JPH03205787A - Electroluminescence device - Google Patents

Abstract

PURPOSE:To improve the intensity of an EL element by laminating an alkaline earth sulfide phosphor and a p-type semiconductor thin film, and forming p-n junction on the interface. CONSTITUTION:A p-type semiconductor thin film 2 is formed on one face of a phosphor 1 such as SrS or CaS, and insulating films 3, 4, an Al electrode 5, an ITO transparent electrode 6 and a glass substrate 7 are provided. When the alkaline earth sulfide phosphor 1 and the p-type semiconductor thin film 2 are laminated, p-n junction is formed on the interface between them. When a forward bias is applied to the p-n junction, carriers are implanted into the phosphor 1 from the p-type semiconductor thin film 2, many carriers are generated on the conducting belt of the phosphor 1, and these carriers are accelerated in the phosphor 1 and hit and excite the center of the phosphor 1. The carriers are captured by a trap, and the energy excites the luminescence center to cause luminescence. The number of conducting electrons flowing in the phosphor 1 is increased, and intensity can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to improvements in electroluminescent devices.

[Summary of the invention] The present invention... In an electroluminescent device using an alkaline earth sulfide phosphor such as SrS or CaS, brightness is improved by increasing the number of conduction electrons flowing through the phosphor.

[Background Art] In today's highly information-oriented society, display devices, which are information terminals, occupy an important position in the human-machine interface. Generally, CRTs, that is, conventional cathode ray tubes, are used as display devices, particularly for personal computers, which have become increasingly popular in recent years. On the other hand, as seen in the spread of laptop-type personal computers, demand for portable terminal devices has increased in recent years. Conventional CRTs, which are heavy and take up a lot of space, are inappropriate for this purpose;
There is a need for a flat display that is thin, lightweight, and capable of displaying large amounts of data to replace this. In addition, not only for portable computers, but also for home use and in-vehicle use.
There is a huge demand for thinner and lighter weight products. Plasma display (FDP) is a flat display.
There are many types such as , liquid crystal (LCD), fluorescent display (VED), and electroluminescence (EL), and they are put into practical use in a wide range of fields depending on their characteristics.

In particular, liquid crystals are widely used in recent portable computer terminals, and have also been put into practical use as small TV devices. However, liquid crystals have many drawbacks, such as being a passive display device, viewing angle dependence, and poor visibility and low display quality. Not only liquid crystals but other flat displays have their advantages and disadvantages, and the current situation is that they do not have performance comparable to current CRTs.

Among the flat displays, especially in terms of visibility,
Electroluminescence (EL) is closest to CRT. There are two types of EL: a dispersion type formed from powder and a thin film type using a thin film. The dispersed type has a long history of practical use, and has recently attracted attention as a backlight for liquid crystals. On the other hand, the thin film type has a short history of research, but has been attracting a lot of attention since the development of an AC voltage EL with a double insulating layer structure that has high brightness and long life. This thin film type allows dot matrix display by cutting the electrode pattern, and has been put into practical use as a graphic display. In the following description, the EL device will be limited to this thin film type. The features of this EL display device are as follows.

A) It is an all-solid-state device and is robust.

B) Excellent visibility.

C) There is no visual angle dependence.

However, on the other hand, there are also drawbacks listed below.

A) There are only two types in practical use: yellow-orange and green, and full color has not been realized.

The most popular material is ZnS doped with Mn as a luminescent center, which exhibits a yellow-orange luminescent color and has satisfactory performance in terms of brightness and stability. On the other hand, blue light emission is farthest from practical application. Conventionally,
The developed ZnS:Tm exhibits favorable blue light emission as a phosphor for CRT, but when used as an EL,
Infrared light emission is large, and blue light emission brightness is much lower than practical brightness. In addition, SrSiCe, a type of alkaline earth phosphor that has been attracting attention recently, can provide a certain level of brightness, but the phosphor is unstable in the environment and the emission color is blue. than,
It is blue-green, and requires filtering to obtain a pure blue color, so it is not a definitive blue light-emitting material.

B) Since a double insulating layer is used, the applied voltage for light emission is large.

The double insulating layer has a structure in which a phosphor that contributes to light emission is sandwiched between two insulating films in a sandwich-like manner. When a voltage higher than the threshold voltage is applied to the phosphor, avalanche collapse occurs, and the initially generated electrons (which are said to be generated mainly through the tunneling effect from the interface between the phosphor and the insulator) multiply. , many electrons flow through the phosphor. These electrons become hot electrons due to the high electric field in the phosphor, collide with the luminescent center, and bring it into an excited state. EL is the light emission observed when excited electrons at the luminescent center fall to the ground state. The above is the light emission mechanism proposed for the ZnS:Mn element. In alkaline earth-based phosphors, which have been attracting attention in recent years, it has been proposed that a different light-emitting mechanism from the above exists, but the details will not be discussed here. In either case, it is necessary to multiply the initially generated electrons in the phosphor, and it is necessary to cause a state like dielectric breakdown. Therefore, when a voltage is simply applied to the phosphor, it emits light, but the brightness increases rapidly.
This causes dielectric breakdown of the phosphor and destroys the device. On the other hand, when a double insulating layer structure is used, the current flowing through the phosphor is limited by the current charged in the insulating film.
The device operates stably without fatal destruction of the phosphor.

On the other hand, since the voltage is divided between the insulating film and the phosphor film, it is necessary to apply a voltage higher than the voltage actually applied to the phosphor to the element. This is the case with the double insulating layer type AC voltage EL.
This causes the applied voltage to increase.

To solve this problem, for example, an element having an MIS type structure has been proposed. This removes one side of the double insulating layer, which reduces the applied voltage accordingly, but there are also attempts to use a dielectric film with a high dielectric constant as the insulating film for the double insulating layer. . Research is currently underway to reduce such driving voltage as much as possible.

[Problems to be Solved by the Invention] What will be discussed here is an alkaline earth sulfide phosphor that has recently attracted attention as a blue- and red-light emitting device.

compounds, especially SrS, which is a sulfide of alkaline earth metals,
Thin-film EL devices using CaS as a fluorescent film are attracting attention as a key material that can realize full-color thin-film EL panels. In particular, it has been actively studied in recent years because it has the potential to realize high-brightness blue EL, which was not possible with conventional ZnS-based fluorescent films.

Here, the current state of the art for each of the three primary colors (red, green, and blue) required for full color production will be described below with respect to ZnS-based materials and alkaline earth metal sulfide materials.

1) Green EL ZnS: Tb and F have excellent color purity and high brightness, and have already reached a practical level. In particular, those produced by the sputtering method have a high brightness, about 137C:d/nf (60℃).

2) Red EL ZnS: In Sm series, SII is S m F 3 e
Methods of introducing in the form of S m C 11 z and SmP have been tried, and SmCQ has the best color purity.

The luminance is about 1 2 Cd/rrr (6 0 Hz), which is a value that needs to be further improved by at least 2 times, preferably about 4 times, to reach a practical level.

CaS:En type also has excellent color purity. Brightness is EB
Approximately 10 Cd/rrr (6
0 cells), and the sputter method is very close to the lowest practical level.

3) Blue EL ZnS: Tm-based ones have been studied for a long time, but although the color purity is good, the fR degree is about 0.14Cd/
m (60 Hz), which is far from practical.

SrS: Although the Ce system has a problem with color purity (blue-green), the brightness is about 40 Cd/bore (60 lights).
has been realized and exceeds the practical level. just. T
For applications such as V-video, it is necessary to pass through a color filter, which reduces the brightness by a factor of 10 (approximately 4 Cd/voice).
0 stop)). Therefore, in this case, an improvement of at least two times, preferably about five times, is required to reach a practical level.

However, if this improvement in brightness could be achieved, it would be possible to produce two primary colors, blue and green, with a single fluorescent film.

Therefore, it is necessary to further improve the brightness of red and blue EL, mainly by examining phosphors. Furthermore, it is necessary to simultaneously proceed with efforts to find effective solutions to the problems of brightness instability and deterioration that are problems associated with these phosphors.

In recent years, research on these light emission mechanisms has progressed, and it has become clear that ZnS:Mn has a different mechanism from that of conventional ZnS:Mn.

That is, in ZnS phosphors, it has been said that electrons injected through tunnels from the interface are accelerated in the phosphor and collide and excite the luminescent center, whereas in alkaline earth sulfides, electrons are injected by tunneling from the interface and excite the luminescent center by collision. It is said that light emission is mainly caused by the ionization of the complex centers of and their recombination.

In view of these points, the present invention is designed to provide a high-luminance element.

When the conduction current is measured using an element using an alkaline earth sulfide phosphor, it is considerably smaller than that of ZnS.

As for the source of these electrons, there are two theories: as with ZnS, they are injected from levels existing at the phosphor-insulator interface, and one theory is that traps and active bodies in the phosphor are injected into the conductor by the action of an electric field. There is a theory. In any case, brightness can be expected to be improved by increasing the number of conduction electrons flowing through the phosphor.

As an attempt in this regard, a structure in which an alkaline earth sulfide phosphor is sandwiched with ZnS has been proposed. This is said to cause electrons to be injected into the ZnS, resulting in improved brightness. However, there are many unknowns about the state of the interface between alkaline earth sulfide and ZnS, and it is unclear whether electrons can actually be efficiently injected into the phosphor.

Furthermore, SrS and CaS are said to be n-type semiconductors with donor levels caused by defects and the like.

[Object of the Invention] Focusing on the above points, the present invention provides an electroluminescence device that increases the number of conduction electrons flowing through an alkaline earth sulfide phosphor such as SrS or CaS, and improves brightness. It is intended to provide.

[Means for Solving the Problems] The present invention solves the above problems by providing an electroluminescence device using an alkaline earth sulfide phosphor with a p-type semiconductor thin film formed on at least one side of the phosphor. This is an attempt to resolve the issue.

[Operation] In the electroluminescent device having the above configuration, a pn junction is formed at the interface between the alkaline earth sulfide phosphor and the p-type semiconductor, thereby producing high-intensity light emission.

[Example] FIG. 1 shows the structure of an EL element according to an example of the present invention.

In the figure, 1 is a phosphor such as SrS, CaS, etc.
A p-type semiconductor thin film 2 is formed on one side (or both sides may be used). The material of this p-type semiconductor thin film 2 is not particularly limited, but it is preferable to use cu2s. In addition,
3 and 4 are insulating films, 5 is an AQ electrode, 6 is an ITO transparent electrode,
7 is a glass substrate.

As described above, by laminating the two films, the alkaline earth sulfide phosphor 1 and the p-type semiconductor thin film 2, a pn junction is formed at the interface between the two.

FIG. 2 shows a predicted appearance of bands at the pn junction. However, here the work function, donor,
Regarding the acceptor level, it is not shown accurately.

When a forward bias is applied to the pn junction, carriers are injected from the p-type semiconductor thin film 2 into the phosphor 1, and a large number of carriers are generated in the conduction band of the phosphor. These carriers are accelerated in the phosphor and can collisionally excite the emitter center. In addition, it is captured by a trap, and its energy excites the luminescent center, potentially causing luminescence.6 Although it is unclear which phenomenon primarily occurs, it results in high-brightness luminescence.

The means for forming the p-type semiconductor thin film is not particularly limited, but physical means such as vapor deposition and sputtering can also be used. Further, as an example, the following forming means is also possible.

After forming the alkaline earth sulfide thin film, a Cu2S thin film is formed and then heat treated, so that Cu diffuses into the sulfide thin film and forms a p-type semiconductor. On the other hand, Sr,C
a etc. diffuse in the opposite direction and form chlorides on the surface. If necessary, remove this by chemical methods. However, this is just an example and does not limit the present invention1.

[Effects of the Invention] As described above, according to the present invention, the alkaline earth sulfide phosphor and the P-type semiconductor thin film are laminated, and a pn junction is formed at the interface. A remarkable effect can be obtained in improving the brightness of the EL element.

[Brief explanation of drawings]

FIG. 1 is a side view of a main part of an electroluminescent device showing an embodiment of the present invention, and FIG. 2 is a band diagram of a pn junction. 1...Alkaline earth sulfide phosphor, 2.
......p-type semiconductor thin film, 3,4...
...Insulating film, 5...AQ electrode, 6...
...... ITO transparent electrode, 7... group... glass substrate.

Claims (1)

[Claims] An electroluminescent device using an alkaline earth sulfide phosphor, characterized in that a p-type semiconductor thin film is formed on at least one side of the phosphor.