Characterization of Photoluminescent (Y1–xEux)2O3 Thin FilmsPrepared by Metallorganic Chemical Vapor Deposition

Joanna McKittrick,* Carlos F. Bacalski,* and G. A. Hirata

Department of Mechanical and Aerospace Engineering and Materials Science Program,University of California at San Diego, La Jolla, California 92093-0411

Kevin M. Hubbard, S. G. Pattillo, Kenneth V. Salazar, and M. Trkula

Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

The purpose of this study was to identify and correlate themicrostructural and luminescence properties of europium-doped Y2O3 (Y1–xEux)2O3 thin films deposited by metallor-ganic chemical vapor deposition (MOCVD), as a function ofdeposition time and temperature. The influence of depositionparameters on the crystallite size and microstructural mor-phology were examined, as well as the influence of theseparameters on the photoluminescence emission spectra. (Y1–x-Eux)2O3 thin films were deposited onto (111) silicon and (001)sapphire substrates by MOCVD. The films were grown byreacting yttrium and europium tris(2,2,6,6-tetramethyl–3,5-heptanedionate) precursors with an oxygen atmosphere at lowpressures (5 torr (1.73 103 Pa)) and low substrate tempera-tures (500°–700°C). The films deposited at 500°C were smoothand composed of nanocrystalline regions of cubic Y2O3, grownin a textured [100] or [110] orientation to the substrate surface.Films deposited at 600°C developed, with increasing depositiontime, from a flat, nanocrystalline morphology into a platelikegrowth morphology with [111] orientation. Monoclinic (Y 1–x-Eux)2O3 was observed in the photoluminescence emissionspectra for all deposition temperatures. The increase in pho-toluminescence emission intensity with increasing postdeposi-tion annealing temperature was attributed to the surface/grainboundary area-reduction effect.

I. Introduction

PHOTOLUMINESCENT and cathodoluminescent thin films havepotential for application in emissive flat-panel displays such as

field-emission and plasma-panel displays. Field-emission flat-panel displays require thinner screens that operate at lowervoltages, without sacrificing brightness or contrast. Thin films, asopposed to the traditional discreet powder screens, offer the benefitof reduced light scattering, a reduction of material waste, and thepotential for fabricating smaller pixel sizes, to enhance resolution.Europium-doped Y2O3, (Y1–xEux)2O3 (x 5 0.01–0.20), is awell-known photoluminescent and cathodoluminescent materialused in cathode ray tube screens and fluorescent lighting.1 Thefluorescent emission of the material results from the 4f 3 4felectronic transitions in the Eu31 ion. Figure 1 shows the energy-

level diagram for the free Eu31 ion. Because of shielding by theouter 5s and 5p electrons, the free ion scheme can be used toanalyze the transitions in an ionic crystal. Excitation of the Eu31

ion results in the promotion of the electrons into the5DJ manifold,with emission occurring during relaxation to the7FJ ground statemanifold.

The luminescence properties of (Y1–xEux)2O3 have been studiedmainly in single crystals2–6 at concentration levels of 0.01# x ,0.05. (Y1–xEux)2O3 is an insulator, with a bandgap energy of 5.6eV, and is strongly absorbing at energies with wavelengths,230nm. Y2O3 has a cubic space group,Th

7, containing 8 yttrium sitesof C3i point symmetry and 24 sites ofC2 point symmetry. Theionic radii of Y31 and Eu31, 0.092 and 0.098 nm, respectively, aresimilar, and the Eu31 ions populate the two different site symme-tries with equal probability.6 The emission spectrum is character-ized by an intense, narrow feature at 611 nm, arising from the5Do 3

7F2 transition from Eu31 in the C2 site. Less intensefeatures occurring in the 580–599 nm region arise from Eu31

occupation of both theC2 andC3i sites and involve the5Do37F1

transitions. Figure 2 shows typical photoluminescent (PL) excita-tion and emission spectra for powders of (Y1–xEux)2O3, with thetransitions labeled.

C. A. Handwerker—contributing editor

Manuscript No. 189941. Received August 20, 1998; approved October 21, 1999.Work conducted under the auspices of the U.S. Department of Energy, supported

(in part) by funds provided by the University of California, for the conduct ofdiscretionary research; by Los Alamos National Laboratory, under project CULARand the Visiting Scholar Program; and through the Phosphor Technology Center ofExcellence, by DARPA Grant No. MDA972-93-1-0030.

*Member, American Ceramic Society.

Fig. 1. Partial energy level diagram for Eu31 in Y2O3. Strong transitioncausing the 611 nm peak in the fluorescence spectra is shown; chargetransfer is the lowest-level absorption band in this system, with thebandgapabsorption occurring at a higher energy level. (Adapted from Ref. 5.)

J. Am. Ceram. Soc.,83 [5] 1241–46 (2000)

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Profound microstructural influences have been observed on thePL spectra of (Y1–xEux)2O3 powders.7 Small crystallites, typically,200 nm, show a weak fluorescence and an increase in PLintensity with an increase in crystallite size. The increase incrystallite size is achieved by postsynthesis, high-temperatureannealing (.1400°C). For extremely fine crystallite sizes, Eilersand Tissue8 reported both monoclinic (Y1–xEux)2O3 and Eu2O3

formation in nanocrystalline powders produced by gas condensa-tion. Grytsivet al.9 found both cubic and monoclinic Y2O3 in thinfilms prepared by reactive radio-frequency diode sputtering.

The monoclinic phase is a high-temperature phase obtained inbulk by quenching under high pressure.10 Nanocrystalline mono-clinic Y2O3 is thought to exist under room-temperature conditionsbecause of the Gibbs–Thompson effect.11 The PL fluorescencespectra of the 12 nm monoclinic (Y1–xEux)2O3 particles consist ofa large feature at 625 nm and two smaller features at 615 and 619nm, attributed to the5Do3

7F2 transitions. Nanocrystalline Eu2O3

shows a fluorescence spectrum similar to that of (Y1–xEux)2O3,except that the feature at 615 nm is split into two features, centeredat ;615 nm.

A later work12 found the solubility of Eu31 to be lower inmonoclinic Y2O3 than in cubic Y2O3: Samples with$0.7 at.%Eu31 were composed of two phases, monoclinic Y2O3 and Eu2O3.Not all nanocrystalline Y2O3 powders are monoclinic. No mono-clinic phase was found in combustion-synthesized (Y1–xEux)2O3

powders 14–26 nm in diameter.13 During combustion synthesis,the reaction temperature usually is.1500°C. Apparently, stabili-zation of the monoclinic phase is dependent on the processingtechnique and more likely to form at lower synthesis temperatures.

Thin-film (Y1–xEux)2O3 has been deposited by low-pressuremetallorganic chemical vapor deposition (MOCVD),14,15 spraypyrolysis,16 and pulsed-laser ablation.17–19 Independent of thedeposition technique, the as-synthesized films were found to beweakly luminescent and required postdeposition annealing toincrease the emission intensity. The increase in luminescence wasrelated to the crystallite size,20 an effect similar to that found inpowder samples. Films containing smaller crystallites had moregrain boundaries, thought to be nonluminescent or evenluminescence-quenching regions.7 It is difficult to decouple thethermal annealing effects, such as diffusion and lattice-parameterchanges, from the increase in grain size. In (Y1–xEux)2O3, theC2

sites (lower symmetry) have stronger transitions than theC3i sites(higher symmetry).6An entirely random distribution of Eu31 onthe Y31 sites may not be present in the as-deposited thin films,thereby altering the fluorescence spectrum. This situation could beespecially true for high concentrations of Eu31, in which Eu31–Eu31 interactions could also modify the luminescent spectrum.

The purpose of this study was to identify and correlate themicrostructural and luminescence properties of (Y1–xEux)2O3 thin

films deposited by MOCVD, as a function of deposition time andtemperature; the influence of deposition parameters on the crys-tallite size and microstructural morphology has not been studiedpreviously in this system. In addition, the present work sought toquantify and separate crystallite-size effects from other thermaleffects on the resulting luminescent properties.

II. Experimental Procedure

Figure 3 depicts a schematic diagram of the stainless-steel,cold-wall vertical MOCVD reactor. The organometallic precursorwas delivered to the reactor through a nozzle with a shower headlocated 5 cm above the hot stage. The shower head was 3 cm indiameter and contained an array of 3 mm circular holes. Sapphiresubstrates 2.54 cm in diameter, with the (001) face exposed, and(111) silicon pieces were placed on the hot stage. Stainless-steelbubblers containing yttrium and europium tris(2,2,6,6-tetramethyl–3,5-heptanedionate) precursors were heated to 146°C,to sublimate the organometallic species. An argon carrier gas waspassed through the heated bubblers, to transport the vapor to thereaction chamber. Ultra-high-purity oxygen gas, used as theoxidant during the growth process, was flowed into the chamberthrough two inlets positioned at660° to the substrates. Duringdeposition, the substrate temperature was maintained between500° and 700°C, monitored by a thermocouple. Mass-flow con-trollers regulated the argon and oxygen flows separately. The totalpressure inside the chamber was maintained by downstreamcontrol. Table I lists the present deposition parameters.

After deposition, the heating stage was turned off, and thesamples were allowed to cool to room temperature inside thechamber. Rutherford backscattering (RBS) was used to determinethe Eu31 concentration in the films. The crystallinity of the filmswas analyzed by X-ray diffractometry (XRD) and observed byscanning electron microscopy (SEM). The crystallite size wasmeasured by X-ray line broadening, using Warren–Averbachanalysis.21 Small scratches were cut into the films by a diamondstylus, and the film thickness was measured with a surfaceprofilometer. PL measurements were made with a charge-coupleddetector (CCD; Fairchild/Weston CCD Imaging, Milpitas, CA)with an ultraviolet mercury excitation source. A spot 2 mm indiameter was applied normal to the film, and the emission spectrawere collected in reflection mode with a 200mm fiber-optic cable.Postdeposition processing consisted of annealing one sample,deposited onto sapphire at 600°C for 8 h, at 850°C for 24 and108 h.

Fig. 2. Typical absorption and emission spectrum for (Y1–xEux)2O3

powders. (Adapted from Ref. 25.)

Fig. 3. Schematic diagram of the MOCVD chamber.

1242 Journal of the American Ceramic Society—McKittrick et al. Vol. 83, No. 5

III. Results and Discussion

All films were luminescent in the as-deposited state, indicatingcrystallinity. The films were smooth and, to the eye, appearedcontinuous. Table II lists the thickness and Eu31 concentration fora series of films deposited for 1, 2, and 4 h at600°C. The growthrate was constant at 0.21mm/h (R5 0.98); however, the europiumconcentration (x) decreased as the deposition time increased,which was not expected. West and Beeson14 determined thatx canbe expressed by

x 5

CEu expS2DHEu

RTEuD

CEu expS2DHEu

RTEuD 1 CY expS2DHY

RTYD (1)

whereCEu andCY are constants,DHEu andDHY the enthalpies ofvaporization,R the gas constant, andTEu and TY the reservoirtemperatures of the precursors. Experimental results indicated thatDHEu 5 DHY and that, at 140°C,CY 5 19CEu. With the reservoirtemperatures held at 147°C, the expected result wasx 5 0.05;however RBS data showed thatx ranged from 0.18 to 0.31. Apossible explanation for the high europium content in all of thefilms, coupled with the decrease in europium concentration withincreasing deposition time, could be related to the nucleationkinetics. RBS data also indicated that the europium concentrationthrough the film thickness was not constant: A higher concentra-tion existed near the substrate/film interface. Apparently, underthese deposition conditions, nucleation of Y2O3 is not as rapid asthat of Eu2O3.

XRD studies revealed a series of compositional and structuralchanges in the films deposited onto (001) sapphire substrates atdifferent deposition temperatures. Depositions for 6 h at500°C or2 h at 600°C were composed of cubic Y2O3, oriented in the [100]or [110] direction perpendicular to the substrate, as evidenced bythe strong (400) and (440) peaks shown in Fig. 4. This result issupported by previous work,17 which showed that MOCVD filmsgrown onto (001) sapphire substrates were oriented in the [100]direction, but that the same films were randomly oriented whendeposited onto In2O3:Sn41-coated glass substrates or polycrystal-line Al2O3 substrates. Other work,14 on MOCVD films deposited

onto polycrystalline Al2O3 at 525°C, revealed polycrystallinecubic (Y1–xEux)2O3.

A 4 h deposition at 600°C showed peaks for cubic Y2O3

oriented in the [100] direction; however additional peaks emergedthat could be indexed to monoclinic Y2O3, as shown in Fig. 4.Monoclinic Y2O3 also was detected on films deposited onto (111)silicon. This report is the first observation of monoclinic(Y1–xEux)2O3 in thin films. After a 4 h deposition at 700°C, theXRD pattern had narrower and more intense peaks, with the mostintense for (222), indicating a [111] texture. Monoclinic Y2O3 wasstill detected in the pattern, as shown in Fig. 4, although thefraction was smaller than in the 600°C sample. No cubic ormonoclinic Eu2O3 was detected. The XRD data are summarized inTable II.

Table I. MOCVD Deposition Parameters

Parameter Description

Substrate Sapphire (001) or silicon (111)Substrate temperature 500°–700°CCarrier gas ArgonMetallorganic precursors Y or Eu tris(2,2,6,6-tetramethyl–3,5-heptanedionate)Y carrier-gas flow rate 100 sccmEu carrier-gas flow rate 10 sccmY reservoir temperature 146°CEu reservoir temperature 146°COxygen gas flow rate 100 sccmChamber pressure 5 torrDeposition time 1–8 h

Table II. Film Thickness, x, in (Y12xEux)2O3, and PhasesPresent

Substratetemperature

(°C)Eu flow(sccm)

Time(h)

Thickness(mm) x

Phases present(on silicon substrates)†

500 10 6 1.38 c-Y2O3 [100] and [110]600 10 1 0.06 0.31‡

600 10 2 0.40 0.29c-Y2O3 [100] and [110]600 10 4 0.73 0.18c [100] 1 m-Y2O3700 10 4 1.51 c [111] 1 m-Y2O3

†m 5 monoclinic,c 5 cubic. ‡Signal too weak.

Fig. 4. XRD pattern of films deposited onto (001) sapphire at 500° (6 h),600°, and 700°C (4 h) ((*) monoclinic Y2O3 peaks (Powder DiffractionFile, No. 44–399, International Centre for Diffraction Data, NewtownSquare, PA). CuKa radiation (l 5 0.154 nm); 2u 5 35°–40° region wasintentionally omitted because of the strong sapphire peak; strong peaks forcubic Y2O3 are labeled (Powder Diffraction File, No. 41–1105).

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These orientation results are in contrast to those from otherwork on films pulsed-laser deposited onto (100) silicon and (111)diamond. Those earlier films grew in a [111] orientation atdeposition temperatures#500°C, while higher deposition temper-atures ($700°C) resulted in a preferential [100] growth direc-tion,18,19opposite the present observations. It has been reported19

that the (111) surface is the low-energy surface in Y2O3, so thatgrowth of these surfaces would be preferred, as was observed forthe 700°C deposits in the present work. However, the initialgrowth direction was along the [100] direction, suggesting that theinterface between the (001) sapphire and (100) Y2O3 should be thelow-energy configuration.

SEM photographs revealed different microstructural features inthe films grown onto silicon at different temperatures. At 500°C,the films were cracked, as shown in Fig. 5(a), likely because of athermal expansion mismatch. At higher magnifications, smallcrystallites,#300 nm, were observed. The thermal stress in a thinfilm is given by Ohring22 as

sf~T! 5~as 2 af!~T 2 To! Ef

1 2 nf(2)

where the subscripts “f” and “s” refer to the film and substrate,respectively; sf is the film stress,a the thermal expansioncoefficient,T the measurement temperature, andTo the depositiontemperature; andE and n are the elastic modulus and Poisson’sratio, respectively. The thermal expansion coefficients for siliconand Y2O3 are 43 1026/°C and 8.13 1026/°C, respectively, andthe elastic modulus and Poisson’s ratio for Y2O3 are 151.7 GPa

and 0.30, respectively.23 The films would be in residual tension,and the film stress is an order of magnitude larger than the flexuralstrength of Y2O3 (123 MPa).23

The films deposited onto sapphire did not exhibit thermalcracking, as shown in Fig. 6, because of the lower thermalexpansion coefficient of (001) sapphire (93 1026/°C). The filmsgrown on sapphire had an appearance similar to that reported forMOCVD (Y1–xEux)2O3 films deposited at 525°C onto polycrys-talline Al2O3,

14 also with no thermal cracking. A 4 h deposition at600°C yielded an entirely different microstructure, as shown inFig. 7. The film was 0.73mm thick, and the microstructureconsisted of an unusual platelet-type growth, not observed previ-ously in films of this composition. Despite the thermal mismatch,the films were not cracked, possibly because of the discontinuity ofthe grains in the microstructure.

Increasing the deposition temperature to 700°C for 4 h yieldeda similar, but coarser, microstructure than that achieved for the600°C deposition, as shown in Fig. 8. Apparently, microstructuraldevelopment consisted of the initial nucleation of cubic Y2O3

oriented in the [100] or [110] directions, because of interfacialenergy constraints between sapphire and Y2O3. Higher depositiontemperatures promoted growth of the low-energy (111) surface ofthe cubic Y2O3, along with the nucleation of monoclinic Y2O3.Possibly, the presence of monoclinic Y2O3 affected the growth andmorphology of the cubic Y2O3. The platelet-type growth also wasobserved on sapphire substrates, indicating that stress relief alonedid not promote this type of growth.

PL measurements taken on films grown onto sapphire revealedthat increasing the substrate temperature increases the intensity ofthe emission spectrum, as shown in Fig. 9. (Y1–xEux)2O3 is cubic,and orientation effects are not expected to influence the PL spectra.The features for all samples were broad (as compared with Fig. 2),indicating a large degree of disorder around the Eu31 ion. Thestrong 611 nm feature is a very good indicator of cubic (Y1–xEux)2O3,verifying that all of the films contained this phase.

The large feature at 628 nm was previously assigned tomonoclinic (Y1–xEux)2O3.

8,12 Although monoclinic (Y1–xEux)2O3

was not detected by XRD in the 500°C deposits, a small amount ofthis phase must have been present in these films, because spectro-scopic techniques are much more sensitive than XRD for detectingsmall amounts of materials. Increasing the deposition temperatureincreased the intensity of the 611 nm feature, indicating anincrease in the volume fraction of cubic Y2O3. The emission-intensity increase can be attributed to both grain growth (minimiz-ing grain-boundary area) and a reduction of disorder around theEu31 in the lattice.

To evaluate the influence of crystallite size on luminescentproperties, while maintaining a constant thermal profile, a filmdeposited onto sapphire at 600°C for 8 h was postannealed at

Fig. 5. SEM photographs of the films deposited onto (100) silicon at500°C for 6 h.

Fig. 6. SEM photograph of the films deposited onto (001) sapphire for6 h at 500°C.

1244 Journal of the American Ceramic Society—McKittrick et al. Vol. 83, No. 5

850°C for 24 or 108 h, to promote crystallite growth. Figure 10shows that longer annealing treatments induced an increased XRDpeak intensity. Sharper peaks also are observed in Fig. 10, alongwith a slight shift in peak location. The sharpening of the peaksindicates an increase in crystallite size, and the shift in the peaklocation indicates a decrease in the lattice parameter.

PL measurements taken on the same films shown in Fig. 10 arepresented in Fig. 11. Here, the PL intensity increases with anincrease in crystallite size. Table III summarizes the structural andluminescent data taken on the as-deposited and annealed samples.The integrated intensity was found by measuring the area under thePL spectra between 550 and 650 nm. A comparison of theas-deposited sample with the 108 h annealed sample revealed thatthe crystallite size increased by 26%, a value closely correlatedwith the increase in PL integrated intensity (20%). The change inlattice parameter was only 0.12%, indicating that this factor hadless influence on the PL emission intensity than did the change incrystallite size. Thermal effects (diffusion, sublimation of volatilespecies, grain growth) have been shown to improve the lumines-cent emission intensity in powders24 and films.14,17 However,

Fig. 7. SEM photographs of the films deposited onto (001) sapphire at600°C for 4 h.

Fig. 8. SEM photograph of the films deposited onto (001) sapphire at700°C for 4 h.

Fig. 9. PL emission spectra of film deposited at 500° (6 h), 600° (4 h),and 700°C (4 h).

Fig. 10. XRD spectra for the film grown at 600°C for 8 h andpostannealed at 850°C for 24 and 108 h. Cobalt radiation (l 5 0.179 nm);(400) and (420) reflections are shown.

Fig. 11. PL spectra of (Y1–xEux)2O3 films deposited for 8 h at600°C andannealed at 850°C for 24 and 108 h.

May 2000 Characterization of Photoluminescent (Y1–xEux)2O3 Thin Films Prepared by Metallorganic CVD 1245

these previous results show that the increased crystallite size,whether induced by an increase in temperature or an increase inannealing time, is the dominant mechanism for improved PLemission intensity. This phenomenon can be attributed to areduction in the grain-boundary area, a region with a high densityof defects that could quench luminescence emission.

IV. Conclusions

Thin films of (Y1–xEux)2O3 were deposited by low-pressureMOCVD onto (001) sapphire and (111) silicon substrates, usingyttrium and europium tris(2,2,6,6-tetramethyl–3,5-heptanedionate)precursors. The growth rate was 0.21mm/h at 600°C, with theyttrium and europium reservoirs at 146°C and flow rates of 100and 10 sccm (standard cubic centimeters per minute), for yttriumand europium, respectively. The films deposited at 500°C weresmooth and composed of nanocrystalline regions, indicating ther-mal cracking on the silicon substrates but not on the sapphiresubstrates. These films were composed of cubic Y2O3, grown in atextured [100] or [110] orientation to the substrate surface. Filmsdeposited onto silicon at 600°C developed, with increasing depo-sition time, from a flat, nanocrystalline morphology (2 h) into aplatelike growth morphology oriented in the [111] direction (4 h).Monoclinic (Y1–xEux)2O3 was observed for all deposition temper-atures on both (111) silicon and (001) sapphire substrates, asdetermined from the PL emission spectra. An increase of thedeposition temperature increased the photoluminescent intensity, achange attributed to a reduction in grain-boundary area and adecrease in ionic disorder around the Eu31. Postdeposition anneal-ing at a fixed temperature increased the crystallite size anddecreased the lattice parameter; crystallite size was more effectivethan the lattice parameter for increasing the PL intensity.

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Table III. Data Summary for (Y 12xEux)2O3 Films onSapphire†

ConditionLattice parameter

(nm)Crystallite size

(nm)Normalized PL

integrated intensity

As-synthesized 1.0632 17 1.0850°C, 24 h 1.0624 20 1.1850°C, 108 h 1.0619 23 1.2

†Deposited for 8 h at600°C and then annealed at 850°C for 24 or 108 h.

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