AC TFEL device having a white light emitting multilayer phosphor

An AC thin film electroluminescent (TFEL) device includes a multilayer phosphor for emitting white light having improved emission intensity in the blue region of the spectrum. The multilayer stack consists of an inverted structure thin film stack having a red light emitting manganese doped zinc sulfide (ZnS:Mn) layer disposed on a first insulating layer; a blue-green light emitting cerium doped strontium sulfide (SrS:Ce) layer disposed on the red light emitting layer; and a blue light emitting cerium activated thiogallate phosphor (Sr.sub.x Ca.sub.1-x Ga.sub.2 S.sub.4 :Ce) layer disposed on the blue-green light emitting layer. The manganese doped zinc sulfide layer acts as a nucleating layer that lowers the threshold voltage, and the cerium activated thiogallate phosphor layer provides a moisture barrier for the hydroscopic cerium doped strontium sulfide layer. The white light from the multilayer phosphor can be appropriately filtered to produce any desired color.

BACKGROUND OF THE INVENTION 
This invention relates to an ac thin film electroluminescent (AC TFEL) 
device and more particularly to an AC TFEL device having a white light 
emitting multilayer phosphor material. 
White light emission can be obtained by combining three primary color 
emissions, for example, by combining red, green and blue light emissions, 
or by combining complimentary color emissions. Thus, white light emission 
can be obtained from a single thin film phosphor layer having emissions in 
the red, green and blue regions of the color spectrum. The emission 
spectrum of a white light emitting phosphor may consist of either narrow 
emission bands at appropriate wavelengths for red, green and blue light or 
a broad emission band extending over the entire visible spectrum. White 
light emissions can also be obtained by combining phosphor layers, each of 
which may emit light primarily in a single region of the color spectrum. 
Current attempts to produce full color thin film electroluminescent panels 
typically include fabricating panels having one of two different basic 
phosphor structures: (a) a patterned structure where stripes of three 
primary color light-emitting phosphors are deposited side by side on a 
common electrode/insulator substrate or (b) a layered structure which may 
include single or multiple phosphor layers emitting either white light, or 
the three main spectral components of white light, combined with patterned 
color filters. It is also known to combine the two basic phosphor 
structures into a hybrid structure having side-by-side patterned red and 
green light-emitting phosphor stripes on one substrate combined with an 
unpatterned blue light-emitting layer on a second substrate. In order to 
fabricate a full color EL device using a broad band white light-emitting 
phosphor, the broad band emitting phosphor must provide significant 
emission intensity over a wide wavelength range in order to achieve three 
sufficiently bright saturated primary colors when combined with suitable 
color filters. 
Tanaka and others, in "Bright White-Light Electroluminescence Based on 
Nonradiative Energy Transfer in Ce- and Eu-doped SrS Thin Films," 51 Appl. 
Phys Lett., 1661 (Nov. 1987), report a single layer white light-emitting 
phosphor, SrS:Ce,Eu,K, which emits electroluminescence over a broad band. 
Tanaka and others, in "White Light Emitting Thin-Film Electroluminescent 
Devices with SrS:Ce,Cl/ZnS:Mn Double Phosphor Layers," 25 Jpn. J. Appl. 
Phys. L225 (Mar. 1986), disclose a multiple-layered white light emitting 
phosphor consisting of a greenish-blue light emitting SrS:Ce,Cl and a 
yellowish-orange light emitting ZnS:Mn. 
Ono and others, in "White-Light Emitting Thin Film Electroluminescent 
Devices with Stacked SrS:Ce/CaS:Eu Active Layers," 66 J. Appl. Phys., 5564 
(Dec. 1989), disclose a white light emitting phosphor obtained by stacking 
layers of blue-green light emitting SrS:Ce and red light emitting CaS:Eu. 
Mauch and others, in "ZnS:Mn/SrS:Ce Multilayer Devices for Full-Color EL 
Applications," SID 93 Digest, 769 (1993), disclose a broad band emitting 
phosphor consisting of multiple layers of manganese-doped zinc sulfide and 
cerium-doped strontium sulfide (ZnS:Mn/SrS:Ce), where nine such double 
layers are employed. 
None of these known phosphors exhibits significant emission intensity in 
the blue region of the spectrum, 450-480 nm. The major peak of cerium 
emission in strontium sulfide is located at 480 nm. Thus, very little deep 
blue emission can be obtained through filtering the emission of a 
cerium-doped strontium sulfide phosphor. Low emission intensity at the 
450-480 nm wavelengths will limit the color gamut in the blue region that 
can be achieved for a color-filtered full color panel and will affect the 
chromaticity of the combined white color. 
Barrow and others, in "A New Class of Blue TFEL Phosphors with Application 
to a VGA Full-Color Display," SID 93 Digest 761 (1993), disclose a hybrid 
phosphor structure for a full-color display panel consisting of patterned 
red light-emitting zinc sulfide doped with manganese (ZnS:Mn) and 
patterned green light-emitting zinc sulfide doped with terbium (ZnS:Tb) 
phosphors and a cerium activated calcium thiogallate phosphor layer 
(CaGa.sub.2 S.sub.4 :Ce) as the unfiltered blue light emitter. However, 
both the patterned phosphor structure and the hybrid phosphor structure 
are difficult to manufacture in the very high resolution structures, about 
1000 lines per inch, required for head mounted display panels. 
What is still needed is a broad band or white light emitting phosphor 
having improved emission intensity in the blue region and a large color 
gamut. 
SUMMARY OF THE INVENTION 
The present invention addresses the problems of the known broad band 
emitting phosphors by providing a multilayer white light-emitting phosphor 
material for an AC TFEL device which has an improved emission intensity in 
the blue region and a wide color gamut. In one embodiment the multilayer 
phosphor material consists of a red light-emitting phosphor layer and an 
alkaline earth thiogallate phosphor layer which includes a rare earth 
activator dopant RE taken from the group cerium and europium, the 
multilayer phosphor material having light emissions in the green portion 
of the spectrum. The alkaline earth thiogallate phosphor layer can be 
represented by the formula Ca.sub.x Sr.sub.1-x Ga.sub.2 S.sub.4 :RE, where 
x is a number between 0 and 1. Calcium and strontium are thus each present 
in the blue light-emitting thiogallate layer. 
The peak wavelength measured for an alkaline earth thiogallate phosphor 
varies as a function of the alkaline earth ion. Peak wavelengths of 459, 
445, and 452 nm, respectively, are obtained for cerium activated calcium 
thiogallate, strontium thiogallate and barium thiogallate. Thus, by 
adjusting the ratio of calcium to strontium and by the selection of the 
rare earth dopant in the thiogallate layer more saturated blue color and 
the desired color gamut is obtained for the phosphor material. 
In a second embodiment of the present invention a white light-emitting 
multilayer phosphor for an AC TFEL device has a phosphor material 
consisting of at least one red light-emitting phosphor layer and at least 
one alkaline earth thiogallate phosphor layer including a dopant. The 
alkaline earth is taken from the group calcium and strontium, and the 
dopant is a rare earth activator dopant taken from the group cerium and 
europium. At least one alkaline earth thiogallate phosphor layer of the 
multilayer phosphor includes more than one dopant. A thiogallate layer 
which is doped with both cerium and europium provides enhanced emission in 
both the blue and green regions of the spectrum. 
In a third embodiment of the present invention, a white light-emitting 
multilayer phosphor material consists of an alkaline earth thiogallate 
phosphor layer including a rare earth dopant, and an alkaline earth 
sulfide phosphor layer including a rare earth dopant. The alkaline earth 
is taken from the group calcium and strontium, and the rare earth dopant 
is taken from the group cerium and europium. The multilayer phosphor 
material has light emissions in at least both the red and green portions 
of the spectrum. Combining an alkaline earth thiogallate phosphor layer 
and an alkaline earth sulfide phosphor layer, each layer including a rare 
earth dopant, provides enhanced emission in the blue region of the 
spectrum. 
Each embodiment of the present invention thus provides a white 
light-emitting phosphor having an improved emission intensity in the blue 
region and a large color gamut. 
An AC thin film electroluminescent (TFEL) device including a white 
light-emitting multilayer phosphor consists of a first electrode deposited 
on a transparent substrate, a second electrode, at least two insulating 
layers located between the first and second electrodes, and a white 
light-emitting multilayer phosphor material of the present invention as 
described above sandwiched between the two insulating layers. 
The foregoing and other objectives, features, and advantages of the 
invention will be more readily understood upon consideration of the 
following detailed description of the invention, taken in conjunction with 
the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a TFEL laminate structure 10 embodying the present 
invention includes a first electrode 12 deposited on a transparent glass 
substrate 14. The first electrode 12 is a film of a transparent material 
such as indium tin oxide (ITO) or a metal which is preferably a refractory 
metal such as molybdenum. A first insulator layer 16, which is composed of 
a layer of aluminum oxide-titanium dioxide (Al.sub.2 O.sub.3 --TiO.sub.2 
or aluminate-titanate, commonly known as ATO), a barium tantalate 
(BaTa.sub.2 O.sub.6), or an insulator material having a perovskite 
structure such as strontium titanate (SrTiO.sub.3), is deposited on the 
first electrode 12. A white light-emitting multilayer phosphor material 18 
is located between the first insulator layer 16 and a second insulator 
layer 20. The second insulator layer 20 is either an aluminate-titanate 
(ATO) or a barium tantalate (BaTa.sub.2 O.sub.6). Other insulating layers 
such as silicon oxynitride (SiON), silicon nitride (Si.sub.3 N.sub.4) or 
Sr(Ti,Zr)O.sub.3, a mixed strontium titanate and zirconate, can also be 
used for the first insulator layer, and insulating layers such as ATO, 
SiON or Si.sub.3 N.sub.4 can also be used for the second insulator layer. 
The insulator layers typically each have a thickness of about 300 nm. A 
second electrode 22 is deposited over the second insulator layer 20. 
Referring to FIG. 1, in a preferred embodiment of the present invention, 
the electrode 12 adjacent the transparent glass substrate 14 is a metal 
electrode such as a molydenum film. The first insulator layer 16 adjacent 
the metal electrode is an aluminum oxide-titanium dioxide (ATO) and the 
second insulator layer 20 is barium tantalate (BaTa.sub.2 O.sub.6). The 
second electrode 22 is a transparent conductor electrode such as an ITO 
electrode. Such an electrode structure, wherein a metal electrode is 
adjacent the transparent substrate, is herein referred to as an inverted 
electrode configuration or structure. 
As depicted schematically in FIG. 1, a full color or multicolor AC TFEL 
device can be fabricated with an inverted electrode configuration using a 
white light-emitting phosphor with patterned red 24, green 26 and blue 28 
filters over each pixel. One of the pixel's electrodes is subdivided so 
that either only part of the pixel under each of the red, green and blue 
filters, or combinations of the subpixels, can be energized at the same 
time. Any color hue within the boundaries of the triangular region 
determined by connecting the CIE coordinates of the respective red, green 
and blue color emissions can be obtained by varying the ratio of the 
voltages supplied to the individual subpixels. 
Referring to FIG. 2, in one embodiment of the present invention the white 
light-emitting multilayer phosphor material 18 is composed of a red 
light-emitting phosphor layer and an alkaline earth thiogallate phosphor 
layer including a rare earth dopant RE, the thiogallate layer being 
represented by the formula Ca.sub.x Sr.sub.1-x Ga.sub.2 S.sub.4 :RE. The 
rare earth dopant is taken from the group cerium and europium, and x is a 
number between 0 and 1. The thickness of each phosphor layer and the ratio 
between strontium and calcium in the thiogallate layer can be adjusted to 
achieve the desired color gamut and luminescence. As shown schematically 
in FIG. 2, an example of such a multilayer phosphor material consists of a 
red light-emitting zinc sulfide phosphor layer 30 having a dopant 
manganese, represented by the formula ZnS:Mn, and an alkaline earth 
thiogallate phosphor layer 32 having the dopant cerium. One such zinc 
sulfide phosphor layer and one such alkaline earth thiogallate phosphor 
layer form a stack 34. Up to 25 stacks may be combined to form the 
multilayer phosphor material 18. When x is 0.5, one stack of such a 
multilayer phosphor material is represented by the formula Ca.sub.0.5 
Sr.sub.0.5 Ga.sub.2 S.sub.4 :Ce/ZnS:Mn. When describing a multi-layer 
structure the use of the slash mark (/) indicates that the compounds are 
separate layers. 
As shown schematically in FIG. 3, an alkaline earth thiogallate phosphor 
layer 36 having the dopant cerium, which provides the blue and some green 
component in the emission spectrum, is sandwiched between a zinc sulfide 
phosphor layer 38 having the dopant manganese, which provides the red 
component, and a zinc sulfide phosphor layer 40 having the dopant terbium, 
which provides a green component of the emission spectrum. These three 
layers form a stack 42 of phosphor material, as represented by the formula 
ZnS:Mn/Ca.sub.x Sr.sub.1-x Ga.sub.2 S.sub.4 :Ce/ZnS:Tb, where 0&lt;x&lt;1. As 
illustrated in FIG. 3, more than one stack of the multilayer phosphor 
material may be deposited to form the white light-emitting multilayer 
phosphor material. Up to 25 stacks of the phosphor material may be 
deposited. 
As shown in FIG. 4, in an alternative embodiment of a white light-emitting 
multilayer phosphor, the multilayer phosphor material 18 consists of at 
least one red light-emitting phosphor layer and at least one alkaline 
earth thiogallate phosphor layer including a dopant. The alkaline earth is 
taken from the group calcium and strontium. The dopant is a rare earth 
activator dopant taken from the group cerium and europium. At least one 
alkaline earth thiogallate layer is doped with both cerium and europium. 
The additional europium doping provides enhanced green emission, The red 
light-emitting phosphor layer can be a zinc sulfide layer 44 having a 
dopant manganese. The alkaline earth thiogallate phosphor layer 46 can be 
represented by the formula Ca.sub.x Sr.sub.1-x Ga.sub.2 S.sub.4 :Ce,Eu, 
where 0.ltoreq.x.ltoreq.1. These two layers form a stack 48 of the 
phosphor material. 
Referring to FIG. 6, in another embodiment of the present invention, the 
white light-emitting multilayer phosphor material 18 includes an alkaline 
earth thiogallate phosphor layer having a dopant RE and an alkaline earth 
sulfide phosphor layer having a dopant RE. The alkaline earth is taken 
from the group calcium and strontium, and the dopant is a rare earth 
activator dopant taken from the group cerium and europium. The multilayer 
phosphor material has light emissions in at least both the red and green 
portions of the spectrum. One such multilayer phosphor material includes 
an alkaline earth thiogallate phosphor layer 54 having the dopant cerium 
and an alkaline earth sulfide phosphor layer 56 having the dopants cerium 
and europium. The multilayer phosphor material can be deposited to form a 
stack 58 of the multilayer phosphor material, where the stack is 
represented by the formula Ca.sub.x Sr.sub.1-x Ga.sub.2 S.sub.4 
:Ce/ZnS/Ca.sub.y Sr.sub.1-y S:Ce,Eu where 0.ltoreq.x.ltoreq.1, and 
0.ltoreq.y.ltoreq.1. The alkaline earth sulfide phosphor layer 56 with 
dopants cerium and europium provides a broad band emission and the 
alkaline earth thiogallate phosphor layer 54 provides enhanced emission in 
the blue region of the spectrum. A layer 60 of zinc sulfide is located 
between the thiogallate phosphor layer and the alkaline earth sulfide 
phosphor layer. 
Alternatively, as shown in FIG. 7, the multilayer phosphor material 18 may 
include the blue light-emitting alkaline earth thiogallate phosphor layer 
62 having the dopant cerium, a blue-green light emitting strontium sulfide 
phosphor layer 64 having the dopant cerium and a red light-emitting 
calcium sulfide phosphor layer 66 having the dopant europium. The 
multilayer phosphor material can be deposited to form a stack 68 
represented by the formula Ca.sub.x Sr.sub.1-x Ga.sub.2 S.sub.4 
:Ce/ZnS/SrS:Ce/CaS:Eu, where 0.ltoreq.x.ltoreq.1. A layer 70 of zinc 
sulfide is located between the thiogallate phosphor layer and the alkaline 
earth sulfide phosphor layers. 
As shown in FIG. 5, another such multilayer phosphor consists of a blue 
light-emitting alkaline earth thiogallate phosphor layer including the 
dopant cerium, a red light-emitting zinc sulfide layer including a dopant 
manganese, and a blue-green light-emitting strontium sulfide phosphor 
layer including the dopant cerium. The phosphor material may be deposited 
in layers to form a stack 56 represented by the formula Ca.sub.x 
Sr.sub.1-x Ga.sub.2 S.sub.4 :Ce/ZnS:Mn/SrS:Ce, where 0.ltoreq.x.ltoreq.1. 
When x is 1, the alkaline earth thiogallate layer 50 having the dopant 
cerium is represented by the formula CaGa.sub.2 S.sub.4 :Ce, the zinc 
sulfide phosphor layer 52 including the dopant manganese is represented by 
the formula ZnS:Mn, the strontium sulfide phosphor layer 54 including the 
dopant cerium is represented by the formula SrS:Ce. The phosphor material 
is represented by the formula CaGa.sub.2 S.sub.4 :Ce/ZnS:Mn/SrS:Ce. 
In general, the layers of the white light-emitting phosphor material are 
deposited using vacuum deposition techniques. Typical thicknesses for the 
phosphor layers are 500-1500 nm for the alkaline earth sulfide layers and 
300-500 nm for the alkaline earth thiogallate layers. However, when the 
multilayer phosphor material consists of more than one stack, each 
phosphor layer is between 5 and 50 nm thick and up to 25 stacks may be 
deposited to form the multilayer phosphor. 
To prepare a single stack of the multilayer white light emitting phosphor 
represented by the formula CaGa.sub.2 S.sub.4 :Ce/ZnS:Mn/SrS:Ce shown in 
FIG. 5, first the blue light-emitting cerium activated calcium thiogallate 
phosphor 50 is deposited in a layer 200 to 400 nm thick. The red 
light-emitting manganese doped zinc sulfide phosphor 52 is deposited in a 
layer 150 to 300 nm thick atop the thiogallate layer. Finally, the 
blue-green light-emitting cerium doped strontium sulfide phosphor layer 54 
is deposited in a layer 600 to 1200 nm thick. Each phosphor layer can be 
deposited by a vacuum deposition technique such as sputtering or atomic 
layer epitaxy. 
FIG. 8 shows a calculated emission spectrum of CaGa.sub.2 S.sub.4 
:Ce/ZnS:Mn/SrS:Ce obtained by superimposing a ZnS:Mn/SrS:Ce spectrum and a 
CaGa.sub.2 S.sub.4 :Ce emission spectrum, both of which were measured 
under the same frequency, pulse width and drive voltage above threshold. 
The relative intensities of the emission at selected wavelengths in the 
blue region are listed in Table 1. The CIE coordinates for the unfiltered 
white light emission of CaGa.sub.2 S.sub.4 :Ce/ZnS:Mn/SrS:Ce are x=0.39, 
y=0.44. 
TABLE 1 
______________________________________ 
CaGa.sub.2 S.sub.4 :Ce/ZnS:Mn/SrS:Ce 
Wavelength Relative Intensity 
______________________________________ 
450 nm 13% 
460 nm 23% 
470 nm 36% 
480 nm 41% 
______________________________________ 
As shown in Table 1, a white light-emitting multilayer phosphor of the 
present invention, CaGa.sub.2 S.sub.4 :Ce/ZnS:Mn/SrS:Ce, exhibits 
significant emission intensity in the blue region of the spectrum. 
The unfiltered emission spectrum of a single stack of the multilayer 
phosphor material represented by the formula ZnS/Ca.sub.0.5 Sr.sub.0.5 
Ga.sub.2 S.sub.4 :Ce/ZnS:Mn is shown in FIG. 9. The relative intensities 
of the emissions in the red, blue and green regions of the spectrum for 
the same phosphor material as viewed through red, green and blue filters 
are also shown in FIG. 9. FIG. 10 shows the CIE 1931 chromaticity diagram 
showing the color gamut available from the multilayer phosphor material 
represented by the formula ZnS/Sr.sub.0.5 Ca.sub.0.5 Ga.sub.2 S.sub.4 
:Ce/ZnS:Mn. The CIE coordinates for the unfiltered phosphor material are 
x=0.54, y=0.42. For comparison, the dashed lines in FIG. 10 show the color 
gamut available from the phosphor represented by the formula 
ZnS:Mn/SrS:Ce. 
Referring now to FIG. 11, a preferred embodiment of the present invention 
is a single stack 80 of the multilayer white light emitting phosphor 
having improved emission intensity in the blue region of the spectrum 
consisting of a red light-emitting zinc sulfide layer 82 including the 
dopant manganese, represented by the formula ZnS:Mn, a blue-green 
light-emitting cerium doped strontium sulfide layer 84, represented by the 
formula SrS:Ce, and a blue light-emitting cerium activated calcium 
thiogallate phosphor 86, represented by the formula CaGa.sub.2 S.sub.4 
:Ce. 
To prepare the stack 80, first the red light-emitting manganese doped zinc 
sulfide phosphor 82, ZnS:Mn, is deposited in a layer 150 to 300 nm thick. 
The blue-green light-emitting cerium doped strontium sulfide phosphor 
layer 84, SrS:Ce, is deposited in a layer 600 to 1200 nm thick atop the 
manganese doped zinc sulfide layer. Finally, the blue light-emitting 
cerium activated calcium thiogallate phosphor 86, CaGa.sub.2 S.sub.4 :Ce, 
is deposited in a layer 200 to 400 nm thick. Each phosphor layer 
preferably is deposited by the vacuum deposition technique of sputtering. 
The terms and expressions which have been employed in the foregoing 
specification are used therein as terms of description and not of 
limitation, and there is no intention, in the use of such terms and 
expressions, of excluding equivalents of the features shown and described 
or portions thereof, it being recognized that the scope of the invention 
is defined and limited only by the claims which follow.