JPH01165165A - switching 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 the Invention] The present invention relates to an MIM device having a polymer insulating layer, and relates to an MIM structure switching device characterized in that the polymer insulating layer has a periodic layer structure.

[Prior art]

Recently, interest in molecular electronics, which seeks to apply the functionality of organic molecules to electronic devices, has increased, and research on Langmuir-Blodgett films (LB films), which is considered to be one of the construction technologies for molecular electronic devices, has become active. ing.

The LB film is made by laminating organic molecules one molecular layer at a time, and the film thickness can be controlled in units of molecular length.
Many attempts have been made to use this as an insulating film because it is possible to form a homogeneous ultra-thin film.For example, (G, L
, Larkins et al Th1nSol
id films 99.1983) Metal-insulator-metal (MIM) structure tunnel junction device (G, L, L
“Electronics Letters J” by Arkinset al.
)'s "Thin Solid Films" (Thin 5
99 (1983)] and light-emitting elements with metal-insulator-semiconductor (Mis) structures [G,
"Electronics Fist J" by Roberts et al.
20, p. 489 (1984)] or switching elements (N., J., Thomas E.
Electronics Letters J (El.
ect'ronics Letters) Volume 20,
838 pages (1984)].

Although device characteristics have been investigated through a series of these studies, the lack of reproducibility and stability, such as variations in characteristics between devices and changes over time, remain unsolved problems.

Conventionally, the above studies have focused on fatty acid LB membranes, which are relatively easy to handle. However, recently, organic materials have been created one after another that overcome heat resistance and mechanical strength, which were previously thought to be inferior.

[Purpose of the invention]

We have conducted extensive research to create MIM devices with excellent reproducibility, stability, and durability using LB films made of these materials as insulators, and have discovered that conventional MIM devices do not.
We have now invented an MIM device that exhibits a completely new switching phenomenon. That is, the present invention has made it possible to provide switching having extremely reliable memory functions.

[Summary of the invention]

The present invention can extend the relatively large π (pi) level to a larger group (
Conventionally known MIM We expected that nonlinear current-voltage characteristics different from those of the device would emerge, and we aimed to realize this. Furthermore, a new MIM element having a switching memory function using such characteristics has been realized.

[Detailed description of aspects of the invention]

Since most organic materials generally exhibit insulating or semi-insulating properties, there is a wide variety of organic materials having a group having a π-electron level that can be applied to the present invention.

Examples of polymeric materials suitable for the present invention include addition polymers such as polyacrylic acid derivatives, condensation polymers such as polyimide, ring-opening polymers such as nylon, and biopolymers such as bacteriorhodopsin.

Regarding the formation of the organic insulating layer, it is possible to specifically apply the vapor deposition method and the cluster ion beam method, but among the known conventional techniques, LB is preferred due to its controllability, ease, and reproducibility
The method is highly preferred.

According to this LB method, a monomolecular film of an organic compound having a hydrophobic site and a hydrophilic site in one molecule or a cumulative film thereof can be easily formed on a substrate, and has a thickness on the order of a molecule. , and can stably supply a uniform and homogeneous ultra-thin organic film over a large area.

The LB method is a molecule with a structure that has a hydrophilic site and a hydrophobic site, and when the balance between the two (amphiphilic balance) is maintained appropriately, the molecule has a hydrophilic group on the water surface. This is a method of creating a monomolecular film or a cumulative film thereof by utilizing the fact that the monomolecular layer forms downward.

Examples of the group constituting the hydrophobic moiety include various hydrophobic groups such as generally widely known saturated and unsaturated hydrocarbon groups, fused polycyclic aromatic groups, and chain polycyclic phenyl groups. Each of these constitutes a hydrophobic portion singly or in combination. On the other hand, the most typical constituent elements of the hydrophilic moiety are, for example, carboxyl groups, ester groups, and acid amide groups.

Imide groups, hydroxyl groups, and even amino groups (1°2.
Examples include hydrophilic groups such as tertiary and quaternary).

These also constitute the hydrophilic portion of the above molecule either singly or in combination.

, : If these hydrophobic groups and hydrophilic groups are present in a well-balanced manner, it is possible to form a monomolecular film on the water surface, making it an extremely suitable material for the present invention.

Specific examples include the following polymers.

[I] Addition polymer l) Polyacrylic acid R.

2) Polyacrylic acid ester 1 3) Acrylic acid copolymer 1 4) Acrylic ester copolymer 5) Polyvinyl acetate 1 OCOCHs 6) Vinyl acetate copolymer R.

[■] Condensation polymer (III) Ring-opening polymer l) Polyethylene oxide Here, R1 corresponds to the group with the above-mentioned σ electron level, and in order to facilitate the formation of a monomolecular film on the water surface. The introduced long-chain alkyl group, the number of carbon atoms n is 5≦n≦
30 is preferred.

Further, R2 is a short-chain alkyl group, and the number of carbon atoms n is 1≦n≦
It is 4. The degree of polymerization m is preferably 100≦m≦5000.

The compounds mentioned above as specific examples are only basic structures,
Needless to say, various substituted forms of these polymers are also suitable in the present invention.

It goes without saying that any polymer material other than those mentioned above is suitable for the present invention as long as it is suitable for the LB method.

For example, biological materials (such as bacteriorhodopsin and cytochrome C) and synthetic polypeptides (such as PBLG), which have been actively researched in recent years, can also be applied. Such amphiphilic molecules form a monomolecular layer on the water surface with the hydrophilic groups facing downward. At this time, the monomolecular layer on the water surface has the characteristics of a two-dimensional system, and when the molecules are sparsely dispersed, the two-dimensional ideal gas equation is expressed between the area A per molecule and the surface pressure π. π A=kT holds true, resulting in a "gas film". Here, k is Boltzmann's constant and T is absolute temperature. If A is made sufficiently small, the intermolecular interaction becomes stronger, resulting in a two-dimensional solid "condensation film (or solid film)." The condensed film can be transferred layer by layer onto the surface of arbitrary objects having various materials and shapes, such as glass and resin. Using this method, a monomolecular film or a cumulative film thereof can be formed and used as an insulating layer having a periodic layer structure for a switching element according to the present invention.

As a specific manufacturing method, for example, the method shown below can be mentioned.

Chloroform and benzene for the desired polymer compound.

Dissolve in a solvent such as acetonitrile or dimethylacetamide. Next, using a suitable apparatus as shown in FIG. 1 of the accompanying drawings, the solution is spread on the aqueous phase 1 to form a film of the organic compound.

Next, a partition plate (or float) 3 is provided to prevent this spread layer from spreading freely on the water phase and spreading too much, and by limiting the spread area, the state of aggregation of the membrane material is controlled, and it is proportional to the state of aggregation. Obtain the surface pressure π. By moving the partition plate 3, the developed area can be reduced to control the aggregation state of the membrane material, and the surface pressure can be gradually increased to set the surface pressure π suitable for membrane production.

While maintaining this surface pressure, the clean substrate 2 is gently raised or lowered vertically to be transferred onto the organic compound monomolecular film substrate 2. Such a monolayer
As schematically shown in Figure a or Figure 2b, it is a film in which molecules are arranged in an orderly manner.

A monomolecular film is produced as described above, and by repeating the above operations, a desired cumulative number of films can be formed. In addition to the above-mentioned vertical dipping method, the monomolecular film can be transferred onto the substrate by methods such as a horizontal deposition method, a 9-turn cylinder method, and the like.

The horizontal deposition method is a method in which a monomolecular film is transferred by bringing the substrate into horizontal contact with the water surface, and the rotating cylinder method is a method in which a cylindrical substrate is rotated above the water surface to transfer the monomolecular film onto the substrate surface. be.

In the vertical immersion method described above, when a substrate with a hydrophilic surface is lifted out of water in a direction across the water surface, a monomolecular film of an organic compound with the hydrophilic groups of the organic compound facing the substrate is formed on the substrate ( Figure 2b).

When the substrate is moved up and down as described above, one monomolecular film is stacked on top of the other at each step, forming a cumulative film. Since the direction of the film-forming molecules is reversed between the pulling process and the dipping process, this method forms a Y-shaped film in which the hydrophobic groups of the organic compound face each other between each layer of the monomolecular film (Figure 3a). ). In contrast, in the horizontal deposition method, a monomolecular film with the hydrophobic groups of the organic compound facing the substrate is formed on the substrate (Figure 2a).
.

In this method, even if monomolecular films are accumulated, there is no change in the direction of the film-forming molecules, and an X-shaped film is formed in which the hydrophobic groups face the substrate in all layers (FIG. 3b).

On the other hand, a cumulative film in which the hydrophilic groups in all layers face the substrate side is called a Z-type film (Figure 3c).

The method of transferring the monomolecular film onto the substrate is not limited to the above method, and when using a large-area substrate, a method of extruding the substrate from a roll into an aqueous phase may also be adopted. Further, the directions of the hydrophilic groups and hydrophobic groups described above toward the substrate are in principle, and can be changed by surface treatment of the substrate, etc.

As described above, a potential barrier layer consisting of a monomolecular film of an organic compound or a cumulative film thereof is formed on the substrate.

In the present invention, the substrate for supporting the thin film in which the above-mentioned inorganic and organic materials are laminated is metal.

Any material such as glass, ceramics, or plastic material may be used, and biomaterials with extremely low heat resistance may also be used.

The above-mentioned substrate may have any shape and is preferably flat, but is not limited to a flat plate.

That is, the film forming method has the advantage that a film can be formed in accordance with the shape of the surface of the substrate, no matter what shape the surface of the substrate has.

Furthermore, according to the LB method, the layer thickness of the insulating layer can be freely controlled on the molecular order, but in the present invention, switching characteristics are expressed in layers with a thickness of several to several thousand layers.
In terms of switching characteristics, it is preferable to have a layer thickness in the range of lOλ to 1000 layers.

On the other hand, the material for the electrodes that sandwich the LB film may be any material as long as it has high conductivity (for example, Au, Pt, etc.).

Ag, Pd, Al4. In, Sn, Pb
There are many materials including metals such as , alloys thereof, graphite, silicide, and even conductive oxides such as ITO, and these materials can be considered to be applied to the present invention. As a method for forming electrodes using such materials, conventionally known thin film techniques are sufficient. Further, as for the electrode material directly formed on the substrate, it is preferable to use a conductive material that does not form an insulating oxide film on the surface when forming the LB film, such as a noble metal or an oxide conductor such as ITO.

A detailed explanation will be given below using examples.

[Example 1] Cr was deposited to a thickness of 500 mm as an undercoat layer by vacuum evaporation on a glass substrate (#7059 manufactured by Corning Inc.) that had been hydrophobically treated by leaving it in saturated vapor of hexamethyldisilazane (HMDS) overnight. Then, Au was further deposited by the same method (
A striped base electrode with a thickness of 1 mm and a width of 1 mm was formed. Details of a method for forming a monomolecular film of polyimide by the LB method using such a substrate as a carrier will be described.

Polyamic acid (molecular weight approximately 200,000) at a concentration of I x 10
-''% (vol/vol) dimethylacetamide solution was spread on an aqueous phase of pure water at a water temperature of 20°C to form a monomolecular film on the water surface.The surface pressure of this monomolecular film was set to 25%.
mN/m, and further immersing the substrate at 5 mm/min in the direction across the water surface while keeping this constant;
Pulling was performed to accumulate a Y-type monomolecular film. By repeating such operations, 12. 18.24°30,
36. Six types of cumulative films with 40 layers were created.

Further, these films were heated at 300° C. for 10 minutes to form polyimide.

Next, a striped AI with a width of 1 mm is formed on the film surface so as to be perpendicular to the base electrode! An electrode (film thickness: 1,500 yen) was vacuum deposited while keeping the substrate temperature below room temperature to form the upper electrode.

The current characteristics (Vl characteristics) when a voltage was applied between the upper and lower electrodes of the sample prepared as described above were measured. In other samples, previously unknown memory switching characteristics were observed (Figure 5).

Furthermore, a stable ON state (resistance value of several tens of Ω) as shown in Figure 6
) and the OFF state (resistance value MΩ or more), switching from 0N to 4OFF shows a constant voltage (about 1 to 2V, 720 layers), and switching from OFF to ON shows a voltage of -2 This occurred at about 5 V, and the switching speed was 100 nsec or less, and the 0N10FF ratio (ratio of resistance values in ON state and OFF state) was more than 5 digits.

The switching threshold voltage tended to increase as the number of insulating layers increased.

The thickness of each polyimide layer was determined to be about 3.6 by ellipsometry.

[Example 2] A monomolecular film of poly-α-n-hexadecyl acrylic acid was deposited by the LB method using as a carrier a substrate in which ITO was etched into stripes of 1 mm width by a conventionally known method. Poly-α-n-hexadecyl acrylic acid (molecular weight: 10,000) was prepared at a concentration of I x 10-3% (vo I
/ v o I) in pure water.

It was developed on a water phase with a water temperature of 20°C, and while keeping this temperature constant, the substrate was spread at a rate of 5 mm/mi in the direction across the water surface.
Dipping and pulling was performed at n, and a Y-type monomolecular film was accumulated. By repeating such operations, 7. 11°15,
Four types of cumulative films with 21 layers were created.

The Vl characteristics of the samples prepared as described above were measured in the same manner as in Example 1, and as a result, the memory switching characteristics of all the samples prepared were observed (FIG. 5).

The threshold voltage remained almost constant regardless of the difference in the upper electrode.

Further, the switching speed was 1 oonsec.

[Example 3] Cr was deposited to a thickness of 500 mm as an undercoat layer by vacuum evaporation on a glass substrate (#7059 manufactured by Corning) that had been hydrophobically treated by being left in saturated vapor of hexamethyldisilazane (HMDS) overnight. Then, Au was further deposited by the same method (
A strip-shaped base electrode with a width of 1 mm was formed. Details of a method for forming a monomolecular film of poly-n-hexyl methacrylic acid (molecular weight approximately 500,000) by the LB method using such a substrate as a carrier will be described.

A benzene solution containing poly-〇-hexyl methacrylic acid (molecular weight approximately 500,000) dissolved at a concentration I x 10-3% (vol/vol) is spread on a water phase of pure water at a water temperature of 20°C, and poured onto the water surface. A monolayer was formed. The surface pressure of this monomolecular film is 1
5 m N/m, and while keeping this constant, the substrate was further increased to 3 mm/mi in the direction across the water surface.
A Y-type monomolecular film was accumulated by dipping and pulling up at n. By repeating such operations, 12, 18.24
．． Six types of cumulative films with 30, 36, and 42 layers were created.

The VI characteristics of the samples prepared as described above were measured in the same manner as in Example 1, and as a result, the memory switching characteristics of all the samples prepared were observed (FIG. 5).

The threshold voltage remained almost constant regardless of the difference in the upper electrode.

Moreover, the switching speed was 100 nsec.

In the embodiments described above, the LB method has been used to form the insulating layer, but any film forming method that can form an extremely thin (uniform insulating) organic thin film can be used other than the LB method. Specifically, vacuum evaporation methods, electrolytic polymerization methods, CVD methods, etc. can be mentioned, and the range of usable organic materials is expanded.

Regarding the formation of the electrode, as already mentioned, any film forming method that can form a uniform thin film on the organic thin film layer can be used, and is not limited to vacuum evaporation or sputtering.

Furthermore, the present invention does not limit the substrate material or its shape in any way.

[Effects of the present invention]

(2) It has been shown that an MIM structure device with an insulating layer made of a thin film of polymer or monomolecular film deposited by the LB method can provide memory-like switching characteristics not seen in conventional MIM devices.

■ Because the insulating layer is formed by accumulating monomolecular films, film thickness control can be easily achieved on the molecular order (several to dozens of people). Furthermore, since the controllability is excellent, the reproducibility and productivity are high when forming elements.

■ It has a higher degree of material freedom than MIM elements made of only inorganic materials, and in the future it will be possible to provide devices with high affinity with living organisms, such as molecular electronics and ■ bioelectronics.

■ Mechanical strength and
Excellent heat resistance and solvent resistance.

[Brief explanation of the drawing]

FIG. 1 is an explanatory view schematically showing a method of forming an organic dye insulating layer of the present invention by the LB method. Figures 2a and 2
Figure b is a schematic diagram of a monomolecular film, and Figures 3a, 3b, and 3c are schematic diagrams of a cumulative film. FIG. 4 shows a schematic configuration diagram of a specific example of the MIM element of the present invention. In addition, Fig. 5 is a characteristic diagram showing the electrical characteristics (Vl characteristics) obtained in the device, and Fig. 6 is a diagram showing the electrical characteristics in the ON state and OFF state confirmed in the device. It is something. l・・・・・・・・・・・・・・・Aqueous phase 2...
・・・・・・・・・・・・・・・ Board 3・・・・・・・
・・・・・・・・・・・・Float 4・・・・・・・・・・・・
...Monolayer film 5...
・Cumulative film 6・・・・・・・・・・・・Hydrophilic part 7
・・・・・・・・・・・・・・・Hydrophobic part 8・・・・・・
・・・・・・・・・Top electrode 9・・・・・・・・・・
・Lower (underlying) electrode lO... Monomolecular cumulative film layer (
LB film layer) Figure 5

Claims (2)

[Claims] (1) A switching element having a periodic layer structure of an organic insulator between a pair of electrodes, and having a memory property for switching characteristics. (2) The switching element according to claim 1, wherein the organic insulator is a polymer compound having a group having a π electron level and a group having only a σ electron level.