Sputtering method of producing and electroluminescent device...

Details Sputtering method of producing and electroluminescent device... Sputtering method of producing and electroluminescent device...

1998-04-13

2001-07-03

Nguyen, Nam (Department: 1753)

Chemistry: electrical and wave energy

Processes and products

Coating, forming or etching by sputtering

C204S192290, C204S192150

Reexamination Certificate

active

06254740

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATION
This application is based upon and claims the benefit of priority of the prior Japanese Patent Applications No. 7-95215 filed on Apr. 20, 1995 and No. 7-103846 filed on Apr. 27, 1995, the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electroluminescent (hereinafter referred to as EL) devices which are used in various instruments of, for example, emissive-type segment displays and matrix displays, or in displays in various information terminal appliances. The present invention also relates to methods for producing the same.
2. Related Arts
Conventional EL devices are formed by laminating a first electrode made of an optically-transparent ITO (indium tin oxide) film, a first insulating layer comprising Ta
2
O
5
(tantalum pentoxide) or the like, a luminescent layer, a second insulating layer, and a second electrode made of an ITO film, on an insulating glass substrate in that order.
The luminescent layer comprises, for example, a host material such as ZnS (zinc sulfide) with a luminescent center such as Mn (manganese) or Tb (terbium) added thereto, or a host material such as SrS (strontium sulfide) with a luminescent center such as Ce (cerium) added thereto.
EL devices give different colors depending on the choice of the additives in ZnS. For example, those having Mn as the luminescent center produce yellowish orange colors, while those having Tb produce green colors. EL devices having Ce as the luminescent center in SrS produce bluish green colors.
To realize full-color EL displays, luminescent layers capable of producing red, green and blue colors must be formed. Of these, SrS with a luminescent center of Ce is generally used as the material for the blue-emitting layers in EL devices. However, this material naturally produces a bluish green color. Therefore, in order to attain pure blue emission, a filter capable of cutting off the green component out of the emission spectrum must be used.
As opposed to this, it is known that an EL device with a CaGa
2
S
4
:Ce luminescent layer having Ce as the luminescent center element added to the host CaGa
2
S
4
(calcium thiogallate) can produce a blue color without using a filter, for example, as so reported in SID 93 Digest, pp. 761-764 (1993).
However, it is reported in the reference that the CIE (Commission Internationale de l'Eclairage) chromaticity coordinates for the EL device with the CaGa
2
S
4
:Ce luminescent layer indicate x=0.15 and y=0.19. On the other hand, the CIE chromaticity coordinates for ZnS:Ag that is used for the blue phosphor in cathode-ray tubes indicate X=0.15 and y=0.07 or so. The purity of the blue color produced by the conventional CaGa
2
S
4
:Ce luminescent layer is low and therefore the luminescent layer was unsatisfactory.
SUMMARY OF THE INVENTION
Considering the above-mentioned problems, the present inventors have accomplished the present invention, and an object of the present invention is to improve the blue color purity in EL devices with thiogallate luminescent layer containing calcium, such as CaGa
2
S
4
:Ce luminescent layer.
The present inventors have found from their extensive research and repetitive experiments that the X-ray diffraction spectrum for the CaGa
2
S
4
:Ce luminescent layer as produced by the conventional method shows a diffraction peak for CaS. The existence of CaS results in the green-emitting component due to CaS:Ce, which lowers the blue color purity. Therefore, the decrease, if possible, in the green-emitting component resulting from CaS:Ce will contributes to the increase in the blue color purity. On the basis of these discoveries, the present inventors have completed the present invention.
According to the present invention, a CaGa
2
S
4
:Ce luminescent layer is selected to have a film quality characterized in that the ratio of the X-ray diffraction peak intensity I
2
for the (200) reflection of CaS to the X-ray diffraction peak intensity I
1
for the (400) reflection of CaGa
2
S
4
as appearing in the X-ray diffraction spectrum for the luminescent layer, I
2
/I
1
is 0.1 or less (including 0).
Such CaGa
2
S
4
:Ce luminescent layers can be deposited by means of a sputtering method using a sintered target which comprises as a main component CaGa
2
S
4
doped with Ce and presents a ratio of the X-ray diffraction peak intensity I
2
for the (200) reflection of CaS to the X-ray diffraction peak intensity I
1
for the (400) reflection of CaGa
2
S
4
as appearing in the X-ray diffraction spectrum, I
2
/I
1
that is 0.5 or less (including 0).
Other objects and features of the present invention will appear in the course of the description thereof, which follows.


REFERENCES:
patent: 4208613 (1980-06-01), Hase et al.
patent: 4437620 (1984-03-01), Ozawa
patent: 5309070 (1994-05-01), Sun
patent: 5482603 (1996-01-01), Kawashima et al.
patent: 5554449 (1996-09-01), Tonomura et al.
patent: 5589040 (1996-12-01), Nishimura
patent: 5598059 (1997-01-01), Sun
patent: 5747929 (1998-05-01), Kato et al.
patent: 7-122363 (1995-05-01), None
S-S. Sun et al., “Electroluminescene and Photoluminescence of Cerium-Activated Alkaline Earth Thiogallate Thin Films and Devices,” J. Electrochem. Soc., vol. 141, No. 10 Oct. 1994, pp. 2877-2883.
Y. Inoue et al., “Blue Electroluminescent Devices with SrGa2S4: Ce Thin Films Grown by Molecular Beam Epitaxy,” Conference Record of the 1994 Int'l Display Research Conference and . . . Materials, Ch2.5, Oct. 1994, pp. 169-171.
Y. Inoue et al., Blue Electroluminescent SrGa2S4:Ce Thin Films Grown by Molecular Beam Expitaxy, Jpn. J. Appl. Phys., vol. 34, Feb.1, 1995, pp. L180-L181.
T.E. Peter and J.A. Baglio, “Luminescene and Structural Properties of Thiogallate Phosphors CE+3and EU+2-Activated Phosphors. Part I,”, J. Electrochem. Soc. vol. 119, No. 9, Feb. 1972, pp. 230-236.
R.T. Tuenge, “Invited Recent Progress in Color Thin Film EL Displays,” ASIA Display '95 Ch. S-12-1, Oct. 17, 1995, pp. 279-282.
W.A. Barrow et al., “A New Class of Blue TFEL Phosphors with Application to a VGA Full-Color Display,” SID 93 Digest, vol. 32.1, 1993 pp. 761-764.

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