Luminescent, cerium doped Lu 2SiO 5 thin films with C2/c symmetry have been prepared by pulsed laser deposition (PLD) at temperatures much lower than the crystallization temperature (2150°C) of the corresponding bulk crystals. The PLD grown films show the typical luminescence resulting from the Ce 3+ 5d-4f transition. Maximum luminescence efficiency was observed for films prepared at an oxygen partial pressure of 200 mTorr at 600°C. These conditions reflect a balance between Ce 4+/Ce 3+ interconversion and the crystalline quality of the films. The results indicate that PLD offers a low temperature deposition technique for complex oxide phosphor materials. © 2006 American Institute of Physics

Bennett, BL

Cooke, DW

Jia, QX

Lee, JK

Lee, JS

Lee, SY

Muenchausen, RE

Nastasi, M

Valdez, JA

Lee, Jung-Kun

Muenchausen, R. E.

Lee, Jang-Sik

Jia, Q. X.

Nastasi, M.

Valdez, J. A.

Bennett, B. L.

Cooke, D. W.

Lee, Sang Yeol

Name not available

Structure and optical properties of Lu2SiO5:Ce phosphor thin films

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Structure and optical properties of Lu 2 Si O 5 : Ce phosphor thin filmsJ.-K. Lee, R. E. Muenchausen, Jang-Sik Lee, Q. X. Jia, M. Nastasi, J. A. Valdez, B. L. Bennett, D. W. Cooke,and Sang Yeol Lee  Citation: Applied Physics Letters 89, 101905 (2006); doi: 10.1063/1.2345373 View online: http://dx.doi.org/10.1063/1.2345373 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/89/10?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Annealing effects on Czochralski grown Lu 2 Si 2 O 7 : Ce 3 + crystals under different atmospheres J. Appl. Phys. 103, 083109 (2008); 10.1063/1.2906348  Scanning electron and cathodoluminescence imaging of thin film Lu 2 Si O 5 : Ce scintillating materials Appl. Phys. Lett. 91, 244102 (2007); 10.1063/1.2821115  Cathodoluminescence degradation of Y 2 Si O 5 : Ce thin films J. Vac. Sci. Technol. A 25, 1226 (2007); 10.1116/1.2429676  Monte Carlo study on the imaging performance of powder Lu 2 SiO 5 : Ce phosphor screens under x-rayexcitation: Comparison with Gd 2 O 2 S : Tb screens Med. Phys. 34, 1724 (2007); 10.1118/1.2724065  Scintillation characteristics of Lu 3 Al 5 O 12 : Ce optical ceramics J. Appl. Phys. 101, 033515 (2007); 10.1063/1.2433706    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  130.49.166.8On: Thu, 01 May 2014 18:51:17APPLIED PHYSICS LETTERS 89, 101905 2006 This article is Structure and optical properties of Lu2SiO5:Ce phosphor thin filmsJ.-K. Lee,a R. E. Muenchausen, Jang-Sik Lee,b Q. X. Jia, M. Nastasi, J. A. Valdez,B. L. Bennett, and D. W. CookeMaterials Science and Technology Division, Los Alamos National Laboratory, New Mexico 87545Sang Yeol LeeDepartment of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, KoreaReceived 15 March 2006; accepted 12 July 2006; published online 5 September 2006Luminescent, cerium doped Lu2SiO5 thin films with C2/c symmetry have been prepared by pulsedlaser deposition PLD at temperatures much lower than the crystallization temperature 2150 °Cof the corresponding bulk crystals. The PLD grown films show the typical luminescence resultingfrom the Ce3+ 5d-4f transition. Maximum luminescence efficiency was observed for films preparedat an oxygen partial pressure of 200 mTorr at 600 °C. These conditions reflect a balance betweenCe4+ /Ce3+ interconversion and the crystalline quality of the films. The results indicate that PLDoffers a low temperature deposition technique for complex oxide phosphor materials. © 2006American Institute of Physics. DOI: 10.1063/1.2345373Phosphors are an integral part of modern technology be-cause of their capability to convert radiation into light. Theyare used in many commercial devices including fluorescentlamps, displays, lasers, and crystal scintillation detectors forradiography. Desirable properties of a phosphor for radiationdetection include high brightness in the blue or green spec-tral region to match existing photomultiplier tube or Si pho-todiode detectors, low optical absorption, nanosecond scaledecay times, and high density.1Among phosphor materials, rare-earth-ion doped oxy-orthosilicates offer excellent scintillating properties and havebeen extensively studied for the potential development ofnew scintillating materials,2,3 for example, Ce dopedLu2SiO5 LSO:Ce activated with around 0.05 at. % of ce-rium relative to the lutetium Lu. Cerium is an excellentactivator because its ground state and excited states liewithin the band gap of about 6 eV for the host LSO lattice.Consequently, LSO:Ce shows good light output,28 000 photons/MeV, fast decay time, 40 ns, a relativelyhigh density, 7.34 g/cm3, and minimal self-absorption.While LSO:Ce is an exceptional phosphor, the materialis difficult to prepare, especially as large single crystals. Thisis at least partly due to the high cost of starting materialssuch as high purity Lu2O3 powder and equipment such asiridium crucibles used for containing the Lu2O3 powder atthe melting temperature of about 2150 °C. Moreover, stressinduced cracking of the boule upon cooling from the highgrowth temperature severely affects the yield of large areadetector elements, i.e., greater than 1 cm2. Additionally, thelow Ce segregation coefficient 0.22 in LSO makes the Ceconcentration and the luminescence efficiency of LSO:Cesingle crystals nonuniform along its growth direction.4Therefore, most of the efforts in the synthesis of LSO:Cescintillators have been devoted to the growth of good singlecrystals.aAuthor to whom correspondence should be addressed; electronic mail:jklee@lanl.govbPresent address: School of Advanced Materials Engineering, KookminUniversity, Seoul 136-702, Korea.0003-6951/2006/8910/101905/3/$23.00 89, 10190copyrighted as indicated in the article. Reuse of AIP content is subject to thOn: Thu, 01 May 2There is also much interest in identifying high perfor-mance and low cost substitutes for single crystals, either asnanocrystalline powder composites or thin films.5 Recently, asol-gel process was successfully employed to prepareLSO:Ce films and nanosize powders.6 However, LSO:Cefilms from sol-gel process require a 1000 °C postannealing,which is still too high for depositing LSO:Ce films on manytechnologically important substrates such as flexible poly-mers and plastic scintillators. In contrast to the sol-gel pro-cess, pulsed laser deposition PLD has been used to facili-tate low temperature deposition of many complex metaloxide films such as high temperature superconductors,7ferroelectrics,8 ferromagnetics,9 and transparent conductiveoxides.10 A general effect for extending the phase stability ofmetal oxides to lower temperatures is through reduction ofthe oxygen partial pressure.11 A decrease in the oxygen par-tial pressure stabilizes the phase of various oxide materialseven at lower temperature.12 This effect is readily manifestedusing PLD which typically operates at oxygen partial pres-sures of 10−4–10−2 atm, and has been reported previouslywith regard to high temperature superconducting oxides.However, thermally activated processes such as diffusionalso diminish with decreasing temperature and there is acritical point balancing a thermodynamic advantage of lowoxygen partial pressure and a kinetic disadvantage of lowtemperature.In this letter, we demonstrate growth of polycrystallineLSO:Ce films at various PLD parameters and investigatetheir optical properties. It is observed that stoichiometricLSO:Ce films can be prepared even at room temperature andthat the x-ray induced luminescence of LSO:Ce films isgreater than that of pure LSO crystals prepared by a hightemperature growth method, when normalized to the activevolume.LSO thin films were deposited using a 308 nm XeClexcimer laser on 50 nm thick Pt/Ti/ 100 Si substrate byusing 0.055 mol % Ce doped Lu2SiO5 crystal pellet as a tar-get. In addition, a 100 MgO single crystal substrate wasused to analyze the composition of as-deposited films viaRutherford backscattering spectrometry RBS. The laser en-2ergy density on the target surface was set to be 1 J /cm and© 2006 American Institute of Physics5-1e terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  130.49.166.8014 18:51:17101905-2 Lee et al. Appl. Phys. Lett. 89, 101905 2006 This article is  to th   130.49.166.8the oxygen pressure was in the range of 210−3–400 mTorr. The substrate temperature was variedfrom room temperature up to 800 °C. After deposition, thefilms were cooled to room temperature at 5 °C/min underthe oxygen flow. The nominal film thickness was 400 nm.The structure of the films was investigated by x-ray dif-fraction. A –2 scan was performed for films in the 2range of 20°–60° using Cu K radiation. The stoichiometryof the thin films was measured using RBS. To investigate theoptical properties of the films, radioluminescence RL emis-sion spectra were measured. The films were x-ray irradiatedat an energy of about 25 keV and an exposure rate of1.75 Gy s−1 in air. RL spectra were collected from the lightemitted through an optical fiber onto the entrance slit of a1 /4 m monochromator with a charge-coupled device detec-tor.Figure 1 shows the results of x-ray diffraction XRDperformed on the films as a function of temperature. Thestrongest diffraction peak, which appears near 40°, is fromthe Pt coated Si substrate. The other reflections can be as-signed to LSO with C2/c monoclinic symmetry, as indi-cated. As the deposition temperature increased, the broadpeaks around 30° started to split and the XRD pattern showsthe most intense peaks of polycrystalline LSO with mono-clinic C2/c symmetry. This indicates that the PLD methodcan form crystalline LSO films even at room temperature.Additionally, the XRD data of the LSO films show a smallshift towards lower 2 values with respect to the standardpowder pattern. The small change in the lattice parameterindicates a residual out of plane tensile strain in the lattice ofabout 0.5%, presumably due to the thermal expansion mis-match between the substrate and the film.Figure 2 shows the result of RBS analysis for a LSO filmgrown on the 100 MgO single crystal substrate at200 mTorr and 400 °C. The substrate was chosen so that theLu and Si peaks could not be buried under a substrate peak.Also shown is a simulation of the experimental data for astoichiometric Lu:Si=2:1 thin film. The good agreementbetween the simulation and the experimental data shows thata stoichiometric LSO film is readily achieved with PLD.Figure 3 shows the RL results of LSO:Ce films as aFIG. 1. X-ray diffraction patterns of Ce doped Lu2SiO5 LSO:Ce films onPt/Ti/Si deposited at 400 °C and 200 mTorr oxygen partial pressure. Insetshows XRD spectra of three LSO films deposited on a Pt/Ti/Si substrate atdifferent conditions.copyrighted as indicated in the article. Reuse of AIP content is subjectfunction of the deposition temperature under nominalOn: Thu, 01 May 2vacuum conditions 10−5 Torr. With increasing growth tem-perature, the RL intensity increased monotonically withdeposition temperature and is essentially maximized at adeposition temperature of 600 °C. The Ce emission peak at420 nm is identical to that observed in the bulk crystals,indicating that the Ce ions are located at optically activepositions in the LSO host lattice.13 Presumably, the increasein the RL intensity with deposition temperature is due toimproved crystalline quality with a concomitant reduction inelectronic defects that promote nonradiative relaxation. Toaddress this point further, RL was examined between 75 and300 K. It is well known that for single crystal LSO:Ce, the5d-4f electronic transition is split into well defined crystal-field levels. Since the 4f level is split due to spin-orbit inter-actions, low temperature RL shows two resolved peaks in themain band emission around 420 nm below 75 K.14 Tempera-ture dependent RL of the PLD LSO:Ce films did not demon-strate this effect, showing only one broad emission peakdown to 75 K. This indicated a significantly more disorderedcrystalline structure, presumably resulting from broadeningFIG. 2. Rutherford backscattering spectrum of Ce doped Lu2SiO5 LSO:Cefilms on MgO and simulation results assuming that the formula is Lu2SiO5.The determination of the ratio of Lu:Si near the expected 2:1 stoichiometryis observed.FIG. 3. RL emission spectra of four Ce doped Lu2SiO5 LSO:Ce filmsdeposited under 210−5 Torr between room temperature and 600 °C. TheRL curve at 800 °C has been omitted for clarity since it essentially overlapse terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:the RL curve at 600 °C.014 18:51:17101905-3 Lee et al. Appl. Phys. Lett. 89, 101905 2006 This article is of the spin-orbit 4f levels due to the stress in films observedin XRD results of Fig. 1.As an additional observation, the RL spectra shown inFig. 3 were integrated and compared to bulk LSO:Ce emis-sion. By defining a relative figure of merit of the measuredintensities normalized to the active volume, it can be inferredthat the thin film brightness 333 000±33 000 / cm3 at ouroptimal deposition conditions is nominally twice that of thesingle crystal LSO:Ce material 167 000±17 000 / cm3.This result suggests that the Ce emission from a polycrystal-line LSO lattice is still quite efficient. This may indicate thatthe Ce is efficiently distributed in each of the monoclinicsites in the C2/c structure, having oxygen coordinations of 6and 7, respectively, and is essentially unaffected by disorderof the isolated silica tetrahedra. Additionally, the Debye-Scherrer broadening of the XRD peaks indicates a grain sizeof nominally 30–50 nm. This compares favorably withatomic force microscopy images not shown that indicated a20 nm grain size and a 5 nm rms surface roughness. Lumi-nescence studies of hydrothermally prepared Ce dopedY2SiO5 nanopowders in a comparable size range have alsodemonstrated enhanced light output consistent with theseobservations.15Another important parameter of the metal oxide filmgrowth is the partial pressure of oxygen during the growth.Figure 4 shows a plot of the RL intensity films as a functionof the oxygen partial pressure at a fixed deposition tempera-ture of 400 °C. An increase in RL intensity was observedwith increasing oxygen partial pressure up to 200 mTorr,above which the RL intensity saturated. The saturation oflight output as a function of oxygen partial pressure may berelated to optical activation of Ce ions. To optimize the lu-minescence of LSO:Ce, Ce4+ in LSO should be converted toCe3+.16 This favors a reducing environment during the filmdeposition. However, the growth of films in vacuum, a strongreducing environment, is expected to decrease the crystallinequality of the films with a concomitant reduction in quantumefficiency. The saturation of the RL intensity suggests that3+ 4+FIG. 4. Radioluminescence intensity vs the oxygen partial pressure for filmsdeposited at 400 °C.the trade-off between Ce /Ce interconversion and defectcopyrighted as indicated in the article. Reuse of AIP content is subject to thOn: Thu, 01 May 2formation gives rise to a maximum in the luminescence ef-ficiency at an oxygen partial pressure of about 200 mTorr.The present study shows that Ce doped LSO films havebeen grown at a temperature ranging from room temperatureto 800 °C, which is much lower than preparation tempera-ture of bulk or powder LSO:Ce. LSO:Ce thin films showefficient luminescence centered at 420 nm at room tempera-ture, which is consistent with LSO:Ce single crystals. Theluminescence of polycrystalline LSO:Ce films exhibits opti-mum performance when the film was grown at 600 °C underan oxygen partial pressure of 200 mTorr. The relative bright-ness for the optimized thin films, normalized to the irradiatedvolume, is higher than that for bulk single crystal LSO:Ce.These parametric studies do not allow us to unambiguouslydistinguish the effects that crystal cation defects and oxygenvacancies may exert on the overall light output. Further workwill explore the local chemical environment of the Ce acti-vator using electron paramagnetic resonance as well as thepossibility of further increasing the thin film brightness bydoping to higher Ce concentrations. The latter point is espe-cially noteworthy since activator doping levels achievablevia PLD are not restricted by the low Ce segregation coeffi-cient in LSO, which limits the equilibrium concentration to0.05 mol % in single crystal bulk LSO.Discussions with Yongqiang Wang of the Los AlamosIon Beam Materials Laboratory are gratefully acknowledged.This work was supported by the U.S. Department of EnergyOffice of Basic Energy Sciences 07SCPE972 and Labora-tory Directed Research and Development at LANL20040213ER.1Phosphor Handbook, edited by S. Shionoya and W. M. Yen CRC, NewYork, 1999, p. 539.2C. D. Brandle, A. J. Valentino, and G. W. Berkstresser, J. Cryst. Growth79, 308 1986.3J. Shmulovich, G. W. Berkstresser, C. D. Brandle, and A. Valentino, J.Electrochem. Soc. 135, 3141 1988.4C. L. Melcher, R. A. Manente, C. A. Peterson, and J. S. Schweitzer, J.Cryst. Growth 128, 1001 1993.5C. Mansuy, J. M. Nedelec, and R. Mahiou, J. Mater. Chem. 14, 32742004.6C. Mansuy, R. Mahiou, and J. M. Nedelec, Chem. Mater. 15, 3242 2003.7M. Lorenz, H. Hochmuth, D. Natusch, H. Borner, G. Lippold, K. Kreher,and W. Schmitz, Appl. Phys. Lett. 68, 3332 1996.8J. K. Lee, C. H. Kim, and K. S. Hong, Appl. Phys. Lett. 80, 3593 2002.9M. Ziese, H. C. Semmelhack, and K. H. Han, Phys. Rev. B 68, 1344442003.10M. A. Morales-Paliza, R. F. Haglund, and L. C. Feldman, Appl. Phys.Lett. 80, 3757 2002.11R. Beyers and B. T. Ahn, Annu. Rev. Mater. Sci. 21, 335 1991.12Y. Niu, F. Gesmundo, and G. Cacciamani, Intermetallics 11, 1361 2003.13H. Suzuki, T. A. Tombrello, C. L. Melcher, and J. S. Schweitzer, IEEETrans. Nucl. Sci. 40, 380 1993.14D. W. Cooke, K. J. McClellan, B. L. Bennett, J. M. Roper, M. T. Whit-taker, and R. E. Muenchausen, J. Appl. Phys. 88, 7360 2000.15D. W. Cooke, J.-K. Lee, B. L. Bennett, J. R. Groves, L. G. Jacobsohn, E.A. McKigney, R. E. Muenchausen, M. Nastasi, K. E. Sickafus, M. Tang,and J. A. Valdez, Appl. Phys. Lett. 88, 103108 2006.16D. W. Cooke, B. L. Bennett, K. J. McClellan, R. E. Muenchausen, J. R.Tesmer, and C. J. Wetteland, Philos. Mag. B 82, 1659 2002.e terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  130.49.166.8014 18:51:17