We demonstrate color shifting from the yellow to the red in the electroluminescence from (ZnGa)S:Mn films. We observe threshold voltages down to about 35 V, extremely low for such devices. We discuss the materials characteristics of the phosphor films, and the potential for improvement of the luminous intensity of the devices

Clark, B. L.

Dimitrova, V.

Draeseke, A.

Keszler, Douglas A.

Tate, J.

Tate, Janet

Yokoyama, T.

Undetermined

ScholarsArchive@OSU

APPLIED PHYSICS LETTERS VOLUME 75, NUMBER 16 18 OCTOBER 1999Red electroluminescence from ZnGaS:Mn thin filmsV. Dimitrova,a) A. Draeseke, and J. Tateb)Department of Physics and Center for Advanced Materials Research, Oregon State University,Corvallis, Oregon 97331T. YokoyamaDepartment of Electrical and Computer Engineering and Center for Advanced Materials Research,Oregon State University, Corvallis, Oregon 97331B. L. Clark and D. A. KeszlerDepartment of Chemistry and Center for Advanced Materials Research, Oregon State University,Corvallis, Oregon 97331~Received 21 June 1999; accepted for publication 16 August 1999!We demonstrate color shifting from the yellow to the red in the electroluminescence from~ZnGa!S:Mn films. We observe threshold voltages down to about 35 V, extremely low for suchdevices. We discuss the materials characteristics of the phosphor films, and the potential forimprovement of the luminous intensity of the devices. ©1999 American Institute of Physics.@S0003-6951~99!01542-9#lmrncnothmortefaityS,m-rsertoina-T-tedpo-eumheedredoutedsform-etrytherateC in-for59ndentLidsera-bye,Full-color displays based on alternating-current thin-fielectroluminescent~ACTFEL! devices are desirable fomany applications because of their high resolution, toleraof temperature extremes, and wide viewing angle. Mochrome displays are based on ZnS:Mn, which emits inyellow @Commission Internationale de l’Eclairage~CIE! co-ordinatesx50.5, y50.5#. With luminanceL4055000 cd/m2and luminous efficiencyE4052 – 4 lm/W at 1 kHz and 40 Vabove threshold, ZnS:Mn is the brightest known electrolunescent ~EL! phosphor.1 Use of the ‘‘color-by-white’’method to filter the radiation to produce full three-colTFEL capability has met with moderate success,2 but clearlynew phosphors that emit pure red, green, and blue atsame intensity that ZnS:Mn produces yellow would be prerable. The search for full three-color TFEL capability hintensified recently with the demonstration of high intensblue emission from SrS:Cu,Ag~L405360 cd/m2 with CIE x50.18, y50.3, andE4050.14 lm/W at 1 kHz!.3 There hasalso been a recent report of strong green emission fromS:Cu,K ~CIE x50.289, y50.596, L40553 cd/m2, E4050.97 lm/W at 60 Hz!.4 In the red region of the spectrumCaS:Eu~CIE x50.68,y50.31,L40512 cd/m2 at 60 Hz!1 isthe most promising candidate, although the best red lunance is still from filtered ZnS:Mn~CIE x50.65, y50.35,andL40565 cd/m2 at 60 Hz!.Very recently, yellow-to-red color shifting in the photoluminescence~PL! of Ga-doped ZnS:Mn phosphor powdehas been reported.5 Li and Keszler showed that by tuning thGa concentration in Zn~123d/2!GadS:Mn0.01, the yellow PLemission from undoped ZnS:Mn~CIE x50.543,y50.455!shifts towards orange for Zn0.7Ga0.2S:Mn0.01 ~CIE x50.620,y50.379! through deep red for Zn0.25Ga0.5S:Mn0.01 ~CIE x50.656, y50.344!. They obtained chromaticity values foZn0.55Ga0.3S:Mn0.01 ~CIE x50.640, y50.360! that are ex-a!Permanent address: Department of Physics, University of RoussStudenska St., 7017 Rousse, Bulgaria.b!Electronic mail: tate@physics.orst.edu2350003-6951/99/75(16)/2353/3/$15.00Downloaded 22 Jan 2010 to 128.193.163.2. Redistribution subject to AIPe-ei-he-sr-i-tremely close to those of the Y2O3:Eu phosphor, often usedas a commercial red standard.The purpose of the work described in this letter isdemonstrate yellow-to-red color shifting of the EL in thfilms of ~ZnGa!S:Mn, to discuss the materials characteriztion of the phosphor films, and to present some of the ACFEL device characteristics.We thermally evaporated~ZnGa!S:Mn powder precur-sors from metal boats, often with additional Mn evaporafrom a metallic source. We also experimented with coevaration of ZnS powder, metallic Ga, and metallic Mn. Wused baffled tantalum boats for the powders, an aluminoxide crucible for Ga, and a flat tantalum boat for Mn. Tevaporation system is a modification of one describpreviously,6 and it is briefly reviewed here.The evaporation rates from each source were monitoby independent quartz crystals; the system stability is ab40% for powders and 5% for metals. Evaporation proceat about 0.5 nm/s, achieving films<500 nm thick for a sub-strate temperature of 120 °C. All results reported here arefilms deposited at 120 °C. Films evaporated at different teperatures can be expected to yield a different stoichiombecause the sticking coefficients and reaction rates ofconstituent elements are different functions of the substtemperature. The glass substrates were heated at 300 °vacuum (<631027 Torr) for about 60 min before the substrate temperature was adjusted to the appropriate valuedeposition. The substrates were 1-mm-thick Corning 70glass coated with about 200 nm of each indium tin oxide aaluminum titanium oxide, which serve as the transparconductor and first insulator, respectively, of the ACTFEdevice.Following deposition, the films were subject to rapthermal annealing~RTA! for 2 min in argon at temperaturebetween 550 and 700 °C. In the range of substrate temptures used~20–400 °C!, post-production RTA provided noparticular improvement to the microstructure as measuredx-ray diffraction ~XRD!, but markedly improved PL. The83 © 1999 American Institute of Physics license or copyright; see http://apl.aip.org/apl/copyright.jsptineteswmswnistecel aictoshithm6–ora1onethatthee-sslowGainguchtedomon-in-tors.sionredsisonistion.heinasn aE2354 Appl. Phys. Lett., Vol. 75, No. 16, 18 October 1999 Dimitrova et al.phosphor films were made into TFEL devices by evaporaaluminum electrodes directly onto the phosphor. This is ctainly sufficient to produce luminescence, but is not expecto produce optimized devices, which require a second inlator between the phosphor and metal electrode. The ELobserved by driving the system at 1 kHz with a system silar to that described by Keir and Wager.7The main result of this letter is in Fig. 1, which showthe CIE coordinates of the EL emission shift from yellothrough orange to deep red~see also Table I!. The lumines-cence is quite variable, but in the better cases, the luminais comparable to those quoted above for CaS:Eu~luminancescales directly with operating frequency in the ranges dcussed here!. In other cases, it is rather weak. We attributhis to the nonoptimized device structure, and all devishould be able to be improved to perform at least as welthe best devices presented here. Interestingly, the devalso exhibited extremely low threshold voltages downabout 30 or 40 V. The electric field in the phosphor at threold, calculated with a simple parallel capacitor model wrelative dielectric constants of 18.5 and 8.3 for aluminutitanium oxide and zinc gallium sulfide, respectively, is 0.0.8 MV/cm for the coevaporated films and 2–4 MV/cm fthe films evaporated from powders. The difference appesignificant, but is not understood in detail. A value ofMV/cm is commonly quoted as necessary for EL operatiThese low voltages may represent a route to much lowFIG. 1. CIE coordinates of electroluminescent emission from~ZnGa!S:Mnfilms in Table I on a chromaticity diagram. The dark circles represent theemission from ZnS:Mn~0.5, 0.5! and the PL emission from Y2O3:Eu31~0.64, 0.36!.Downloaded 22 Jan 2010 to 128.193.163.2. Redistribution subject to AIPgr-du-asi-ce-sses-rs.r-power consumption devices. However, it is also possiblesuch low-voltage devices are inherently limited becausehot tail of the distribution of tunnel-injected electrons rsponsible for the impact excitation of the Mn is much lepopulated at these lower voltages. The issue of thethreshold voltage of the EL devices could be related tobeing incorporated as a shallow donor rather than beproperly alloyed. Compensation by a shallow acceptor sas nitrogen might resolve the problem.Figure 2 shows the EL emission spectra from selecdevices listed in Table I. These were all coevaporated frZnS powder, Ga metal, and Mn metal under the same cditions except for the percentage of Ga. As more Ga iscluded, the shift in the central EL emission peak shiftslarger wavelength, just as in the PL spectra from powde4The asymmetric shape and the broadening of the emisalso follow the same pattern. The stoichiometry, as measuby wavelength-dispersive electron probe microanaly~EPMA! at 15 kV, does not reflect the relative evaporatirates, but it should be noted that the EPMA compositionan average one; there is no information about segregaAlso, one of the coevaporated films does not fit into tpattern. The difficulty with Ga control was also reflectedthe films evaporated from the~ZnGa!S:Mn powder precur-sors; the concentration of Ga relative to Zn in the films whigher than in the precursors, sometimes by more thaLFIG. 2. Electroluminescence emission spectra of some of the~ZnGa!S:Mnfilms ~see Table I!.etersTABLE I. Characteristics of EL emission from~ZnGa!S:Mn ACTFEL devices. All films were deposited at a substrate temperate of 120 °C, and parammeasured with the device operating at 1 kHz.FilmNo. PrecursorThickness~nm! Composition CIEx CIE ylmax~nm!Vth~V!f th~MV/cm!L40~cd/m2!32 Zn0.925Ga0.05S:Mn0.01 160 Zn0.35Ga0.49S:Mn0.03 0.539 0.453 584 95 3.8 1.61powder59 Zn0.85Ga0.1S:Mn0.01 480 0.557 0.438 592 140 2.5 9.71powder27 ZnS powder, Ga metal 660 Zn1.04Ga0.03S:Mn0.003 0.525 0.423 578 60 0.80 1.5810 Å/s, 0.3 Å/s23 ZnS powder, Ga metal 450 Zn0.36Ga0.44S:Mn0.03 0.546 0.452 585 31 0.57 17010 Å/s, 0.5 Å/s33 ZnS powder, Ga metal 500 Zn0.37Ga0.37S:Mn0.08 0.620 0.372 639 47 0.80 1.1510 Å/s, 0.7 Å/s30 ZnS powder, Ga metal 600 Zn0.31Ga0.55S:Mn0.005 0.593 0.405 592 45 0.65 13.910 Å/s, 1 Å/s license or copyright; see http://apl.aip.org/apl/copyright.jsprshaucaue-eaorin0dinthlfoloedeetItthLatde-e inwi-akeGautoflt,di-icaofroupiza-hisda-er,onalurthhos-s.2355Appl. Phys. Lett., Vol. 75, No. 16, 18 October 1999 Dimitrova et al.factor of two. Efforts to improve the Ga control are undeway. EPMA also revealed that the Mn content of filmevaporated entirely from powder precursors was less t0.01%, so we generally coevaporated Mn metal to prodabout 1 at. % Mn. Rutherford backscattering analysis walso employed on some films. It confirmed the cation to sfur stoichiometry obtained from EPMA, but it does not rsolve Zn from Ga in this system.XRD of the ~ZnGa!S:Mn in theu–2u geometry revealeda strong peak at 2u528.5°, indicating that the films arstrongly textured, and not random polycrystalline. This pecorresponds to diffraction from the~111! planes in a cubicstructure or from~0002! planes in the hexagonal structure fpure ZnS:Mn. None of our films showed any channelingRBS.Transmission through the~ZnGa!S:Mn films in the vis-ible region of the spectrum is extremely good. Down to 5nm, the transmission of the film and glass substrate is intinguishable from the transmission of the substrate aloThe transmission drops to about 80% at 400 nm, andfilms are strongly absorbing below 350 nm.Several authors have discussed the need for a suoverpressure to grow high-quality ZnS films.8 Although thiswas not implemented in our system, we have found thatZnS films are nearly stoichiometric~with even a slight sulfurexcess!, are strongly oriented with the~111! plane parallel tothe substrate surface, and are highly resistive at dc andfields. Also, EL devices made from similar ZnS films dopwith a few percent of Mn21 luminesce with about 50% of thintensity of much thicker, optimized devices with a sputterphosphor layer. The main drawback appears to be thatevaporated films are very thin~,500 nm from about 2 g ofstarting material! and this contributes to the weaker EL.appears that an ALE-type mechanism is at work in thatlow sulfur sticking coefficient is rate limiting.Downloaded 22 Jan 2010 to 128.193.163.2. Redistribution subject to AIP-nesl-k0s-e.eururwdheeChromaticity control of PL and EL is possible in TFE~ZnGa!S:Mn devices. The color coordinates indicate thsaturated red can be obtained. The potential for usefulvices needs to be further evaluated. The threshold voltagmost of the devices is below 50 V, which bodes well for lopower drives, but this may also be responsible for low lumnance. Thermal coevaporation is a useful method to mgood quality, highly transparent phosphor films, and thedoping can in principle be controlled continuously withoneed to produce many sputter or laser ablation targetsparticular stoichiometry. In practice, Ga control is difficuand the system can be improved by incorporation of adtional sulfur sources.The authors thank Dr. Sey-Shing Sun of Planar Amerfor supplying substrates, Professor Arthur Sleight for useequipment, Professor John Wager and members of his gfor useful discussions, Beau Baukol for device charactertion, and Professor L. C. McIntyre for the RBS analysis. Twork was supported in part by the National Science Fountion under Grant No. DMR-9710207.1Y. A. Ono, Electroluminescent Displays~World Scientific, Singapore,1995!.2S. Tanaka, SID 88 Dig. 293~1988!.3H. M. Menkara, W. Park, M. Chaichimansour, T. C. Jones, B. K. WagnC. J. Summers, and S.-S. Sun, Proceedings of the Ninth InternatiWorkshop on Inorganic and Organic Electroluminescence and the FoInternational Conference on the Science and Technology of Display Pphors, Bend, OR, 14–17 September 1998.4P. D. Keir, J. F. Wager, B. L. Clark, D. Li, and D. A. Keszler, Appl. PhyLett. 75, 1398~1999!.5D. Li, Ph.D. thesis, Oregon State University, 1999.6D. W. Tom, R. Platzer, J. A. Gardner, and J. Tate, Appl. Phys. Lett.63,3224 ~1993!.7J. F. Wager and P. D. Keir, Annu. Rev. Mater. Sci.27, 223 ~1997!.8D. Poelman, R. Vercaemst, R. L. Van Meirhaeghe, W. H. Lafle`re, and F.Cardon, J. Lumin.75, 175 ~1997!. license or copyright; see http://apl.aip.org/apl/copyright.jsp

Red electroluminescence from ZnGaS:Mn thin films

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Red electroluminescence from ZnGaS:Mn thin films

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