JPH0845666A - Thin film EL device - Google Patents

Abstract

(57) Abstract: A thin film EL device of high brightness and low voltage drive is provided. The present invention provides a thin film EL device having a columnar polycrystalline layer in which a light emitting layer satisfies a stoichiometric composition and an emission center impurity is uniformly distributed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film EL (electroluminescence) device, and more particularly to a high brightness and low voltage drive thin film EL device.

[0002]

2. Description of the Related Art Zinc sulphide (ZnS), which has many problems in terms of brightness and has been forced to abandon its development as a light source for illumination.
Instead of the dispersion type EL element using the S) type phosphor powder,
A thin film type EL device using a thin film phosphor layer has been receiving attention in recent years because it can obtain high brightness.

In the thin film EL device, the light emitting layer is composed of a transparent thin film, and light incident from the outside and light emitted inside the light emitting layer are not scattered to cause halation or bleeding, and are clear. Because of its high contrast, it has been spotlighted for use in vehicles, display devices such as computer terminals, and lighting.

For example, the basic structure of a thin film EL element using manganese (Mn) as a light emission center in ZnS is such that a transparent electrode composed of a tin oxide (SnO ₂ ) layer and the like are formed on a transparent substrate. Dielectric layer, a light emitting layer made of a crystalline thin film having ZnS as a base material and Mn as an emission center impurity, that is, a ZnS: Mn thin film, and a back electrode made of a second dielectric layer, an aluminum (Al) layer, and the like. It has a double-dielectric structure that is sequentially laminated. The light emitting process is as shown below. When a voltage is applied between the transparent electrode and the back electrode, the electrons trapped in the interface state are extracted and accelerated by the electric field induced in the light emitting layer to obtain sufficient energy. Mn (emission center)
And collide with orbital electrons of to excite it. Light is emitted when the excited luminescence center returns to the ground state.

Conventionally, in such a thin film EL device, an electron beam vapor deposition method has been usually used for forming a light emitting layer such as ZnS: Mn as described above (Japanese Patent Publication No. 52-1035).
No. 8 and Japanese Patent Publication No. 54-8080).

As shown in FIG. 8, a pellet 2 formed by mixing ZnS and 0.1 to 1% manganese (Mn) in a vacuum chamber 1 and sintering is mixed from an electron gun 3. The electron beam 4 is used to heat the pellet, evaporate it, and deposit it on the substrate 5.

According to this method, the vapor pressures of the base material of the light emitting layer, the vapor pressures of the elements constituting the base material, and the vapor pressures of the emission center impurities (for example, PZnS, PZn, PS, and PMn) greatly differ (PZnS < Since PMn <PZn <PS, the evaporation method is not uniform, or the elements adhering to the substrate are re-evaporated, so that the base material of the light emitting layer to be formed deviates from the stoichiometric composition. However, there are inconveniences such as poor crystallinity and non-uniform distribution of emission center impurities. (Note that PZnS, PMn, PZn, and PS are ZnS, Mn, and Z, respectively.
The vapor pressures of n and S are shown. ) Therefore, the e-beam
As shown in FIG. 9, the light-emitting layer formed by the vapor deposition method had a granular polycrystalline structure or a so-called dead layer in which many small crystal grains were formed in the initial stage of growth.

[0008]

In a thin film EL element using such a light emitting layer, electrons E in the light emitting layer accelerated by an externally applied electric field collide with the emission center impurity Im and contribute to light emission. Before it does, it is scattered by the crystal grain interface B, so that the electric field applied from the outside does not effectively contribute to light emission.

Therefore, in the thin film EL device having such a structure,
A high voltage of about 200 V is required to obtain a brightness (20 ft-L) that can be put to practical use.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a thin film EL element of high brightness and low voltage drive.

[0011]

In view of the above, the present invention is characterized in that the light emitting layer is composed of a columnar polycrystalline layer.

Preferably, the light emitting layer is composed of columnar polycrystals made of a material obtained by adding an element or a compound constituting a luminescence center to a II-VI compound.

Preferably, the light emitting layer is composed of a columnar polycrystal formed by adding manganese (Mn) to zinc sulfide (ZnS).

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION That is, as shown in FIG. 1, when a columnar polycrystal having a stoichiometric composition and an emission center impurity uniformly distributed is used as a light emitting layer, an external electric field is applied. The electrons E in the light emitting layer are accelerated, collide with the emission center impurity Im, and effectively emit light.

When forming the light emitting layer, as shown in FIG. 2, the structure of ZnS, which is the base material of the light emitting layer, is placed in a vacuum chamber 11 set to a vacuum degree of, for example, 10 ⁻³ to 10 ⁻⁶ Torr. An element and an emission center impurity Mn (for example, Zn, S, and Mn) are placed in separate crucibles 12, 13 and 14 and heated independently under temperature control, so that the formed emission layer has a stoichiometric composition. By controlling the amount of evaporation of each of these so that the emission center impurities are evenly distributed, columnar crystals are formed on the substrate 15.
It can be deposited on top.

This is due to the following growth process.

For example, it is assumed that the substance A and the substance B are put in different crucibles and the substance AB is formed on the substrate by a multi-source vapor deposition method using independent evaporation sources.

The vapor pressure of substance A at a certain temperature TA is defined as P
Assuming A, the degree of vacuum (pressure) P0 in the vacuum chamber is P0 <P
When A, the substrate temperature TS is such that the vapor pressure PAS of the substance A at TS becomes PAS> PA so that PAS> P0, and AAS is vaporized to adhere to the substrate. Even if it tries, it hardly adheres to the substrate.

At this time, the vapor pressure PA ABS of the compound AB of the other substance B and the substance A at the temperature TS is PA ABS <P0.
If Ts is set so that the substance B exists on the substrate (PBS <P0), the substance A and the substance B react with each other on the substrate to grow the compound crystal AB. At this time, the element B (or A) existing on the substrate and the element A flying over
When (or B) is bonded, the element is accommodated in the position with the lowest potential, so that only a certain crystal plane grows and grows to form a columnar crystal.

Of the elements that reach and bond on the substrate,
In particular, since sulfur easily evaporates, there is a problem that the sulfide thin film formed in a high vacuum has a deviation from the stoichiometric ratio due to the formation of S holes.

The present invention has been made in order to solve this problem. For example, solid sulfur is used as another independent evaporation source, from which sulfur is excessively supplied to increase the pressure. By doing so, collision of molecular beams easily occurs, and even if they are not immediately captured by the substrate, they are not immediately carried away by the vacuum system but remain and contribute to growth, satisfying the stoichiometric composition. Columnar polycrystals are formed.

[0022]

Embodiments of the present invention will now be described in detail with reference to the drawings.

As shown in FIG. 3, this thin film EL device has
The light emitting layer 21 has a double dielectric structure, and the light emitting layer 21 is 0.4 at.% As an emission center impurity in ZnS as a base material.
And a thin film layer having a columnar polycrystal structure with a film thickness of 500 nm containing Mn (hereinafter referred to as ZnS: 0.4 at.% Mn).

That is, a transparent electrode 23 made of a tin oxide (SnO ₂ ) layer having a film thickness of 0.3 μm and a tantalum oxide film having a film thickness of 0.5 μm (T) are formed on a translucent glass substrate 22 having a thickness of 1 mm.
a ₂ O ₅ ) layer, the first dielectric layer 24, the light emitting layer 21, a second dielectric layer 25 made of a tantalum oxide (Ta ₂ O ₅ ) layer having a thickness of 0.5 μm, and a thickness of 0. A back electrode 26 made of an aluminum thin film of 5 μm is sequentially laminated.

The equivalent circuit of this thin film EL element is, as shown in FIG. 4, connected in series with three capacitors each composed of a first dielectric layer 24, a light emitting layer 21, and a second dielectric layer 25. It can be represented as a body.

Next, a method of manufacturing the thin film EL element of the embodiment of the present invention will be described.

First, as shown in FIG. 5 (a), SnO _{2 is} sputtered on a transparent glass substrate 22.
The transparent electrode 23 composed of layers is formed.

Then, as shown in FIG. 5B, a first dielectric layer 24 made of a tantalum oxide layer is formed by a sputtering method.

Then, using the vapor deposition apparatus as shown in FIG. 2, Zn, S, and Mn were put in different crucibles, and the vapor pressure in the vacuum chamber was set to 10 ^-5 T0rr. The Zn, S, and Mn are heated and evaporated while the temperature of the crucible is independently controlled so that the formed light emitting layer has a stoichiometric composition, and the temperature TS of the glass substrate (substrate 15) is controlled. By setting an appropriate range of 100 to 1000 ° C., Zn in which the emission center impurity Mn is uniformly distributed
A light-emitting layer 21 made of S-columnar polycrystal is deposited (FIG. 5 (c)).

Further, as shown in FIG. 5D, a second dielectric layer 25 made of a tantalum oxide layer is formed by a sputtering method.

Finally, as shown in FIG. 5 (e),
After forming an aluminum thin film by the vacuum deposition method, patterning is performed by the photolithographic etching method and the back electrode 2
6 is formed.

The light emitting layer of the thin film EL element thus formed has extremely good crystallinity as shown in the result of X-ray diffraction in FIG. 6 (a). For comparison, ZnS formed by a conventional electron beam evaporation method:
The result of X-ray diffraction of the Mn layer (light emitting layer) is shown in FIG. 6 (b). As is clear from these comparisons, it can be seen that the light emitting layer formed by the method of the present invention has better crystallinity. Further, this thin film EL element is driven by applying an alternating electric field between the transparent electrode and the back electrode, and its voltage-luminance characteristic curve a is combined with the voltage-luminance characteristic curve b of the conventional thin film EL element. It is shown in FIG. From these comparisons as well, it can be seen that the thin film EL element of the present invention can obtain the same luminance at a voltage of about 1/2 that of the thin film EL element of the conventional example, and has high luminance characteristics at a low voltage. Recognize.

Here, for the light emitting layer 21 of the present invention, a dielectric film having a larger dielectric constant, for example, barium titanate BaTi.
By using O3 for the first dielectric layer 24 and the second dielectric layer 25, the voltage-luminance characteristic becomes as shown by the curve (c) in FIG. 7, and the thin film EL device of the conventional example (curve (b )), It is possible to drive at a voltage of about 1/3 to 1/4. This is the first dielectric layer 24, the second dielectric layer 25,
Assuming that the relative permittivity of the light emitting layer 21 is εr1, εr2, εl, respectively, the relationship between them becomes εr1, εr2 >> εl (see FIG. 4), and therefore the electric capacities Cr1, Cr2, Cl are Cr.
Since 1, Cr2 >> Cl, most of the voltage applied from the outside to this element is applied only to the light emitting layer.

As described above, according to the thin film EL device of the embodiment of the present invention, a low voltage of about 100 V or less is required to obtain a brightness of about 20 ft-L (foot run bart), which is extremely practical. Has become.

In the examples, the case where the ZnS: Mn thin film is used for the light emitting layer has been described. ₃ ), ZnS replaced with samarium fluoride (SmF ₃ ), etc .: 0.1-1a
t.% TbF ₃ , ZnS: 0.1 to 1 at.% SmF _{3 and} other columnar polycrystals, calcium sulfide (Ca
S): 0.1-1 at.% Europium (Eu), strontium sulfide (SrS): 0.1-1 at.% Cerium fluoride (CeF ₃ ) columnar polycrystals of other substances such as columnar polycrystals But needless to say.

Further, in the examples, three crucibles containing Zn, S, and Mn, respectively, were used as vapor deposition sources to form the columnar polycrystalline thin film of ZnS: Mn.
Three of S and Mn, three of Zn, S, and MnS, Zn, S, ZnS, and Mn can be appropriately adjusted.

In addition, this thin film EL element is used for illumination other than a display device, for writing a signal to an optical recording medium,
It can also be used as a light source for reading and erasing.

[0038]

As described above, according to the thin film EL device of the present invention, the light emitting layer is composed of columnar polycrystals which are vapor-deposited while a plurality of evaporation sources are independently temperature-controlled. Therefore, high brightness can be obtained with a low driving voltage.

[Brief description of drawings]

FIG. 1 is a diagram showing a crystal structure of a light emitting layer of a thin film EL device of the present invention.

FIG. 2 is a principle diagram of a method for forming a light emitting layer according to the present invention.

FIG. 3 is a diagram showing a thin film EL element of an example of the present invention.

FIG. 4 is an equivalent circuit diagram of the same element.

FIG. 5 is a manufacturing process diagram of the same device.

FIG. 6 is a comparative diagram showing the results of X-ray diffraction of the light emitting layers of the thin film EL devices of the present invention and the conventional example.

FIG. 7 is a comparison diagram of voltage-luminance characteristics of the thin film EL element of the present invention and the conventional example.

FIG. 8 is a diagram showing a principle of a method for forming a light emitting layer of a conventional thin film EL element.

FIG. 9 is a diagram showing a crystal structure of a conventional light emitting layer.

[Explanation of symbols]

 1, 11 Vacuum tank 2 Pellet 3 Electron gun 4 Electron beam 5,15 Substrate E Electron Im Emission center impurity B Crystal grain interface 12, 13, 14 Crucible 21 Emission layer 22 Glass substrate 23 Transparent electrode 24 First dielectric Layer 25 Second dielectric layer 26 Back electrode

[Procedure amendment]

[Submission date] September 5, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Claims

[Correction method] Change

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[Claims]

[Procedure Amendment 2]

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[Correction target item name] 0011

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[0011]

Therefore, in the first aspect of the present invention, the light emitting layer is constituted of a columnar polycrystalline layer in which a stoichiometric composition is satisfied and emission center impurities are uniformly distributed. Characterize. Desirably, the light emitting layer is II
-A columnar polycrystal composed of a material obtained by adding an element or a compound constituting a luminescence center to a group VI compound. Further preferably, the light emitting layer is composed of a columnar polycrystal formed by adding manganese (Mn) to zinc sulfide (ZnS).
In the second aspect of the present invention, the light emitting layer satisfies a stoichiometric composition,
Calcium sulfide (C
aS) composed of a columnar polycrystalline layer. In the third aspect of the present invention, the luminescent layer satisfies the stoichiometric composition and strontium sulfide (Sr
It is composed of a columnar polycrystalline layer made of S). According to a fourth aspect of the present invention, in a thin film EL device in which a transparent electrode, a dielectric layer, a light emitting layer, and an electrode are sequentially laminated on the surface of a transparent substrate, the light emitting layer has a stoichiometric composition. The columnar polycrystalline material is formed so that the emission center impurities are uniformly distributed and the grain boundaries are parallel to each other along the voltage application direction. A fifth aspect of the present invention comprises a transparent electrode, a dielectric layer, a light emitting layer, and a back electrode, wherein the light emitting layer increases the number of collisions of sulfur molecular beams and remains without being carried away from the vacuum chamber for growth. 10 ^-3 Torr to 10 ^-6 Torr to contribute
In the vacuum of, the base material of the light emitting layer or the constituent elements of the base material and the emission center impurities are evaporated from a plurality of evaporation sources made of different solids including the evaporation source made of solid sulfur, and are combined on the substrate. It is characterized by being constituted by a sulfide layer formed by a multi-source vapor deposition method.

Claims (3)

[Claims] 1. A thin film EL device, wherein the light emitting layer is composed of a columnar polycrystalline layer. 2. The light emitting layer is composed of a columnar polycrystal composed of a material obtained by adding an element or a compound constituting a luminescence center to a II-VI group compound. The thin film EL device according to the item (1). 3. The thin film EL element according to claim (2), wherein the light emitting layer is composed of a columnar polycrystal formed by adding manganese (Mn) to zinc sulfide (ZnS).