FI105643B - Thin-film electroluminescent device and method for its manufacture - Google Patents

Description

105643

THIN-FILM ELECTROLUMINENSING DEVICE AND METHOD FOR MANUFACTURING IT

The present invention relates to AC thin-film electroluminescent devices. In particular, the invention relates to thin-film sandwich structures having a steep voltage-luminance characteristic, improved light symmetry, and a more stable aging luminescence, and methods for producing these structures.

10 Thin-film electroluminescent components (TFELs) are used to display information, such as words or numbers, in place of cathode tubes, plasma displays, liquid crystal devices, and light emitting diodes (LEDs). They are especially used in applications where wide angle, wide temperature range and solid structure are important. The state of the art electroluminescent display is easy to read under different light conditions. This feature is advantageously achieved by using adjustable luminance for good contrast and optimum image quality.

There are many ways to adjust the luminance level. In order to achieve a wide dynamic range for light emission, the luminance-voltage dependence (LV-20 curve) of the TFEL component must be efficiently utilized using different amplitudes of voltage. The full LV curve of the TFEL component must be available and have good enough properties throughout the life of the display.

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High voltages and frequencies are used to ensure high overall luminance under 25 daylight conditions. This maximum load sets the LV curve to the test. On the other hand, this usually increases electricity consumption, which limits the use of higher voltages. Electricity consumption is particularly affected by line and column voltages. Voltage levels are limited by the driver's electronic technology, which results in the use of lower voltages.

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An advanced driver program for lower luminance levels also utilizes the ability to adjust the drive frequency or frame shift. These methods have set new demands on the symmetry of the light emission for different polarities to avoid information: vibration on the screen.

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Considering all these requirements, high performance EL components are expected to have high luminance, good display efficiency, sharp and very stable LV curve and good enough light emission symmetry with different polarities. Regardless of the growth method, no commercial electroluminescent component using thin layers of calcium can meet these requirements.

The EL display devices generally comprise a translucent substrate, such as a glass surface, a translucent electrode layer on the substrate, and a second electrode layer spaced from the first electrode layer, between the electrodes of the phosphor layer and at least one dielectric layer of electrode on both sides. One of the most common lining layers used in EL devices today in commercial TFEL products is ALE-grown ZnS.Mn. The strengths of the ALE-grown ZnS .Mn layer have been its high luminous intensity and efficiency. At the same time, stability and light symmetry are somewhat compromised.

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The stability of ZnS: Mn fluorescent material has been extensively studied. It is generally believed that aging occurs through a variety of mechanisms and that thin films grown under different growing techniques age very differently. However, it is not clear which mechanisms determine the obsolescence of different devices. Even as a result of the same breeding technique, different dominant aging properties can be obtained [Mikami, A. et al., J.Appl.Phys. 72 (2): 773-782 (1992)].

Various solutions have been proposed to improve stability, including changes in load, processing, and thin-film structure [Mikami, A. et al., J. Appl. Phys. 72 (2): 25773-782 (1992); Muller, G. et al., SID 88 Digest, 23-26 (1988); U.S. Patent No. 4,869,973; Nishikawa, M. et al., SID 88 Digest, 19-22; and Khormaei, R. et al., SID 89 Digest, 65-67 (1989)]. Most of these have other drawbacks, are difficult to implement, or do not provide sufficient stability.

• 30 With respect to changes in thin film structure, U.S. Patent No. 4,869,973 and Nishikawa, M. et al., Have proposed to provide a TFEL display layer with a thin layer of calcium sulfide film produced by increasing the electron beam vaporization time to ZnS: to ensure stable operation. The Nishikawa EL display device consists of dielectric layers of strontium titanium or barium tantalum binary oxides. Khormaei R. et al.

3 105643 investigated ACTFEL devices with dielectric layers composed of silicon oxide nitrides and found that the location of the buffer layer was negligible. According to Khormaei et al., The results suggest that the instability mechanisms (i.e., ionic migration) previously proposed by Nishikawa et al., Would not work in their devices, although in both cases the ZnS layer was obtained by physical vapor deposition techniques. This conclusion is consistent with the above-mentioned observation by Mikami et al. That different structures have different aging properties. Nishikawa or Khormaei did not investigate the effect of insulating layers or spark propagation technology.

Atomic Layer Epitaxy (hereinafter referred to as "ALE") is a process for producing homogeneous well structures by contacting the substrate with evaporated source materials under surface-binding selective conditions. The most commonly used chemical to produce ZnS membranes in the ALE process is ZnCh. According to the generally accepted theory of aging of these devices, Cl-15 residues of ZnS membranes play a key role in the aging of ZnS devices grown on ALE [Neyts, K. and Viljanen, J., Proc. Int. EL Workshop, Beijing, 359-366 (1992); Muller, G. et al., 1988, SID 88 Digest, 23-26. Generally, such Cl residues do not exist on most other ZnS membranes grown, mainly by physical vaporization techniques. One solution to this problem is to use non-chlorinated chemicals for the deposition of ZnS (FI Patent Publication No. 20,100,758).

However, the use of chlorinated zinc source materials in the cultivation of ZnS-based thin film lubricants has its own benefits. In particular, zinc chloride generally produces clear effective films and the production methods used are good. The use of 25 zinc chloride also avoids the safety risks associated with the use of some alternative pyrophoric chemicals in ZnS ALE cultivation.

It is an object of the present invention to eliminate the problems of the state of the art and to provide a new thin film structure with improved stability and symmetry in the light emission of TFEL components. In particular, it is an object of the present invention to improve the stability of EL displays with Cl residues having ZnS fluorescent layers. These include, for example, ZnS fluorescents grown from zinc chloride and volatile sulfide source material by ALE.

Another object of the invention is to provide a thin film structure consisting of a Mn-4 105643 doped ZnS fluorescent layer and an insulating layer of binary oxides of aluminum and titanium.

A third object of the invention is to provide a method for producing novel thin film structures having improved stability.

These and other objects, as well as the advantages derived from the following description with respect to known thin film structures, are achieved by the invention as defined in this specification and the appended claims.

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The invention is based on the idea that it is possible to improve the photometry of the AC thin film electroluminescence contained in a ZnS layer grown on a substrate by atomic layer epitaxy and to increase its luminance stability against aging by increasing at least one layer containing a second metal sulfide, ZnS layer, the layers are formed, with respect to the substrate, only on the opposite side of the ZnS layer.

Surprisingly, and contrary to what both Nishikawa and Khormaei state in the above articles, our results suggest that for existing ALE-EL displays, the inclusion of 20 additional metal (e.g., alkaline earth metal) sulfides between the light emitting phosphor layer and the last grown insulator clearly, whereas fitting a corresponding layer between the lower dielectric and the phosphor layer, either alone or in combination with a second alkaline earth metal sulfide layer to be fitted between the phosphor layer and the upper dielectric, impairs the performance of the device.

Thus, according to the present invention, there is provided a new EL display device which comprises: - first electrodes applied to a substrate, preferably translucent, - a first dielectric layer applied to the first electrodes, - a ZnS layer doped with a suitable with an activator grown by the ALE method on the first dielectric layer: an intermediate layer containing additional metal sulfide on the ZnS layer; 35 - a second dielectric layer on top of the interlayer; and 105643 - second electrodes on the second dielectric layer.

More specifically, the solution according to the invention is characterized by what is stated in the characterizing part of claim 1.

The EL display device, in turn, is characterized by what is stated in the characterizing part of claim 12.

The present invention achieves significant advantages. Thus, the additional sulfur-containing rosin grown between the doped, atomic layer epitaxially produced ZnS fluorescent layer and the most recently grown dielectric layer will largely eliminate the latency problem of the AC thin-film electroluminescence component, particularly in the LV curve in the low-voltage region. .

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The present thin-film structures greatly improve both anti-aging stability and light symmetry, without sacrificing the benefits of current ALE ZnS processes (especially high brightness and high efficiency). These thin film structures can be used in applications that combine both peak load and different levels of luminance.

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The invention will now be further explored by means of a detailed description with reference to the accompanying embodiments.

Fig. 1 is a cross-sectional view of a thin-film electroluminescent device showing a layer structure with an additional metal sulfide-containing intermediate layer according to one embodiment of the invention, Figures 2a and 2b showing luminance-voltage curves for pre-aged and further aged ALE-increased ZnS: for a grown ZnS: Mn device containing an interlayer according to Example 1, Figure 3 shows a latency function as a function of luminance for a conventional ALE-grown ZnS: Mn device and a new ZnS: Mn device including an interlayer according to Example 1; shows differences in light emission symmetry in accelerated aging for the conventional ZnS: Mn device and the present interlayer 35 ZnS: Mn device, 6 105643 Figures 5A and 5B show luminance-voltage curves of a very thin SrS interlayer (lnm), v a layer of (100 nm, 5B) of its own devices, and illustrated in Figure 6 the pre aged for SrS intermediate layers apparatus to the luminance-voltage curves for the different cases: the intermediate layer is either the first or the last grown in 5 phosphor-dielectric layer, or in both or in neither.

The present invention provides a method of growing ZnS: Mn EL thin films. Atomic layer epitaxy (ALE method) is used to grow ZnS: Mn thin films, meaning that the anion and cation precursors are fed in a pulsed sequence 10 so that the surface of the substrate in the reaction chamber is not simultaneously exposed to both source materials. The thin film growth process comprises the steps of growing zinc sulphide layers and subsequent manganese doping steps such that each step is repeated alternately until the desired film strength is achieved. The dielectric layers and the sulfide intermediate layer are also preferably formed by atomic layer septum (ALE). Details of the ALE-15 method are disclosed in U.S. Patent Nos. 4,058,430 and 4,389,973, the contents of which are incorporated herein by reference.

In Figure 1, a sectional view of a thin film EL device according to the invention is numbered as follows: 20 1 Substrate 2 Ion Diffusion Barrier 3 (Optional) 3 First Electrodes 4 First Insulation Layer 5 5 ZnS Fluorescent Film with Suitable Activator / Activators 6 Intermediate Layer 7 Second insulation layer 8 Second electrodes 9 Voltage source 30

Preferably, glass, Al 2 O 3, silicon or a suitable ceramic compound is used as substrate 1. For many applications, it is preferable that the substrate is transparent. The ion diffusion barrier layer 2 is preferably a metal oxide or nitride, e.g. Al 2 O 3. The first electrodes 3 may: be either a transparent material, e.g., indium tin oxide or ZnO, or a suitable metal, such as aluminum or molybdenum, or a metal alloy, such as TiW. The first insulating layer may be metal oxide or nitride or a mixture thereof, such as AlxTiyO2 or SiON, particularly preferred are oxides of aluminum and titanium.

The phosphor layer 5 consists of an ALE-grown ZnS layer preferably utilizing ZnCl2, zinc acetate or diethylene zinc (DEZ), but other volatile zinc concentrates may be used. As mentioned above, in the ALE method, the membrane is grown by exposing the substrate to alternating pulses of two source materials. For example, ZnS can be grown by first exposing the surface to vaporized ZnCh, after which excess material is removed by an inert gas stream. Subsequently, the substrate is subjected to 10 H 2 SO 2 which reacts with ZnCl 2 adsorbed on the substrate to form Zn S and HCl. The process temperature and pressure are selected according to the source material, but typical values are between 200 and 550 ° C and 0.1 and 10 Torr.

ZnS is doped with a suitable activator using a suitable source. As examples of activators or dopants, Mn, Tb and Eu may be mentioned. In the case of Mn, MnCl2 or a manganese β-diketonate compound or other may be used. Other Akti observers may employ suitable source materials such as organometallic compounds.

When the ZnS layer is complete, the buffer layer 6 is grown on the ZnS fluorescent layer. This media layer 20 contains another sulfur-containing metal compound. Typically, an alkaline earth sulfide such as SrS, MgS, CaS, or BaS, or a mixture of two or more alkaline earth sulfides is used for this layer. The most preferred sulfide is strontium sulfide. As explained in more detail in Example 3 below, the latter three sulfides, illustrated in the example with CaS, produce interlayer effects very similar to those of the 25 SrS interlayer, but the low-voltage light emission symmetry is neither as good as the SrS layer nor the luminance voltage curve is not as good as with SrS layer devices.

The thickness of the sulfur-containing layer is preferably 1 to 100 nm, as shown in Example 2 below, although a wider range of about 0.1 to 200 nm is possible.

Oxysulfides and mixtures of sulfides with oxides or nitrides may also be considered. According to a preferred embodiment, the last (i.e., upper) part of the metal sulfide-containing intermediate layer is made of metal oxide. The excess sulfur-containing layer may also consist of a layer structure having separate, alternately alternating layers of metal sulfides and metal oxides. In particular, the metal oxide comprises ALCl and the metal sulfide SrS. Typically, the same layer growth method is used for the interlayer as for the ZnS layer, but not necessarily under the same conditions.

Next, the second dielectric 7 is grown in the same manner as the first dielectric layer, but the dielectrics do not need to be identical in their composition or growth conditions. The second dielectric is formed with second patterned electrodes 8. The second electrodes may be, for example, indium tin oxide, Mo or Al, and may contain the same material as the first electrodes, or the electrodes may be of different material. The electrodes 3, 8 10 are connected to a voltage source 9.

The following non-limiting examples further illustrate the invention.

Example 1 15 Characteristics of a ZnS: Mn Device Using an Addition of SrS and Al 2 O 3 Between the Fluorescent and Upper Insulating Layer Compared to the Properties of ZnS: Mn Devices traditionally used

ZnSiMn TFELs having the structure shown in Fig. 1 were prepared by growing an Al 2 O 3 ion diffusion barrier on a soda lime substrate. The transparent ITO conductor 3 was grown over this layer and patterned by a lithographic process. The A xTiyOz composite dielectric layer 4 was grown on the ITO patterned ALE method. The light emitting ZnSiM fluorescent layer 5 was grown by ALE at 500 ° C using chlorinated source materials, ZnCl 2 and MnCl 2, and FLS. On top of these 25 layers, an SrS-containing intermediate layer was grown at 400 ° C using bis (2,2,6,6-tetramethyl-3,5-heptanedione) strontium (Sr (thd) 2) and FLS. The AlxTiyOz dielectric layer 7 was then grown by ALE method starting from the Al2O3 layer using trimethylaluminum and H2O. Finally, this film stack was grown and patterned with an Al electrode 8.

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The ZnSiMn fluorescent layer was traditionally grown by the ALE method (cf. U.S. Patent 4,058,430). The SrS layer on top of the ZnS.Mn fluorescent layer consisted of 550 alternating Sr (thd) 2 and H2S pulses to provide a strong film of about 25 nm. The interlayer was grown by ALE at 400 ° C and 0.7 torr.

The additional TMA layer of Al2O3 consisted of 400 cycles, which corresponds to approximately 105643 films of 25 nm.

As the ALE device produced by the conventional method gets older, its luminance-voltage curve becomes less steep and LV curve softening can be seen particularly at low luminance levels (near threshold voltage) and at the same high luminance levels

The luminance decreases at a certain voltage (when the device is switched on). The same goes for the new ZnS: Mn. In the case of the traditional ZnS: Mn device, the lowering of the threshold and the lowering of the luminance continue over time. This is not the case for the new ZnS: Mn device with SrS interlayer. After a few hours of accelerated aging, the LV curve 10 reaches its steady-state and the luminance-voltage curve does not change significantly with aging (Figure 2).

As a result of the above-mentioned softening of the luminance-voltage curve following aging, and hence the change in luminosity caused by aging, a latent image is formed which can be seen on the screen after prolonged use. The magnitude of this latent image can be defined as the difference between the pre-aged point luminance and the highly aged point luminance: ((LVanh-Lesivanh) / Lesivanh) * 100. Although high luminance levels remain reasonably constant, the latency image is clearly visible with the traditional ZnS .Mn structure. low luminance levels, and in particular 20 luminance levels near the threshold voltage, as shown in Figure 3. This phenomenon is no longer observed in ZnS: Mn devices with a SrS interlayer. At medium voltages and snow nuances, there is a slight change as a result of long aging, but also threshold thresholds. - nitrous range that high voltage and luminance zones very stable. The above ratio, (LVanh-Lesivanh) / T, eSivanh, is generally less than 0.1 (i.e., the latency of the image is less than 25 10%).

Another advantage achieved by using an SrS interlayer on a ZnS: Mn fluorescent is that the light emission from both pulse polarities is highly symmetrical. In the case of traditional ZnS: the light emission symmetry is good initially, but as the device is used for a longer period, the asymmetry increases to 40%. When the SrS interlayer is used, the luminoussoasymmetry remains below or close to 10% regardless of how long the device has been used (Fig. 4). In general, the light emission asymmetry of the device of the present invention is less than 20% after 20 hours.

This example demonstrates how much better performance is achieved with the new Srn-105643 interlayer ZnS: Mn device compared to the conventional ZnS: Mn device in terms of aged luminance-voltage curve steepness, aging stability, and light emission symmetry. These features can significantly improve the performance of TFEL displays in a variety of demanding applications, as noted in general section 5 of the specification.

Example 2

Sensitivity of New SrS Interlayer ZnS: Mn to Interlayer Strength 10

Several devices were prepared according to the method described in Example 1. The only difference between these devices was the strength of the SrS interlayer. The strength range tested was between 20 and 2200 turns, corresponding to film thicknesses between about 1 and 100 nm.

15 The effect of even a single nanometer-thick SrS layer on LV properties is clear. The lowering of the threshold of the voltage threshold during pre-aging corresponds to the values of traditional ZnS: Mn devices, but as the aging progresses, the threshold voltage remains stable. The softening of the luminance-voltage curves of aged devices resembles the typical softening of traditional ZnS: Mn devices, but is far from co-20 corroded. It seems that the SrS interlayer should preferably be 1 nm stronger (Fig. 5A), although very thin layers of 0.1 to 1 nm already slightly improve the resistance to aging.

t

If the SrS layer is 100 nm, the sharpness of the luminance-voltage curve disappears and the luminance level begins to decrease with the device turned on. However, further increasing the SrS layer (e.g., to about 200 nm) does not significantly reduce the intrinsic stability of the SrS layer-containing ZnS: Mn devices (Fig. 5B).

As shown above, the SrS layer should preferably be between 1 nm and about 100 to 30 nm in order to achieve the better performance shown in Example 1. This is a very large production area which indicates the considerable rigidity of the method used. In practical applications, a particularly preferred range is from three to about fifty nm.

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Example 3

Replacing the SrS intermediate layer with a CaS thin layer

The ZnS: Mn containing CaS interlayer is manufactured as a SrS interlayer • 5 ZnS: Mn as described in Example 1. In this case, the CaS interlayer is grown on bis (2,2,6,6-tetramethylene-3,5-heptanedione) calcium and H 2 SO 5 at 400 ° C and 0.7 Torr using 600 cycles corresponding to a film thickness of 30 nm.

Devices containing CaS interlayer work very similarly to devices in SrS interlayer 10, but at low voltages the light emission symmetry is not as good as

The SrS case and the luminance-voltage curve are more complex, which affects the latent image behavior, which is slightly worse, especially after prolonged aging, than with the SrS device.

This example shows that instead of SrS., CaS can be used in the interlayer to achieve the same luminance-voltage behavior, but the device performance is then somewhat weaker than that of the SrS interlayer ZnS: Mn.

EXAMPLE 4 Changing the Location of the SrS Interlayer with respect to the ZnS: Mn Layer In this example, the processing of the ZnS: Mn layer was carried out as in Example 1 except that. that in the case of A, the SrS layer was grown before the ZnS: Mn phosphor, between the lower dielectric layer 4 and the ZnS.Mn phosphor layer 5. At position B, the SrS layer was grown on both interface surfaces, below and above the ZnS: Mn layer, forming a sandwich structure with the phosphor layer sandwiched between the SrS layers.

As shown in Figure 6, at both A and B, the slope of the luminance-voltage curve is reduced even compared to conventional ZnS: Mn structures. Also, the high voltage luminance is significantly lower.

This example demonstrates that in order to improve the performance of the device described in the above embodiments, it is not necessary to increase the SrS layer before or on either side of the ZnS.Mn fluorescent layer. If the SrS intermediate layer is not on the ZnS.Mn fluorescent layer, .

Claims (22)

105643 A method for improving the luminance-vanity stability of an AC thin-film electroluminescent device grown on a substrate by a suitable activator-doped ZnS layer (5), characterized in that at least one layer (6) is formed adjacent to said ZnS layer (5), containing a second metal sulfide, the layer (s) being formed only on the opposite side of the ZnS layer to the substrate. Method according to Claim 1, characterized in that the layer comprising the second metal sulfide is formed between the ZnS layer (5) and the dielectric layer (7) as an intermediate layer (6). The method according to claim 1, characterized in that the second metal sulfide layer (6) is grown on top of the ZnS layer (5) using atomic layer epitaxy (ALE method). Process according to Claim 3, characterized in that the metal sulfide is MgS, CaS, SrS or BaS or a mixture thereof. 20 Method according to one of Claims 1 to 4, characterized in that the layer (6) comprising the second metal sulfide also contains metal oxide. Process according to Claim 5, characterized in that the layer (6) is formed by alternating growth of the alkali metal sulfide and the metal oxide on the ZnS layer. Process according to Claim 4 or 5, characterized in that the metal oxide is alumina. 30 Process according to claim 7, characterized in that the alumina is grown from metal alkyl precursors. ';' The process according to claim 1, characterized in that *. 35 - forming first electrodes (3) on the transparent substrate (1), forming a dielectric layer (4) on the first electrodes (3), - increasing the ZnS layer (5) activated by an activator by atomic layer epitaxy (ALE) on the dielectric layer (4). An intermediate layer (6) comprising an alkaline earth metal sulphide is grown on the ZnS layer (5), - a second dielectric layer (7) is formed on the intermediate layer (6), and - second electrodes (8) are formed on the second dielectric layer. Method according to one of the preceding claims, characterized in that the ZnS layer (5) is grown from a volatile chloride-containing zinc compound. Method according to one of Claims 1 to 12, characterized in that the ZnS layer (5) is grown from zinc chloride and doped with Mn or Tb. An AC thin-film electroluminescent device, characterized in that it comprises: - first electrodes (3) produced on a transparent substrate (1), - a first dielectric layer (4) formed on the electrodes (3), - ALE method grown on the first dielectric layer (4). , an activator-doped ZnS layer (5), Device according to claim 12, characterized in that the ZnS layer is doped with Mn or Tb. Device according to claim 12, characterized in that the ZnS layer contains Cl residues. 30 Device according to Claim 12, characterized in that the second intermediate layer (6) consists of an alkaline earth metal sulphide which is MgS, CaS, SrS or BaS. . Device according to Claim 12, characterized in that the interlayer (6) has a thickness of 1 to 100 nm. Device according to Claim 12, characterized in that at least a part of the metal sulphide-containing intermediate layer (6) consists of metal oxide. 105643 Device according to Claim 16, characterized in that the metal oxide layer consists of AbCl2. Device according to Claim 17 or 18, characterized in that the second sulfide-containing layer (6) is a layer structure consisting of separate metal sulfide and metal oxide layers. Device according to one of Claims 12 to 19, characterized in that it has at least one of the following properties: improved luminance stability against aging or better light emission symmetry. 15 Apparatus according to claim 20, characterized in that the light emission asymmetry is less than 20% after 20 hours. A second dielectric layer (7) formed on the ZnS layer (5), - an additional dielectric layer (7) formed on the intermediate layer (6), - a second electrode formed on the second dielectric layer (7), wherein the ZnS layer (5) and the intermediate layer (6) are in that order. Device according to claims 20 or 21, characterized in that the ratio (Lvanh-20 Lesivanhj / Lesivanh is less than 0.1. Is 105643