JPH0645069A - Thin film el element - Google Patents

Abstract

PURPOSE:To increase luminance of EL element by using an EL element in which the distributions of luminescence center in low density area and high density area are continuous. CONSTITUTION:A light emitting layer 4 containing luminescence center such as a small amount of a rare earth element in a SrS base material is formed of a low density area 4a of luminescence center and high density area 4c to the film thickness direction, and it is also provided with a transition area 4b of 10nm or more in which the density of luminescence center is continuously increased or decreased with a proper gradient. Thus, since electrons are efficiently accelerated in the highly crystallized area 4a, and these electrons excite the luminescence center of the area 4c, a high luminance EL element can be provided. By providing the area 4b, crystallinities of the area 4a, 4c become continuous, the level for trapping the electrons on the critical surface of both the areas is minimized, the electrons accelerated in the area 4a efficiently excite the luminescence center of the area 4c, and the luminance can be improved.

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) element which emits light in response to an applied electric field.

[0002]

2. Description of the Related Art M is used for compound semiconductors such as ZnS and ZnSe.
The phenomenon of electroluminescence in which light is emitted by applying a high voltage to a material doped with a light-emitting center such as n has been known for a long time. In recent years, due to the development of the double insulating layer type EL device, the brightness and the life have been dramatically improved, and the thin film EL device has been applied to a thin display, and has come to the market now.

The emission color of the EL element is determined by the combination of the semiconductor matrix forming the light emitting layer and the doped emission center.
For example, when the ZnS host is doped with Mn as an emission center, yellow-orange emission is obtained, and when Tb is added, green emission is obtained. However, only the yellow-orange system in which the ZnS matrix is doped with Mn has reached the brightness of a practical level at present. When manufacturing a full-color thin-film display using EL elements, EL elements that emit the three primary colors of blue, green, and red are required, and the development of EL elements that emit each color with high brightness is energetically advanced. ing. In developing such a color EL device, Sr
S is useful as a base material, for example, SrS: Ce light-emitting layer in which SrS is doped with Ce is blue green, and SrS is Eu.
It is known that red emission can be obtained in a SrS: Eu light emitting layer doped with.

Conventionally, in a thin film EL element, a luminescent base material such as SrS having a luminescent center such as Ce at a constant concentration is used as a luminescent layer. However, there is a problem that the emission brightness is insufficient. As described above, when a light emitting layer made of a compound semiconductor such as SrS is doped with a light emitting center as a cause of not obtaining sufficient light emission brightness, the lattice position of the atoms forming the light emitting layer may be changed to a light emitting center having different ion radii. By the substitution or the inclusion of an emission center between the lattices, the emission center itself showing EL emission hinders the crystallization of the emission layer from becoming highly crystallized, and electrons are efficiently accelerated by the electric field applied to the emission layer. It can be hindered. Another reason is that the emission center becomes a scattering center of accelerated electrons in the light emitting layer, which hinders high acceleration of electrons.

The light emitting layer has a multi-layer structure in which an undoped layer having a light emitting center and a doped layer, or a low-concentration layer containing a low concentration of a light emitting center and a heavily doped layer are stacked to form a light emitting center. A structure in which an unaccelerated layer or a low-concentration layer is used as a high-quality electron acceleration layer is disclosed in JP-A-63-63
-29489 and JP-A-62-97295. However, in these, the electrons accelerated in the electron accelerating layer do not efficiently excite the emission center of the light emitting layer, and an EL element having sufficient brightness for practical use has not been obtained.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an EL device which emits light with higher brightness.

[0007]

[Means for Solving the Problems] Under such circumstances,
As a result of diligent studies on the concentration distribution of emission centers in the emission layer of an EL element for exhibiting high-luminance emission, the present inventors have found an element in which the distribution of emission centers in a low concentration region and a high concentration region is continuous. It was found that the brightness of the EL element was significantly increased by the fabrication, and the present invention was completed.

That is, the present invention has a structure in which both sides of a light emitting layer in which a luminescent center is doped in an SrS base material is sandwiched by insulating thin films, and both sides thereof are sandwiched by electrodes, and at least one of the electrodes is light transmissive. In the thin film EL element as an electrode, the light emitting layer is composed of a low concentration region and a high concentration region of the emission center in the film thickness direction, and the emission center is between the low concentration region and the high concentration region of the emission center. Is a thin film EL element having a transition region of 10 nm or more in which the concentration of C increases or decreases continuously.

The present invention will be described in detail below. Figure 1
It is a thin film EL element having a double insulation structure showing one specific example of the present invention. In the figure, 1 is a transparent substrate made of a glass plate or the like, 2
Is a transparent electrode made of an ITO thin film having a thickness of about 100 to 300 nm, 3 is a back electrode made of an Al thin film or an ITO thin film having a thickness of about 100 to 500 nm, and is patterned into a shape corresponding to a display pattern. 4 is Sr
S base material containing a small amount of a luminescent center such as a rare earth element, for example, SrS: Ce, SrS: Eu, SrS: C
The light emitting layer is made of e, Eu or the like. In the light emitting layer, 4a is a low concentration region of the emission center and 4c is a high concentration region of the emission center.

In the emission center, continuity of distribution in the low concentration region and the high concentration region is important, and 4b is a transition region in which the concentration of the emission center continuously increases from the low concentration region to the high concentration region. In the present invention, the transition region refers to a region where the emission center concentration continuously increases or decreases at the boundary between the low concentration region and the high concentration region, and refers to the point from the start point to the end point of the increase or decrease.

The order of the low concentration region and the high concentration region is not particularly limited. The thickness of the entire light-emitting layer including the low-concentration region and the high-concentration region of the luminescence center is not particularly limited, but if it is too thin, the emission brightness is low, and if it is too thick, the light emission starting voltage is high, so that it is preferably 50 to 3000 nm. The range is more preferable, and the range of 100 to 1500 nm is more preferable. The film thickness in the low concentration region of 4a is in the range of 10 to 2000 nm,
More preferably, it is in the range of 50 to 1000 nm. The film thickness of the high-concentration region 4c is in the range of 10 to 1000 nm, more preferably 50 to 800 nm. The film thickness of the transition region where the emission center concentration continuously increases from the low concentration region 4b to the high concentration region 4b is required to be 10 nm or more. In the present invention, the upper limit of the film thickness of the transition region is not particularly limited, but the practical range is preferably 800 nm or less, more preferably 50 nm or more and 500 nm or less.

The concentration of the emission center in the low concentration region in the light emitting layer is not particularly limited, but if it is too high in the low concentration region, the crystallinity deteriorates and the luminance does not rise. Preferably 0 to 2 mol% with respect to the base material, more preferably 0 to
It is in the range of 1 mol%. The concentration of the luminescent center in the high-concentration region in the light-emitting layer is not particularly limited, but if it is too low, the emission brightness does not increase, and if it is too high, concentration quenching occurs and the brightness does not increase. It is preferably 0.
01-5 mol%, more preferably 0.05-2 mol
% Range. Regarding the emission center, it is necessary to have a low-concentration region and a high-concentration region in the film thickness direction and a region that continuously changes with an appropriate gradient at the boundary.

The method for producing a light emitting layer such as the above 4a, 4b, 4c is not particularly limited, but for example, a method in which the film forming rate of the base material is kept constant and the doping rate of the luminescence center is continuously changed, and the like. can give. After the film formation, heat treatment may be performed to improve crystallinity. The method for forming the light emitting layer is not particularly limited, and many methods such as electron beam heating vapor deposition, sputter vapor deposition, MBE, MOCVD, and ALE can be selected.

Reference numerals 5 and 6 are adjacent to the front surface and the back surface of the light emitting layer 4.
Insulating layer in contact. Insulation used in the EL device of the present invention
The edge layer is not particularly limited. For example, SiO₂, Y
₂O ₃, TiO₂, Al₂O₃, HfO₂, Ta
₂O_Five, BaTa₂O_Five, PbTiO₃, Si₃N_Four,
ZrO₂Etc. or a mixed film of these or a laminated film of two or more kinds
Can be mentioned. In addition, between the insulating layer and the light emitting layer,
A buffer to prevent both reactions during film formation and heat treatment
Layer is preferably used. Special as a buffer layer
Metal sulfides, especially ZnS, Cd
S, SrS, CaS, BaS, CuS, etc. are mentioned.
The thickness of the buffer layer is not particularly limited, but is 10-100
The range is 0 nm, and more preferably 50 to 300 nm.
Is the range.

The feature of the present invention is that the light-emitting layer is composed of a low-concentration region and a high-concentration region at the emission center in the film thickness direction, and further at the boundary between the low-concentration region and the high-concentration region at the emission center
That is, it has a transition region of 10 nm or more in which the concentration of the emission center continuously increases or decreases with an appropriate gradient. With such a structure, electrons are efficiently accelerated in a highly crystallized low-concentration region, and the accelerated electrons efficiently excite emission centers in the high-concentration region, so that an EL element that emits light with high luminance can be obtained. Can be made.

The light emitting layer has a multi-layered structure in which an undoped layer having a light emitting center and a doped layer, or a low-concentration layer containing a low concentration of a light emitting center and a heavily doped layer are stacked to form a light emitting center. When a structure in which an unaccelerated layer or a low-concentration layer is formed as a high-quality electron acceleration layer has a tendency to prevent the electrons accelerated in the electron acceleration layer from efficiently exciting the emission center of the light-emitting layer, sufficient brightness is obtained. It is difficult to manufacture an EL element having In the present invention, the film thickness of the transition region where the concentration of the emission center is continuously increased or decreased at the boundary between the low concentration region and the high concentration region is 10
When the thickness is not less than nm, the crystallinity of the low-concentration region and the high-concentration region of the emission center becomes continuous, and there are relatively few levels for trapping electrons at the interface between the two regions, which accelerates in the low-concentration region. It is thought that the brightness of the EL element can be improved because the electrons efficiently excite the emission center in the high concentration region.

[0017]

EXAMPLES Examples of the present invention will be specifically described below.

[0018]

[Example 1] On a glass substrate [Hoya Co., Ltd., NA-4
0], an ITO electrode having a thickness of about 100 nm was formed by the reactive sputtering method. Then, a Ta target and a SiO ₂ target are used to form a Ta ₂ film having a thickness of 400 nm.
O ₅ and SiO ₂ having a thickness of 100 nm were sequentially formed by a sputter deposition method to form an insulating layer. Subsequently, a ZnS thin film having a thickness of about 100 nm was prepared as a buffer layer by sputter deposition in an argon gas using a ZnS target. Next, as a light emitting layer, a target of SrS and CeF
Ar containing ₂ vol% H ₂ S using ₃ targets
While introducing a gas and keeping the substrate temperature at 450 ° C., two-source simultaneous sputtering vapor deposition was performed to form a thin film having a thickness of about 1000 nm. At this time, the emission center Ce as shown in FIG.
The sputter rate of SrS was kept constant and the sputter rate of CeF ₃ was continuously changed so as to have a concentration distribution of. The produced light emitting layer is composed of a low-concentration region and a high-concentration region of Ce, which are emission centers in the film thickness direction, and the concentration of the emission center Ce is continuously between the low-concentration region and the high-concentration region. It has an increasing transition region of 200 nm. The concentration of CeF ₃ with respect to SrS was 0.1 mol in the low concentration region.
%, And the high concentration region was set to 0.5 mol% at the maximum point. Further, on the light emitting layer, ZnS, SiO ₂ , Ta
_A laminated film was formed by the above method in the order of ₂ O ₅ to construct a double insulating structure. Finally, Al electrodes were formed in a stripe shape by a resistance heating vapor deposition method using a metal mask. As the lower electrode, a part of the light emitting layer and the insulating layer was peeled off to expose the ITO electrode, which was used. The maximum brightness of the EL device manufactured from this light emitting layer is 6 when driven by a 5 kHz sin wave.
It was 000 cd / m ² .

[0019]

[Comparative Example 1] CeF ₃ is 0.3 mol% with respect to SrS.
Thus, an element was produced in the same manner as in Example 1 except that the light emitting layer was produced while keeping the sputtering rates of SrS and CeF ₃ constant. As shown in FIG. 3, the produced light emitting layer had a uniform Ce concentration distribution in the film thickness direction. The maximum brightness of the EL device made from this light emitting layer is
It was 1000 cd / m ^{2 when} driven by a 5 kHz sin wave.

[0020]

[Comparative Example 2] CeF ₃ was 0.5 mo with respect to SrS while keeping the sputtering rates of SrS and CeF ₃ constant.
After forming a thin film containing 1% and having a thickness of about 300 nm, S
sputter rate of rS is intact, kept constant and with low only sputter rate of CeF _3, a thin film CeF ₃ is contained 0.1 mol% relative to SrS, further about 700
nm except that the light emitting layer is formed by forming
An element was produced in the same manner as in Example 1. Ce in the light emitting layer
4 is as shown in FIG. 4, and the film thickness of the transition region between the low concentration region and the high concentration region of Ce is less than 10 nm.
The maximum brightness of the EL device manufactured from this light emitting layer is 5 kHz.
It was 2500 cd / m ^{2 when} driven by z sin wave.

[0021]

Example 2 A light emitting layer having a Ce concentration distribution as shown in FIG. 2 was prepared by EB vapor deposition while using SrS pellets and CeF ₃ pellets as evaporation sources and simultaneously evaporating sulfur by resistance heating. A device was manufactured in the same manner as in Example 1. The maximum brightness of the EL element manufactured from this light emitting layer is 6500 cd / s when driven by a 5 kHz sin wave.
It was m ² .

[0022]

Example 3 A target of SrS was used when manufacturing a light emitting layer.
To and CeF₃And EuF₃Target with equimolar mixture of
A device was manufactured in the same manner as in Example 1 except that
did. The maximum brightness of the manufactured EL element is 5 kHz si
5000 cd / m with n-wave drive ²Met.

[0023]

[Comparative Example 3] An element was produced in the same manner as Comparative Example 1 except that a target of SrS and a target of equimolar mixture of CeF ₃ and EuF ₃ were used in producing the light emitting layer. The maximum brightness of the manufactured EL element is 5 kHz si
It was 800 cd / m ^{2 when} driven by n-wave.

[0024]

According to the present invention, a low-concentration region of the emission center concentration in the thickness direction of the light-emitting layer, that is, a high-quality electron accelerating layer is provided, and electrons accelerated there are efficiently increased. It is possible to obtain a light emitting layer that excites the light emission center in the concentration region, and as a result, it is possible to manufacture an EL element that emits light with high brightness.

[Brief description of drawings]

FIG. 1 is a sectional view showing an EL device to which the present invention is applied.

FIG. 2 is a graph showing the concentration distribution of Sr and Ce in the light emitting layer in the film thickness direction in Example 1.

FIG. 3 is a graph showing a concentration distribution of Sr and Ce in a light emitting layer in a film thickness direction in Comparative Example 1.

FIG. 4 is a graph showing a concentration distribution of Sr and Ce in a light emitting layer in a film thickness direction in Comparative Example 2.

[Explanation of symbols]

 1. Glass substrate 2. Transparent electrode 3. Back electrode 4. Light emitting layer 4a. Low concentration region of emission center 4b. Transition region where emission center concentration changes continuously 4c. High concentration region of luminescence center 5. Insulating layer 6. Insulation layer

Claims (1)

[Claims] 1. A structure in which an SrS base material is doped with an emission center at both sides with an insulating thin film sandwiching both sides of the light emitting layer, and electrodes sandwiching both sides thereof, and at least one of the electrodes is a light transmissive electrode. In the thin film EL element, the light emitting layer is composed of a low concentration region and a high concentration region of the emission center in the film thickness direction, and the concentration of the emission center is continuous between the low concentration region and the high concentration region of the emission center. A thin-film EL device having a transition region of 10 nm or more that increases or decreases with time.