09/804,206, filed March 12,2001, and a continuation-in-part of U. S. Application No. 09/804,211, filed March 12,2001, which are hereby incorporated herein by reference in their entirety.

Field of the Invention The present invention relates to light emitting devices and, in particular, to light emitting devices useful in alternating current electroluminescent displays.

Background of the Invention The next generation of flat panel displays is seeking to provide advances in brightness, efficiency, color purity, resolution, scalability, reliability and reduced costs. One such technology is thin film electroluminescence (TFEL) of inorganic phosphors. TFEL displays can provide high brightness, outstanding durability and reliability. Current inorganic TFEL phosphors are composed of group Il-VI wide band gap semiconductor hosts such as zinc sulfide and strontium sulfide which provide hot carriers (greater than two

electron volts of energy) which impact luminescent centers such as manganese, cerium, and copper.

Sufficient hot carrier generation requires high field exceeding the break down field of the phosphor thin film. An alternating current biased dielectric/phosphor/dielectric layered structure allows reliable high field operation by current limiting of the electrical breakdown of the phosphor layer.

Generally these dielectric layers are thin film dielectric layers which are applied by sputtering or the like. As such, the thickness of the dielectric layers is generally limited. The thinness of the dielectric layer limits the voltage which can be applied and, further, limits the reliability of the TFEL.

Thin dielectric layers have other significant deficiencies.

Specifically, any irregularities, such as pinholes, in thin dielectric layers can result in premature electrical breakdown of the dielectric layer. This requirement for an irregularity-free thin film places difficult constraints on the deposition equipment and the cleanliness of the surrounding environment if adequate process yields are to be achieved.

One way to evaluate the performance of a dielectric layer is to calculate its figure of merit i. e., the product of dielectric permittivity and electrical breakdown field. Thin film dielectrics generally have a figure of merit (-5-50) which is one magnitude lower than the figure of merit (-50-500) for thick film dielectric layers based on materials containing primarily ferroelectric ceramics along with glass or fluxing agents which facilitate lower temperature (-850 °C) sintering of the dielectric layer. There are thick film dielectrics which can be formed at near room temperature but they suffer from a low figure of merit.

These thicker dielectric layers can be applied with a thickness in the neighborhood of 10-100 microns. At such thicknesses, the thick film dielectric layer is not susceptible to premature electrical breakdown due to uniformity irregularities which would cause a thin film dielectric to break down.

Furthermore, thick film dielectrics also possess the advantage of application by screen printing, which is a simple, high yield, and easily saleable process.

The thick film dielectrics generally must be screen-printed as a paste and subsequently sintered at high temperature in order to bring about desired dielectric properties. However, this sintering of the dielectric layer generally exceeds the breakdown temperature of the phosphor layer. Thus in order to utilize a thick dielectric layer, one must apply the dielectric layer to the electroluminescent device and sinter this layer prior to application of the phosphor layer. As a result, the phosphor layer is applied to the thick dielectric film whose surface is generally irregular and not necessarily suitable for formation of an electroluminescent device. Therefore, before the thin film phosphor layer is deposited, the surface of the thick film dielectric must be smoothed by an additional planarization layer applied by techniques such as sol-gel dip-coating.

Forming this electroluminescent device on, and emitting light through, a standard flat panel display glass substrate is not possible since the thick dielectric film layer is semi-transparent at best. This approach for forming the thick dielectric layer, before thin film phosphor layer deposition, has been patented (U. S. Patent 5,756,147) and is being pursued for commercialization of flat-screen TVs among other information display products.

Fig. 1 is a conventional electroluminescent device structure 200 incorporating a thick film dielectric 202 of lead niobate that is illustrative of the disadvantages of such device structures. Because of the roughness of the thick dielectric 202, an intermediate planarization layer 204 of lead zirconium titanium oxide must be incorporated between the thick film dielectric 202 and an overlying phosphor layer 206 of manganese-doped zinc sulfide. Therefore, the prior art electroluminescent device structure 200 has a peculiar disadvantage in that the fabrication sequence for the layers forming structure 200 is complicated by the need to provide the additional planarization layer. Another disadvantage of the device structure 200 is that it is impossible to emit light in the opposite direction from the exposed outer surface of the ceramic substrate 210 at least partially because the rear electrode 212 and thick dielectric film layer 202 lack the requisite optical transparency.

Summary of the Invention In one aspect, the present invention is premised on the realization that a doped wide band gap semiconductor material can be used in forming an alternating current electroluminescent device. More particularly the present invention is premised on the realization that wide band gap semiconductors such as gallium nitride, aluminum nitride or indium nitride doped with an emitting rare earth or other metal can be isolated and subjected to an alternating current to provide electroluminescence. These nitride-based electroluminescent semiconductors have the advantage of high brightness red, green, or blue emission. The nitride-based semiconductors are also extremely rugged which allows them to be electrically driven at high input powers without

significant semiconductor degradation. Furthermore, the nitride-based semiconductor is chemically stable and can be processed up to temperatures as high as 900°C. This high temperature stability allows compatibility of the nitride semiconductor with harsh electroluminescent device fabrication techniques. Fabrication techniques such as screen printing a high performance and high yield thick film dielectric layer requires a high sintering temperature of >800°C. It is the ruggedness of the nitride-based semiconductor that allows high temperatures and reactive chemicals to be utilized in device fabrication.

In another aspect, the present invention is premised on the realization that with proper selection of material and implementation of phosphor and dielectric material, an electroluminescent device utilizing a thick film dielectric layer can be formed on and emit through a glass substrate. More particularly the present invention is premised on the realization that a high temperature stable phosphor can be formed on a glass substrate prior to formation and sintering of a thick film dielectric layer. The by-products produced during, and high temperature required for, sintering of the dielectric does not adversely impact the semiconductor phosphor of the present invention. This permits the use of the present invention for electroluminescent flat panel display devices using relatively inexpensive manufacturing technology.

The objects and advantages of the present invention will be further appreciated in light of the following detailed description and drawings in which:

Brief Description of the Drawings Fig. 1 is a sectional view of a prior art alternating current electroluminescent device formed on the ceramic substrate in which light is emitted away from the ceramic substrate.

Fig. 2 is a sectional view of an alternative embodiment of the present invention formed on a glass substrate in which light is emitted through the glass substrate.

Fig. 3 is a sectional view of an alternate embodiment of the present invention.

Fig. 4 is a sectional view of an alternate embodiment of the present invention.

Fig. 5 is a graphical representation of brightness vs. applied voltage for an electroluminescent device of the present invention.

Detailed Description of Preferred Embodiments As shown in Fig. 2 and in accordance with the principles of the present invention, an electroluminescent device 12 of the present invention has an electroluminescent phosphor 14 applied over a glass substrate 16. In this embodiment, the initial substrate 16 is simply a glass substrate such as Corning 1737 glass substrate. Other glasses, such as fused silica, can be utilized.

Substrate 16 is coated with a transparent first electrode 18 followed by the electroluminescent phosphor 14. Suitable materials for forming transparent first electrode 18 include metal oxides such as zinc oxide, tin oxide, indium oxide and mixtures thereof. The phosphor layer 14 is any phosphor that is compatible, or can be made compatible, with sintering of the thick dielectric

layer. The phosphor layer preferably comprises gallium nitride doped with a transition or rare earth metal. However, phosphor layer 14 may be formed using other doped compounds that can withstand the high temperatures of, and byproducts produced during, sintering of the thick film dielectric layer. To that end, the material of the phosphor layer should not break down at a temperature of 400oC and, preferably, at a temperature as a high as 500°C.

Generally, phosphor layer 14 may be formed from a first element selected from Groups II, III and IV of the periodic table and a second element selected from group V, VI and Vil of the periodic table. Such stable doped compounds include gallium nitride, aluminum indium gallium nitride, aluminum gallium nitride, aluminum indium oxide, aluminum gallium indium oxide, aluminum gallium indium sulfide, gallium oxide, zinc silicon germanium oxide, zinc silicate/germanate, and zinc galates. Zinc, calcium and strontium sulfides would also be suitable as a doped compound for phosphor layer 14 if annealed or hot-deposited to stabilize their structural integrity and encapsulated with protective barriers before depositing and sintering the thick film dielectric layer.

The stable electroluminescent phosphor is covered with a thick coating 22 of dielectric. Thick dielectric films are generally greater than about 5 microns, preferably about 10 microns or more but usually less than about 100 microns in thickness, and, in particular, are applied by physical coating methods as opposed to chemical (i. e., gas phase or plasma phase) coating methods.

The dielectric layer 22 is applied as a gel. The preferred method of applying the dielectric is simply screen printing. Alternate methods include spraying, sol-gel dip-coating, and tape casting. The thick-film dielectric layer 22 can be formed by a single or multiple applications until the desired final thickness is reached.

A reduced dielectric firing temperature allows for compatibility with a larger variety of electroluminescent phosphors. Thus, the thick film dielectric 22 must be one which can be sintered at a temperature which will not adversely effect the phosphor or glass and which possess the requisite electrical properties. In certain embodiments, the dielectric layer 22, after sintering, has a dielectric constant greater than about 500. In other embodiments, the dielectric layer has a dielectric constant greater than about 100.

Suitable dielectrics for thick film dielectric 22 include, but are not limited to, barium titanate, lead zirconate titanate, and lead niobate. Some dielectrics such as barium titanate have additional glass and fluxing agents which facilitate low temperature (<900°C) sintering and densification of the dielectric layer. Preferably the dielectric can be sintered at temperatures compatible with standard display glass substrates (<700 °C).

Dielectric layer 22 once applied is then heated up to dry, densify, partially sinter, or fully sinter the dielectric, preferably but not limited to temperature below the strain temperature of the glass substrate. Barium titanate is preferably heated to about 200-800°C for about 1-10 minutes.

Following drying, densifying, partial sintering, or full sintering additional layers of barium titanate can be applied and dried, densified, partially sintered, or fully sintered. As a final application step, multiple thick dielectric layers can be fired together at a final temperature of about 400-800°C for about 1-10 minutes. The dielectric layer 22 is coated with a metal electrode 24 by means of screen printing or other suitable method. The metal electrode can be of the thin (-0. 1- 1 um) or thick film (~1-10, um) form. If the metal electrode 24 is applied a gel, such as screen printing, it is preferably heated to about 200-800°C for about 1-

10 minutes in order to bring about requisite electrical properties. As a final application step, multiple thick dielectric layers and metal electrode can be fired together at a final temperature of about 400-800°C for about 1-10 minutes. The final device structure is in turn coated with an encapsulant 25 such as Dupont 8185 or Honeywell Aclar film.

When alternating current is applied between the first electrode 18 and the rear electrode 24, light will be emitted from the layer phosphor 14 through the first electrode 18 and through the glass substrate 16 as indicated by arrows 26.

This structure has the advantage that without any additional processing to the lower surface the phosphor is applied to a relatively smooth surface as opposed to the much rougher surface found when forming a thick film dielectric layer before phosphor layer deposition. It further reduces processing steps and requires a minimum of only one dielectric. It allows for light emission through the glass substrate on which the structure is formed.

Further, the dielectric has significantly greater effectiveness than can be practically achieved using two thin layer dielectrics.

Although the transparent surfaces underlying the phosphor layer are generally considered smooth in comparison to the surface of the thick film dielectric layer, slight roughness of the underlying surfaces, or the phosphor layer itself, can improve the device efficiency and brightness. Surface roughness on the order of 100 nm allows for light to be scattered out of the light emitting device more efficiently than the case of a light emitting device with perfectly smooth transparent surfaces. The present invention allows for such roughening of the transparent electrode or underlying glass substrate since the

10 to 100 um thick dielectric layer is reliable at high voltages for surface roughness values even about 1 um. Such is not the case for thin film dielectrics where 100 nm surface roughness would adversely effect the high voltage reliability of the 200-400 nm thin film dielectric.

Figure 3, in which like reference numerals refer to like features in Fig. 2, shows a slightly modified embodiment of an electroluminescent device 41 constructed according to the principles of the invention. Shown in Fig. 3, the glass substrate 30 of device 41 is coated with a transparent zinc oxide aluminum electrode 32 in turn coated with a stable gallium-based phosphor 34.

This is coated with a thin dielectric 36 (0.1 to 1 micron) followed by a thick dielectric 38. The thin dielectric 36 allows one to modify the electrical contact or adhesion of the thick film dielectric layer 36 to the phosphor layer 34. The thin dielectric film 36 is applied by sputtering or the like as is the phosphor 34 as previously described. The thick film dielectric 38 is then applied using a screen printing technique followed by sintering and screen printing of a silver electrode layer 40. Additionally, use of oxide phosphors, oxide dielectrics, and oxide transparent electrodes allows of entirely vacuum free device fabrication through methods such as sol-gel dip coating of all thin films and screen printing of all thick films. This can then be covered with a well-known encapsulant 42. In this embodiment likewise once current supply between the silver electrode 42 and the zinc oxide aluminum electrode 32, the phosphor 34 film will be excited causing it to fluoresce and emitting light as indicated by arrows 44.

The thick film dielectric layer 38 allows formation of a high yield, high capacitance, and high voltage stable device structure. An additional thin film dielectric (not shown) may be inserted between the phosphor layer 34 and

transparent front electrode 32 with a different primary purpose than that served by the thick film dielectric layer. Sputtering of an Al203 thin film dielectric is an example of such a thin film dielectric (0.1 to 1 microns). This transparent thin film dielectric layer can result in improved efficiency and brightness of the device through charge trapping of electrons at the phosphor/thin film dielectric interface. The effects of the dielectric/phosphor interface on device performance is well known by those skilled in the art.

As shown with both the embodiments in Figs. 2 and 3, the phosphor layers are applied on a very smooth surface providing significant uniformity. Further, they are applied transparent thin films on glass substrates so that the emission is directly through the original substrate.

The present invention can utilize the thick film dielectric layer of the present invention because of the chemical and thermal stability of the GaN- based phosphor layer. The light emitting performance of the GaN phosphor layer is not adversely affected by the high temperature or reactive by-products produced during sintering of the thick film dielectric layer. As shown in Fig. 4, in which like reference numerals refer to like features in Figs. 2 and 3, an electroluminescent device 51 according to the present invention can incorporate phosphors in layer 52 that are structurally but not chemically stable at the required sintering temperature of the thick film dielectric layer 54. In order to prevent degradation of phosphor layer 52 due to reactive by-products produced during dielectric sintering, a thin film protective barrier 56 may be placed between the structurally stable phosphor layer 52 and thick film dielectric 54.

Preferably, the thin film protective barrier 56 also functions as a dielectric layer

and allows one to modify the electrical contact or adhesion of the thick film dielectric layer 54 to the phosphor layer 52.

The present invention can also be utilized with phosphor layers which, as deposited, are neither chemically or structurally stable at the required sintering temperature of the thick film dielectric layer. Near-room temperature deposited phosphors such as ZnS: Mn and SrS: Ce are examples. Such phosphor layers are preferably made structurally stable at the sintering temperature of the dielectric layer. Then a protective barrier must be placed over the phosphor layer. Without structural stability, the integrity of the protective barrier can be compromised during sintering of the thick film dielectric layer. To complete such a structure, a phosphor layer 52 ZnS: Mn is sputtered at near room temperature (over protective barrier 58), then annealed near or above the sintering temperature of the thick film dielectric layer 54. This anneal allows for structural stability of the ZnS: Mn phosphor layer 52 at high temperature.

After anneal, an aluminum oxide protective barrier 56 can be applied to the ZnS : Mn phosphor layer 52 of device 51. The protective barrier 56 functions as a dielectric and protective barrier layer whereas the protective barrier 58 need only function as a dielectric layer for optimum electrical contact or adhesion between phosphor layer 52 and transparent electrode 18.

Generally, the protective barriers 56,58 may be any suitable metal oxide, metal nitride, semiconductor oxide, or semiconductor nitride. More specifically, the protective barriers 56,58 may be aluminum oxide aluminum oxide, strontium titanium oxide, barium titanium oxide, and or titanium oxide, each of which can be sputtered deposited onto the phosphor layer 52. The protective barriers 56,

58 may be formed of the same dielectric material, from among the dielectric materials previously listed, or may be formed of different dielectric materials, as shown in Fig. 4. The thick film dielectric layer 54 is then screen printed and sintered at a temperature which is near or below the annealing temperature of the ZnS: Mn phosphor layer 52. This embodiment allows the use of less stable phosphors in a structure which emits light through a transparent glass substrate 16.

The structures shown in the figures are exemplary and show adjacent layers in contact with each other without intervening layers. However, additional layers can be utilized to the extent they do not interfere with the recited layers. Therefore, the terms"coating"and"in contact"do not exclude the possibility of additional intervening but non-interfering layers.

In accordance with another aspect of the present invention, the electroluminescent phosphor layer will be a wide band gap semiconductor material doped with a light-emitting rare earth element, transition metal or other metal. The preferred wide band gap semiconductors include gallium nitride, aluminum nitride and indium nitride. These can be alloyed with other compounds such that the wide band gap semiconductor is composed of primarily gallium nitride, aluminum nitride, or indium nitride and mixtures thereof. These can be doped with a variety of elements. Preferred elements for doping include : Pr, Eu, Tb, Er, and Tm. According to this aspect of the present invention, the semiconductor phosphor layer can be utilized in any device structure which can reliably provide high voltage excitation to the semiconductor phosphor layer which emits light during electrical breakdown. The preferred device structure 12 for the semiconductor phosphor layer is shown in Fig. 2.

Any production method which forms polycrystalline semiconductors can be used to apply the semiconductor phosphor layer.

Suitable techniques include molecular beam epitaxy, metalo-organic chemical vapor deposition, chemical vapor deposition, plasma enhanced chemical vapor deposition, hydride vapor phase epitaxy, plasma enhanced chemical deposition, sputtering and evaporation. This aspect of the present invention is unlike nitride light emitting diodes, which do not operate as a phosphor does and generally require costly crystalline substrates instead of display glass.

The desired thickness of the semiconductor material should be from about 0.2 to about 2 microns with about 0.5 to about 1 micron being preferred. For the rare earth or metal dopant to be strongly optically active in the wide band gap semiconductor, the dopant should substitute for the group III element (Ga, Al or In). This should permit the light emitting element to sit in an optically active site which promotes visible light emission.

Typically the rare earth dopant will be Tm for a blue display, Pr or Eu for a red display and Er or Tb for a green display. These can be added to the semiconductor by either in situ methods or post growth doping using ion implantation or diffusion. Generally the concentration of the dopant is relatively high from less than 0.1 at. % up to about 10 at. % or higher. The dopant concentration can be increased until the emission stops. Generally the preferred concentration will be 0.1 at. % to 10 at. %.

The method of forming the light emitting semiconductor layer in the present invention is further explained in Patent No. 6,255,66 entitled"Visible Light Emitting Device Formed from Wide BandGap Semiconductor Dopant with a Rare Earth Element"is further the subject of published PCT Application

US00/10283, both of which are incorporated herein by reference in their entirety.

The invention will be further appreciated in light of the following examples.

Example 1 Dupont 9970 Ag: Pt paste was screen printed onto alumina substrates, dried, and fired at 850°C for ten minutes. One or two layers of Dupont 5540 dielectric paste were screen printed, dried and sintered at 850°C for ten minutes. The Dupont 5540 paste contains barium titanate along with glass or fluxing agents that facilitate lower temperature (<900°C) sintering of the dielectric. A scanning electron microscope photograph showed that the thick- film dielectric layer has a surface roughness of-1 micron and is granular with high porosity. The resulting dielectric layer thickness was 20-40 microns with a dielectric constant of £ ~6000. A GaN: Er phosphor film of-1 micron thickness was then deposited by solid-source molecular beam epitaxy onto the dielectric/metal/ceramic substrates.-300 nm thick indium-tin-oxide transparent electrodes were then sputtered through a stencil mask onto the GaN: Er phosphor film. The resulting structure was biased with a 1 kHz, 200V square wave and exhibited a low luminance of <1 cd/m2. The poor luminance was due to the roughness of the thick-film dielectric layer on which the GaN: Er phosphor layer was deposited. Also, since both the dielectric layer, Ag: Pt metal electrode, and ceramic substrate were opaque, light emission was not possible through the substrate.

Example 2 -300 nm indium-tin-oxide films were deposited on Corning 1737 glass substrate which has a thermal strain point of 666°C, sufficiently above the ~600°C substrate temperature used during gallium nitride phosphor deposition.

Corning 1737 is a widely utilized display glass due to its compatibility with low temperature (500-600°C) poly-Si processing used for active-matrix liquid crystal displays. Approximately 1 micron thick erbium doped gallium nitride phosphor film was deposited by solid-source molecular beam epitaxy onto the indium-tin- oxide coated 1737 glass substrates. Following phosphor deposition, 1 layer of Dupont 5540 dielectric paste was screen printed, dried, densified for 10 minutes at-600°C, and sintered at-800°C for 4 minutes. The resulting dielectric layer had a thickness of-20 microns, breakdown strength >200 V, and dielectric constant of E ~500-1000. Rear electrodes were formed by sputtering of tantalum through a stencil mask.

A scanning electron microscope photograph of the completed device showed that the underlying phosphor and transparent electrode layers were structurally intact after sintering the dielectric layer. The phosphor layer was unaffected and the substrate was only slightly warped by the high temperature sintering of the thick film dielectric layer. When viewed under a high magnification microscope, the light emission from the electroluminescent device is uniform well below 10 microns, which is beyond the requirements of a flat panel display. When biased with an alternating voltage source, the erbium doped gallium nitride electroluminescent device exhibited a green emission with a maximum luminance value of-20 cd/m2 at 200 V, 1 kHz biasing. Unlike the device of Example 1, this brightness value is sufficient for a flat panel display.

Further, unlike previous art the device utilized a gallium nitride phosphor instead of a sulfide or oxide phosphor. Accelerated aging tests of the electroluminescent device resulted in a 60 Hz operational lifetime in excess of 1000 hrs at >50% initial brightness.

Example 3 A device similar to that of Example 2 was formed with the following changes to fabrication of the thick-film dielectric layer. Following the phosphor deposition, 1 layer of Dupont 5540 dielectric paste was screen printed, dried, and densified for 10 minutes at-600°C. A second layer of Dupont 5540 dielectric paste was then screen printed, dried, and densified for 10 minutes at-600°C. This stack of two dielectric layers was then sintered at --800°C for 4 minutes. The resulting dielectric layer had a thickness of-40 microns, breakdown strength >300 V, and permittivity of s-500-1000. The device exhibited similar luminance characteristics to that of Example 2 but showed improved reliability at high voltages (>300 V breakdown).

Example 4 A device similar to that of Example 2 was formed with the following changes to fabrication of the thick-film dielectric layer. Following the phosphor deposition, 1 layer of Dupont 5530 dielectric paste was screen printed, and dried for 10 minutes at-200°C. A second layer of Dupont 5530 dielectric paste was then screen printed and dried for 10 minutes at-200°C.

This stack of two dielectric layers was then sintered at-650°C for 5 minutes.

The resulting dielectric layer had a thickness of-10 microns, breakdown

strength >400 V, and dielectric constant of s--500-1000. The device exhibited similar luminance characteristics to that of Example 2 but showed improved reliability at high voltages and exhibited no warping of the glass substrate due to a reduced dielectric sintering temperature.

Example 5 A device structure similar to that of Examples 2,3 and 4 was formed with an approximately 1 micron thick europium-doped gallium nitride phosphor film deposited onto an indium-tin-oxide coated 1737 glass substrate.

Following the phosphor deposition a thin layer of aluminum oxide was sputtered in order to enhance electrical contact and adhesion of the thick dielectric layer.

Following the aluminum oxide deposition, 1 layer of Dupont 5530 dielectric paste was screen printed, and dried for 10 minutes at-200°C. A second layer of Dupont 5530 dielectric paste was then screen printed and dried for 10 minutes at-200°C. This stack of two dielectric layers was then sintered at --650°C for 5 minutes. The resulting dielectric layer had a thickness of-10 microns, breakdown strength >400 V, and dielectric constant of g ~500-1000.

When biased with an alternating voltage source, the completed europium-doped gallium nitride electroluminescent device exhibited a deep red emission with a maximum brightness value of-40 cd/m2 at 200 V, 1 kHz biasing. Due to the addition of the thin aluminum oxide layer between the phosphor and thick dielectric layer, the device exhibited improved accelerated aging tests of the electroluminescent device resulting in 120 Hz operational lifetime in excess of 50,000 hrs at >50% initial brightness.

Example 6 A device structure similar to that of Example 5 was formed with the following changes. A thin layer of aluminum oxide was included between the phosphor and transparent electrode in order to improve device efficiency.

In place of the GaN: Eu phosphor layer a structurally stable ZnS: Mn phosphor layer was utilized. Following the phosphor deposition a thin layer of aluminum oxide was sputtered in order to enhance electrical contact and adhesion of the thick dielectric layer. The thin layer of aluminum oxide also serves as a protective barrier layer for the ZnS: Mn layer which is not chemically stable when exposed to the by-products and temperatures of the thick dielectric sintering process. Following the aluminum oxide deposition, 1 layer of Dupont 5530 dielectric paste was screen printed, and dried for 10 minutes at-200°C. A second layer of Dupont 5530 dielectric paste was then screen printed and dried for 10 minutes at-200°C. This stack of two dielectric layers was then sintered at-650°C for 5 minutes. In place of the Ta electrodes utilized in Examples 2-4, an Ag thick-film back electrode was screen printed, dried, and sintered at ~550°C for 5 minutes. The resulting device structure operated in a similar manner to that obtained for sulfide phosphors in conventional TFEL device structure. Unlike previous art with sulfide phosphors, the device emitted through a glass substrate on which it was formed and included a screen printed and sintered thick dielectric and rear electrode.

The present invention can then be used to form a flat screen display device by simply forming red, green and blue electroluminescent devices adjacent to each other. Basically tens to millions of the electroluminescent devices would be formed over an area preferably utilizing

the embodiment shown in Figs. 2,3, and 4. This would then enable a flat screen display device such as a television, computer monitor or the like.

Further, the cost and yield of manufacturing is significantly improved due to the ability to screen print the dielectric onto the structure. Further, utilizing the thick dielectric film significantly enhances the reliability of the electroluminescent device of the present invention.

This has been a description of the present invention along with a preferred method of practicing the invention. However, the invention itself should be defined only by the appended claim, wherein we claim :