Cathodoluminescence of Mn2+ centers in MgAl2O4 spinels

Journal of Luminescence 46 (1990) 391—396 North-Holland

391

CATHODOLUMINESCENCE OF Mn~CENTERS IN MgAI2O4 SPINELS R. MLCAK and A.H. KITAI

a

Departments of Materials Science and Engineering and S Engineering Physics, McMaster University, Hamilton, L8S 414 Canada Received 7 November 1989 Revised 10 January 1990 Accepted 30 January 1990

MgAI2O4: Mn spinels containing between 0 and 8% Mn by weight were prepared (a) by grpwlh from an induction melt, and (b) by Mn~ implantation into nonstoichiometric spinel crystals. Cathodoluminescence blightness measurements (520 nm emission band) were made as a function of Mn concentration on both (a) and (b), revealing increasing brightness to even the highest Mn concentrations. This suggests that MgAI204: Mn is not susceptible to strong concentration quenching phenomena, and might be useful for highly efficient phosphors.

1. Infroduction Luminescence of Mn-activated phosphors of the ZnS : Mn and ZnSe : Mn types have been studied in detail [1]. These systems are particularly desirable since the emission occurs to ais spin2~ion,due which subflip transition within thelattice Mn (tetrahedral coordistitutional for Zn in the nation). Since no charge enters or leaves the Mn ion the phosphors are very stable in electroluminescence (EL), and ZnS: Mn thin film EL devices achieve yellow (580 nm) luminescence with no observable degradation after 20000 h of operation. The limitation of efficiency in ZnS : Mn occurs due to a concentration quenching effect in which energy is believed, to transfer among Mn2~sites before it is in either emitted, or in trapped andabove lost, l’his results a rapid decrease efficiency 1 mol% Mn doping [2], due to strong coupling between excited and unexcited Mn2t Another popular Mn-activated phosphor is willemite (Zn 2SiO4: Mn). It exhibits coupling effects between Mn atoms at concentrations of above 1% Mn, although segregation of Mn atoms in this material makes it difficult to discuss in terms of concentration quenching [3]. —

0022-2313/90/$03.50 © 1990

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MgA12O4 spinels, when doped with Mn, exhibit green emission at 520 nm. This is believed to originate from Mn substituting for tetrahedrally coordinated Mg. The same transition as 4T spin-flip 6A in ZnS: Mn takes place ( 1— 1) [4]. The change in wavelength due phc~noncoupling to differences between crystal field strengthsisand in the two host crystals. It was thought worthwhile to study the dependence of luminescence tipon Mn concentration in the MgAI 204 : Mn system, since Mg sites in the spinel crystal are inherently isolated from one another. This comes about because (i) the nearest neighbour atoms do not share a common anion, resulting in very little electronic interaction, and (ii) the concentration of Mg sites is relatively dilute: the average volume of MgA1 3, compared to 39.42O4 A3 per per Mg Zn atoms is 65.9 A atom in ZnS [5] (see fig. 1). This would have an effect on concentratiofl quenching: theoretical work with ZnS: Mn materials suggests that efficiency should increase if, for a given Mn doping 2~ ions. level, average [2]. distance between Mnthat for could the be increased It should be noted a given molar doping le’~el,the average separation between Mn centres in MgAI 204: Mn is larger by a factor of 1.19 than that in ZnS: Mn.

Elsevier Science Publishers B.V. (North-Holland)

392

R. Micak, A. H. Kitai

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saturated nonstoichiometric MgO(Al 203) spinel solutions. purchased followed spinel was with Mn by ionThe implantation, by doped a thermal diffusion.

Zn2”

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2 + centers

Cathodoluminescence of Mn

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Fig. 1. Lattice parameters of MgAl 2O4 spinel and ZnS, 2~ions, ilNote that there lustrating the crystalline are 8 Mg2~,16 environment A13~and of resident 32 02- Mn ions in the spinel unit cell shown, resulting in a relatively dilute concentration of Mg sites [5].

To test the process, two samples were progressively polished down to a 1 ~tm surface feature size and implanted with 2 x 1015 cm2 150 keV Mn’~’ ions. One of these samples was then annealed at 1455°Cfor 20 mm (see fig. 3). Although surface appear on the unannealed sample, X-ray elemental analysis revealed a homogeneous cornto 15 ~.tmalumina precipitates (identified by X-ray position throughout. After annealing, however, 10 elemental analysis) are clearly evident as expected [8] and appear to have nucleated from the surface defects. Surface coverage of Al 203 is estimated at 25% (see fig. 3). In2~concentrations order to dope the spinels0.1with between and the 8% desired of Mg Mn an implantation data interpolation program sites, was used to obtain an estimate of the projected range R~ 0.065 ~tm and range distribution L~R~ 0.021 ~tm of the implanted Mn2” (150 keY) ions in the spinel. Since the cathodoluminescence measurements require a sample thickness of 0.3 ~m, it was necessary to diffuse the Mn2+ implant —

—

In addition to searching for more efficient phosphors, it is hoped that studies of this type will lead to a better understanding of concentration quenching phenomena in general.

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2. Sample preparation

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A single, crystal boule (2.5 cm in diameter and 5 cm long) the verneuil of magnesium method wasaluminate purchased. spinel Thegrown verneuil by

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method cannot produce stgichiometric MgAI 203 crystals of adequate size due to low mechanical stability [6], and hence nonstoichiometric crystals rich in A1 2O3 are grown [7]. Magnesium alununate spinels with surplus Al203 up to MgO(A1203)53 are stable at 1900°Cas shown by the phase diagram in fig. 2. An RBS analysis revealed that the purchased crystal composition is MgA135067, or MgO(A12O3)19. Panda et al. [8] have reported on the kinetics of alumina precipitation in super-

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Fig. 2. Equilibrium phase diagram for the system MgAl2O4— ~.J2Ø3 showing stable concentrations and temperatures for the nonstoichiometric spine! [61.

R. Micak, A.H. Kitai

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2 + centers

393

Cathodolu,ninescence ofMn

The ion implantation doping density (S), diffusion temperature, and diffusion time required to yield the final Mn2± concentrations could then be calculated from the following equation: _____

S

ifx—R ______

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(em2), t — CMfl time 2= +(s), 2Dt}1/2, diffusion SX = iO~ doping(cm’3). constant density (cm2/s), A diffusion and with Mn2 ~ +Dconcentration == 3.3 cm was where [(~R~) sufficiently 1 x+R~ ~ uniform 2]~ (3) found Zto= provide Mn concentrations. Hence nine samples of MgAI 2°4 were polished and implanted with Mn between 3 x 1014 cm’2 and 8 x 1015 cm”2, and then annealed at ~/ii~ 3.3 x iO~cm. It was realized that uncertainties in R ~,, ~, and D exist, and evaporation of Mn from the surface may occur during annealing as well as enhanced Mn2+ diffusion along the spinel/ precipitated alumina interface. Possible consequences of alumina precipitation on brightness measurements were dealt with in the discussion. Polished samples of the original, unprocessed spmel as well as unimplanted, annealed spine! were also prepared. -~(

Fig. 3. (a) Polished, implanted spinel showing homogeneous composition. The features are pits in the surface. (b) Sample of fig. 3(a), but after annealing at 1455°C for 20 mi showing ~ 203 precipitates that nucleated from surface defects.

to a relatively constant concentration (variation not to exceed ±35% of the desired value) over this range. The diffusivity of Mn2~ in MgAl 2O3 is 2~provided mangathought that of Mg nese sits to on equal the magnesium sites as assumed. Two 2~self-diffusion [9,10] references the rate Mg a temperature range agreed to on within 20%ofover between 1250 and 1850 K: D = 200 exp [(—86 000 cal/mol)/RT] cm2/s, (1) D

=

469 exp[( —88500 cal/mol)/RT] cm2/s. (2)

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Since commercially available single crystal stoichiometnc MgA1 204 spinels were not found, predoped MgA! 204: Mn spinel crystals were grown by induction melting. The apparatus (fig. 4) consisted of an RF soUrce, an internally watercooled work coil, and a water-cooled copper skull which maintains a 2 mm layer of the oxide —

powders and solid spinel between itselfconsisting and the melt, reducing contamination. A batch of weighted amounts of MgO, Al 203 and MnO powders was prepared by ballmilling in distilled2 water for 24 h and calcining at 1000°C. The resulting powder was then packed into the skull with three 5 g pellets of’ equal atomic percent Mg and Al metals as shown in fig. 4. The heat of oxidation of the pellets produced a small melt to which the 50 MHz, 10 kY RF field coupled. A graphite ring was placed on top of the skull to generate heat near the melt surface where radia-

394

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R. Micak, A.H. Kitai WORK COIL

2 + centers

Cathodoluminescence of Mn

were sampled and analyzed by EXAFS for manganese form Mn concentration concentration. to within Data a confirmed system unceruni-

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tainty of ±10%,indicating no Mn segregation in

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the doped spinel during solidification and therefore allowed the use of the expected Mn con-

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centrations (1.85This and permitted 3.75% Mn)absolute to calibrate SIMS measurements. measure-

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ments of Mn concentration and depth profile using

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SIMS in implanted samples. A schematic diagram of the apparatus used for

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cathodoluminescent brightness measurements is shown in fig. 5. Up to 8 samples were affixed with

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1, II +~I II I Fig. 4. Growth system for spine! crystals. RF excitation provided by the water cooled work coil heats the mix. Arrows indicate water flow in the skull,

silver dag and to a avertically movable in ethe beam main chamber, transparent at an angle window. of sample 45° Sorption toholder both and the ion connected to the main generatedpumps a vacuum in the system up tochamber iO-~ton. A 5 keY electron beam (2 mA gun emission current) was supplied by a Perkin-Elmer (model 04-015)

tive cooling was excessive. The ring was designed so that powder could be added into the skull to compensate for the powder-to-liquid volume reduction during melting. Polycrystalline spinels of 0.5 mm crystal size were obtained in two separate growths with expected manganese concentrations of 1.85 and 3.75% of Mg sites as calculated from the ratios of oxide powders and metals used.

electron gun with a PHI 11-110 gun control unit, which contains a beam current modulator circuit. The beam was focused to a 3 mm diameter spot and positioned on the brightest part of each sampie with the xy beam deflector. Emitted photons exited the chamber through the window and were focused by a lens into a motorized Oriel (model 77250) grating monochromator. The brightness of the cathodoluminescent over by the an spectral range of 350 to 700 nm emission was detected Oriel

—

3. Measurements

(model 77346) photomultiplier tube and a Stanford Research Systems model SR51O lock-in

In order to check the composition of induction melted spinels, various locations in the melt zone

amplifier which synchronously measured the modulated luminescence signal.

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Fig. 5. Block diagram of experimental system for cathodoluminescence measurements.

2 + centers

it Mkak, A.H. Kiral / Cathodoiwninescence ofMn

395

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500

550

600

650

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X(nm) Fig. 6. Cathodoluminescence spectrum of a typical implanted and annealed spiel, showing the measurement of the Mn2~ luminescence (520 nm) superimposed on the background level.

4. Results

versus Mn2’~concentration is shown in fig. 7, for both melt grown and implanted spinel samples.

Spectral analysis of implanted and annealed samples revealed luminescence in most of the visible spectrum as well as the resolved peak of Mn2~ luminescence at 520 nm, as shown in fig. 6. The spinel brightness measurements required spectral response curves from which the Mn2~ cathodoluminescent peak height could be extracted by subtracting the interpolated baseline. The spectral emission curve of the 3.75% Mn induction melted spinel (fig. 7) shows little emission apart from the 520 nm peak, indicating superior crystallinity and stoichiometry was expected. A graph of the cathodoluminescent brightness 2$

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Brightness and cathodoluminescent efficiency of Mn2~doped spinels increases to at least 8% Mn ______________________________

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5. Discussion

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Fig. 7. CathodoluminescenceX(nm) spectrum of melt grown spiel with 3.75% Mn. Note the lack of background signal. This is due to improved stoichiometry.

10

Mn CONCENTRATION 1% of Mg sites) Fig. 8. Plot of MgAI 2~ concentration. The Mn concentration 2O4: Mn cathodoluminescence intensity as a function of Mn values are taken from SIMS measurements. Open squares: melt-grown sample. Closed squares: implanted samples. Only the green line (520 nm) was measured.

396

K Micak, A.H. Kitai

/

2’ centers

Cathodoluminescence of Mn

substitution for Mg sites, with no evidence of concentration quenching as seen in ZnS : Mn at 1—2 mol% Mn. Relative brightness and Mn2 + concentrations for the melt grown samples are considered reliable. More uncertainty exists in the implanted values, as seems evident by the scatter of relative brightness measurements at low manganese doping concentrations. It is not certain whether alumina precipitation affected the SIMS Mn2~concentration measurements, by enhanced Mn sputtering at precipitate/ spinel interfaces, Also the interaction of defects (introduced by ~ 203 precipitation) with the luminescent Mn2+ centres is unknown, and may lead to errors in relative brightness values, Three possibilities are proposed to explain the weakness of concentration quenching in MgA1 2 + luminescent centres2 04: Mn. Shielding of efficient Mn from each(i)other may be due to the nature of the tetrahedral site: 0~ in the spine! is highly electronegative, and should confine the spatial extent of the Mn—0 bond electrons. Furthermore, although nearest neighbour Mn2~sites in ZnS : Mn share a common anion, this is not the case in a spinel, which further isolates the space charge associated with the luminescent centres. This suppresses non-radiative energy transfer between them, and hence the excitation energy available for luminescence does not migrate to the traps. (n) Similar shielding may directly suppress energy transfer from an excited centre to a trap, and (iii) the MgA.l 204 spinel may contain fewer suitable impurities and/or 2~centre. defects which can act as traps to At the manganese excited Mnconcentrations of 8% of Mg sites, most Mn2+ ions are surrounded by at least one other Mn2~ion on the next closest Mg sites. Thus it seems likely that if shielding of Mn2+ centres and hence suppression of energy loss to traps occurs to C~,= 8%, further increases in manganese concentration will not significantly increase energy transfer rates to traps. Cathodoluminescent studies of spinels with higher Mn content could therefore test the validity of the shielding explanation.

6. Conclusions MgA1 204: Mn spinels of Mn2 + concentrations varying between 0 and 8% Mn substituted for Mg were prepared by growth from an induction melt and by Mn + implantation into a nonstoichiometric spinel. Mg2~ions are believed to reside on the same site in samples of either method of preparation. Cathodoluminescent brightness was found to increase to CMfl of at least 8%, suggesting only weak concentration quenching. It appears that the MgAl 204: Mn spinel crystal, perhaps due to isolation of tetrahedral sites, or lack of coupling to traps, is an interesting host for potentially high efficiency, highly doped phosphor materials.

Acknowledgements The authors wish to thank Jim Garrett for his help in spinel growth by induction melting, and to Frank Szostak, Doris Stevanovic, Brett McClelland and John Hudak for their help in SIMS, RBS, ion implantation and SEM work, respectively. EXAFS work by Adam Hitchcock and Tolek Tyliszczak is also acknowledged. Funding is from both the National Science and Engineering Research Council of Canada and the Ontario Center for Materials Research.

References [1] R. Mach and G.O. Muller, Phys. Stat. Sol. (a) 69 (1982) 11. [2] A.H. Kital, J. AppL Phys. 67 (1990) 1492. [3] Di. Robbins et al., J. Electrochem. Soc. 131 (1984) 141. [4] Bazilevskaya et al., J. AppL Spectrosc. 39(1983) 818. [51T.A. W.D. Kigery et al., Introduction to Ceramics (Wiley, New York, 1976) p. 64. [6] R.H. Arlett, J. Am. Ceram. Soc. 45 (1962) 523. [7] R. Falckenber& J. Cryst. Growth 13 & 14 (1972) 723. [8] P.C. Panda and R. Raj, J. Am. Cer. Soc. 69 (1986) 365. [9] R. Linder and A. Akerstrom, Z. Phys. Chem. 18 (1958) 303. [10] KJ.D. MacKenzie and Mi. Ryan, J. Mater. Sc!. 16 (1981) 579.