JPS6025197A - Thin film electric field light emitting element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an electroluminescent device, in particular a thin film,
The present invention relates to an electroluminescent device driven by alternating current.

Conventional Structure and Problems There is known a thin film electroluminescent device as shown in FIG. 1, which emits light upon application of an alternating current electric field. In the figure, reference numeral 1 denotes a glass substrate, on which ITO transparent electrodes 2 are formed in a striped shape, and a lower dielectric thin film 3. Phosphor thin film 4. Y2O3 thin film for protection of the fluorescent thin film 4.5. Upper dielectric thin film 6. Striped counter electrodes 7 are formed in sequence. There are also devices with only one side of the dielectric thin film, which is characterized by a simple structure and low driving voltage.

A device with two dielectric thin film layers is characterized by being less prone to dielectric breakdown and having particularly high brightness.

The phosphor material used here is ZnS, Z
n5e, ZnF2, etc. are known, and in particular, in an element with ZnS as the base material and Mn added as the luminescent center, the maximum value is 3,500 to 5,000 cd/rr? (D brightness has been achieved.Dielectric material is Y2O3, S10.

513N4. At203. A typical example is Ta206.

3.

The Zn8 layer 4, which is a phosphor thin film, has a thickness of 500 nm to 7
00 nm and a relative permittivity of approximately 9, and the dielectric thin film 3°6 has a thickness of 400 nm to 800 nm and a relative permittivity of 4 to 2.
It is 6. When driving with AC, the voltage applied to the element is divided between the Zn8 layer 4 and the dielectric thin films 3 and 6, and the former has 4
~ It only costs about 60%. Therefore, the voltage required to emit light has become much higher.

In an element in which dielectric thin films are provided on both sides of a ZnS film,
Currently, a voltage of 200 V or more is applied by pulse drive of several KHz. This high voltage places a heavy burden on the drive circuit, requiring a special high voltage IC.
This also leads to an increase in costs.

On the other hand, in order to lower the driving voltage, P b with a high dielectric constant is used.
T i Os , P b (T i 1-x Z r
X) It has been proposed to use materials mainly composed of Oa, B a TiO3, etc. for dielectric thin films. Also, 5
It has also been proposed to use rTio3. All of these thin films have a perovskite titanate crystal structure, and the product of the dielectric constant, which is a figure of merit for a dielectric thin film, and the dielectric breakdown electric field strength (hereinafter referred to as Eb) is large, so it is difficult to apply an electric field to the device. Most of the applied voltage can be divided into the ZnS film, and the driving voltage can be significantly reduced. Among these materials, PbTi
O3゜Pb(Ti1-!Zr,)03 thin film has a relative dielectric constant (
(hereinafter referred to as ε1) is 1oO to 150IEb is O, sMV
/cm. B a T i 03 has ε1 of 90 to 1
10. Eb is 1.6MV/crn, S r T i
03 has ε1 of 140. Eb is 1.6 to 2 MV/cm, and these two thin films have particularly excellent properties.

However, a thin film electroluminescent device using these two thin films operates almost stably if it is a small device of a few minutes or less, but if it is made into a large area device such as a large X-Y Dora Tomato 992 screen, the propagation property Dielectric breakdown occurs, resulting in a fatal defect in the display element.

This propagating dielectric breakdown reflects the inherent properties of the thin film used in the device, and does not occur under normal driving conditions in the case of conventionally used dielectric thin films with a low dielectric constant. The nature of this dielectric breakdown will be explained below, focusing only on the dielectric thin film.

Dielectric thin films are produced by 5. methods such as vapor deposition, sputtering, and CVD, but various defects such as pinholes and dust are generated in the film. These defects are more likely to cause dielectric breakdown at lower electric field strengths than areas without defects. There are roughly two types of dielectric breakdown in films.

One type is called self-healing dielectric breakdown, as shown in FIG. In the figure, 11 is a substrate;
On top of that, a lower electrode 12. Dielectric film 13a, upper electrode 14
is formed. 15a indicates the location where dielectric breakdown occurred;
The upper mold around the dielectric breakdown point 15a lol! , 14 are scattered in a range of tens of 1177+ by the discharge energy, and the upper electrode 14 and the lower electrode 12 are open.

The other type is a non-self-healing type, in which the upper electrode 14 is not sufficiently scattered as shown in FIG. 3, and the two-part electrode 14 and the lower electrode 1
2 is shorted. If voltage continues to be applied in this state, dielectric breakdown will propagate throughout the dielectric film 13b and the entire device will be destroyed.

As the upper electrode is made thinner, this self-healing type becomes 6 ・＼
' Although dielectric breakdown is less likely to occur, if it is made too thin, the resistance increases and it is not desirable as an electrode, so the minimum thickness is about several tens of nanometers. The electrode material is Au,
Zn, AL, etc. are most prone to self-healing dielectric breakdown. However, there are dielectric thin films that do not undergo self-healing dielectric breakdown even when using an electrode made of Au, Zn, At, etc. several nanometers thick, and this dielectric breakdown is caused by the inherent properties of the material. Although the cause is not clear, it is thought that there is a large difference in the behavior of the arc discharge that occurs during dielectric breakdown, which scatters the upper electrode, between films that undergo self-healing dielectric breakdown and films that do not.

When this self-healing non-breakdown dielectric thin film is used as a dielectric thin film between a light-reflecting electrode such as At and a phosphor thin film in a thin-film electroluminescent device, it is stable in small devices except for a few defective devices. However, when large-sized elements were used, the following problems occurred. When driving a large light-emitting element, dielectric breakdown occurs in the initial stage, and some of the breakdown occurs only in an area of several tens of micrometers, with the light-reflecting electrode scattering and not spreading further7. In this case, dielectric breakdown that occurs at one point spreads in a short period of time and destroys the entire picture element. For this reason, either the picture element does not emit light at all, or the light reflecting electrode in the x-y matrix is disconnected, creating a linear non-light-emitting portion. Such propagating dielectric breakdown is considered to reflect the property of the dielectric film, which does not undergo self-healing dielectric breakdown.

Defects in the film can be reduced by improving the cleanliness of the fabrication environment 1 by improving the fabrication process, but if the area is several tens to hundreds of centimeters
It is almost impossible to eliminate defects with a high yield in a display element having tr1 or more. Dielectric thin films that do not undergo self-imposed dielectric breakdown have not been used in practice because even the slightest defect can cause fatal consequences for the entire device.

OBJECTS OF THE INVENTION An object of the present invention is to provide a thin film electroluminescent device in which propagating dielectric breakdown does not occur even when using a dielectric thin film with a high dielectric constant that does not cause self-healing dielectric breakdown.

According to the present invention, a dielectric thin film that does not undergo self-healing dielectric breakdown is provided only on the substrate side of the phosphor thin film, and the phosphor thin film is formed of a material that causes self-healing dielectric breakdown. As a result, the upper electrode is scattered due to the dielectric breakdown of the phosphor thin film, thereby preventing the propagation of the dielectric breakdown.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings.

In FIG. 4, 21 is a glass substrate, and I
TO transparent electrodes 22 are formed in a stripe shape. 2
3 is a lower dielectric thin film made of a self-healing material that does not cause dielectric breakdown. For example, 5rTio3. BaTio3.
A perovskite titanate such as is used. A phosphor thin film 24 is formed on the lower dielectric thin film 23. This phosphor thin film 24 is made of, for example, ZnS',
It consists of T b F s and exhibits self-healing dielectric breakdown. A Y2O3 thin film 26 is formed thereon, and a counter electrode 26 is further formed in a stripe shape. This counter electrode 26 is made of a 9.sup.At film. Furthermore, the Y2O3 film undergoes self-healing dielectric breakdown.

The ITO transparent electrode 22 and the At counter electrode 26 are perpendicular to each other, and when an electric field is applied to the intersection point, light is emitted. According to the structure described above, the lower dielectric thin film 23 has a high dielectric constant. The driving voltage of the element can be lowered. On the other hand, since the dielectric breakdown is not self-healing, it is easy to cause propagating dielectric breakdown. However, since the phosphor thin film 24 undergoes self-healing dielectric breakdown, and the dielectric film that does not self-heal and dielectric breakdown exists only under the phosphor thin film 24, the counter electrode 26 and the transparent electrode 22 may be short-circuited. There isn't. Therefore, propagation of dielectric breakdown can be prevented.

FIG. 6 shows another embodiment, in which the same parts as in FIG. 4 are given the same numbers and their explanations are omitted. In this embodiment, the Y2O3 film 26 in FIG. 4 is omitted.

FIG. 6 shows yet another embodiment, and similarly, the same parts as in the embodiment of FIG.

In this embodiment, the Y2O3 film 26 in the embodiment of FIG.
An upper dielectric thin film 27 is further provided between the electrode 26 and the counter electrode 26 . The upper dielectric thin film 27 is made of a material that exhibits self-healing dielectric breakdown, such as B a T a 2 .
Consists of 0 e.

Experiments were conducted to confirm the effects of specific samples of the above examples. For comparison, an experiment was also conducted on an element having the structure of the conventional example shown in FIG. 1, and this will be referred to as Sample I.

The embodiments of the present invention shown in FIGS. 4 to 6 are sample 1,
Called ■ and ■.

As the lower dielectrics 3 and 23, sample 1. ll and ■
A SrTiOs film was used, and the thicknesses were set to 0.5 μm, 1 μm, and 1 μm, respectively. For sample ■, a B a Ti Os film was used with a thickness of 1 μm.
And so. The film was formed by magnetron RF sputtering using a ceramic target. The sputtering atmosphere was o2:Ar=1=4 and the pressure was 0.5 P.
It was a. The substrate temperature was 380°C.

11: As the phosphor thin films 4 and 24, ZnS:TbF 3 films were used for all samples. The film was formed by electron beam evaporation using ceramics of ZnS:TbF3 as an evaporation source. The substrate temperature was 20° C., and after the deposition, heat treatment was performed at 350° C. for 1 hour in a vacuum.

The thicknesses of the Y2O3 thin films in Samples I, ■ and ■ are 35 nm, 45 nm and 50 nm, respectively. For film formation, electron beam evaporation using Y2O3 ceramics as an evaporation source was performed.

The two-part dielectric thin film 6 in sample ① is 5rTio3.
using the same method as the lower dielectric thin film 3 to a thickness of 0.5
It was formed in μm. Upper dielectric thin film 2 in sample ■
7 uses B a Ta 206 with a thickness of 0.2 and 1 m.
.

was formed. The film was formed by magnetron sputtering using a ceramic target. The atmosphere is o2
:Ar-1:4 and the pressure was 0.3 Pa.

The substrate was not heated.

The counter electrodes 7 and 26 have a thickness of about 150 nm for all samples, and are formed into thin films by electron beam evaporation.
Stripes were formed using the lift-off method.

Table 1 shows the properties of the thin film itself used in the above samples. Z n S :T b F 3 also has high insulation resistance and can be considered as a dielectric thin film.

Table 1 The light emitting part of the device is 150 x 200 words and made of ITO.

Together with At, the pitch is 6oμm. Driving conditions were AC pulses with a pulse width of 26 μB and a frame frequency of 60 μB. The driving voltage was set so that the brightness was approximately 40 c d/m''. When the voltage was applied, dielectric breakdown occurred in each element.
The hole for the four electrodes is less than 60 μn1 in size and is completely inconspicuous.

On the other hand, part of the dielectric breakdown that occurred in the conventional sample (2) remained at a size of 50 μIn or less and did not spread any further, but part of it spread over the entire picture element and broke the A4 stripe electrode. This is a self-healing type that does not cause dielectric breakdown.
This is considered to be because the dielectric breakdown state of the thin film electroluminescent device was dominated by the S r T 103 thin film and became propagated because the r TiO3 thin film was exposed on the surface.

On the other hand, samples according to the present invention ■~! IJ: Self-healing non-breakdown S r TiO3 thin film, B a T
Yo thin film with self-healing breakdown on top of iOs thin film, B a T a 20 e thin film, Z n S:
Since the T b F 33 thin films were formed thickly, it is thought that the state of dielectric breakdown of the thin film electroluminescent device was controlled by the properties of these thin films and reached a small hole. Table 2 shows the driving voltage of each element, the number of dielectric breakdowns, and the number of disconnections in the At stripes.

It can be seen from Table 2 that the structure of the present invention provides a thin film electroluminescent device that can be driven at low voltage and is free from any fatal defects. In the description of the above embodiment, an example was shown in which ZnS:TbF3 was used as the phosphor thin film, but
For example, ZnS:Mu.

It goes without saying that the present invention produces similar effects with other phosphor thin films such as Zn5e:Mn.

Among materials that do not undergo self-healing dielectric breakdown, perovskite titanate is the most excellent dielectric material because it has a large product of the figure of merit Eb and ε.

Among Samples 1 to 2, Samples 1 and 15, which had a dielectric thin film on top of the phosphor thin film, and Sample 15 were more stable against humidity and the like.

Effects of the Invention As described above, the structure of the present invention makes it possible to use a self-healing type material that does not cause dielectric breakdown, which has a large figure of merit (Ebx permittivity) as a dielectric thin film, but could not be used because propagating dielectric breakdown occurs. As a result, it is possible to obtain a thin film electroluminescent element that can be driven stably at low voltage and without causing large defects due to dielectric breakdown even in large display elements.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view showing the structure of a conventional thin film electroluminescent device, Figure 2 is a schematic diagram showing self-healing dielectric breakdown in a dielectric, and Figure 3 is a schematic diagram showing non-self-healing dielectric breakdown in a dielectric. A schematic diagram shown in FIG. 4. FIGS. 5 and 6 are cross-sectional views showing the structure of a thin film electroluminescent device according to an embodiment of the present invention. 22... Transparent electrode, 23... Lower dielectric thin film, 24... Fluorescent thin film, 26...
・Counter electrode. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 Figure 15α 14 Figure 3 Figure 4 Figure 5

Claims (3)

[Claims] (1) A dielectric thin film that does not self-heal dielectric breakdown is provided only on the substrate side of the phosphor thin film that causes self-healing dielectric breakdown, and two electrodes, at least one of which is optically transparent, are provided on at least both of the thin films. A thin film electroluminescent device characterized in that it is configured to apply a voltage. (2) A thin film electroluminescent device according to claim (1), characterized in that a dielectric thin film that undergoes self-healing dielectric breakdown is provided on the opposite side of the phosphor thin film from the substrate. (3) The thin film electroluminescent device according to claim (1), wherein the self-healing non-breakdown dielectric thin film is a perovskite titanate film.