JP2004247529A - Switching element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching element that has an organic bistable material between two electrodes a greater ON/OFF current ratio with a small leakage current. <P>SOLUTION: In the switching element, the organic bistable material is placed at least between two electrodes. The organic bistable material has two kinds of stable resistance values with respect to an applied voltage, and employs an insulating substrate the side of which in contact with the electrodes uses an organic substance such as a glass substrate coated with an organic substance or an organic group high polymer film. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

また、図３の（ａ）、（ｂ）及び（ｃ）には、それぞれ、実施例１および従来例のスイッチング素子についての原子間力顕微鏡で観察した有機膜の表面形状を示す。
図３（ａ）に示す有機膜の表面形状写真から明らかなように、実施例１のポリビニルアルコール膜上の有機膜は、アルミ電極上の有機膜と遜色のない、緻密な膜となっている事がわかる。これに対し、（ｃ）の従来例では、ガラス基板上の有機膜は非常に大きな粒径となっており、その結果粒界は非常に疎になっている事がわかる。
【００２２】
さらに、それぞれ実施例及び従来例のスイッチング素子についての電流‐電圧特性を示す図４、５の結果より、実施例１及び従来例のスイッチング素子いずれにおいても高抵抗状態７１、８１、及び低抵抗状態７２、８２と双安定性が得られているが、高抵抗状態における電流値（ｏｆｆ電流）の値は明らかに本発明による実施例１の方が小さく抑えられている事がわかる。
すなわち、図４の実施例１において、高抵抗状態７１ではｏｆｆ電流（高抵抗状態での電流）は３．９×１０^−８Ａ／ｃｍ^２程度となっているが、Ｖｔｈ１において高抵抗状態７１から低抵抗状態７２へ遷移し、低抵抗状態におけるｏｎ電流（低抵抗状態での電流）は１．１×１０^−４Ａ／ｃｍ^２となっている。この際の低抵抗状態／高抵抗状態の比として、約１０^３が得られた。
【００２３】
これに対し従来例では、高抵抗状態８１から低抵抗状態へと双安定性が得られているものの、ｏｆｆ電流（高抵抗状態での電流）は９．０×１０^−６Ａ／ｃｍ^２程度と実施例１と比較しても２桁大きく、このために低抵抗状態／高抵抗状態の比も約１０と大変小さくなっている。
また、実施例１〜４のすべてのスイッチング素子において双安定性は得られており、表１に示すようにｏｆｆ電流はおおよそ（４〜１３）×１０^−８Ａ／ｃｍ^２の範囲であり従来例と比較して約２桁小さくなっている。
【００２４】
【発明の効果】
以上説明したように、本発明によれば、単一の分子組成においてドナーとアクセプタの両方の機能を持った有機双安定材料を用いたスイッチング素子においても、絶縁性基板と金属電極との境界に存在する有機膜の粒界に起因した漏れ電流を低減させることができ、オン電流とオフ電流との比が大きなスイッチング素子を得る事ができる。したがって、このスイッチング素子は、有機ＥＬディスプレーパネルの駆動用スイッチング素子や、高密度メモリ等に好適に利用できる。
【図面の簡単な説明】
【図１】本発明のスイッチング素子の一実施形態を示す概略構成図
【図２】本発明のスイッチング素子の他の実施形態を示す概略構成図
【図３】実施例１、および従来例におけるスイッチング素子の有機膜表面の原子間力顕微鏡写真
【図４】実施例１におけるスイッチング素子の電流−電圧特性を示す図
【図５】従来例におけるスイッチング素子の電流−電圧特性を示す図
【図６】従来のスイッチング素子の電圧−電流特性の概念図
【符号の説明】
１０：基板
２１ａ、２１ｂ：電極層
２２：第３電極層
３０：双安定材料層
５１、７１、８１：高抵抗状態
５２、７２、８２：低抵抗状態
Ｖｔｈ１：低閾値電圧
Ｖｔｈ２：高閾値電圧[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a switching element for driving an organic EL display panel, and a switching element used for a high-density memory or the like, in which an organic bistable material is disposed between two electrodes.
[0002]
[Prior art]
In recent years, the properties of organic electronic materials have made remarkable progress. In particular, some low-dimensional conductors such as charge-transfer complexes have characteristic properties such as metal-insulator transition, and their application to switching elements for driving organic EL display panels and high-density memories has been studied. I have.
As a material applicable to the above-described switching element, an organic bistable material has been attracting attention. An organic bistable material is an organic material exhibiting a so-called non-linear response in which, when a voltage is applied to a material, a current of a circuit rapidly increases at a certain voltage or more and a switching phenomenon is observed.
[0003]
FIG. 6 shows an example of the voltage-current characteristics of the organic bistable material exhibiting the switching behavior as described above.
As shown in FIG. 6, the organic bistable material has two current-voltage characteristics, a high resistance characteristic 51 (off state) and a low resistance characteristic 52 (on state). When the voltage is Vth2 or more in the applied state, the state transits from the off state to the on state, and when the voltage is Vth1 or less, the state transits from the on state to the off state and the resistance value changes, and has a nonlinear response characteristic. . That is, a so-called switching operation can be performed by applying a voltage of Vth2 or higher or Vth1 or lower to this organic bistable material. Here, Vth1 and Vth2 can be applied as pulsed voltages.
[0004]
Various organic complexes are known as organic bistable materials exhibiting such a nonlinear response. For example, R. S. Have prototyped a switching element having two stable resistance values with respect to voltage using a Cu-TCNQ (copper-tetracyanoquinodimethane) complex (Non-Patent Document 1).
Kumai et al. Have observed switching behavior due to non-linear response using a single crystal of a K-TCNQ (potassium-tetracyanoquinodimethane) complex (Non-Patent Document 2).
Furthermore, Adachi et al. Formed a Cu-TCNQ complex thin film using a vacuum deposition method, clarified the switching characteristics thereof, and studied the applicability to an organic EL matrix (Non-Patent Document 3).
[0005]
Recently, in contrast to the above-described two-component charge transfer complex, Yang et al. Disclose a very thin Al film with a thin film of 2-amino-4,5-imidazole-dicarbonitrile to form a single component. A switching characteristic similar to that of the above-described charge transfer complex is obtained by the organic film (Non-Patent Document 4).
In this configuration, since each is a thin film composed of a single component, the composition controllability is remarkably improved as compared with a conventional two-component charge transfer complex.
In addition, as a result of intensive studies, we have obtained switching characteristics similar to those of the above charge transfer complex with an organic film using only a single-component organic material without using a thin Al film (see Japanese Patent Application Laid-Open No. H10-163,873). 2002-271911).
[0006]
[Non-patent document 1]
R. S. Potember, 1 other, Appl. Phys. Lett. 34 (6), 15 March 1979, p. 405-407
[Non-patent document 2]
Reiji Kumai and 2 others, "Solid State Physics", vol. 35, No. 1 (2000), P.K. 33-40
[Non-Patent Document 3]
Chihaya Adachi and 3 others, Proceedings of the Japan Society of Applied Physics, Spring 2002, 3rd volume, p. 1236
[Non-patent document 4]
Yang Yang, 3 others, Appl. Phys. Lett. , Vol. 80, no. 3, (21 / Jan / 2002) p. 362-364
[0007]
[Problems to be solved by the invention]
However, a switching element using an organic material composed of a single molecule as described above also has the following problems.
That is, when applied to an organic EL display panel or a memory, the above bistable material is usually used in a simple matrix configuration. In such a simple matrix configuration, an insulating substrate (usually, a glass substrate is used) and an electrode alternately appear as bases, so the organic bistable material is deposited on two different substances. .
[0008]
Here, since the organic bistable material is a molecule having both an electron donating group and an electron accepting group and having a relatively small molecular weight, the organic bistable material has a feature that it is easy to grow in a granular form. Although a dense film is formed, there is a feature that large grains are easily formed, particularly on a substrate having a flat and amorphous structure such as glass.
For this reason, in the conventional simple matrix configuration, since the organic bistable film on the glass substrate forms large grains, a large leakage current due to the grain boundary of the organic bistable film on the glass substrate adjacent to the electrode is generated. This causes a problem that the ratio of the on-current to the off-current of the switching becomes very small because the off-current increases.
[0009]
SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems of the related art, and has as its object to obtain a switching element having a small leakage current and a large ratio between an on-state current and an off-state current.
[0010]
[Means for Solving the Problems]
That is, the switching element of the present invention includes an organic bistable material having two kinds of stable resistance values with respect to an applied voltage between an electrode disposed on an insulating substrate and another electrode. In the switching element, the surface of the insulating substrate in contact with the electrode is made of an organic material.
Further, as the insulating substrate of the switching element of the present invention, glass having an organic substance coated on a surface in contact with an electrode, or an organic polymer film can be used.
[0011]
Further, the organic bistable material preferably contains at least a compound having an electron donating functional group and an electron accepting functional group in one molecule.
According to the switching element of the present invention, the organic bistable material is formed on the organic substance of the electrode, without forming large grains on the insulating substrate as in the related art, and the insulating substrate, metal A dense film can be formed with small particles on both electrodes.
For this reason, it is possible to reduce the leakage current caused by the grain boundary of the organic film existing at the boundary between the insulating substrate and the metal electrode, and to obtain a switching element having a large ratio between the on-current and the off-current. .
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram showing one embodiment of the switching element of the present invention.
As shown in FIG. 1, the switching element has a configuration in which an electrode layer 21a, a bistable material layer 30, and an electrode layer 21b are sequentially stacked on a substrate 10.
In the present invention, the surface of the substrate 10 in contact with the electrode 21a is made of an organic material. For this purpose, glass coated with an organic material, an organic polymer film, or the like is used.
[0013]
As the electrode layers 21a and 21b, a metal material such as aluminum, gold, silver, nickel, and iron; an inorganic material such as ITO and carbon; an organic material such as a conjugated organic material, a liquid crystal; and a semiconductor material such as silicon are appropriately used. It is selectable and is not particularly limited.
For the bistable material layer 30, a compound containing at least a compound having an electron-donating functional group and an electron-accepting functional group in one molecule is used. As such a compound, an aminoimidazole compound, Dicyano compounds, pyridone compounds, imine compounds, quinone compounds and the like.
Preferably, the electrode layer 21a, the bistable material layer 30, and the electrode layer 21b are sequentially formed as thin films on the substrate 10. As a method of forming a thin film, a conventionally known method such as a vacuum deposition method is preferably used for the electrode layers 21a and 21b, and is not particularly limited.
[0014]
As a method for forming the bistable material layer 30, a method for producing an organic thin film such as a spin coating method, an electrolytic polymerization method, a chemical vapor deposition method (CVD method), and a monomolecular film accumulation method (LB method) is used, and is particularly limited. Although not required, it is preferable to use a vacuum evaporation method that can use the same film formation method as the above-described electrode layer.
The substrate temperature at the time of vapor deposition is appropriately selected depending on the electrode material and the bistable material to be used. However, in forming the electrode layers 21a and 21b, 0 to 150? Preferably, in the formation of the bistable material layer 30, 0-100? Is preferred.
The thickness of each layer is preferably 50 to 200 nm for the electrode layers 21 a and 21 b, and 20 to 150 nm for the bistable material layer 30.
[0015]
In the switching element of the present invention obtained by the above-described manufacturing method, the organic bistable material forms a film having a small particle size without forming a large particle, so that a dense film can be obtained over the entire film.
As a result, even in a switching element using an organic bistable material having both functions of a donor and an acceptor in a single molecular composition, it is caused by a grain boundary of an organic film present at a boundary between an insulating substrate and a metal electrode. Leakage current can be reduced, and a switching element having a large ratio of on-current to off-current can be obtained.
FIG. 2 shows another embodiment of the switching element of the present invention. This embodiment is different from the above-described embodiment of FIG. 1 in that a three-terminal element in which a third electrode 22 is further provided in a bistable material layer 30 is provided. Thereby, the bias Vb in FIG. 6 described above is applied by using the electrode layers 21 a and 21 b as electrodes through which an additional current flows, and the third electrode 22 is used as an electrode for controlling the resistance state of the bistable material layer 30. 6, the low threshold voltage Vth1 or the high threshold voltage Vth2 can be applied.
[0016]
【Example】
Hereinafter, the switching element of the present invention will be described in more detail using examples.
Example 1
By the following procedure, a switching element having a configuration as shown in FIG. 1 was created.
A substrate having a polyvinyl alcohol film formed on the surface of glass by spin coating is used as the substrate 10, and aluminum is used as the electrode layer 21 a and 2-amino-4,5-imidazole-diethyl is used as the bistable material layer 30 by vacuum evaporation. A thin film was successively formed from carbononitrile and aluminum as the electrode layer 21b to form a switching element of Example 1.
[0017]
Note that the polyvinyl alcohol film was formed by applying a solution in which 3 wt% was dissolved in a solvent using water as a solvent so as to have a thickness of 300 nm by a spin coating method, and dried at 120 ° C. for 1 hour.
The electrode layer 21a, the bistable material layer 30, and the electrode layer 21b were formed to have thicknesses of 100 nm, 80 nm, and 100 nm, respectively. In addition, the deposition apparatus was evacuated with a diffusion pump to a degree of vacuum of 3 × 10 ⁻⁶ torr. The deposition rate of aluminum was 3 A / sec by a resistance heating method, and the deposition rate of 2-amino-4,5-imidazole-dicarbonitrile was 2 A / sec by a resistance heating method. . The vapor deposition of each layer was continuously performed by the same vapor deposition apparatus, and was performed under the condition that the sample did not come into contact with air during the vapor deposition.
[0018]
Example 2
The polyvinyl alcohol film is coated so as to have a thickness of 100 nm, and the thickness of each vapor deposition layer is set so that the electrode layer 21a, the bistable material layer 30, and the electrode layer 21b have a thickness of 100 nm, 80 nm, and 100 nm, respectively. A film was formed under the same conditions as in Example 1 to obtain a switching element of Example 2.
Example 3
As a substrate 10, a substrate in which a polystyrene film is formed on a glass surface by a spin coating method is used, and the thickness of each vapor deposition layer is set such that the electrode layer 21a, the bistable material layer 30, and the electrode layer 21b have a thickness of 100 nm, 80 nm, and 100 nm, respectively. A film was formed under the same conditions as in Example 1 so as to have a thickness, and a switching element of Example 3 was obtained.
[0019]
The polystyrene film was prepared by applying a solution prepared by dissolving 3% by weight in a solvent using dichloromethane as a solvent so as to have a thickness of 300 nm by a spin coating method, and dried at 90 ° C. for 1 hour.
Example 4
A polyimide film (Kapton film: manufactured by Du Pont-Toray Co., Ltd.) was used as the substrate 10, and the thickness of each vapor-deposited layer was set to 100 nm, 80 nm, and 100 nm, respectively, for the electrode layer 21a, the bistable material layer 30, and the electrode layer 21b. Thus, a film was formed under the same conditions as in Example 1 to obtain a switching element of Example 4.
[0020]
For comparison, a glass substrate was used as the substrate 10, and the thickness of each deposition layer was adjusted so that the electrode layer 21a, the bistable material layer 30, and the electrode layer 21b had a thickness of 100 nm, 80 nm, and 100 nm, respectively. A switching element of a conventional example formed under the same conditions as in Example 1 was also prepared.
Test Example Regarding the switching elements of Examples 1 to 4 and the conventional example, current-voltage characteristics were measured at room temperature using a matrix electrode, and a transition from a high resistance state to a low resistance state was observed. At this time, the current value (on-current) Ion in the low-resistance state and the current value (OFF) in the high-resistance state at the threshold voltage Vth2 for transition from the high-resistance state to the low-resistance state and the bias voltage Vb in FIG. Table 1 summarizes the measurement results of the current (Ioff). As a measurement condition, an electric resistance in a range of 1 MΩ was connected in series to each switching element, and a current in an ON state was limited to suppress damage to the element due to an overcurrent. Further, a voltage half of the threshold voltage Vth2 was used as Vb.
[0021]
[Table 1]

FIGS. 3A, 3B, and 3C show the surface shapes of the organic films of the switching elements of Example 1 and the conventional example, respectively, observed by an atomic force microscope.
As is clear from the photograph of the surface shape of the organic film shown in FIG. 3A, the organic film on the polyvinyl alcohol film of Example 1 is a dense film comparable to the organic film on the aluminum electrode. I understand that. On the other hand, in the conventional example (c), it can be seen that the organic film on the glass substrate has a very large particle size, and as a result, the grain boundaries are very sparse.
[0022]
Further, from the results of FIGS. 4 and 5 showing the current-voltage characteristics of the switching element of the embodiment and the conventional example, respectively, the high resistance states 71 and 81 and the low resistance state of each of the switching elements of the embodiment 1 and the conventional example are shown. Although the bistability is obtained as 72 and 82, it is apparent that the value of the current value (off current) in the high resistance state is clearly smaller in Example 1 according to the present invention.
That is, in the first embodiment of FIG. 4, the off current (current in the high resistance state) is about 3.9 × 10 ⁻⁸ A / cm ² in the high resistance state 71, but the high resistance state 71 in Vth1. To the low-resistance state 72, and the on-current (current in the low-resistance state) in the low-resistance state is 1.1 × 10 ⁻⁴ A / cm ² . As the ratio of the low-resistance state / high-resistance state at this time, about ¹⁰³ were obtained.
[0023]
On the other hand, in the conventional example, although the bistability is obtained from the high resistance state 81 to the low resistance state, the off current (current in the high resistance state) is about 9.0 × 10 ⁻⁶ A / cm ^2. In comparison with Example 1, the ratio is two orders of magnitude larger, and therefore, the ratio of the low resistance state / high resistance state is very small, about 10.
Bistability was obtained in all the switching elements of Examples 1 to 4. As shown in Table 1, the off current was approximately in the range of (4 to 13) × 10 ⁻⁸ A / cm ^2. It is about two orders of magnitude smaller than the example.
[0024]
【The invention's effect】
As described above, according to the present invention, even in a switching element using an organic bistable material having both functions of a donor and an acceptor in a single molecular composition, even at a boundary between an insulating substrate and a metal electrode, It is possible to reduce a leakage current caused by a grain boundary of an existing organic film, and to obtain a switching element having a large ratio between an on-state current and an off-state current. Therefore, the switching element can be suitably used for a switching element for driving an organic EL display panel, a high-density memory, and the like.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing one embodiment of a switching element of the present invention. FIG. 2 is a schematic configuration diagram showing another embodiment of a switching element of the present invention. FIG. 3 Switching in Example 1 and a conventional example. Atomic force micrograph of the organic film surface of the device. FIG. 4 is a diagram showing current-voltage characteristics of a switching device in Example 1. FIG. 5 is a diagram showing current-voltage characteristics of a switching device in a conventional example. Conceptual diagram of voltage-current characteristics of conventional switching element [Explanation of symbols]
10: substrate 21a, 21b: electrode layer 22: third electrode layer 30: bistable material layer 51, 71, 81: high resistance state 52, 72, 82: low resistance state Vth1: low threshold voltage Vth2: high threshold voltage

Claims (4)

In a switching element, an organic bistable material having two kinds of stable resistance values with respect to an applied voltage is disposed between an electrode disposed on an insulating substrate and another electrode. A switching element characterized in that the surface of the conductive substrate in contact with the electrode is made of an organic substance. 2. The switching element according to claim 1, wherein the insulating substrate is glass having a surface in contact with an electrode coated with an organic substance. 2. The switching element according to claim 1, wherein the insulating substrate is an organic polymer film. The organic bistable material according to any one of claims 1 to 3, wherein at least one compound having an electron-donating functional group and an electron-accepting functional group is contained in one molecule. Switching element.

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