GB2286081A

GB2286081A - Thin film light-emitting element

Info

Publication number: GB2286081A
Application number: GB9500767A
Authority: GB
Inventors: Yukihiro Maruta
Original assignee: Fuji Electric Co Ltd
Current assignee: Fuji Electric Co Ltd
Priority date: 1994-01-17
Filing date: 1995-01-16
Publication date: 1995-08-02
Also published as: FR2715262A1; DE19501229A1; JPH07211458A; GB9500767D0

Description

1 - W 2286081 THIN FILM LIGHT-EMITTING ELEMENT The present invention

relates to an intermediate insulation layer of a thin film light emitting element. in particular. an intermediate insulation layer for preventing reflection of light emitted from a light emitting layer.

A double insulation type thin film electro-luminescence display (hereinafter called a thin film light emitting element), in which a light emitting layer of a fluorescent substance with Hn as a light emitting centre is sandwiched between a transparent electrode and a back electrode through insulation layers, has attracted much attention as a thin display panel, since it is able to provide improved brightness. resolution and display capacity.

Figure 8 is a perspective view showing a prior art double insulation type thin-film light emitting element.

A thin film light emitting element comprises a glass substrate 1, on which are formed sequentially an intermediate insulation layer 7, a plurality of parallel transparent electrodes 2, and a first insulation layer 3 of alumina A1203, silica SiO2 or silicon nitride Si3N4. A light emitting layer 4, a second insulation layer of the same material as the first insulation layer, and a plurality of back electrodes 6 of Al arranged parallel and orthogonally to the transparent electrodes 2 are then formed thereon. The thickness of each layer is arranged to be from 20 to 1000 nm. The transparent electrodes 2, the first insulation layer 3 and the second insulation layer 5 are generally formed by the sputtering method. The light emitting layer 4 is formed either by the sputtering method or by the electron beam vapour deposition method.

The light emitting layer 4 of the conventional thin film light emitting element is composed of zinc sulphide ZnS as a base material to which a small amount of Mn or TbOF is added as a light emitting centre. The light emitting layer is deposited, with the concentration of its light emitting centre kept at an optimum value 2 (Manganese Mn: 0.4 to 0.6 wt% of zinc sulphide US). Subsequently, the layer is heat-treated at a high temperature of about 55COC to improve the crystalline structure of the light emitting layer and to increase the dispersity of the light emitting centre.

In such a thin film light emitting element. light produced in the light emitting layer 4 and directed to the glass substrate 1 is partly transmitted through the transparent electrode 2 and intermediate insulation layer 7 via the first insulation layer 3, and partly reflected at respective boundary surfaces of the transparent electrode 2, intermediate insulation layer 7 and glass substrate 1.

To improve the visibility of the thin film light emitting element, it is necessary to minimise reflections and to increase transmission for various angles of incidence at the aforesaid boundary surfaces. The reflectance can be expressed as a function of the refractive index, as will be described later.

Figure 9 is a graph showing the refractive index of the intermediate insulation layer of the prior art thin film light emitting element.

The refractive index of the intermediate insulation layer was found to be of an intermediate value between those of the glass substrate and transparent electrode. The refractive index of the intermediate insulation layer was equal to the square root of the product of the refractive indices of the glass substrate and the transparent electrode. An optical thickness of the intermediate insulation layer. a product of the refractive index and a film thickness of the intermediate insulation layer, was set at 114 of a centre wave length of the emitted light.

With the refractive index as determined above, the reflectance is minimised to be zero only for that light which impinges perpendicularly on the glass substrate (angle of incidence = 0) from the light emitting layer. In this case, however, there is almost no problem for the visibility, since the element is generally viewed from a direction substantially perpendicular to the plane of the light emitting layer, for which direction a reflectance for light from the outside becomes small.

On the other hand, for light which does not impinge perpendicularly on the glass substrate (angle of incidence > 0) the reflectances increase at the boundary surfaces. In this case, the reflectance for light from the outside also increases, and visibility deteriorates.

The object of the present invention is to provide a thin film light emitting element with excellent visibility over a wide viewing angle, by optimising the refractive index and optical thickness of the intermediate insulation layer, so that it has a reduced reflectance even for light incident at angles greater than zero degrees.

The aforementioned object is achieved with a thin film light emitting element according to a first embodiment of the invention. which element comprises: (1) a glass substrate, (2) an intermediate insulation layer, (3) a transparent electrode, (4) a first insulation layer, (5) a light emitting layer, (6) a second insulation layer, and (7) a back electrode in which the intermediate insulation layer, transparent electrode, first insulation layer, light emitting layer, second insulation layer and back electrode are layered one by one on the glass substrate; the glass substrate comprises soda glass; the intermediate insulation layer has a refractive index whose value lies between the refractive indices of the glass substrate and the transparent electrode, and has an optical thickness such that the product of the refractive index and the thickness of the intermediate insulation layer, is equivalent to an intermediate value between 0.25 and 0.5 times as large as a centre wave length of an emission spectrum; a voltage is applied between the transparent electrode and the back electrode; the first and second insulation layers comprise inorganic insulation material; the light emitting layer comprises an inorganic light emitting material, and is excited to emit light by electrons which propagate from boundary surfaces between the light emitting layer and the first and second insulation layers.

4 - IF Further, the object is also achieved with a thin film light emitting element according to a second embodiment, which element comprises (1) a glass substrate, (2) an intermediate insulation layer. (3) a transparent electrode, (4) a first insulation layer. (5) a light emitting layer, (6) a second insulation layer, and (7) a back electrode, in which the intermediate insulation layer. transparent electrode, first insulation layer. light emitting layer. second insulation layer and back electrode are layered one by one on the glass substrate; the glass substrate comprises soda glass; the intermediate insulation layer has a retractive index which continuously changes from that of the glass substrate at the boundary of the intermediate layer with the glass substrate to that of the transparent electrode at the boundary of the intermediate layer with the transparent electrode, and has a mean optical thickness, the product of the refractive index and the layer thickness of the intermediate insulation layer, being equivalent to at least 0.25 times a centre wave length of an emission spectrum; the first and second insulation layers comprise inorganic insulation material; and a voltage is applied between the transparent electrode and the back electrode; the light emitting layer comprising an inorganic light emitting material which is excited to emit light by electrons which propagate from boundary surfaces between the light emitting layer and the first and second insulation layers.

The optimisation of a refractive index and an optical thickness results in reduced reflectance for light within a wide range of incidence angles, due to a phase difference in the interference of the incident and reflected light.

The present invention will now be explained in detail with reference to the accompanying drawings, which illustrate preferred embodiments of the present invention. In the drawings:

Figure 1 is an isometric view of a first embodiment of a thin film light emitting element according to the present invention; b i 1 p Figure 2 is a diagram showing the dependance of reflection coefficient on angles of incidence with optical thicknesses nd as parameters; Figure 3 is a schematic drawing of a radio frequency sputtering apparatus used for manufacturing the embodiment of the thin film light emitting element according to the present invention; Figure 4 is a diagram showing an example of refractive index of the intermediate insulation later of a second embodiment of the thin film light emitting element according to the present invention; Figure 5 is a diagram showing a dependance of reflectance on a mean optical thickness (product of mean refractive index and film thickness) of the intermediate insulation layer in the second embodiment of the thin film light emitting element; Figure 6 is a diagram showing a dependance of a refractive index on an oxygen gas ratio in the sputtering gas for forming the intermediate insulation layer in the second embodiment of the present invention; Figure 7 is a diagram showing a wave length dependance of the spectral transmittance of the intermediate layer in the second embodiment of the present invention; Figure 8 is a perspective view showing a conventional double insulation type thin film light emitting element; and Figure 9 is a graph showing the refractive index of the intermediate insulation layer of the conventional thin film light emitting element.

Figure 1 is an isometric view of a thin film light emitting element according to a first embodiment of the present invention.

Those components and parts which are the same as in the conventional element shown in Figure 8 are designated by the same reference numerals. An intermediate insulation layer 7A situated between the glass substrate 1 and the transparent electrode 2 is the only difference from the prior art thin film light emitting element.

A perpendicular reflectance R. of a glass surface in contact with air is given by the following formula:

6 -IF R. = (na. - 1)2 1 (ni + 1)2 Where n, is an index of refraction of glass. With a glass substrate made of NA40 glass, made by HOYA Company, the refractive index of this glass substrate is 1.573 and thus the reflectance R, is 4.97%. A reflectance Roe for an angle of incidence of 600 is 20.1%. On the other hand, when soda glass (na. = 1.51) is used for the substrate. R, is 0.41%. and the reflectance Roe for an angle of incidence of 600 is 18%. Therefore. soda glass is a little more favourable for a thin film light emitting element with a wide viewing angle. Soda glass is used as a glass substrate.

The reflectance R is expressed by the following formula:

R = 1 rz + r2 exp(-2i6) 1' (1) Wherein the refractive index of a transparent electrode is no, the refractive index of the intermediate insulation layer is n, the refractive index of the glass substrate is ni, the film thickness of the intermediate insulation 'layer is d, the angle of incidence of the light is e, and a centre wavelength of emitted light is 1.

rj., r2 and 6 are respectively expressed by the following formulae:

rm = (no - n) 1 (no + n) r2 = (n - ni) / (n + ni) 6 = 2. nd. cos E)/X (4) Table 1 lists reflection coefficients in relation to angles of incidence with optical thicknesses as a parameter. The reflection coefficients are calculated from phase difference of light by substituting formulae (2), (3) and (4) into formula (1) with no=2.00, ni=2.51, and n=1.74.

k 1 7 IF Table 1

Optical thickness 72.5 100 145 205 290 nd (nm) Angle 0 0.707 1 0.468 0 1 0.5 1 of 30 0.779 0.592 0.209 0.345 0.913 incidence 45 0.850 0.721 0.444 0 0.605 e 60 0.924 0.857 0.707 0.444 0 (0) 90 ca 1 ca 1 ca 1 ca 1 ca 1 These reflection coefficients are normalised with a unit of reflectance being that for light having a centre wave length 1 of 580 nm emitted without an intermediate insulation layer.

Table 1 shows that the reflection coefficient of the intermediate insulation layer is reduced over a wide range of angles of incidence for optical thicknesses within from 145 nm to 290 nm. These thicknesses are between 0.25 and 0.5 times as large as the centre wave length.

The aforementioned results can be applied to the emitted light from ZnS:Tb, ZnS:Sm and ZnS:Tm, with centre wave lengths of 540 nm, 650 nm and 470 nm.

Figure 2 is a diagram showing the dependance of reflection coefficient on angle of incidence with optical thicknesses nd as a parameter.

The optical thicknesses selected are: (1) 72.5 nm, (2) 100 nm, (3) 145 nm, (4) 205 nm and (5) 290 nm.

It is found that the reflectances become small over a wide range of angles of incidence at an optical thickness of 205 nm. The embodiment of the thin film light emitting element according to the present invention is prepared by the following method. Figure 3 is a schematic drawing of a radio frequency sputtering apparatus used for manufacturing the thin film light emitting element. A radio frequency (RF) power supply 13, a target 12, a soda glass substrate 11 and a gas inlet tube 14 were provided in a chamber 15. A sputtering gas, which included argon gas, oxygen gas and nitrogen gas in predetermined ratios, flowed through the gas inlet tube 14.

8 - P SiALON was used as the target. By SiALON is meant any insulating material comprising the elements Silicon, Aluminium, Nitrogen and Oxygen.

With a mixture of argon gas, oxygen gas and nitrogen gas in the ratio of 10:0.1:0.05, an RF voltage was applied to sputter the SiALON target. An intermediate insulation layer 7A of SiALON with a refractive index of 1. 73 was formed on the soda glass substrate 11. Intermediate layers of six optical thicknesses were prepared, with thicknesses of (1) 72.5 nm, (2) 100 nm, (3) 145 nm, (4) 205 nm, (5) 290 nm, and (6) 400 nm.

The transparent electrode 2 was then formed on the intermediate insulation layer 7A to a thickness of 200 nm, using an ITO (indium oxide with 10 wt% of tin oxide) target. The refractive index of the transparent electrode was about 2.

Aluminium oxide A1203 and tantalum oxide 11a20s were then filmed to a total thickness of 350 nm by the sputtering method as the first insulation layer, over the transparent electrode 2.

Next, the light emitting layer of MS:Mn was formed to a thickness of 700 nm by the MOM method.

The second insulation layer was then formed on the light emitting layer by sputtering, using the same material as the first insulation layer.

The brightness in a direction perpendicular to a pixel of a prepared thin film light emitting element, driven with 60 Hz, was 350 cd/M2. The brightness of the emitted light was so high that only the slightest difference in visibility of the thin film light emitting elements (1) to (6) was seen when observed from the front (an angle of incidence of 00).

Contrary to this, a reflection of room light at the glass substrate was observed, for angles of incidence larger than 0-. The reflection of room light became remarkable particularly for angles of incidence of room light larger than 300. When the aforementioned optical thickness was within from J1.45 to 290 nm, the element had a good visibility, even for light with a wide range of angles of incidence. When the optical thickness was 205 nm, a k 9 particularly good result was obtained, showing agreement with the theoretical calculation.

With the optical thickness of the insulating layer set at 205 nm, a reflectance for the emitted light becomes minimum at angles of incidence of approximately 450. The reflectance for room light can also be minimised at angles of incidence of approximately 450. This minimum reflectance enables an outside insulation layer to be provided on the glass substrate surface opposite to the intermediate insulation layer. The refractive index of the outside insulation layer is chosen to be between those of the glass substrate and intermediate insulation layer, and the film thickness of the outside insulation layer is chosen to make its optical thickness 205 nm.

The optical thickness of the intermediate insulation layer is determined by the required viewing angle of the thin film light emitting element. For example, a comparatively small optical thickness may be adopted to meet the requirement of a narrow viewing angle as is required in personal computers. Contrary to this, when a wide viewing angle is required as in an FA apparatus, a comparatively large optical thickness may be adopted.

SiALON is used for the aforementioned intermediate insulation layer, but insulators such as aluminium oxide A1203 or others with refractive indices are near 1.74 may be used.

Referring now to Figure 6, there is seen a diagram showing the dependance of the refractive index of an intermediate insulation layer on the oxygen gas ratio in a second embodiment of a thin film light emitting element according to the present invention.

In the second embodiment, the intermediate insulation 'Layer is formed with the concentration of oxygen in the sputtering gas changed during sputtering from 0.3 to 0, the respective ratios of argon gas and nitrogen gas being held at 10 and 0.05 [Ar:O:N.= (10:0.3:0.05) to (10:0:0.05)1. It is found that the refractive index continuously changes from 1.51 of the glass substrate to 2.00 of ITO.

Figure 4 is a diagram showing an example of the refractive index of the intermediate insulation layer of the second embodiment of the present invention.

The refractive index of the intermediate insulation layer continuously changes from that of ITO to that of the glass substrate.

Figure 5 is a diagram showing the dependance of reflectance on the mean optical thickness (a product of a mean index of refraction and film thickness) in the intermediate insulation layer of the thin film light emitting element of the second embodiment of the present invention.

It is found that the continuously changing the reflectance is significantly reduced when the mean optical thicknesses are equivalent to 1/4 or more of the centre wave length. The thicker the mean optical thickness, the larger is the effect. A linear change of the refractive index is most preferable.

Figure 7 is a diagram showing a dependance of a spectral transmittance of the intermediate insulation layer on the wave length in the second embodiment of the invention.

Sputtering gas mixing ratios of argon, oxygen and nitrogen are variously changed as (a) 10:0.005:0, (b) 10:0:0.02, (c) 10:0:0.1, (d) 10:0.2:0.05, and (e) 10:0A.05.

It is found that a flat spectral transmittance characteristic is obtained when the sputtering gas mixing ratio is (d) or (e). These ratios are in a range for continuously changing the reflectance from 1.51 of the glass substrate to 2.00 of ITO. The light transparency becomes poor in the cases of (a), (b) and (c), due to poor spectral transmittance in the short wave length range.

Secondary ion mass spectrometric analysis revealed that the aforementioned intermediate insulation layer, whose refractive index is continuously changed by using SiALON, also prevents alkali diffusion from a soda glass substrate, and is more effective to prevent alkali diffusion than insulation layers of aluminium oxide Al=03 or silicon oxide SiO=.

7 11 v An intermediate layer with a continuously changing refractive index is, as aforementioned, able to reduce the reflectance when the optical thickness is equivalent to 1/4 or more of the centre wave length of the emission spectrum. Such a layer with a larger optical thickness can also reduce reflectance irrespective of the centre wave length, and can thus be applied to a colour thin film light emitting element.

The first embodiment thus provides an insulation layer with a specific value of refractive index between those of the glass substrate and the transparent electrode, and an optical thickness such that the product of the refractive index and the film thickness of the intermediate insulation layer is from 0.25 to 0.5 times as large as the centre wave length of the emitted light. In the second embodiment, the intermediate insulation layer has the value of its refractive index continuously changing from that of the glass substrate to that of the transparent electrode, and has a mean optical thickness of the intermediate insulation layer arranged to be 0.25 times or more as large as the centre wave length of the emitted light. The intermediate layers significantly reduce reflectance for light incident from a wide range of angles, enabling the thin film elements excellent results over wide viewing angles.

Further, efficient production of an intermediate insulation layer with a continuously changing refractive index is achieved by RF sputtering of SiALON using a mixture of argon gas, oxygen gas and nitrogen gas, in which the concentration of oxygen is continuously changed.

Moreover, the.technique of sputtering with a predetermined ratio of argon gas, oxygen gas and nitrogen gas provides a thin film light emitting element with an excellent 'Light transparency and resistance to diffusion of sodium.

12 1 13

Claims

1. A thin film light emitting element comprising a glass substrate, an intermediate insulation layer, a transparent electrode, a first insulation layer, a light emitting layer, a second insulation layer, and a back electrode, wherein: the intermediate insulation layer. transparent electrode. first insulation layer, light emitting layer, second insulation layer and back electrode are layered one by one on the glass substrate the glass substrate comprises soda glass and the first and second insulation layers comprise inorganic insulation material; the intermediate insulation layer has a specific value of refractive index lying between the refractive indices of the glass substrate and the transparent electrode. and the optical thickness of the intermediate insulating layer is such that the product of the refractive index and the film thickness of the intermediate insulation layer is between 0.25 to 0. 5 times the centre wave length of the emitted spectrum; a voltage is applied between the transparent electrode and the back electrode; the light emitting layer comprises an inorganic light emitting material, and is excited to emit light by electrons which propagate from boundary surfaces between the light emitting layer and the first and second insulation layers.

2. A thin film light emitting element including a glass substrate, an intermediate insulation layer, a transparent electrode, a first insulation layer, a light emitting layer, a second insulation layer, and a back electrode, wherein: the intermediate insulation layer, transparent electrode. first insulation layer, light emitting layer, second i nsulation layer and back electrode are layered one by one on the glass substrate; the glass substrate comprises soda glass and the first and second insulation layers comprise inorganic insulation material; 1 13 the intermediate insulation layer has a refractive index which continuously changes from the refractive index of the glass substrate at the boundary of the intermediate insulation layer with the glass substrate to that of the transparent electrode at the boundary of the intermediate insulation layer with the transparent electrode, and the optical thickness of the intermediate layer is so arranged that the product of the mean refractive index and film thickness of the intermediate insulation layer is 0.25 times or more as large as a centre wave length of the emitted spectrum; a voltage is applied between the transparent electrode and the back electrode; and wherein the light emitting layer comprises an inorganic light emitting material, and emits light by being excited by electrons which propagate from boundary surfaces of the light emitting layer with the first and second insulation layers.

3. The thin film light emitting element as claimed in claim 2.

wherein the intermediate insulation layer comprises SiALON.

4. The thin film light emitting element as claimed in claim 3, wherein the intermediate insulation layer is formed by a high-frequency sputtering method.

5. The thin film light emitting element as claimed in claim 4, wherein the intermediate insulation layer is formed using a mixture of argon gas, oxygen gas and nitrogen gas as the sputtering gas.

6. The thin film light emitting element as claimed in claim 5, wherein the intermediate insulation layer is formed by changing the mixing ratio of the argon gas, oxygen gas and nitrogen gas mixture from 10:0.2:0.05 to 10:0:0.05 during the sputtering process.

7. A thin film light emitting element as claimed in any preceding claim, substantially as herein described with reference to Figure 1 of the accompanying drawings.

8. A method of producing a thin film light emitting element substantially as herein described.