phor 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 pho-tosynthetic 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 possibilitiesTable1Optical properties of Ce3+doped CaSiN2substituted with different cationson the Ca and Si sitesCe3+dopedcompoundEmissionmaximumExcitationmaximumBody colorCaSiN2625535Pink/redCa(Al,Si)N2560475Orange(Ca,Sr)SiN2640500Orange(Ca,Mg)SiN2540460YellowTable2Optical properties and unit cell parameters(refinement in space group F23)for CaSiN2doped with different rare-earth ions(RE)Ca1Àx RE x SiN2RE=Ce3+RE=Pr3+RE=Sm3+RE=Eu2+RE=Tb3+RE=Er3+ Body color Pink/red White White Orange White Grey Cubic unit cell parameter(A˚)14.882214.872014.846314.841914.858714.8517 k Emission maximum(nm)625425610605550420k Excitation maximum(nm)535330400400275325The RE doping level is3%.For line emitter rare earths,only the strongest peak is presented.R.Le Toquin,A.K.Cheetham/Chemical Physics Letters423(2006)352–356355for 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 a460nm 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.AcknowledgementsWe 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.References[1]S.Nakamura,G.Fasol,The Blue Laser Diode:GaN Based LightEmitters 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.SPIE4776(2002)122.[4]N.Sharma,N.Kijima,A.K.Cheetham,Chem.Phys.Lett.387(2004)2;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.[5]G.Blasse,B.C.Grabmeier,Luminescent Materials,Springer,Berlin,1994.[6]N.Kodama,Y.Tanii,M.Yamaga,J.Luminescence87–89(2000)1076.[7]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.[8]G.Blasse,A.Brill,Appl.Phys.Lett.11(1967)53.[9]G.Blasse,A.Brill,Chem.Phys.47(1967)5139;A.K.Cheetham et al.,unpublished results.[10]J.W.H.van Krevel,H.T.Hintzen,R.Metselaar,A.Meijerink,J.Alloys Compd.268(1998)272.[11]G.Gauthier,S.Jobic,M.Evain,H.J.Koo,M.H.Whangbo,C.Fouassier,R.Brec.Chem.Mater.15(2003)828.[12]J.W.H.van Krevel,J.W.T.van Rutten,H.Mandal,H.T.Hinzen,R.Metselaar,J.Solid State Chem.165(2002)19.[13]K.Uheda,N.Hirosaki,H.Yamamoto,H.Yamane,Y.Yamamoto,W.Inami,K.Tsuda.Proceedings of the2004Joint International Meeting of the Electrochemical Society Oct.3–Oct.8.[14]E.A.Pugar,J.H.Kennedy,P.E.D.Morgan,J.H.Porter,J.Am.Ceram.Soc.71(1988)C-288.[15]Z.A.Gal,P.M.Mallinson,H.J.Orchard,S.J.Clarke,Inorg.Chem.43(2004)3998.[16]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.[17]F ULLPROF J.Rodriguez-Carvajal,T.Roisnel,F ULLPROF98and Win-plotr,New windows95/NT Application for diffmission for Powder Diffraction,International Union of Crystallography.Newsletter No.20(May–August)Summer1998.356R.Le Toquin,A.K.Cheetham/Chemical Physics Letters423(2006)352–356。