JPS6315718B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention is a flat thin display device that displays characters, symbols, and graphics on computer output display terminal equipment and other various display devices. The present invention relates to thin film EL devices used as means for displaying still images and moving images, and more specifically relates to thin film EL devices with improved contrast.

[Conventional technology]

Conventionally, this type of thin film EL device has been introduced as shown in Figures 1a, b, and c, in which 1 is a glass substrate and 2 is a transparent substrate made of In ₂ O ₃ , SnO ₂ , etc. electrode, 3 is a first dielectric layer made of Y ₂ O ₃ , Ta ₂ O ₅ , etc., 4 is a luminescent center of 0.1 to 2.0wt
% Mn (or Tb, Sm, Cu, Al, Br, etc.) doped ZnS (or ZnSe, etc.) light emitting layer, 5 is Y ₂ O ₃ ,
A second dielectric layer made of Ta ₂ O ₅ or the like, 6 a back electrode made of Al or the like, and 7 a light absorption layer made of CdTe. Here, the transparent electrodes 2 are arranged in parallel in a plurality of strips on the glass substrate 1, and the back electrodes 6 are arranged in parallel in a plurality of strips in a direction perpendicular to the transparent electrodes 2. Graphically, the crossing position corresponds to one pixel on the panel. Then, by applying an AC voltage to electrodes 2 and 6, electrons are excited to the conduction band by the electric field generated in the light emitting layer 4 and accelerated to obtain sufficient energy. When the excited Mn luminescent center returns to the ground state, it emits yellow light.

Then, the thin film EL device according to FIG.
As shown by curve a in the figure, this structure has the advantage of having a relatively low light emission starting voltage and the absence of the aforementioned insulating light absorption layer 7 in the area where the electric field is applied, making it difficult for dielectric breakdown to occur. Although it has advantages, it has the disadvantage that display quality deteriorates because contrast cannot be obtained.

The thin-film EL device shown in FIGS. 1b and 1c has the light absorption layer 7 interposed between the light emitting layer 4 and the second dielectric layer 5 and between the second dielectric layer 5 and the back electrode 6, respectively. By absorbing external light incident from the glass substrate 1 side, the contrast of the EL element can be increased. However, the thin film EL according to Fig. 1b
As shown by curve b in Figure 2, the luminance of the device rises slowly with respect to the applied voltage, and in order to increase the luminance, the applied voltage must be considerably high. Since the absorption layer 7 is interposed, there is a drawback that dielectric breakdown is likely to occur when a high electric field is applied. Next, Figure 1c
As shown by curve c in FIG. 2, the thin-film EL device according to the present invention had the disadvantage of a high emission starting voltage, and the disadvantage of easily causing dielectric breakdown when a high electric field was applied for the same reason as mentioned above.

[Problem that the invention seeks to solve]

An object of the present invention is to provide a novel thin film EL element that simultaneously maintains good applied voltage-emission brightness characteristics, makes dielectric breakdown less likely to occur, and improves contrast.

[Means for solving problems]

In order to achieve this purpose, the light absorption layer of the present invention uses a lower oxide of germanium, and its light absorption coefficient is varied continuously or in stages along the film thickness direction from the light emitting layer side to the back electrode side. It is increasing. According to such a light absorption layer, the reflectance of returned light on the glass substrate side at the interface with the light emitting layer side (in the examples described later, the interface between the first light absorption layer and the etching-resistant dielectric layer) It is possible to reduce the amount of light and absorb enough light.

As an embodiment of such a light absorption layer, it is desirable to form it only under the back electrode.

As a means of controlling the light absorption coefficient α from small to large continuously or stepwise along the film thickness direction from the light-emitting layer side to the back electrode side, the temperature It is sufficient to change the temperature from high temperature to low temperature, the oxygen partial pressure from high to low (the amount of oxygen gas introduced from high to low), and the deposition rate from low to high.

〔Example〕

The thin film EL element according to the present invention is made of aluminosilicate glass (manufactured by Hoya Glass Co., Ltd.:
Transparent electrode 2 made of indium oxide mixed with tin oxide on glass substrate 1 made of NA-40)
(thickness: 2000 Å), first dielectric layer 3 (thickness: 3000 Å) consisting of Y ₂ O ₃ and 0.5 wt% as active material.
A light-emitting layer 4 (thickness: 6000 Å) made of ZnS:Mn sintered pellets doped with Mn and a second dielectric layer 5 (thickness: 3000 Å) made of Y ₂ O ₃ are sequentially deposited using a vacuum evaporation method. The film was formed by And the temperature is 350℃, 1
Under oxygen partial pressure of ×10 ^-4 Torr (after high vacuum,
Oxygen gas is introduced to create an oxygen atmosphere of 1×10 ^-4 Torr. ), an etching-resistant dielectric layer 8 (thickness: 1000 Å) made of an oxide of Hf was formed on the second dielectric layer 5 by reactive vacuum deposition.

Next, Ge was heated with an electron beam as a target at a temperature of 350°C and an oxygen partial pressure of 1×.
The first light absorbing layer 9 (thickness: 500 Å) made of a lower oxide of Ge (GeOx: 0<x<2) is formed by reactive vacuum deposition at 10 ^-4 Torr and a deposition rate of 1 Å/sec. 8, and then reactive vacuum deposition was performed at a temperature of 350°C, an oxygen partial pressure of 3 × 10 ^- ₅ Torr, and a deposition rate of 2 Å/sec.
<2) Second light absorption layer 10 (thickness: 800 Å)
was deposited. First light absorption layer 9 and second light absorption layer 1
The optical absorption coefficient α of 0 was 0.7×10 ⁵ cm ⁻¹ and 3×10 ⁵ cm ⁻¹ at a wavelength of 580 nm, respectively. Next, a back electrode 6 (thickness: 3000 Å) made of Al is placed on the second light absorption layer 10, and an antireflection layer 11 made of MgF ₂ is placed on the second light absorption layer 10 to reduce the light reflectance from the surface of the glass substrate 1. (film thickness: 1000 Å) was formed on the light extraction side of the glass substrate 1 by vacuum evaporation. In addition, the transparent electrode 2 and the back electrode 6
As in the case shown in FIG. 1, a plurality of strips were arranged perpendicularly to each other.

An alternating current voltage (frequency 100Hz
peak wavelength by applying a sine wave)
It emits yellow light at 580nm, and the luminance at that time is
It was 140cd/ ^m2 . The sheet resistance of the first and second light absorbing layers 9 and 10 consisting of two layers is approximately 10 ⁸
Since the Ω/□ is relatively low, areas other than the area where the back electrode 6 is present tend to emit light. Therefore, if you want to form a pixel without crosstalk or leakage, use the back electrode 6 as a mask to By selectively etching the second light absorbing layers 9 and 10 with nitric acid, the first and second light absorbing layers 12 and 13 can be formed only under the back electrode 6, as shown in FIG. 3b. desirable.

In addition, in the embodiment described above, the second light absorption layer 1
For example, if the oxygen partial pressure is 5×
10 ^-5 Torr, and the deposition rate is 1 Å/sec.
By making the sheet resistance of the second light absorption layer 10 as low as 10 ⁹ Ω/□,
The aforementioned selective etching can be dispensed with (that is, the etching-resistant dielectric layer 8 can be dispensed with). In this case, the light absorption coefficient of the second light absorption layer 10 was 2×10 ⁵ cm ⁻¹ .

The reflectance characteristics of the thin film EL device of this example on the glass substrate 1 side were low, as shown by curve f in FIG. 5, and the average reflectance in the wavelength range of 400 to 700 nm was 4%. This low reflectance is due to reflections at the interfaces between the etching-resistant dielectric layer 8 and the first light absorption layers 9 and 12, and between the first light absorption layers 9 and 12 and the second light absorption layers 10 and 13. This is because the absorption rate is low and these light absorption layers effectively absorb light.

Further, the contrast characteristics are shown by curve g in FIG. 6, and the contrast characteristics are shown by curve g in FIG.
It can be seen that the contrast is significantly improved compared to the curve h of the EL element in which (or 12, 13) is not formed.

On the other hand, regarding the applied voltage-luminance characteristics, the first light absorption layer 9, 10 and the second light absorption layer 12,
The resistivity of 13 is 10 ⁵ Ω-cm and 10 ² Ω-cm, respectively.
The conventional light absorption layer made of CdTe is 10 ⁸
Since the applied voltage is sufficiently lower than that of Ω-cm, the applied voltage substantially acts between the first dielectric layer 4 and the second dielectric layer 5, and the curve a and the curve a in FIG. Similarly, the light emission starting voltage can also be maintained low, and dielectric breakdown can be prevented.

In the above embodiments, the light absorption coefficient α of the first light absorption layer 9 and the second light absorption layer 10 was increased stepwise, but it may be increased continuously. That is, the fourth
As shown in the oxygen partial pressure curve d and vapor deposition rate curve e in the figure, the oxygen partial pressure is controlled as 1.5×10 ^-4 → 0.5×10 ^-4 Torr, and the miscellaneous deposition rate is controlled as 0 → 2 Å/sec. As the film thickness increases, the light absorption coefficient α increases,
The light absorption layer may be made of substantially one lower oxide of Ge. When compared with the previous example, the light absorption layer according to this example has a deposition time of 0 to 10 min and 10 to 10 min.
Each portion after 20 minutes has passed corresponds to the first light absorption layer and the second light absorption layer. Further, in the case of such a light absorption layer in which the light absorption coefficient α is continuously increased, as described above, the sheet resistance can be increased to eliminate the need for selective etching, that is, the need for the etching-resistant dielectric layer 8. (Under the vapor deposition conditions mentioned above, for example, the oxygen partial pressure is 1.5×10 ^-4 →
The deposition rate may be set to 0.5×10 ⁻⁴ Torr and the deposition rate from 0 to 2 Å/sec. ).

In addition to the materials used in the above embodiments, the present invention uses multi-component glass such as soda lime glass or quartz glass for the glass substrate, and In ₂ O ₃ or its W-added material or SnO for the transparent electrode. ₂ to which Sb, F, etc. are added, and the first dielectric layer and the second dielectric layer are Ta ₂ O ₅ , Al ₂ O ₃ ,
Oxides such as HfO ₂ or nitrides such as Si ₃ N _4. For the light emitting layer, ZnS:Cu, ZnS:Al (all emit yellow-green light), Zn(S/Se): Cu, Zn(S/Se). :Br (all emit green light), ZnS:Sm (emit red light), ZnS:Tb
(green light emission), ZnS:Tm (blue light emission), and metals such as Ta, Mo, Fe, Ni, and NiCr may be used for the back electrode. Further, when an etching-resistant dielectric is used, it may also be used as the second dielectric layer, and Ta ₂ O ₅ or the like may be used instead of HfO ₂ as the material.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to improve contrast, maintain good applied voltage-emission brightness characteristics, and prevent dielectric breakdown.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a conventional thin-film EL device, FIG. 2 is an applied voltage-emission luminance characteristic diagram of a conventional thin-film EL device, FIG. 3 is a cross-sectional view of a thin-film EL device of the present invention, and FIG. 5 is a diagram showing oxygen partial pressure characteristics and evaporation rate characteristics according to the present invention, FIG. 5 is a reflectance characteristic diagram in the thin film EL device of the present invention, and FIG. 6 is a diagram showing contrast characteristics in the thin film EL device of the present invention. . DESCRIPTION OF SYMBOLS 1...Glass substrate, 2...Transparent electrode, 3...First dielectric layer, 4...Light emitting layer, 5...Second dielectric layer, 6...Back electrode, 8...Etching-resistant dielectric layer , 9, 12... first light absorption layer, 10, 13...
Second light absorption layer.

Claims (1)

[Claims] 1. A light-emitting layer that emits EL light upon application of a voltage;
In a thin film EL element in which a light absorption layer that absorbs incident light is interposed between a transparent electrode and a back electrode,
The light absorption layer is a lower oxide of germanium, and the light absorption coefficient of the light absorption layer increases continuously or stepwise along the film thickness direction from the light emitting layer side to the back electrode side. A thin film characterized by
EL element. 2. The thin film EL device according to claim 1, wherein the light absorption layer is formed only under the back electrode.