JPH0645068A - Thin film electroluminescent element - Google Patents

Abstract

PURPOSE:To provide a high luminance EL element by using a transparent electrode as an upper electrode, and taking out EL emission from the upper electrode side. CONSTITUTION:In the element structure of a thin film electroluminescent element having a light emitting layer 4 consisting of a low density area 4a of luminescence center and high density area 4b, in order, to the film thickness direction directing from the upper side to the lower side, a transparent electrode 3 patterned to the form according to a display pattern is used as an upper electrode, and EL emission is taken out from the electrode 3 side. When a transparent base is used as a base 1, a transparent electrode is used as a lower electrode 2, and EL emission is taken out from the electrode 2 side in the element having the light emitting layer 4, the taking out is arrested by the high density area 4b of luminescence center. By using the transparent electrode in the upper electrode 3 and taking out EL emission from the upper electrode 3 side, EL emission can be efficiently taken out, and the luminance of the EL element is also enhanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film electroluminescent device (hereinafter referred to as EL device) which emits light in response to the application of an electric field.
It is abbreviated as an element. ) Related to the device structure.

[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 ZnS host is doped with Mn as an emission center, yellow electroluminescence is obtained, and when Tb is added, green electroluminescence emission (hereinafter abbreviated as EL emission) is obtained. EL light emission is obtained when the SrS host is doped with Ce as the emission center and blue is emitted when the CaS host is doped with Eu as the emission center.

However, only the yellow-orange system in which the ZnS matrix is doped with Mn has reached the brightness of the practical level at present. EL full-color thin film display
In the case of manufacturing using an element, an EL element that emits three primary colors of blue, green, and red is required, and E that emits each color with high brightness is required.
The development of the L element is being actively pursued. The light emitting layer is formed by a resistance heating vapor deposition method, an electron beam heating vapor deposition method, a sputter vapor deposition method, a MOCVD method (metal organic gas vapor phase epitaxy method), an MBE method (molecular beam epitaxial method), an ALE method (atomic method). Layer epitaxial method is used. As a relationship between the crystallinity of the light emitting layer formed by these methods and the brightness of the EL element, it is known that the EL element having a highly crystallized light emitting layer has high brightness. It is presumed that this is because the electrons accelerated by the electric field applied to the light emitting layer efficiently excite the luminescence center. A highly crystalline thin film is obtained from a ZnS: Mn light emitting layer formed by using the MOCVD method, the ALE method, and the MBE method, and an EL element which emits light with high brightness is manufactured. But Z
In a system using a compound semiconductor other than nS as a host, for example, a SrS: Ce light emitting layer that emits blue light, an element that emits light with high brightness has not been obtained.

The MOCVD method, the ALE method, and the MBE method are promising methods for producing a highly crystalline thin film, but it is difficult to uniformly disperse the emission centers, and a large-area EL emission layer is formed. There is a problem that it is inferior to the electron beam heating vapor deposition method and the sputter vapor deposition method in that it is economically difficult to manufacture. In order to increase the crystallization of the light-emitting layer, which is one of the promising conditions for manufacturing an EL device exhibiting high-brightness emission, the substrate temperature during the production of the light-emitting layer is increased, or after the light-emitting layer is produced in a vacuum or at Methods such as high temperature heat treatment in an active gas atmosphere have been taken. However, when a sulfide called SrS is used as a host material of the light emitting layer, the amount of S in the film is decreased by the high temperature heat treatment to deviate from the stoichiometric composition, and the light emission is highly crystallized due to defects due to the escape of S. The inability to create layers was also a major problem.

In a light emitting layer having SrS as a matrix, as a means for highly crystallizing the light emitting layer, heat treatment in H ₂ S after forming the light emitting layer is disclosed in JP-B-63-46117 and JP-A-1-272093. Japanese Patent Laid-Open No. 3-2257
92 specification. In particular, JP-A-3-225
According to the specification of No. 792, the maximum brightness of SrS: Ce system is 12000 cd / m by heat treatment at 650 ° C. or higher for 1 hour or longer.
² and 7000 cd / m ^{2 in} SrS: Ce, Eu system
It is described that a very high brightness can be obtained. However, these do not mention the concentration distribution in the film thickness direction of the emission center after heat treatment and the EL emission extraction direction. Also in the examples, generally, a glass substrate and a lower transparent electrode are used, and EL light emission is taken out from the lower electrode side. However, the emission centers are uniformly dispersed in the film thickness direction in the SrS emission layer during the film formation step, and then H ₂
When the heat treatment is performed in S, the distribution of the emission centers in the thickness direction in the light emitting layer is not uniform, that is, the emission centers are diffused toward the interface between the light emitting layer and the lower insulating layer and concentrated there, so that the emission center There is a tendency to have a high density region and a high density region.

Further, when the light emitting layer made of a compound semiconductor such as SrS is doped with the light emitting center, the lattice positions of the atoms forming the light emitting layer are replaced by the atoms serving as the light emitting centers having different ionic radii, or light is emitted between the lattices. Since it is considered that the center of the light emission causes the light emitting center itself which exhibits EL light emission to prevent the light emitting layer from being highly crystallized, the light emitting layer may be an undoped layer and a doped layer of the light emitting center, or A multilayer structure is formed by stacking a low-concentration layer containing a low concentration of an emission center and a heavily doped layer, and a structure in which an undoped layer or a low-concentration layer of the emission center is a high-quality electron acceleration layer. 63-29489, JP-A-6-63
2-97295. However, also in these cases, regarding the direction of taking out the EL light emission, in the embodiment, the EL light emission is generally taken out from the lower electrode side by using the glass substrate and the lower transparent electrode.

In an element having a high-concentration region of a light emission center in the lower part of the light emitting layer as described above, conventionally, generally, a glass substrate and a lower transparent electrode are used to take out EL light emission from the lower electrode side. However, when EL emission is taken out from the lower electrode side, absorption and scattering occur when EL emission from the low concentration region of the emission center passes through the high concentration region of the emission center,
There is a problem that the EL intensity that can be taken out to the outside is reduced.

[0009]

In order to solve the above-mentioned problems, the present invention provides an EL device which emits EL light with higher brightness by using a transparent electrode as the upper electrode and extracting EL light emission from the upper electrode side. The purpose is to do.

[0010]

[Means for Solving the Problems] Under such circumstances,
As a result of diligent studies on the element structure of an EL element that exhibits high-luminance emission, the present inventors have found that the luminance of the EL element is significantly improved by using the upper electrode as a transparent electrode and extracting the EL emission from the upper electrode side. The present invention has been completed.

That is, according to the present invention, a lower electrode, a lower insulating layer, a light emitting layer doped with an emission center, an upper insulating layer, and
In a thin film electroluminescent element produced by stacking in the order of an upper electrode, the light emitting layer has a low concentration region and a high concentration region of emission centers in order with respect to the thickness direction from the upper side to the lower side, and The upper electrode is a transparent electrode, which is a thin film electroluminescent element for extracting light from the upper electrode side.

The present invention will be described in detail below with reference to the drawings. FIG. 1 is a thin-film EL element having a double insulation structure showing one embodiment of the present invention. In FIG. 1, 1 is a substrate made of a glass plate or the like, and 2 is an IT having a thickness of about 100 to 300 nm.
O, electrodes made of a conductive thin film such as MoSi _x, 3 is an upper thin transparent electrode is patterned into a shape corresponding to the display pattern. The material of the upper transparent electrode is not particularly limited, and I
TO, ZnO: Al, GaN, SnO ₂ or the like can be selected. The film thickness of the upper transparent electrode is not particularly limited, but is preferably in the range of 50 to 1000 nm, more preferably 100 nm to 500 nm. Reference numeral 4 is composed of a Group IIb sulfide such as ZnS, CdS, or Zn _x Cd _1-x S, an alkaline earth metal sulfide such as SrS, CaS, or BaS, or a mixed composition such as Zn _x Sr _1-x S. A semiconductor matrix doped with a small amount of a rare earth element or an emission center such as Mn, for example, SrS: Ce, SrS: Ce,
It is a light emitting layer made of Eu, CaS: Eu, BaS: Eu, etc., and from the low-concentration region (4a) and the high-concentration region (4b) of the emission center in order in the film thickness direction from the upper side to the lower side. It is a light emitting layer. The distribution of emission centers at the interface between the low-concentration region and the high-concentration region is not particularly limited, and may be continuous or discontinuous. The thickness of the entire light emitting layer including the low concentration region and the high concentration region is not particularly limited. , And more preferably in the range of 100 to 1500 nm. 5,
Reference numeral 6 is an insulating layer adjacent to the lower and upper portions of the light emitting layer 4. The insulating layer used in the EL device of the present invention is not particularly limited, but examples thereof include SiO ₂ , Y ₂ O ₃ , and TiO _2.
2 , Al ₂ O ₃ , HfO ₂ , Ta ₂ O ₅ , BaTa ₂ O
₅ , PbTiO ₃ , Si ₃ N ₄ , ZrO _{2 and the} like, a mixed film thereof, or a laminated film of two or more kinds can be mentioned.

A film is formed between the insulating layer and the light emitting layer during film formation.
It is preferable to use a buffer layer in order to prevent the reaction between the two during heat treatment. The buffer layer is not particularly limited, but metal sulfides, especially ZnS and 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.

As the film forming method of the upper transparent electrode, electron beam heating vapor deposition, sputter vapor deposition, MBE, MOCVD, ALE
Many methods such as method can be selected. The light emitting layer is formed by electron beam heating vapor deposition, sputter vapor deposition, MBE, MOC.
Many methods such as VD and ALE can be selected. Used for heat treatment in a sulfide gas atmosphere to highly crystallize the SrS light-emitting layer thin film, and to fabricate a light-emitting layer composed of a low-concentration region and a high-concentration region of the emission center in the film thickness direction from the upper side to the lower side. Examples of the sulfide gas include hydrogen sulfide, carbon disulfide, sulfur vapor, ethyl mercaptan, methyl mercaptan, dimethyl sulfur, diethyl sulfur, and the like. Among them, hydrogen sulfide gas is preferable because it has a large effect of improving brightness. The heat treatment concentration in the sulfide gas atmosphere is not particularly limited, but is 0.01 to 100%, more preferably 0.1 to 30.
%. An inert gas such as argon or helium is used as the diluent gas. Further, in order for the effect of the sulfurizing gas to be prominent, the heat treatment temperature must be 650 ° C. or higher and the time must be 1 hour or longer. Further, the heat treatment at a temperature of 800 ° C. or higher has problems such as strain of the substrate glass, high resistance of ITO used as a transparent electrode, and necessity of using an expensive quartz glass substrate.

A feature of the present invention is that in the element structure of a thin film electroluminescent element having a light emitting layer consisting of a low concentration region and a high concentration region of an emission center in the thickness direction from the upper side to the lower side, the upper electrode As a transparent electrode,
EL emission is taken out from the upper electrode side. In a thin film electroluminescent device having a light emitting layer as described above, when the substrate is a transparent substrate and the lower electrode is a transparent electrode, and EL emission is taken out from the lower electrode side, EL emission is taken out due to the presence of a high-concentration region at the emission center. Rather, it tends to be disturbed, and rather, by using a transparent electrode as the upper electrode and taking out EL light emission from the upper electrode side, EL light emission can be efficiently taken out, and the brightness of the EL element is further increased.

[0016]

EXAMPLES Examples of the present invention will be specifically described below. Luminance of the EL element in the examples was measured in a dark room using LUMINANCE COLORIMTER.
It was performed using BM-7 (manufactured by TOPCON).

[0017]

Example 1 Glass substrate [Hoya Co., Ltd., NA-4
0] on top of the Mo layer with a thickness of about 100 nm by sputtering.
A Si _x lower electrode was formed. On top of that, a Ta target and a SiO ₂ target are used to form a T film having a thickness of 400 nm.
a ₂ 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, using a target in which SrS and SrS were mixed with 0.3 mol% CeF ₃ and KCl, sputter deposition was performed while maintaining the substrate temperature at about 300 ° C.
A thin film having a thickness of about 800 nm was formed. Then 2 mol%
4 at 700 ℃ in an argon gas atmosphere containing hydrogen sulfide
Heat treatment was performed for an hour. Sr constituting the base material of the light emitting layer
The results obtained by SIMS analysis of the distributions of the atoms and Ce atoms, which are the luminescence centers, in the thickness of the light emitting layer before and after the heat treatment in the light emitting layer are shown in FIGS. 2 and 3, respectively. The luminescence center moves by heat treatment, and a low-concentration region and a high-concentration region of the luminescence center are formed in the film thickness direction. Further, a laminated film was formed on the light emitting layer in the order of ZnS, SiO ₂ , and Ta ₂ O ₅ by the above method to construct a double insulating structure. Finally, an ITO transparent electrode was formed into a stripe shape by using an electron beam heating vapor deposition method using a metal mask.
L emission was taken out.

The maximum brightness of this EL element is 5 kHz s
It was 16500 cd / m ² by in-wave drive.

[0019]

[Comparative Example 1] An element was manufactured in the same manner as in Example 1 except that an ITO electrode having a thickness of about 100 nm formed by a reactive sputtering method was used as the lower electrode and EL light emission was taken out from the lower electrode side. Then, the brightness was measured. The maximum brightness of this EL element is 12000 cd with 5 kHz sin wave drive.
/ M ² .

[0020]

Example 2 As a method for forming a light emitting layer, a dual sputtering method at a substrate temperature of 450 ° C. using a target of SrS and a target of equimolar mixture of CeF ₃ and KCl is used.
Keeping the SrS sputtering rate constant, CeF ₃ and KC
The emissive layer thin film having a thickness of 800 nm composed of a low-concentration region and a high-concentration region at the emission center was sequentially formed in the film thickness direction from the upper side to the lower side by continuously changing the sputtering rate of l. Further, an element was produced in the same manner as in Example 1 except that the heat treatment after forming the light emitting layer was not performed, and the luminance was measured. The maximum brightness of the manufactured EL element is 5 kHz
It was 3000 cd / m ^{2 when} driven by z sin wave.

[0021]

[Comparative Example 2] An element was prepared in the same manner as in Example 2 except that an ITO electrode having a thickness of about 100 nm formed by the reactive sputtering method was used as the lower electrode, and EL emission was taken out from the lower electrode side. Then, the brightness was measured. The maximum brightness of this EL element is 1500 cd / s when driven by a 5 kHz sin wave.
It was m ² .

[0022]

Example 3 A device was prepared in the same manner as in Example 1 except that a ZnO: Al thin film prepared by the reactive sputtering method was used as the upper transparent electrode, and the brightness was measured.
The maximum luminance of this EL device was 16500 cd / m ^{2 when} driven by a 5 kHz sin wave.

[0023]

Example 4 When forming a light emitting layer by sputtering deposition,
0.3 mol% CeF ₃ and K for SrS and SrS
Sr was used in the same manner as in Example 1 except that a target containing Cl and 0.02 mol% EuF ₃ was used.
A S: Ce, Eu white EL device was prepared and the brightness was measured. The maximum luminance of this device was 9200 cd / m ^{2 when} driven by a 5 kHz sin wave.

[0024]

[Comparative Example 3] An element was manufactured in the same manner as in Example 4 except that an ITO electrode having a thickness of about 100 nm formed by a reactive sputtering method was used as the lower electrode, and EL emission was taken out from the lower electrode side. Then, the brightness was measured. The maximum brightness of this EL element is 7000 cd / in 5kHz sin wave drive.
It was m ² .

[0025]

Fifth Embodiment When forming a light emitting layer by sputtering deposition,
0.3 mol% EuF ₃ and K for CaS and CaS
A CaS: Eu red EL device was produced in the same manner as in Example 1 except that a target mixed with Cl was used, and the brightness was measured. The maximum brightness of this device is 5 kHz si
It was 3500 cd / m ^{2 when} driven by n-wave.

[0026]

[Comparative Example 4] An element was manufactured in the same manner as in Example 5 except that an ITO electrode having a thickness of about 100 nm formed by a reactive sputtering method was used as the lower electrode and EL emission was taken out from the lower electrode side. Then, the brightness was measured. The maximum brightness of this EL element is 2500 cd / s when driven by a 5 kHz sin wave.
It was m ² .

[0027]

According to the present invention, E emitted inside the device
The L light emission can be efficiently extracted to the outside, and as a result, an EL element which emits light with higher brightness can be manufactured as compared with an element manufactured using a conventional technique.

[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 result obtained by SIMS analysis of distributions of Sr and Ce in a light emitting layer before heat treatment in a sulfide gas atmosphere in Example 1.

FIG. 3 shows SIMS distribution of Sr and Ce in the light emitting layer after heat treatment for 4 hours in a sulfide gas atmosphere in Example 1.
This is the result obtained by analysis.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Glass substrate 2 ... Transparent electrode 3 ... Back electrode 4 ... Light emitting layer (4a: Low concentration region of emission center 4b: High concentration region of emission center) 5, 6 ... Insulating layer

Claims (1)

[Claims] 1. A thin film electroluminescent device produced by stacking a lower electrode, a lower insulating layer, a light emitting layer doped with an emission center, an upper insulating layer, and an upper electrode on a substrate in this order from the upper side. A thin-film electroluminescent device which has a low-concentration region and a high-concentration region in the order of emission in the film thickness direction toward the lower side, and the upper electrode is a transparent electrode for extracting light from the upper electrode side. element.