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Rare earth complex dopants in a.c. thin-film electroluminescent cells
Journal of Luminescence 23 (1981) 175—190 North-Holland Publishing Company
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RARE EARTH COMPLEX DOPANTS IN a.c. THIN-FILM ELECTROLUMINESCENT CELLS J. BENOIT and P. BENALLOUL Laborazoire de Luminescence ~, Université P. et M. Curie, Tour 13, 4 p/ace Jussieu, 75230 Paris Cedex 05, France
B. BLANZAT Laboratoire de Physico-Chimie Minéra/e ~, Eco/e Centra/e, Grande Voie des Vignes, 92290 Chatenay-Ma/abry, France Received 18 February 1981
The authors have carried out some comparative studies between the different dopants used in thin-film a.c. electroluminescent devices; as well as the MnS+ ion high 3+)leading and raretoearth luminosity and long-lived devices, the use of trivalent rare earth ions (Ln fluoride centers LnF 3 (lumocen center) has been investigated. In the first part the author explain the nature of LnF3 centers and they have shown that these centers are a new type of luminescent center, which are strongly linked to the ZnS matrix: the authors propose labeling these centers “complex centers”. In the second part they have searched for other promising centers and have chosen the rare earth oxysulphides Y202S (Ln) and La2O2S (Ln). These compounds have been introduced as complex centers 3 ~, in Th3 the ZnS Eu3matrix ~, thebyauthors RF sputtering have realized methods. some Using devices the followingrespectively emitting trivalent ions: the three Tm basic colours (blue, green, red). The authors have presented some properties of these centers.
1. Introduction In the past few years, we have seen a renewed interest in this layered electroluminescent systems; this is due for the most part to results of research by Inoguchi et al. [1] who obtained high luminosity (1000 fL) and long-lived devices (more than 1O~h) by using a structure carrying an active layer of ZnS: Mn placed between two insulating layers, the whole device operating at high electric alternating field. Studies on these devices show that the excitation mechanism is a mechanism of acceleration of electrons followed by an impact phenomenon on the *
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luminescent center [2]; the result being that these centers should fulfil two contradictory conditions [3]: on the one hand, a large cross section which induces.quite a wide wavefunction; on the other hand the high electric fields used, require “isolated centers”. The experiment shows that the centers of the associated donor—acceptor type are of a low level efficiency with high-field EL. In contrast the Mn2 ions have shown themselves to be the most appropriate. The manganese emission is a band centered at 5800 A and it is necessary to use other centers to cover the rest of the visible domain. Rare carth ions are one possibility. When we dope a thin layer of zinc sulphide with a metallic rare earth, in the form of a trivalent ion, except for europium; the charge compensation is obtained most probably by the intermediary of a lattice defect. The 4f” states of trivalent rare earth ions are very stable under a high field, but the light efficiency is weak, in fact the best devices obtained with these ions gave a brightness of only 30 fL for ZnS: Th3 and a few fL for the other rare earths [4]. The ZnS : Th3 emission consists of a green emission centered about 5400 A, corresponding to the transition 5D 7F 4 —~ 5. Another type of center was proposed in 1968 by Kahng et al. [5,6] who introduced rare earth fluorides in zinc sulphide. To distinguish these centers from the aforementioned ionic centers the authors proposed the name lumocen: “Luminescence from molecular center”. the brightness obtained in the systems ZnS: LnF3 have not 3~ shown themselves to be much higher than systems. The strongest brightness has those been obtained with with ThF the ZnS: Ln obtained 3 of the order of 50 fL (an improvement of the order of 50%). Krupka and Mahoney [4] have carried out some comparative studies between the lumocen and ionic centers. the principal observations being the following: (a) The optimal concentration of the dopant in zinc sulphide for electroluminescence is the same for a rare earth ions as for the molecule LnF3, about 2%; the idea has been put forward that the fluoride enters in the same manner as the ions in the zinc sulphide matrix. are different (for carried example 5D (b) The transition 3+) butintensities a precise study as not been out.the transitions 3(c) The ~ of Th reproducibility of the results is better in the case of the LnF 3 centers. 3~ cannot be obtained when the europium is (d) The in emission of Eu introduced ionic form in zinc sulphide; on the contrary, this emission appears up to a certain point in the case where europium is introduced as EuF 3. In addition, Krupka and Rochkind [7] have ascertained the absence of a vibrational spectrum of the ThF3 molecule in the ZnS : ThF3 layer, in the range of 400 to 900 cm~ corresponding to the isolated molecule ThF3. They +
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concluded that the molecule LnF3 lost its integrity and henceforth they do not use the appellation lumocen center although they retain the fact that the fluorine controls the valence state of the rare earth suggesting the use of LnF3 rather than the isolated ion introduced by metal evaporation. Kobayashi et al. [8] continued the preparation and study of electroluminescent ZnS: LnF3 devices, no longer distinguishing the LnF3 center from that of a trivalent rare earth ionic center, but interesting themselves especially in the electrical mechanism and the properties of electroluminescence. In 1979, Okamoto and Hamakawa [9] improved these electroluminescent devices by using a structure with a brightness of 200 fL for ZnS : ThF3. In addition to the work relative to the rare earth fluorides we can also cite the work of Yamauchi et al. [10] on manganese. After a coevaporation of 2~by ZnS, ThF3 and Mn the authors obtained a red shift of the emission of Mn introducing fluorine ions near the manganese ion (band emission centered on 7000 A, 80 fL). They thought that the terbium ion does not enter into the emission mechanism. To summarize, in the works we have cited, it clearly appeared that the LnF 3 + more or less compensated by3 centers are considered as ionic centers with Ln fluorine. In addition, the cited authors ascertain that LnF 3 centers have special properties but do not present in general an improvement of the brightness. To our knowledge no research other that that of rare earth fluorides has been carried out on molecular centers for EL. The work we are going to present is divided in two parts: (i), a summary of the results we have obtained on the LnF3 centers proving the existence of “complex centers” [11,12]; (ii), a complementary study has led to the realisation of new efficient centers based on rare earth oxysulphides [131. 2. Study of the LnF3 centers 2.1. Introduction The introduction of the rare earth fluorides in zinc sulphide is carried out by co-evaporation of ZnS and LnF3 under a vacuum of 10—6 torr. This encourages us to make some remarks: (a) ZnS and LnF3 have appreciable vapor pressure when they are separately sublimated [5];on the contrary in a coevaporation we cannot exclude chemical reactions between vapors and it is necessary to prove that the entity LnF3 is effectively conserved in zinc sulphide. This is the first necessary condition for the possible existence of a “lumocen” center; no previous experiments having been effected on this point. (b) The optimum concentration of LnF3 in zinc sulphide for a good electroluminescence is about 2%; we can thus think that the LnF3 molecule is
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statistically distributed in the zinc sulphide. This remark would allow us to eliminate the idea of agglomerates. No previous experiment having been made on this point, it must be checked that this is so. Before speaking of the existence of a new center it is necessary to check beforehand that these two conditions are satisfied, a chemical reaction between the vapors can give rise to a center, different from that which we expected. 2.2. Microprobe analysis of ZnS: TbF3 A layer of ZnS: ThF3 has been examined with a scanning microprobe (MEB) and we have observed a uniform distribution of terbium and fluorine in the zinc sulphide layer. It has been checked that the ratio between zinc and sulphur is 1: 1, when that between terbium and fluorine is 1: 3. This last point leads towards showing that the chemical formula ThF3 is preserved. The resolution (250 A) being of the same order as that of the crystallites size of the polycrystalline layer ZnS : ThF3, we have also prepared epitaxial layers of this material on a NaC1 support. The ZnS : ThF3 single crystal has a cubic structure with dimensions of 10 to 30 ~m and a thickness of 2 jsm. On the epitaxial layers, we have been able to verify that the distribution of terbium and fluorine stays uniform. As the concentration of terbium is the same inside the grains as at the boundaries there is no preferential condensation. 2.3. Luminescence studies The basis of the study consists of the comparison of emission spectra of 3~ it is possible to different materials; in the case of ZnS : LnF3 or ZnS: Ln observe the electroluminescence, the photoluminescence or the cathodoluminescence. We have established that only the electroluminescence and catholuminescence spectra are identical. This remark is important when we need to have the emission of a compound which cannot be electroluminescent, as for example LnF 3, for which we can also use the cathodoluminescence. For reasons which will be clear later we have chosen to draw on fig. 1 a the emission spectra 3~, in electroluminescence and3Pcathodoluminescence of thin films ZnS: PrF 3H of PrF3,ZnS: Pr 3 (transition 0 —* 4). We can observed that the three emissions are different which allows us to establish that the ZnS: PrF3 emission is not due to an ionic center or PrF3 agglomerate. On fig. lb we can establish that the emissions of PrF3 and powders of Pr2 S3 and PrSF are different from the emission of ZnS : PrF3. This eliminates any possibility of agglomerates and confirms the microprobe observations. 3+ the other hand with we observed the emissions areOn shifted compared the emission of PrF of ZnS : PrF3 and ZnS: Pr 3. This so-called “nephelauxetic
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effect” is due to the more covalent bonding of the praesodynium ion. A similar effect is observed with the samarium centers (fig. ‘2). In order to compare this effect for all the lanthanides we have drawn the displacement i~v/v versus different rare earth fluorides (fig. 3). We can ascertain that the nephelauxetic effect is even more important when we move away from the half-filled shells. For europium, gadolinium and terbium the effect was not observed. In fig. 3 we observe a nephelauxetic effect stronger for Tm3 + than for Er3~ a phenomenon previously observed by Jorgensen [14] on other compounds. We can profit from the nephelauxetic effect to detect eventual agglomerates; SmF 3 has been chosen in preference to PrF3 even though the nephelauxetic effect is weaker because of the fact that the brightnesses in electroluminescence
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and in cathodoluminescence of ZnS: SmF3 are one order of magnitude higher than the corresponding bnghtnesses of ZnS: PrF3. 6H 3 + in SmF Fig. 4a shows the emission of the transition 2/2 of Sm 3 and in ZnS doped with 2% SmF3. Above concentrations of 30% mole/mole (fig. 4b), peaks at 5940, 5925 and 5890 A corresponding to the emission of SmF3 agglomerates, are observed in the cathodoluminescence spectra. When we increase the concentration of SmF3 to 55% mole/mole, emission of the agglomerate increases compared with the emission of the isolated SmF3 center. In fact the level of 30% is not absolute as the emission of the agglomerate has an efficiency lower than the emission of the isolated SmF3 center (non-radiative recombination due to the presence of —~
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Sm3 + as nearest neighbours). On the contrary this experiment gives additional evidence that the emission observed at concentration of 2% has really originated from an isolated LnF 3 center. 2.4. Particular properties of the EuF3 center In general all evaporated lanthanides in a metallic form exist in the zinc sulphide matrix in the trivalent state with the exception of europium which is divalent (low intensity blue emission band). As previously indicated [6] the red trivalent emission appears in addition to the blue divalent band when europium is introduced in the EuF3 form. We can attribute this blue emission to a decomposition due to a chemical reaction between the vapors of ZnS, Zn, S and EuF3. In order to avoid this problem we have separated the zinc sulphide and europium fluoride vapours and have deposited them successively by means of a rotating substrate holder and appropriate masks. A vacuum annealing at 450°Cstrongly enhances the electroluminescence, in spite of this precaution the brightness of the ZnS: EÜF3 systems stays weak and of the same order as that indicated by Chase (some fL). The only point of interest of these systems remains in the possibility of using trivalent europium as a structural point symmetry probe, due to 5D 7F the value J = 0 of the resonant level 0 in emission and of the ground level 0 in absorption. Furthermore, in this case, Judd [15] has defined some simple rules ~J 2, 4 or 6 for electric dipole transitions. The 3spectrum obtained for ZnS: EuF3 + site symmetry. The observation (fig. 5) leads to 5780 the determination of the line at A for the 5Dof the7FEu 0 —* 0 transition is important because it proves the uniqueness of the rare earth site but also, as shown by Niewport and Blasse [16] this strictly forbidden transition can only be observed in the case of symmetry such as C~,C~or ~ Let us remember that the point symmetries for the rare earth ion in the starting materials are C3~for5DZnS and C9 for 5DEuF3 [17]. The number of lines observed for the transitions 0 and 0 ‘F~enable these hypothetical symmetries 5D 7F 5D 7F~ Symmetry 1 1 0 —* ~F0 2 3 0 -+ C 3~, 1 3 4 1 3 5 -+
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2.5. Summary The LnF3 entity is conserved when LnF3 is evaporated into ZnS. There are no agglomerates for the LnF3 .concentration used in the electroluminescent experiments. The emission of ZnS : LnF3 is quite special. LnF3 is a dopant existing in the form of one lanthanide and three fluorines, the whole being strongly linked to the 3~in ZnSZnS : LnFmatrix (nephelauxetic effect). The point symmetry of Ln 3 is probably C3,,. — LnF3 in the matrix ZnS being neither a molecular center nor an ionic center, we put forward the designation of complex center but the exact nature of this center cannot be given at the moment. For such studies the best probe is given by a europium center because the presence of red trivalent emission proves the existence of a complex center in ZnS. —
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3. Study of the complex center Ln 2°2~ 3.1. Intrpduction On analyzing how the cells are working, it is possible to distinguish some different steps [3]: Inside the zinc sulphide layer, working as an electron source, an electron process first appears, followed by an excitation, similar to cathodoluminescence, by hot electrons of the luminescent centers. It seems logical to search for promising centers among cathodoluminescent materials, especially the efficient luminophors which are used for television. We have chosen the rare earth oxysulphides, in particular the three following compounds for a trichromic application: For blue emission: La2O2S (Tm) ‘G4 —63H6 of Tm3~at 4800 A. For green emission: La 5D 7F 3~at 5400 A. 47F—* 5 of 3~at Th 6100 A. For red emission: Y 2O2S (Th) 5D 202S (Eu) of Eu In the cathodoluminescence studies 0on—6 the2rare earth oxysulphides the highest efficiencies are obtained for concentrations of the activators from 0.5 to 5%; beyond this value the interaction between the activators gives rise to nonradiative recombinations leading to a decrease of light efficiency. In active electroluminescent layers, a complex center consists of an isolated molecule coupled to the ZnS lattice, the interaction between the luminescent centers not being active. As it is necessary to heve only one active rare earth ion in the local center, the optimal solution would be realized with the lanthanum or yttrium oxysuiphide doped at 50% by an active rare earth oxysulphide such as compounds with the chemical formulae LaTmO 2S, LaTh02S and YEuO2 S. Such solid solutions at 50% are still not possible for all the rare earth elements because of the differences in ionic radii [18], however the synthesis procedure seems to have a certain influence on the respective solubilities [19]. The compound YEuO2S may be obtained whereas the LaTmO2S and LaThO2S compounds cannot be obtained. Therefore we have chosen not to use the compounds with the best light efficiencies but the well-crystallized compounds doped at 30% such as La14Tm06O2S, La1 4Th06 02S and Y1 .4 Eu06 02S. 3.2. Preparation of the active layers The rare earth oxysulphides are decomposed with a loss of sulphur during the heating under vacuum. Therefore, the rare earth oxysuiphide thin films are only obtained by diode-radiofrequency sputtering methods. To preserve the stoichiometry, sulphur compensation is realised either using an atmosphere of Ar + H2S [20] for sputtering, or by a determined excess of LnS3 added in the composition of the target [21]. So, for the elaboration of the luminescent ZnS
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thin film doped with rare earth oxysuiphide, coevaporation of ZnS and oxysulphide or evaporation of doped powder by joule heating or electron gun are not suggested. The diode-radiofrequency sputtering method is adequate to prepare the luminescent thin film. The target was obtained by careful mixing of the starting powders = 97.5% zinc sulphide, and 2.5% mole/mole doped rare earth oxysuiphide. We have previously emphasized the sensitivity of the europium complex center to the stoichiometry. When the thin film of zinc sulphide doped with Y14Eu06O2S is prepared under a pure argon atmosphere a weak and broad emission centered around 4900 A due to the europium in its divalent form is 3+ emission in cathodoluminescence. When also observed in addition to the Eu preparing a thin layer of rare earth oxysulphide alone, adding a small excess of sulphur is necessary; in the former case, the zinc sulphide provides the oxysuiphide with sulphur and the presence of divalent europium is due to a lack of oxygen. In the case of sputtering under an atmosphere of argon and 5 %o oxygen the one and only emission of trivalent europium is observed in cathodoluminescence. Annealing the active layers at 450°C under vacuum or pure nitrogen atmosphere for one hour enhances the, light efficiency. During sputtering, the rate of deposition, of about 40 A/mn, is laser optically controlled and the substrate is maintained at a temperature of 180°C. The electroluminescent device is composed of the active layer (4500 A thick) included in a sandwich of insulating films of yttrium oxide (3300 A).
4. Experimental results Fig. 6 gives the applied voltage dependence of the brightnesses of ZnS: La 2O2S (Th) and ZnS: Y202S (Eu) active layers compared to those of 2). The ZnS : ThF3 and ZnS: Mn devices (the electrode having an area of 25 mm form of the curves is similar but the rare earth oxysulphide devices show a greater resistivity since these materials work at a higher voltage. Actually the quality of our insulating layers does not allow the extension of the activity field in the “saturation area”, permitting a gain of the brightness by a factor of more than 2. Terbium center: green emission. Our own fabrication ZnS : ThF 3 devices present at best a brightness of 60 fL when they are optimized; on the contrary the ZnS: La2O2S(Th) device already shows a brightness of 70 fL without any special research of the optimization. Europium center: red emission. The brightness of the ZnS : Y202S(Eu) devices reaches a value of 20 fL which is clearly higher than those with ZnS: EuF3. Furthermore the complete absence of the divalent europium emission shows the real stability of this center.
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150
200
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3 in ZnS : Y The luminescent spectrum of Eu 2O2S(Eu) is drastically different from 7F those in ZnS:7FEuF3 (fig. 7). One additional line is present in the 5J)~—+ 1 and ~l)~ —~ 2 transitions. The point symmetry of the trivalent europium which was C3,, in the ZnS: EuF3 compound is reduced to a C2,, site symmetry in the rare earth oxysuiphide case. In this compound we also observe broad lines; a more precise 5D ‘characterization is in progress by means of a selective excitation of the 0 levels. The Y2O2S(Eu) center is a very promising one because it is the only center actually allowing consideration of trichromic devices which need a bright red emitter around 6100 A. Thulium center: blue emission. We have not succeeded in making ZnS : TmF3 devices with a significant blue emission. In the past Chase [61encountered the same difficulty. In the case of ZnS : La2O2S (Tm) the characteristic blue emission is still weak but a higher luminescence intensity is soon expected with new and better solid solutions of thulium in lanthanum oxysulphide. +
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ZnS:Y202S((u) 300°C
1 D,—.’F2
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5
00
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5. Conclusions The study of the LnF3 centers allow us to show the existence of a particular center which we designate as a complex center. The presentation of the new rare earth oxysuiphide complex centers opens future prospects for high-field electroluminescent devices mainly because of the new possibility of developing trichromic display. Many other complex centers are considered, the main problem being to know the best technique for the introduction of these kinds of centers in the zinc sulphide matrix. Currently, no device procce~sesa brightness equal to that of the ZnS: Mn devices; however, devices based on the rare earth oxysulphide complex centers are substantially brighter than previous rare earth doped EL cells.
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Acknowledgments We wish to thank Mrs Gouy and Mrs Nappey for their participation in the cell realization and Mr Baherze for microprobe analysis. Some of the starting rare earth oxysulphides were kindly provided by Messrs Dougier and Browaeys, Rhône-Poulenc Society. We also wish to thank Professor F. Williams for helpful and fruitful discussions. References [II T. Inoguchi and S. Mito, Phosphor Films, Topics in Appl. Phys. Electroluminescence, Vol. 17 ed. J.I. Pankove (Springer-Verlag, Berlin, 1977) p. 197.
[Ill J. Benoit, P. Benalloul, Y. Charreire and B. Blanzat, Mat. Res. Bull. 11(1976)1463. [12] J. Benoit, thesis (1979). [13] J. Benoit, P. Benalloul, J. Mattler, M. Gouy and A. Nappey, Brevet CNRS-ANVAR No. 79 24927. [14] Jorgensen, private communication. [IS] BR. Judd, Phys. Rev. 127 (1962) 750. [16] W.C. Niewport and G. Blasse, Solid State Comm. 4 (1966) 227. [171A. Zalkin and D.H. Templeton, J. Am. Chem. Soc. 75 (1953) 2453. [18] M. LeskelS and L. NiinistO, J. Solid State Chem. 19 (1976) 245. [19] M. Leskela, L. NiinistO, H. Dexpert and Y. Charreire, Met. Res. Bull. 14 (1979) 455. [201 T.G. Mapple and R.A. Buchanan, J. Vac. Sci. Technol. 10 (1973) 616. [21] C. Sella, J.C. Martin, Y. Charreire and J. Loners, Le Vide, Les Couches Minces, CIP 79, p.201.
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RARE EARTH COMPLEX DOPANTS IN a.c. THIN-FILM ELECTROLUMINESCENT CELLS J. BENOIT and P. BENALLOUL Laborazoire de Luminescence ~, Université P. et M. Curie, Tour 13, 4 p/ace Jussieu, 75230 Paris Cedex 05, France
B. BLANZAT Laboratoire de Physico-Chimie Minéra/e ~, Eco/e Centra/e, Grande Voie des Vignes, 92290 Chatenay-Ma/abry, France Received 18 February 1981
The authors have carried out some comparative studies between the different dopants used in thin-film a.c. electroluminescent devices; as well as the MnS+ ion high 3+)leading and raretoearth luminosity and long-lived devices, the use of trivalent rare earth ions (Ln fluoride centers LnF 3 (lumocen center) has been investigated. In the first part the author explain the nature of LnF3 centers and they have shown that these centers are a new type of luminescent center, which are strongly linked to the ZnS matrix: the authors propose labeling these centers “complex centers”. In the second part they have searched for other promising centers and have chosen the rare earth oxysulphides Y202S (Ln) and La2O2S (Ln). These compounds have been introduced as complex centers 3 ~, in Th3 the ZnS Eu3matrix ~, thebyauthors RF sputtering have realized methods. some Using devices the followingrespectively emitting trivalent ions: the three Tm basic colours (blue, green, red). The authors have presented some properties of these centers.
1. Introduction In the past few years, we have seen a renewed interest in this layered electroluminescent systems; this is due for the most part to results of research by Inoguchi et al. [1] who obtained high luminosity (1000 fL) and long-lived devices (more than 1O~h) by using a structure carrying an active layer of ZnS: Mn placed between two insulating layers, the whole device operating at high electric alternating field. Studies on these devices show that the excitation mechanism is a mechanism of acceleration of electrons followed by an impact phenomenon on the *
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luminescent center [2]; the result being that these centers should fulfil two contradictory conditions [3]: on the one hand, a large cross section which induces.quite a wide wavefunction; on the other hand the high electric fields used, require “isolated centers”. The experiment shows that the centers of the associated donor—acceptor type are of a low level efficiency with high-field EL. In contrast the Mn2 ions have shown themselves to be the most appropriate. The manganese emission is a band centered at 5800 A and it is necessary to use other centers to cover the rest of the visible domain. Rare carth ions are one possibility. When we dope a thin layer of zinc sulphide with a metallic rare earth, in the form of a trivalent ion, except for europium; the charge compensation is obtained most probably by the intermediary of a lattice defect. The 4f” states of trivalent rare earth ions are very stable under a high field, but the light efficiency is weak, in fact the best devices obtained with these ions gave a brightness of only 30 fL for ZnS: Th3 and a few fL for the other rare earths [4]. The ZnS : Th3 emission consists of a green emission centered about 5400 A, corresponding to the transition 5D 7F 4 —~ 5. Another type of center was proposed in 1968 by Kahng et al. [5,6] who introduced rare earth fluorides in zinc sulphide. To distinguish these centers from the aforementioned ionic centers the authors proposed the name lumocen: “Luminescence from molecular center”. the brightness obtained in the systems ZnS: LnF3 have not 3~ shown themselves to be much higher than systems. The strongest brightness has those been obtained with with ThF the ZnS: Ln obtained 3 of the order of 50 fL (an improvement of the order of 50%). Krupka and Mahoney [4] have carried out some comparative studies between the lumocen and ionic centers. the principal observations being the following: (a) The optimal concentration of the dopant in zinc sulphide for electroluminescence is the same for a rare earth ions as for the molecule LnF3, about 2%; the idea has been put forward that the fluoride enters in the same manner as the ions in the zinc sulphide matrix. are different (for carried example 5D (b) The transition 3+) butintensities a precise study as not been out.the transitions 3(c) The ~ of Th reproducibility of the results is better in the case of the LnF 3 centers. 3~ cannot be obtained when the europium is (d) The in emission of Eu introduced ionic form in zinc sulphide; on the contrary, this emission appears up to a certain point in the case where europium is introduced as EuF 3. In addition, Krupka and Rochkind [7] have ascertained the absence of a vibrational spectrum of the ThF3 molecule in the ZnS : ThF3 layer, in the range of 400 to 900 cm~ corresponding to the isolated molecule ThF3. They +
+
+
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concluded that the molecule LnF3 lost its integrity and henceforth they do not use the appellation lumocen center although they retain the fact that the fluorine controls the valence state of the rare earth suggesting the use of LnF3 rather than the isolated ion introduced by metal evaporation. Kobayashi et al. [8] continued the preparation and study of electroluminescent ZnS: LnF3 devices, no longer distinguishing the LnF3 center from that of a trivalent rare earth ionic center, but interesting themselves especially in the electrical mechanism and the properties of electroluminescence. In 1979, Okamoto and Hamakawa [9] improved these electroluminescent devices by using a structure with a brightness of 200 fL for ZnS : ThF3. In addition to the work relative to the rare earth fluorides we can also cite the work of Yamauchi et al. [10] on manganese. After a coevaporation of 2~by ZnS, ThF3 and Mn the authors obtained a red shift of the emission of Mn introducing fluorine ions near the manganese ion (band emission centered on 7000 A, 80 fL). They thought that the terbium ion does not enter into the emission mechanism. To summarize, in the works we have cited, it clearly appeared that the LnF 3 + more or less compensated by3 centers are considered as ionic centers with Ln fluorine. In addition, the cited authors ascertain that LnF 3 centers have special properties but do not present in general an improvement of the brightness. To our knowledge no research other that that of rare earth fluorides has been carried out on molecular centers for EL. The work we are going to present is divided in two parts: (i), a summary of the results we have obtained on the LnF3 centers proving the existence of “complex centers” [11,12]; (ii), a complementary study has led to the realisation of new efficient centers based on rare earth oxysulphides [131. 2. Study of the LnF3 centers 2.1. Introduction The introduction of the rare earth fluorides in zinc sulphide is carried out by co-evaporation of ZnS and LnF3 under a vacuum of 10—6 torr. This encourages us to make some remarks: (a) ZnS and LnF3 have appreciable vapor pressure when they are separately sublimated [5];on the contrary in a coevaporation we cannot exclude chemical reactions between vapors and it is necessary to prove that the entity LnF3 is effectively conserved in zinc sulphide. This is the first necessary condition for the possible existence of a “lumocen” center; no previous experiments having been effected on this point. (b) The optimum concentration of LnF3 in zinc sulphide for a good electroluminescence is about 2%; we can thus think that the LnF3 molecule is
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statistically distributed in the zinc sulphide. This remark would allow us to eliminate the idea of agglomerates. No previous experiment having been made on this point, it must be checked that this is so. Before speaking of the existence of a new center it is necessary to check beforehand that these two conditions are satisfied, a chemical reaction between the vapors can give rise to a center, different from that which we expected. 2.2. Microprobe analysis of ZnS: TbF3 A layer of ZnS: ThF3 has been examined with a scanning microprobe (MEB) and we have observed a uniform distribution of terbium and fluorine in the zinc sulphide layer. It has been checked that the ratio between zinc and sulphur is 1: 1, when that between terbium and fluorine is 1: 3. This last point leads towards showing that the chemical formula ThF3 is preserved. The resolution (250 A) being of the same order as that of the crystallites size of the polycrystalline layer ZnS : ThF3, we have also prepared epitaxial layers of this material on a NaC1 support. The ZnS : ThF3 single crystal has a cubic structure with dimensions of 10 to 30 ~m and a thickness of 2 jsm. On the epitaxial layers, we have been able to verify that the distribution of terbium and fluorine stays uniform. As the concentration of terbium is the same inside the grains as at the boundaries there is no preferential condensation. 2.3. Luminescence studies The basis of the study consists of the comparison of emission spectra of 3~ it is possible to different materials; in the case of ZnS : LnF3 or ZnS: Ln observe the electroluminescence, the photoluminescence or the cathodoluminescence. We have established that only the electroluminescence and catholuminescence spectra are identical. This remark is important when we need to have the emission of a compound which cannot be electroluminescent, as for example LnF 3, for which we can also use the cathodoluminescence. For reasons which will be clear later we have chosen to draw on fig. 1 a the emission spectra 3~, in electroluminescence and3Pcathodoluminescence of thin films ZnS: PrF 3H of PrF3,ZnS: Pr 3 (transition 0 —* 4). We can observed that the three emissions are different which allows us to establish that the ZnS: PrF3 emission is not due to an ionic center or PrF3 agglomerate. On fig. lb we can establish that the emissions of PrF3 and powders of Pr2 S3 and PrSF are different from the emission of ZnS : PrF3. This eliminates any possibility of agglomerates and confirms the microprobe observations. 3+ the other hand with we observed the emissions areOn shifted compared the emission of PrF of ZnS : PrF3 and ZnS: Pr 3. This so-called “nephelauxetic
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effect” is due to the more covalent bonding of the praesodynium ion. A similar effect is observed with the samarium centers (fig. ‘2). In order to compare this effect for all the lanthanides we have drawn the displacement i~v/v versus different rare earth fluorides (fig. 3). We can ascertain that the nephelauxetic effect is even more important when we move away from the half-filled shells. For europium, gadolinium and terbium the effect was not observed. In fig. 3 we observe a nephelauxetic effect stronger for Tm3 + than for Er3~ a phenomenon previously observed by Jorgensen [14] on other compounds. We can profit from the nephelauxetic effect to detect eventual agglomerates; SmF 3 has been chosen in preference to PrF3 even though the nephelauxetic effect is weaker because of the fact that the brightnesses in electroluminescence
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and in cathodoluminescence of ZnS: SmF3 are one order of magnitude higher than the corresponding bnghtnesses of ZnS: PrF3. 6H 3 + in SmF Fig. 4a shows the emission of the transition 2/2 of Sm 3 and in ZnS doped with 2% SmF3. Above concentrations of 30% mole/mole (fig. 4b), peaks at 5940, 5925 and 5890 A corresponding to the emission of SmF3 agglomerates, are observed in the cathodoluminescence spectra. When we increase the concentration of SmF3 to 55% mole/mole, emission of the agglomerate increases compared with the emission of the isolated SmF3 center. In fact the level of 30% is not absolute as the emission of the agglomerate has an efficiency lower than the emission of the isolated SmF3 center (non-radiative recombination due to the presence of —~
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Sm3 + as nearest neighbours). On the contrary this experiment gives additional evidence that the emission observed at concentration of 2% has really originated from an isolated LnF 3 center. 2.4. Particular properties of the EuF3 center In general all evaporated lanthanides in a metallic form exist in the zinc sulphide matrix in the trivalent state with the exception of europium which is divalent (low intensity blue emission band). As previously indicated [6] the red trivalent emission appears in addition to the blue divalent band when europium is introduced in the EuF3 form. We can attribute this blue emission to a decomposition due to a chemical reaction between the vapors of ZnS, Zn, S and EuF3. In order to avoid this problem we have separated the zinc sulphide and europium fluoride vapours and have deposited them successively by means of a rotating substrate holder and appropriate masks. A vacuum annealing at 450°Cstrongly enhances the electroluminescence, in spite of this precaution the brightness of the ZnS: EÜF3 systems stays weak and of the same order as that indicated by Chase (some fL). The only point of interest of these systems remains in the possibility of using trivalent europium as a structural point symmetry probe, due to 5D 7F the value J = 0 of the resonant level 0 in emission and of the ground level 0 in absorption. Furthermore, in this case, Judd [15] has defined some simple rules ~J 2, 4 or 6 for electric dipole transitions. The 3spectrum obtained for ZnS: EuF3 + site symmetry. The observation (fig. 5) leads to 5780 the determination of the line at A for the 5Dof the7FEu 0 —* 0 transition is important because it proves the uniqueness of the rare earth site but also, as shown by Niewport and Blasse [16] this strictly forbidden transition can only be observed in the case of symmetry such as C~,C~or ~ Let us remember that the point symmetries for the rare earth ion in the starting materials are C3~for5DZnS and C9 for 5DEuF3 [17]. The number of lines observed for the transitions 0 and 0 ‘F~enable these hypothetical symmetries 5D 7F 5D 7F~ Symmetry 1 1 0 —* ~F0 2 3 0 -+ C 3~, 1 3 4 1 3 5 -+
—~
—*
The electroluminescent 5D 7F spectrum of ZnS: EuF3 possesses broad lines even for the transition 0 —~ 0 its width is quite but somewhat than that 3~.similar Examination of the less spectrum of observed ZnS: EuF in glasses doped with Eu 3 is C 3 allows us however to propose that the surrounding of Eu 3~in this compound. +
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0
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2.5. Summary The LnF3 entity is conserved when LnF3 is evaporated into ZnS. There are no agglomerates for the LnF3 .concentration used in the electroluminescent experiments. The emission of ZnS : LnF3 is quite special. LnF3 is a dopant existing in the form of one lanthanide and three fluorines, the whole being strongly linked to the 3~in ZnSZnS : LnFmatrix (nephelauxetic effect). The point symmetry of Ln 3 is probably C3,,. — LnF3 in the matrix ZnS being neither a molecular center nor an ionic center, we put forward the designation of complex center but the exact nature of this center cannot be given at the moment. For such studies the best probe is given by a europium center because the presence of red trivalent emission proves the existence of a complex center in ZnS. —
—
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3. Study of the complex center Ln 2°2~ 3.1. Intrpduction On analyzing how the cells are working, it is possible to distinguish some different steps [3]: Inside the zinc sulphide layer, working as an electron source, an electron process first appears, followed by an excitation, similar to cathodoluminescence, by hot electrons of the luminescent centers. It seems logical to search for promising centers among cathodoluminescent materials, especially the efficient luminophors which are used for television. We have chosen the rare earth oxysulphides, in particular the three following compounds for a trichromic application: For blue emission: La2O2S (Tm) ‘G4 —63H6 of Tm3~at 4800 A. For green emission: La 5D 7F 3~at 5400 A. 47F—* 5 of 3~at Th 6100 A. For red emission: Y 2O2S (Th) 5D 202S (Eu) of Eu In the cathodoluminescence studies 0on—6 the2rare earth oxysulphides the highest efficiencies are obtained for concentrations of the activators from 0.5 to 5%; beyond this value the interaction between the activators gives rise to nonradiative recombinations leading to a decrease of light efficiency. In active electroluminescent layers, a complex center consists of an isolated molecule coupled to the ZnS lattice, the interaction between the luminescent centers not being active. As it is necessary to heve only one active rare earth ion in the local center, the optimal solution would be realized with the lanthanum or yttrium oxysuiphide doped at 50% by an active rare earth oxysulphide such as compounds with the chemical formulae LaTmO 2S, LaTh02S and YEuO2 S. Such solid solutions at 50% are still not possible for all the rare earth elements because of the differences in ionic radii [18], however the synthesis procedure seems to have a certain influence on the respective solubilities [19]. The compound YEuO2S may be obtained whereas the LaTmO2S and LaThO2S compounds cannot be obtained. Therefore we have chosen not to use the compounds with the best light efficiencies but the well-crystallized compounds doped at 30% such as La14Tm06O2S, La1 4Th06 02S and Y1 .4 Eu06 02S. 3.2. Preparation of the active layers The rare earth oxysulphides are decomposed with a loss of sulphur during the heating under vacuum. Therefore, the rare earth oxysuiphide thin films are only obtained by diode-radiofrequency sputtering methods. To preserve the stoichiometry, sulphur compensation is realised either using an atmosphere of Ar + H2S [20] for sputtering, or by a determined excess of LnS3 added in the composition of the target [21]. So, for the elaboration of the luminescent ZnS
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thin film doped with rare earth oxysuiphide, coevaporation of ZnS and oxysulphide or evaporation of doped powder by joule heating or electron gun are not suggested. The diode-radiofrequency sputtering method is adequate to prepare the luminescent thin film. The target was obtained by careful mixing of the starting powders = 97.5% zinc sulphide, and 2.5% mole/mole doped rare earth oxysuiphide. We have previously emphasized the sensitivity of the europium complex center to the stoichiometry. When the thin film of zinc sulphide doped with Y14Eu06O2S is prepared under a pure argon atmosphere a weak and broad emission centered around 4900 A due to the europium in its divalent form is 3+ emission in cathodoluminescence. When also observed in addition to the Eu preparing a thin layer of rare earth oxysulphide alone, adding a small excess of sulphur is necessary; in the former case, the zinc sulphide provides the oxysuiphide with sulphur and the presence of divalent europium is due to a lack of oxygen. In the case of sputtering under an atmosphere of argon and 5 %o oxygen the one and only emission of trivalent europium is observed in cathodoluminescence. Annealing the active layers at 450°C under vacuum or pure nitrogen atmosphere for one hour enhances the, light efficiency. During sputtering, the rate of deposition, of about 40 A/mn, is laser optically controlled and the substrate is maintained at a temperature of 180°C. The electroluminescent device is composed of the active layer (4500 A thick) included in a sandwich of insulating films of yttrium oxide (3300 A).
4. Experimental results Fig. 6 gives the applied voltage dependence of the brightnesses of ZnS: La 2O2S (Th) and ZnS: Y202S (Eu) active layers compared to those of 2). The ZnS : ThF3 and ZnS: Mn devices (the electrode having an area of 25 mm form of the curves is similar but the rare earth oxysulphide devices show a greater resistivity since these materials work at a higher voltage. Actually the quality of our insulating layers does not allow the extension of the activity field in the “saturation area”, permitting a gain of the brightness by a factor of more than 2. Terbium center: green emission. Our own fabrication ZnS : ThF 3 devices present at best a brightness of 60 fL when they are optimized; on the contrary the ZnS: La2O2S(Th) device already shows a brightness of 70 fL without any special research of the optimization. Europium center: red emission. The brightness of the ZnS : Y202S(Eu) devices reaches a value of 20 fL which is clearly higher than those with ZnS: EuF3. Furthermore the complete absence of the divalent europium emission shows the real stability of this center.
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B(FLj 3
i0
Zn5,Mn
102
— -
Zn5 L~O 2S(Tb)
ZnS :Y2025 (Eu) 10
Zn5 ThF3
100
150
200
V
Fig. 6. Brightness (fL) as a function of voltage for the various dopants.
3 in ZnS : Y The luminescent spectrum of Eu 2O2S(Eu) is drastically different from 7F those in ZnS:7FEuF3 (fig. 7). One additional line is present in the 5J)~—+ 1 and ~l)~ —~ 2 transitions. The point symmetry of the trivalent europium which was C3,, in the ZnS: EuF3 compound is reduced to a C2,, site symmetry in the rare earth oxysuiphide case. In this compound we also observe broad lines; a more precise 5D ‘characterization is in progress by means of a selective excitation of the 0 levels. The Y2O2S(Eu) center is a very promising one because it is the only center actually allowing consideration of trichromic devices which need a bright red emitter around 6100 A. Thulium center: blue emission. We have not succeeded in making ZnS : TmF3 devices with a significant blue emission. In the past Chase [61encountered the same difficulty. In the case of ZnS : La2O2S (Tm) the characteristic blue emission is still weak but a higher luminescence intensity is soon expected with new and better solid solutions of thulium in lanthanum oxysulphide. +
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ZnS:Y202S((u) 300°C
1 D,—.’F2
5
5
00
7
____
Fig. 7. Emission spectrum of ZnS : Y202S(Eu) (T
-P.
5
7
F~ Dó—P.F0
77K).
5. Conclusions The study of the LnF3 centers allow us to show the existence of a particular center which we designate as a complex center. The presentation of the new rare earth oxysuiphide complex centers opens future prospects for high-field electroluminescent devices mainly because of the new possibility of developing trichromic display. Many other complex centers are considered, the main problem being to know the best technique for the introduction of these kinds of centers in the zinc sulphide matrix. Currently, no device procce~sesa brightness equal to that of the ZnS: Mn devices; however, devices based on the rare earth oxysulphide complex centers are substantially brighter than previous rare earth doped EL cells.
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Acknowledgments We wish to thank Mrs Gouy and Mrs Nappey for their participation in the cell realization and Mr Baherze for microprobe analysis. Some of the starting rare earth oxysulphides were kindly provided by Messrs Dougier and Browaeys, Rhône-Poulenc Society. We also wish to thank Professor F. Williams for helpful and fruitful discussions. References [II T. Inoguchi and S. Mito, Phosphor Films, Topics in Appl. Phys. Electroluminescence, Vol. 17 ed. J.I. Pankove (Springer-Verlag, Berlin, 1977) p. 197.
[Ill J. Benoit, P. Benalloul, Y. Charreire and B. Blanzat, Mat. Res. Bull. 11(1976)1463. [12] J. Benoit, thesis (1979). [13] J. Benoit, P. Benalloul, J. Mattler, M. Gouy and A. Nappey, Brevet CNRS-ANVAR No. 79 24927. [14] Jorgensen, private communication. [IS] BR. Judd, Phys. Rev. 127 (1962) 750. [16] W.C. Niewport and G. Blasse, Solid State Comm. 4 (1966) 227. [171A. Zalkin and D.H. Templeton, J. Am. Chem. Soc. 75 (1953) 2453. [18] M. LeskelS and L. NiinistO, J. Solid State Chem. 19 (1976) 245. [19] M. Leskela, L. NiinistO, H. Dexpert and Y. Charreire, Met. Res. Bull. 14 (1979) 455. [201 T.G. Mapple and R.A. Buchanan, J. Vac. Sci. Technol. 10 (1973) 616. [21] C. Sella, J.C. Martin, Y. Charreire and J. Loners, Le Vide, Les Couches Minces, CIP 79, p.201.