JP5160078B2 - Field effect transistor - Google Patents

Description

  The present invention relates to a field effect transistor. More specifically, the present invention relates to a field effect transistor using a specific organic heterocyclic compound as a semiconductor material.

  A field effect transistor generally has a structure in which a semiconductor material on a substrate is provided with a source electrode, a drain electrode, and a gate electrode or the like through these electrodes and an insulator layer. Besides being used, it is also widely used for switching elements. Currently, inorganic semiconductor materials centering on silicon are used for field effect transistors, and thin film transistors formed on a substrate such as glass using amorphous silicon are used for displays and the like. When such an inorganic semiconductor material is used, it is necessary to process the field effect transistor at a high temperature or in a vacuum, and the cost is very high because it requires expensive equipment investment and a lot of energy for manufacturing. Become. In these, since the substrate is exposed to a high temperature during the production of the field effect transistor, a substrate having insufficient heat resistance such as a film or plastic cannot be used as the substrate, and its application range is limited.

On the other hand, research and development of field effect transistors using organic semiconductor materials that do not require high-temperature processing during the manufacture of field effect transistors have been conducted. By using an organic material, manufacturing by a low temperature process becomes possible, and the range of substrate materials that can be used is expanded. As a result, it has become possible to produce a field effect transistor that is more flexible, lighter, and less likely to break. Further, in the field effect transistor creation process, a large area field effect transistor may be manufactured at a low cost by employing a technique such as solution coating or ink jet printing. In addition, various compounds can be selected as the compound for the organic semiconductor material, and an expression of an unprecedented function utilizing the characteristics is expected.
As an example of using an organic compound as a semiconductor material, various studies have been made so far. For example, a material using pentacene, thiophene, or an oligomer or polymer thereof is already known as a material having a hole transport property. (See Patent Documents 1 and 2). Pentacene is an acene-based aromatic hydrocarbon in which five benzene rings are linearly condensed. A field effect transistor using this as a semiconductor material has a charge mobility comparable to amorphous silicon currently in practical use. It has been reported to show (carrier mobility). However, its performance is greatly influenced by the purity of the compound, and further, its purification is difficult, and the production cost is high for use as a transistor material. Furthermore, a field effect transistor using this compound is deteriorated due to the environment and has a problem in stability. In addition, when thiophene compounds are used, there are similar problems, and it is difficult to say that the materials are highly practical.

On the other hand, a compound in which X ₁ and X ₂ are both sulfur atoms in the structure of the formula (1) described later has been described in the literature, but no application using the compound has been carried out (Non-patent Document 1). And 2). In addition, there has been no report on derivatives in which the analogs X ₁ and X ₂ are selenium atoms.

JP 2001-94107 A JP-A-6-177380 J. et al. Sci. Industr. Res. , Vol. 17B (1958), 260 J. et al. Chem. Soc. Perkin I, (1973) 1099 Org. Lett. , Vol. 2, no. 1,2000 J. et al. Am. Chem. Soc. 2004, 126, 1338

  An object of the present invention is to provide a field effect transistor having excellent carrier mobility and excellent stability using an organic compound as a semiconductor material.

  As a result of intensive studies to solve the above problems, the present inventors have demonstrated excellent carrier mobility and excellent stability by using a heterocyclic compound having a specific structure as a semiconductor material. Has been found, and the present invention has been completed.

That is, the configuration of the present invention is as follows.
[1] A field effect transistor using a compound represented by the following formula (1) as a semiconductor material.

(In Formula (1), X ₁ and X ₂ each independently represent a sulfur atom or a selenium atom, and R ₁ and R ₂ each independently represent a hydrogen atom, a halogen atom, or an optionally substituted aliphatic hydrocarbon group. Represents.)
[2] A compound represented by the following formula (2).

(In formula (2), R ₁ and R ₂ each independently represent a hydrogen atom, a halogen atom, or an optionally substituted aliphatic hydrocarbon group.)

The structure of the formula (2) is included in the definition of the formula (1). That is, in the definition of the formula (1), the compound represented as one of options regarding X ₁ and X ₂ is the formula (2).

  By using the specific heterocyclic compound represented by the formula (1), an organic field effect transistor having excellent carrier mobility and excellent stability could be provided.

The present invention will be described in detail.
The present invention is an organic field effect transistor using a specific organic compound as a semiconductor material, and the compound represented by the formula (1) is used as the organic compound. First, the compound of formula (1) will be described.

X ₁ and X ₂ are each independently a sulfur atom or a selenium atom. R ₁ and R ₂ each independently represent a hydrogen atom, a halogen atom or an optionally substituted aliphatic hydrocarbon group. Preferred as R ₁ and / or R ₂ is a hydrogen atom.

In the above, preferred examples of the halogen atom in R ₁ and R ₂ include fluorine, chlorine, bromine and iodine, more preferably fluorine, chlorine and bromine, and still more preferably fluorine and bromine.
Examples of the aliphatic hydrocarbon group of the aliphatic hydrocarbon group which may be substituted include a saturated or unsaturated linear, branched or cyclic aliphatic hydrocarbon group, and the number of carbon atoms is preferably 1 to 20. . Here, examples of the saturated or unsaturated linear or branched aliphatic hydrocarbon group include, for example, methyl group, ethyl group, propyl group, isopropyl group, n-butyl group, iso-butyl group, allyl group, t -Butyl group, n-pentyl group, n-hexyl group, n-octyl group, n-decyl group, n-dodecyl group, n-stearyl group, n-butenyl group and the like can be mentioned. Examples of the cyclic aliphatic hydrocarbon group include cycloalkyl groups having 3 to 12 carbon atoms such as a cyclohexyl group, a cyclopentyl group, an adamantyl group, and a norbornyl group.

Examples of the substituent that the aliphatic hydrocarbon group of R ₁ and R ₂ which may be substituted can have are not particularly limited, but for example, a halogen atom, a hydroxyl group, a mercapto group, a nitro group, an alkoxyl group , Alkyl-substituted amino group, aryl-substituted amino group, unsubstituted amino group, aryl group, acyl group and the like.

  The compound represented by the formula (1) can be synthesized by a known method disclosed in Non-Patent Documents 1 and 2. In addition, in order to obtain a highly pure compound, for example, as shown in the following reaction formula, the desired product can be obtained through 6 steps from 1,4-dibromo-2,5-diiodobenzene (3) (non-patent document). References 3 and 4).

In the above reaction formula, as described above, X ₁ and X ₂ each independently represents an oxygen atom or a sulfur atom. Further, the compound having the substituents R ₁ and R ₂ of this compound is obtained by a known method of Non-Patent Document 2, or after obtaining a derivative having a halogen group as described in Non-Patent Document 3. It can be obtained through a coupling reaction.
The purification method of the compound represented by the above formula (1) is not particularly limited, and known methods such as recrystallization, column chromatography, and vacuum sublimation purification can be employed. Moreover, you may use these methods in combination as needed.

  Next, specific examples of the compound represented by the formula (1) are shown. First, Table 1 shows examples (compound No. 100 to compound No. 140) of compounds represented by the following formula (15) among the compounds represented by the formula (1). In Table 1, the phenyl group is abbreviated as Ph.

  Specific examples (Compound No. 150 to Compound No. 159) other than the compound represented by Formula (15) among the compounds represented by Formula (1) are shown below.

  A field effect transistor (hereinafter sometimes abbreviated as FET) of the present invention has two electrodes (source electrode and drain electrode) in contact with a semiconductor, and a current flowing between the electrodes, It is controlled by a voltage applied to another electrode called.

  In general, a field-effect transistor often has a structure in which a gate electrode is insulated by an insulating film (Metal-Insulator-Semiconductor: MIS structure). An insulating film using a metal oxide film is called a MOS structure. In addition, there is a structure (MES) in which a gate electrode is formed through a Schottky barrier, but in the case of an FET using an organic semiconductor material, a MIS structure is often used.

Hereinafter, the organic field effect transistor according to the present invention will be described in more detail with reference to the drawings. However, the present invention is not limited to these structures.
FIG. 1 shows some examples of the field effect transistor (element) of the present invention. In each example, 1 represents a source electrode, 2 represents a semiconductor layer, 3 represents a drain electrode, 4 represents an insulator layer, 5 represents a gate electrode, and 6 represents a substrate. In addition, arrangement | positioning of each layer and an electrode can be suitably selected according to the use of an element. A to D are called lateral FETs because a current flows in a direction parallel to the substrate. A is called a bottom contact structure, and B is called a top contact structure. C is a structure often used for producing an organic single crystal FET. A source and drain electrodes and an insulator layer are provided on a semiconductor, and a gate electrode is further formed thereon. D has a structure called a top & bottom contact type transistor. E is a schematic diagram of a FET having a vertical structure, a static induction transistor (SIT). According to this SIT structure, a large amount of carriers can move at a time because the current flow spreads in a plane. Further, since the source electrode and the drain electrode are arranged vertically, the distance between the electrodes can be reduced, so that the response is fast. Therefore, it can be preferably applied to uses such as flowing a large current or performing high-speed switching. In FIG. 1, E does not indicate a substrate, but in the normal case, a substrate is provided outside the source and drain electrodes represented by 1 and 3 in FIG. 1E.

Each component in each embodiment will be described.
The substrate 6 needs to be able to hold each layer formed thereon without peeling off. For example, insulating materials such as resin plates and films, paper, glass, quartz, and ceramics, materials in which an insulating layer is formed on a conductive substrate such as metal and alloy by coating, materials made of various combinations such as resin and inorganic materials, etc. Can be used. Examples of the resin film that can be used include polyethylene terephthalate, polyethylene naphthalate, polyethersulfone, polyamide, polyimide, polycarbonate, cellulose triacetate, polyetherimide, and the like. When a resin film or paper is used, the element can have flexibility, is flexible and lightweight, and improves practicality. The thickness of the substrate is usually 1 μm to 10 mm, preferably 5 μm to 5 mm.

The source electrode 1, the drain electrode 3, and the gate electrode 5 are made of a conductive material. For example, platinum, gold, silver, aluminum, chromium, tungsten, tantalum, nickel, cobalt, copper, iron, lead, tin, titanium, indium, palladium, molybdenum, magnesium, calcium, barium, lithium, potassium, sodium, etc. And alloys containing them; conductive oxides such as InO ₂ , ZnO ₂ , SnO ₂ , ITO; conductive polymer compounds such as polyaniline, polypyrrole, polythiophene, polyacetylene, polyparaphenylene vinylene, polydiacetylene; silicon, germanium, Semiconductors such as gallium arsenide; carbon materials such as carbon black, fullerene, carbon nanotube, and graphite can be used. In addition, the conductive polymer compound or the semiconductor may be doped. As the dopant at that time, for example, acids such as hydrochloric acid, sulfuric acid and sulfonic acid, Lewis acids such as PF ₅ , AsF ₅ and FeCl ₃ , halogen atoms such as iodine, metal atoms such as lithium, sodium and potassium are used. It is done. In addition, a conductive composite material in which carbon black, metal particles, or the like is dispersed in the above material is also used.
The distance between the source and drain electrodes (channel length) is an important factor that determines the characteristics of the device, but is usually 100 μm or less, preferably 50 μm or less, and the width between the source and drain electrodes (channel width) is usually 2000 μm. Hereinafter, it is preferably 1000 μm or less. The channel width may be longer when the electrode structure is a comb structure.
The structure (shape) of each of the source and drain electrodes will be described. The structure of the source and drain electrodes may be the same or different. When it has a bottom contact structure, it is generally preferable to create it using a lithography method and form it in a rectangular parallelepiped. The length of the electrode may be the same as the previous channel width. The width of the electrode is not particularly specified, but is preferably shorter in order to reduce the area of the element within a range where the electrical characteristics can be stabilized. Usually, it is 1000 μm or less, preferably 500 μm or less. The thickness of the electrode is usually 1 nm to 1 μm, preferably 5 nm to 0.5 nm, and more preferably 10 nm to 0.2 μm.
A wiring is connected to each of the electrodes 1, 3, and 5, and the wiring is also made of the same material as the electrode.

An insulating material is used for the insulator layer 4. For example, polymers such as polyparaxylylene, polyacrylate, polymethyl methacrylate, polystyrene, polyvinylphenol, polyamide, polyimide, polycarbonate, polyester, polyvinyl alcohol, polyvinyl acetate, polyurethane, polysulfone, epoxy resin, phenol resin, and combinations thereof Copolymers; oxides such as silicon dioxide, aluminum oxide, titanium oxide and tantalum oxide; ferroelectric oxides such as SrTiO ₃ and BaTiO ₃ ; nitrides such as silicon nitride and aluminum nitride; sulfides; fluorides and the like Or a polymer in which particles of these dielectrics are dispersed can be used. The film thickness of the insulator layer 4 varies depending on the material, but is usually 0.1 nm to 100 μm, preferably 0.5 nm to 50 μm, more preferably 5 nm to 10 μm.

As the material of the semiconductor layer 2, the compound represented by the formula (1) is used. The organic material may be a mixture, but the organic material preferably contains 50% by weight or more, preferably 80% by weight or more, more preferably 95% by weight or more of the compound represented by the formula (1). In order to improve the characteristics of the field effect transistor or to impart other characteristics, other organic semiconductor materials and various additives may be mixed as necessary. The semiconductor layer 2 may be composed of a plurality of layers.
The thickness of the semiconductor layer 2 is preferably as thin as possible without losing necessary functions. In lateral field effect transistors as shown in A, B, and D, the device characteristics do not depend on the film thickness if the film thickness exceeds a predetermined value, while the leakage current may increase as the film thickness increases. This is because there are many. In order to show a necessary function, it is usually 1 nm to 10 μm, preferably 5 nm to 5 μm, more preferably 10 nm to 3 μm.

In the field effect transistor of the present invention, other layers can be provided between the layers or on the outer surface of the element as necessary. For example, when a protective layer is formed on the semiconductor layer directly or via another layer, the influence of outside air such as humidity can be reduced, and the ON / OFF ratio of the device can be increased. There is also an advantage that the mechanical characteristics can be stabilized.
Although it does not specifically limit as a material of a protective layer, For example, the film | membrane which consists of various resins, such as acrylic resins, such as an epoxy resin and polymethylmethacrylate, polyurethane, polyimide, polyvinyl alcohol, a fluororesin, polyolefin, silicon oxide, aluminum oxide, A film made of a dielectric such as an inorganic oxide film or a nitride film, such as silicon nitride, is preferably used, and a resin (polymer) having a low oxygen or moisture permeability and a low water absorption rate is particularly preferable. In recent years, protective materials developed for organic EL displays can also be used. Although the film thickness of a protective layer can employ | adopt arbitrary film thickness according to the objective, it is 100 nm-1 mm normally.

  In addition, it is possible to improve device characteristics by performing surface treatment in advance on a substrate or an insulator layer on which a semiconductor layer is stacked. For example, by adjusting the degree of hydrophilicity / hydrophobicity of the substrate surface, the film quality of the film formed thereon can be improved. In particular, the characteristics of organic semiconductor materials can vary greatly depending on the state of the film, such as molecular orientation. Therefore, the surface treatment on the substrate or the like controls the molecular orientation at the interface between the substrate and the semiconductor layer to be formed thereafter, and reduces the trap sites on the substrate and the insulator layer. It is considered that characteristics such as carrier mobility are improved. The trap site refers to a functional group such as a hydroxyl group present in an untreated substrate. When such a functional group is present, electrons are attracted to the functional group, and as a result, carrier mobility is lowered. . Therefore, reducing trap sites is often effective for improving characteristics such as carrier mobility. Examples of such substrate treatment include hydrophobization treatment with hexamethyldisilazane, cyclohexene, octadecyltrichlorosilane, acid treatment with hydrochloric acid, sulfuric acid, acetic acid, sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonia, etc. Electrical treatment such as alkali treatment with ozone, ozone treatment, fluorination treatment, plasma treatment with oxygen and argon, Langmuir / Blodgett film formation process, other insulator and semiconductor thin film formation process, mechanical process, corona discharge, etc. Examples thereof include a rubbing treatment using a treatment or fibers.

  As a method of providing each layer in these embodiments, for example, a vacuum deposition method, a sputtering method, a coating method, a printing method, a sol-gel method, or the like can be appropriately employed.

Next, a method for manufacturing a field effect transistor according to the present invention will be described below with reference to FIG. 2 by taking a bottom contact type field effect transistor (FET) shown in the embodiment example A of FIG.
This manufacturing method can be similarly applied to the field effect transistors of the other embodiments described above.

(Substrate and substrate processing)
The field effect transistor of the present invention is manufactured by providing various layers and electrodes necessary on the substrate 6 (see FIG. 2A). As the substrate, those described above can be used. It is also possible to perform the above-described surface treatment or the like on this substrate. The thickness of the substrate 6 is preferably thin as long as necessary functions are not hindered. Although it varies depending on the material, it is usually 1 μm to 10 mm, preferably 5 μm to 5 mm. If necessary, the substrate may have an electrode function.

(Formation of gate electrode)
A gate electrode 5 is formed on the substrate 6 (see FIG. 2B). The electrode material described above is used as the electrode material. As a method for forming the electrode film, various methods can be used. For example, a vacuum deposition method, a sputtering method, a coating method, a thermal transfer method, a printing method, a sol-gel method, and the like are employed. It is preferable to perform patterning as necessary so as to obtain a desired shape during or after film formation. Various methods can be used as the patterning method, and examples thereof include a photolithography method in which patterning and etching of a photoresist are combined. Patterning can also be performed using a printing method such as ink jet printing, screen printing, offset printing, letterpress printing, soft lithography such as a microcontact printing method, and a combination of these methods. The thickness of the gate electrode 5 varies depending on the material, but is usually 0.1 nm to 10 μm, preferably 0.5 nm to 5 μm, more preferably 1 nm to 3 μm. Moreover, when it serves as a gate electrode and a board | substrate, it may be larger than said film thickness.

(Formation of insulator layer)
An insulator layer 4 is formed over the gate electrode 5 (see FIG. 2 (3)). As the insulator material, those described above are used. Various methods can be used to form the insulator layer 4. For example, spin coating, spray coating, dip coating, casting, bar coating, blade coating, etc., screen printing, offset printing, ink jet printing, vacuum deposition, molecular beam epitaxial growth, ion cluster beam method, ion plating Examples thereof include dry process methods such as a coating method, a sputtering method, an atmospheric pressure plasma method, and a CVD method. In addition, a method of forming an oxide film on a metal such as a sol-gel method or alumite on aluminum is employed.
In the portion where the insulator layer and the semiconductor layer are in contact with each other, in order to satisfactorily orient the molecules constituting the semiconductor at the interface between the two layers, for example, the molecules of the compound represented by the above formula (1), A predetermined surface treatment can also be performed. As the surface treatment method, the same surface treatment as that of the substrate can be used. The thickness of the insulator layer 4 is preferably as thin as possible without impairing its function. Usually, it is 0.1 nm-100 micrometers, Preferably it is 0.5 nm-50 micrometers, More preferably, it is 5 nm-10 micrometers.

(Formation of source electrode and drain electrode)
The source electrode 1 and the drain electrode 3 can be formed in accordance with the case of the gate electrode 5 (see FIG. 2 (4)).

(Formation of semiconductor layer)
As described above, as the semiconductor material, an organic material containing a total amount of one or more kinds of the compound represented by the formula (1) by 50% by weight or more is used. Various methods can be used for forming the semiconductor layer. Formation method in vacuum process such as sputtering method, CVD method, molecular beam epitaxial growth method, vacuum deposition method, coating method such as dip coating method, die coater method, roll coater method, bar coater method, spin coating method, ink jet method In addition, it is roughly classified into formation methods in solution processes such as screen printing, offset printing, and microcontact printing. Hereinafter, a method for forming a semiconductor layer will be described in detail.

First, a method for obtaining an organic semiconductor layer by forming an organic material by a vacuum process will be described.
A method (vacuum deposition method) in which the organic material is heated in a crucible or a metal boat under vacuum and the evaporated organic material is attached (deposited) to a substrate (exposed portions of the insulator layer, the source electrode and the drain electrode). Preferably employed. Under the present circumstances, a vacuum degree is 1.0 * 10 < ^-1 > Pa or less normally, Preferably it is 1.0 * 10 <^-4> Pa or less. Further, since the characteristics of the organic semiconductor film, and hence the field effect transistor, vary depending on the substrate temperature during vapor deposition, it is preferable to select the substrate temperature carefully. The substrate temperature during vapor deposition is usually 0 to 200 ° C, preferably 10 to 150 ° C. The deposition rate is usually 0.001 nm / second to 10 nm / second, preferably 0.01 nm / second to 1 nm / second. The film thickness of the organic semiconductor layer formed from an organic material is usually 1 nm to 10 μm, preferably 5 nm to 5 μm, more preferably 10 nm to 3 μm.
Instead of heating and evaporating the organic material for forming the organic semiconductor layer and attaching it to the substrate, a sputtering method in which accelerated ions such as argon collide with the material target to knock out material atoms and attach them to the substrate. It may be used.

  Since the semiconductor material in the present invention is an organic substance and a relatively low molecular compound, such a vacuum process can be preferably used. Although such a vacuum process requires somewhat expensive equipment, there is an advantage that a uniform film can be easily obtained with good film formability.

  Next, a method for obtaining an organic semiconductor layer by forming an organic semiconductor material by a solution process will be described. The manufacturing method by coating is advantageous because it can be realized at a low cost by using a relatively inexpensive facility, without requiring the environment during manufacturing to be in a vacuum or at a high temperature, and at a low cost.

First, an ink for producing a semiconductor device is prepared by dissolving a compound of the formula (1) in a solvent. The solvent at this time is not particularly limited as long as the compound dissolves and a film can be formed on the substrate. Specifically, halogeno hydrocarbon solvents such as chloroform, methylene chloride and dichloroethane; alcohol solvents such as methanol, ethanol, isopropyl alcohol and butanol; fluorinated alcohol solvents such as octafluoropentanol and pentafluoropropanol; ethyl acetate Ester solvents such as butyl acetate, ethyl benzoate and diethyl carbonate; aromatic hydrocarbon solvents such as toluene, xylene, benzene, chlorobenzene and dichlorobenzene; such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone and cyclohexanone Ketone solvents; amide solvents such as dimethylformamide, dimethylacetamide and N-methylpyrrolidone; ether solvents such as tetrahydrofuran and diisobutyl ether It can be used. These can be used alone or in combination.
The concentration of the total amount of the compound of the above formula (1) or a mixture thereof in the ink varies depending on the type of solvent and the thickness of the semiconductor layer to be produced, but is about 0.001% to 50%, preferably 0.01. % To about 20%. Additives and other semiconductor materials can be mixed for the purpose of improving the film formability of the semiconductor layer and doping described later.
When using the ink, a material such as a semiconductor material is dissolved in the above solvent, and if necessary, a heat dissolution treatment is performed. Furthermore, the obtained solution is filtered using a filter to remove solids such as impurities, thereby obtaining an ink for producing a semiconductor device. When such an ink is used, the film formability of the semiconductor layer is improved, which is preferable for manufacturing the semiconductor layer.

  The ink for manufacturing a semiconductor device prepared as described above is applied to the substrate (exposed portions of the insulator layer, the source electrode, and the drain electrode). Coating methods include casting, spin coating, dip coating, blade coating, wire bar coating, spray coating, and other coating methods, inkjet printing, screen printing, offset printing, letterpress printing, and other micro contact printing methods. The method of soft lithography, etc., or a method combining a plurality of these methods may be employed. Furthermore, as a method similar to the coating method, the Langmuir project method in which a monomolecular film of a semiconductor layer produced by dropping the above-described ink on the water surface is transferred to a substrate and laminated, and two substrates of liquid crystal or melted material are used. It is also possible to adopt a method of sandwiching between the substrates or introducing between the substrates by capillary action. The film thickness of the organic semiconductor layer produced by this method is preferably thinner as long as the function is not impaired. There is a concern that the leakage current increases as the film thickness increases. The film thickness of the organic semiconductor layer is usually 1 nm to 10 μm, preferably 5 nm to 5 μm, more preferably 10 nm to 3 μm.

The characteristics of the semiconductor layer thus formed (see FIG. 2 (5)) can be further improved by post-processing. For example, it is considered that the heat treatment reduces strain in the film generated during film formation, reduces pinholes, etc., and can control the arrangement and orientation in the film. The characteristics can be improved and stabilized. In order to improve the characteristics, it is effective to perform this heat treatment when producing the field effect transistor of the present invention. This heat treatment is performed by heating the substrate after forming the semiconductor layer. The temperature of the heat treatment is not particularly limited, but is usually about room temperature to 150 ° C, preferably 40 ° C to 120 ° C, more preferably 45 ° C to 100 ° C. The time at this time is not particularly limited, but is usually from 1 minute to 24 hours, preferably from about 2 minutes to 3 hours. The atmosphere at that time may be air, but may be an inert atmosphere such as nitrogen or argon.
As another post-treatment method for the semiconductor layer, a property change due to oxidation or reduction may be induced by treatment with an oxidizing or reducing gas such as oxygen or hydrogen or an oxidizing or reducing liquid. it can. This is used, for example, for the purpose of increasing or decreasing the carrier density in the film.

In addition, in a technique called doping, semiconductor layer characteristics can be changed by adding a trace amount of elements, atomic groups, molecules, and polymers to the semiconductor layer. For example, oxygen, hydrogen, hydrochloric acid, sulfuric acid, sulfonic acid and other acids, PF ₅ , AsF ₅ , FeCl _{3 and} other Lewis acids, iodine and other halogen atoms, sodium and potassium metal atoms and the like can be doped. This can be achieved by bringing these gases into contact with the semiconductor layer, immersing them in a solution, or performing an electrochemical doping process. These dopings may be added during the synthesis of the semiconductor material, even after the semiconductor layer is not formed, or may be added to the ink in the process of manufacturing the semiconductor layer using the ink for manufacturing the semiconductor device. It can be added in the process step of forming the precursor thin film disclosed in Document 2. In addition, a material used for doping is added to the material for forming the semiconductor layer during vapor deposition, and co-evaporation is performed, or the semiconductor layer is mixed with the surrounding atmosphere when the semiconductor layer is formed (in the environment where the doping material exists) It is also possible to perform doping by accelerating ions in a vacuum and colliding with the film.

  These doping effects include changes in electrical conductivity due to increase or decrease in carrier density, changes in carrier polarity (p-type and n-type), changes in Fermi level, and the like. Such doping is often used particularly in semiconductor elements using inorganic materials such as silicon.

(Protective layer)
When the protective layer 7 is formed on the organic semiconductor layer, there is an advantage that the influence of outside air can be minimized and the electrical characteristics of the organic field effect transistor can be stabilized (see FIG. 2 (6)). The above-mentioned materials are used as the protective layer material.
Although the film thickness of the protective layer 7 can employ | adopt arbitrary film thickness according to the objective, it is 100 nm-1 mm normally.
Various methods can be used to form the protective layer. When the protective layer is made of a resin, for example, a method of applying a resin solution and then drying to form a resin film, or applying or depositing a resin monomer The method of polymerizing after that is mentioned. Cross-linking treatment may be performed after film formation. When the protective layer is made of an inorganic material, for example, a formation method in a vacuum process such as a sputtering method or a vapor deposition method, or a formation method in a solution process such as a sol-gel method can be used.
In the field effect transistor of the present invention, a protective layer can be provided between the layers as necessary in addition to the organic semiconductor layer. These layers help to stabilize the electrical properties of the organic field effect transistor.

  According to the present invention, since an organic material is used as a semiconductor material, it can be manufactured in a relatively low temperature process. Accordingly, flexible materials such as plastic plates and plastic films that could not be used under conditions exposed to high temperatures can be used as the substrate. As a result, it is possible to manufacture a light, flexible, and hard-to-break element, which can be used as a switching element for an active matrix of a display. Examples of the display include a liquid crystal display, a polymer dispersion type liquid crystal display, an electrophoretic display, an EL display, an electrochromic display, a particle rotation type display, and the like. Further, since the field effect transistor of the present invention can be manufactured by a coating method or a printing process, it is also suitable for manufacturing a large area display.

  The field effect transistor of the present invention can also be used as digital elements and analog elements such as memory circuit elements, signal driver circuit elements, and signal processing circuit elements. Further, by combining these, it is possible to produce an IC card or an IC tag. Furthermore, since the field effect transistor of the present invention can change its characteristics by an external stimulus such as a chemical substance, it can be used as an FET sensor.

  The operating characteristics of the field effect transistor are determined by the carrier mobility of the semiconductor layer, the conductivity, the capacitance of the insulating layer, the element configuration (distance and width between source and drain electrodes, film thickness of the insulating layer, etc.), and the like. As a semiconductor material used for the field effect transistor, the higher the carrier mobility, the better the semiconductor layer. The compound of the formula (1) in the present invention has good film forming properties and large area applicability. Further, pentacene derivatives and the like are unstable and difficult to handle compounds such as decomposition in the atmosphere due to moisture contained in the atmosphere. When used as a material, there is an advantage that the semiconductor layer is highly stable and has a long lifetime even after it is manufactured.

EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated still in detail, this invention is not limited to these examples. In the examples, unless otherwise specified, parts represent parts by mass, and% represents mass%.
The various compounds obtained in the synthesis examples are ¹ H-NMR, ¹³ C-NMR (NMR is a nuclear magnetic resonance spectrum), MS (mass spectrometry spectrum), mp (melting point), and elemental analysis as necessary. The structural formula was determined by performing various measurements. The measuring equipment is as follows.
NMR: JEOL Lambda 400 spectrometer
MS: Shimadzu QP-5050A
Elemental analysis: Parkin Elmer2400 CHN type elemental analyzer

Synthesis example 1
Synthesis of 1,4-Dibromo-2,5-diodo-benzene (3)

1,4-Dibromobenzene (34.8 g, 148 mmol) and solid iodine (148 g, 583 mmol) were added to 98% concentrated sulfuric acid solution (480 ml) and heated at 120-135 ° C. for 6 hours. After completion of the reaction, the reaction mixture was cooled in an ice bath and the precipitate was filtered. It was ground into powder with a mortar breast, washed in order with aqueous sodium hydrogen sulfite solution, saturated aqueous sodium hydrogen carbonate solution, and water and dried. Recrystallization from benzene gave colorless needle crystals (61 g, 85%).
¹ H-NMR (60 MHz, CDCl ₃ ) δ 7.9 (s, 2H, 3, 6 positions)
m. p. 159-164 ° C (163-165 ° C ¹⁾ )

Synthesis example 2
Synthesis of 1,4-Dibromo-2,5-bis (trimethylsilylethyl) benzene (4)

In a nitrogen atmosphere, Compound 3 (10 g, 21 mmol) was dissolved in anhydrous diisopropylamine (30 ml) and anhydrous benzene (120 ml), and then degassed for 30 minutes. PdCl ₂ (PPh ₃ ) ₂ (0.45 g, 640 μmol), CuI (0.25 g, 1.3 mmol) and TMSA (5.6 ml, 40 mmol) were added at room temperature, and the mixture was stirred at room temperature for 3 hours. After the stirring, the reaction was terminated with water (300 ml), extracted with chloroform (50 ml × 3), washed with brine (200 ml × 3), and dried over anhydrous magnesium sulfate. Chloroform was distilled off under reduced pressure and purified by column chromatography (silica gel, batch, methylene chloride: hexane = 1: 3, Rf = 0.5). Recrystallization from hexane gave colorless needle crystals (5.61 g, 64%).
¹ H-NMR (60 MHz, CDCl ₃ ) δ 7.67 (s, 2H, 3, 6 positions) δ 0.27 (s, 18H, TMS)
m. p. 127-132 ° C

Synthesis example 3
Synthesis of 1,4-Bis (methylthio) -2,5-bis (trimethylsilylethyl) benzene (5)

Under a nitrogen atmosphere, compound 4 (4.0 g, 9.3 mmol) was dissolved in anhydrous THF (55 ml), cooled to −78 ° C., and a 1.59 M t-butyllithium / pentane solution (24 ml, 37 mmol) was added. . The mixture was stirred at that temperature for 30 minutes and warmed to room temperature. Dimethyl disulfide (2.0 ml, 22 mmol) was added and stirred for 1 hour. Thereafter, extraction was performed with chloroform (80 ml × 3). It was washed with an aqueous ammonium chloride solution (50 ml × 3) and water (300 ml × 3), dried over anhydrous magnesium sulfate, and the solvent was distilled off under reduced pressure. Purification by column chromatography (silica gel, batch, methylene chloride: hexane = 1: 3, Rf = 0.4) and recrystallization from hexane gave yellow needles (1.78 g, 53%).
¹ H-NMR (400 MHz, CDCl ₃ ) δ 7.17 (s, 2H, 3, 6 positions) δ 2.47 (s, 6H, SMe) δ 0.28 (s, 18H, TMS)
¹³ C-NMR (100 MHz, CDCl ₃ ) δ137.74, 127.93, 121.78, 103.37, 101.54, 13.36, −0.16
M.M. S. (70 ev, DI) 362 m / z
m. p. 175-177 ° C
Anal. _{Calcd for C 18 H 26 S 2} Si 2: C, 59.61; H, 7.23 Found: C, 59.68; H, 7.14

Synthesis example 4
Synthesis of 3,7-Diiodo-2,6-dimethylsilylbenzene [1,2-b: 4,5-b ′] dithiophene (6)

Compound 5 (4.0 g, 11 mmol) and iodine (11.2 g, 44 mmol) were added to methylene chloride (90 ml), stirred for 9 hours, and then treated with an aqueous sodium thiosulfate solution. Extraction was performed with chloroform (50 ml × 3). After washing with water (100 ml × 3), it was dried over anhydrous magnesium sulfate and the solvent was distilled off under reduced pressure. Purification by column chromatography (silica gel, batch, methylene chloride: hexane = 1: 3, Rf = 0.8), dissolved in a minimum amount of hot methylene chloride, and then reprecipitated from methanol gave colorless needle crystals. (6.4 g, 98%).
¹ H-NMR (400 MHz, CDCl ₃ ) δ 8.23 (s, 2H, 4th, 8th) δ 0.53 (s, 18H, TMS)
¹³ C-NMR (100 MHz, CDCl ₃ ) δ 143.30, 141.59, 138.95, 85.51, −0.73
M.M. S. (70 ev, DI) 586 m / z
m. p. 263-265 ° C
Anal. _{Calcd for C 16 H 22 I 2} S 2 Si 2: C, 32.77; H, 3.44 Found: C, 32.66; H, 3.24

Synthesis example 5
2,3,6,7-Tetraiodobenzo [1,2-b: 4,5-b ′] dithiophene (7) Synthesis

Under a nitrogen atmosphere, compound 6 (3.0 g, 5.1 mmol) was dissolved in anhydrous methylene chloride (120 ml), and 1.56 M iodine monochloride / methylene chloride solution (7.88 ml, 12.3 mmol) was added at −10 ° C. It was dripped. It stirred for 12 hours after completion | finish of dripping. The precipitated solid was filtered and washed with ethanol and hexane (3.5 g, 98%).
¹ H-NMR (400 MHz, CDCl ₃ , CS ₂ ) δ 8.16 (s, 2H, 4th, 8th positions)
M.M. S. (70 ev, DI) 694 m / z
m. p. > 300 ° C
Anal. _{Calcd for C 10 H 2 I 4} S 2: C, 17.31; H, 0.29 Found: C, 17.39; H, 0.23

Synthesis Example 6
2,3,6,7-Tetrakis (trimethylsilyl ethyl) benzo [1,2-b: 4,5-b ′] dithiophene (8)

Under a nitrogen atmosphere, Compound 7 (2.0 g, 2.9 mmol) was dissolved in anhydrous diisopropylamine (40 ml) and anhydrous benzene (80 ml), and then degassed for 30 minutes. At room temperature, 10 mol% PdCl ₂ (PPh ₃ ) ₂ (202 mg), 20 mol% CuI (56 mg) and TMSA (1.96 ml, 13.8 mmol) were added and stirred at room temperature for 6 hours. After completion of stirring, the reaction was terminated with water (100 ml), extracted with ether (40 ml × 3), washed with brine (100 ml × 3), and dried over anhydrous magnesium sulfate. Evaporate the solvent under reduced pressure, purify by column chromatography (silica gel, batch, methylene chloride: hexane = 1: 3, Rf = 0.8), dissolve in the minimum amount of hot hexane and reprecipitate from methanol. Yielded a yellow solid (1.4 g, 83%).
¹ H-NMR (400 MHz, CDCl ₃ ) δ 8.17 (s, 2H, 4th, 8th) δ 0.34 (s, 18H, TMS) δ 0.31 (s, 18H, TMS)
¹³ C-NMR (100 MHz, CDCl ₃ ) δ 137.6, 136.1, 128.4, 122.8, 116.6, 107.2, 102.5, 97.1, 96.8, 0.09, -0.20
M.M. S. (70 ev, DI) 574 m / z
m. p. 265-266 ° C
Anal. _{Calcd for C 30 H 38 Si 4} S 2: C, 62.55; H, 6.66 Found: C, 62.65; H, 6.62

Synthesis example 7
Synthesis of Benzo [1,2-b: 4,5-b ′] bis [d] benzothiophene (9)

Compound 8 (200 mg, 0.35 mmol) was dissolved in anhydrous benzene (14 ml) in a container, and then phase transfer catalyst Aliquat 336 (3 drops) was added thereto, followed by deaeration for 30 minutes. A 10% aqueous sodium hydroxide solution (14 ml), sodium borohydride (53.2 mg, 1.4 mmol), and hydrazine monohydrate (0.21 ml) were added to another container, and deaeration was performed for 30 minutes. Tellurium (98 mg, 0.77 mmol) and a benzene solution were added to the aqueous solution, and the mixture was stirred at 45 ° C. for 6 hours under a nitrogen atmosphere. Water was added to terminate the reaction, the organic solvent was distilled off under reduced pressure, and the resulting solid was filtered and continuously extracted from carbon disulfide. The extracted solution was distilled off under reduced pressure, and the resulting solid was filtered and washed with methylene chloride (72 mg, 71%). Finally, the compound 9 was obtained by sublimation purification (48 mg, 48%).
¹ H-NMR (400 MHz, CDCl ₃ ) δ 8.61 (s, 2H, 6, 12th position) δ 8.23 (d, J = 5.88 Hz, 2H, 2, 8th position) δ 7.88 (d, J = 6.00 Hz, 2H, 5th, 11th) δ 7.50 (t, J = 5.84 Hz, 2H, 3rd, 9th) δ 7.49 (t, J = 5.88 Hz, 2H, 4th, 10th)
M.M. S. (70 ev, DI) 290 m / z
m. p. > 300 ° C
Anal. _{Calcd for C 18 H 10 Se 2} : C, 74.45; H, 3.47 Found: C, 74.16; H, 3.22

Synthesis Example 8
Synthesis of 1,4-Bis (methylseleno) -2,5-bis (trimethylsilylethyl) benzene (10)

Under a nitrogen atmosphere, compound 4 (2.0 g, 4.6 mmol) was dissolved in anhydrous THF (40 ml), cooled to −78 ° C., and 1.46 M t-butyllithium / pentane solution (13 ml, 18.4 mmol) was added. added. The mixture was stirred at that temperature for 15 minutes and warmed to room temperature. Black Serene powder (0.80 g, 10 mmol) was added and stirred for 15 minutes. Then, iodomethane (0.86 ml, 13.8 mmol) was added and stirred for 15 minutes. Extraction was performed with chloroform (40 ml × 3). It was washed with an aqueous ammonium chloride solution (30 ml × 3) and water (100 ml × 3), dried over anhydrous magnesium sulfate, and the solvent was distilled off under reduced pressure. Purification by column chromatography (silica gel, batch, methylene chloride: hexane = 1: 3, Rf = 0.4) and recrystallization from hexane gave yellow needle crystals (1.0 g, 47%).
¹ H-NMR (400 MHz, CDCl ₃ ) δ 7.24 (s, 2H, 3, 6 positions) δ 2.33 (s, 6H, SeMe) δ 0.29 (s, 18H, TMS)
¹³ C-NMR (100 MHz, CDCl ₃ ) δ 133.0, 130.6, 124.0, 102.8, 102.3, 6.42, −0.20
m. p. 161-162 ° C
Anal. _{Calcd for C 18 H 26 Se 2} Si 2: C, 47.36; H, 5.74 Found: C, 47.28; H, 5.78

Synthesis Example 9
Synthesis of 3,7-Diiodo-2,6-dimethylsilylbenzo [1,2-b: 4,5-b ′] diselenophene (11)

Compound 10 (4.0 g, 8.8 mmol) and iodine (9.0 g, 35 mmol) were added to anhydrous methylene chloride (90 ml), stirred for 9 hours, and treated with an aqueous sodium thiosulfate solution. Extraction was performed with chloroform (40 ml × 3). After washing with water (100 ml × 3), it was dried over anhydrous magnesium sulfate and the solvent was distilled off under reduced pressure. Purification by column chromatography (silica gel, batch, methylene chloride: hexane = 1: 3, Rf = 0.8), dissolution in a minimum amount of hot methylene chloride, and reprecipitation from methanol gave a colorless solid (5 .3 g, 89%).
¹ H-NMR (400 MHz, CDCl ₃ ) δ 8.38 (s, 2H, 4th, 8th positions) δ 0.52 (s, 18H, TMS)
¹³ C-NMR (100 MHz, CDCl ₃ ) δ 146.7, 143.4, 138.8, 123.8, 88.1, -0.38
m. p. 274-275 ° C
Anal. _{Calcd for C 16 H 22 I 2} Se 2 Si 2: C, 28.25; H, 2.96 Found: C, 28.19; H, 2.97

Synthesis Example 10
Synthesis of 2,3,6,7-Tetraiodobenzo [1,2-b: 4,5-b ′] diselenophene (12)

Under a nitrogen atmosphere, Compound 11 (1.0 g, 1.5 mmol) was dissolved in anhydrous methylene chloride (30 ml), and 1.9 M iodine monochloride / methylene chloride solution (0.19 ml) was added dropwise at −10 ° C. It stirred for 12 hours after completion | finish of dripping. The precipitated solid was filtered and washed with ethanol hexane (1.1 g, 91%).
¹ H-NMR (400 MHz, CDCl ₃ , CS ₂ ) δ 8.20 (s, 2H, 4th, 8th positions)
M.M. S. (70 ev, DI) 790 m / z
m. p. > 300 ° C
Anal. _{^{Calcd for C 10 H 2 I 4}} Se 2: C, 15.25; H, 0.26 Found: C, 15.53; H, 0.26

Synthesis Example 11
Synthesis of 2,3,6,7-Tetrakis (trimethylsilylethyl) benzo [1,2-b: 4,5-b ′] diselenophene (13)

Under a nitrogen atmosphere, Compound 12 (2.0 g, 2.6 mmol) was dissolved in anhydrous diisopropylamine (40 ml) and anhydrous benzene (80 ml), and then degassed for 30 minutes. At room temperature, 10 mol% PdCl ₂ (PPh ₃ ) ₂ (178 mg), 20 mol% CuI (48 mg) and TMSA (1.68 ml, 11.9 mmol) were added and stirred at room temperature for 6 hours. After stirring, the reaction was terminated with water (30 ml), extracted with ether (40 ml × 3), washed with brine (100 ml × 3), and dried over anhydrous magnesium sulfate. Evaporate the solvent under reduced pressure, purify by column chromatography (silica gel, batch, methylene chloride: hexane = 1: 3, Rf = 0.7), dissolve in the minimum amount of hot hexane and reprecipitate from methanol. Yielded a yellow solid (1.36 g, 80%).
¹ H-NMR (400 MHz, CH ₂ Cl ₂ ) δ 8.24 (s, 2H, 4th, 8th positions) δ 0.34 (s, 18H, TMS) δ 0.31 (s, 18H, TMS)
¹³ C-NMR (100 MHz, CDCl ₃ ) δ 139.5, 137.0, 129.9, 121.9, 108.9, 101.5, 98.8, 98.6, 0.11, −0.15
M.M. S. (70 ev, DI) 670 m / z
m. p. 255-256 ° C
Anal. _{Calcd for C 30 H 38 Si 4} Se 2: C, 53.87; H, 5.73 Found: C, 53.81; H, 5.53

Synthesis Example 12
Synthesis of Benzo [1,2-b: 4,5-b ′] bis [d] benzoselenophene (14)

Compound 13 (468 mg, 0.70 mmol) was dissolved in anhydrous benzene (14 ml) in a container, and then phase transfer catalyst Aliquat 336 (3 drops) was added and degassed for 30 minutes. A 10% aqueous sodium hydroxide solution (14 ml), sodium borohydride (106 mg, 2.8 mmol), and hydrazine monohydrate (0.42 ml) were added to another container, and deaeration was performed for 30 minutes. Tellurium (196 mg, 1.54 mmol) and a benzene solution were added to the aqueous solution, and the mixture was stirred at 45 ° C. for 6 hours under a nitrogen atmosphere. Water was added to terminate the reaction, the organic solvent was distilled off under reduced pressure, and the resulting solid was filtered and continuously extracted from carbon disulfide. The extracted solution was distilled off under reduced pressure, and the resulting solid was filtered and washed with methylene chloride (100 mg, 71%). After recrystallization from chlorobenzene (69 mg, 26%), compound 14 was obtained by purification by sublimation (53 mg, 20%).
¹ H-NMR (400 MHz, CDCl ₃ ) δ 8.62 (s, 2H, 6, 12th position) δ 8.18 (d, J = 7.80 Hz, 2H, 2, 8th position) δ 7.91 (d, J = 8.08Hz, 2H, 5th, 11th) δ 7.50 (t, J = 7.87Hz, 2H, 3rd, 9th) δ 7.49 (t, J = 7.91Hz, 2H, 4th, 10th)
M.M. S. (70 ev, DI) 386 m / z
m. p. > 300 ° C
Anal. _{Calcd for C 18 H 10 Se 2} : C, 56.27; H, 2.62 Found: C, 56.15; H, 2.43

Example 1
An n-doped silicon wafer with 200 nm SiO ₂ thermal oxide film (surface resistance 0.02 Ω · cm or less) subjected to hexamethylene disilazane treatment was placed in a vacuum deposition apparatus, and the degree of vacuum in the apparatus was 5.0 × 10. ^It exhausted until it became ^-3 Pa or less. Compound 9 (No. 100 in Table 1) was vapor-deposited on this electrode to a thickness of 80 nm at room temperature (25 ° C.) by resistance heating vapor deposition to form semiconductor layer 2. Next, this substrate is placed in a vacuum vapor deposition apparatus, evacuated until the degree of vacuum in the apparatus becomes 1.0 × 10 ⁻³ Pa or less, and gold electrodes (source and drain electrodes) are formed to 40 nm by resistance heating vapor deposition. The field effect transistor of the present invention was obtained. In the field effect transistor in this embodiment, the thermal oxide film in the n-doped silicon wafer with the thermal oxide film has the function of the insulating layer 4, and the n-doped silicon wafer has the functions of the substrate 6 and the gate electrode 5. (See FIG. 3).

The obtained field effect transistor was installed in a prober, and semiconductor characteristics were measured using a semiconductor parameter analyzer 4155C (manufactured by Agilent). For semiconductor characteristics, the gate voltage was scanned from 10 V to -100 V in 20 V steps, the drain voltage was scanned from 10 V to -100 V, and the drain current-drain voltage was measured. As a result, current saturation was observed, and from the obtained voltage-current curve, the device showed a p-type semiconductor, and the carrier mobility was 2 × 10 ⁻³ cm ² / Vs. The on / off ratio was 1 × 10 ⁶ and the threshold voltage was −17V.

Example 2
In Example 1, the organic field effect transistor of this invention was produced like Example 1 except having changed the compound 9 into the compound 14 (No. 131 in Table 1). For semiconductor characteristics, the gate voltage was scanned from -10 V to 100 V in 20 V steps, the drain voltage was scanned from -10 V to 100 V, and the drain current-drain voltage was measured. As a result, current saturation was observed, and from the obtained voltage-current curve, the device showed an n-type semiconductor, the carrier mobility was 3 × 10 ⁻³ cm ² / Vs, and the on / off ratio was 1 × 10 ^6. The threshold voltage was −23V.

It is the schematic which shows the structural example of the field effect transistor of this invention. It is the schematic of the process for manufacturing the example of 1 aspect of the field effect transistor of this invention. 1 is a schematic view of a field effect transistor of the present invention obtained in Example 1. FIG.

Explanation of symbols

The same number is attached | subjected to the same name in FIGS. 1-3.
1 source electrode 2 semiconductor layer 3 drain electrode 4 insulator layer 5 gate electrode 6 substrate 7 protective layer

Claims (4)

A field effect transistor using a compound represented by the following formula (1) as a semiconductor material.

(In formula (1), X ₁ and X ₂ are each independently a sulfur atom or a selenium atom, and R ₁ and R ₂ are each independently a hydrogen atom, a halogen atom, or an aliphatic hydrocarbon group having 1 to 12 carbon atoms. Represents.) The field effect transistor according to claim 1, wherein R ₁ and R ₂ each independently represent a hydrogen atom or an aliphatic hydrocarbon group having 1 to 3 carbon atoms. A compound represented by the following formula (2).

(In Formula (2), R ₁ and R ₂ each independently represent a hydrogen atom, a halogen atom, or an aliphatic hydrocarbon group having 1 to 12 carbon atoms.) The compound according to claim 3, wherein R ₁ and R ₂ each independently represent a hydrogen atom or an aliphatic hydrocarbon group having 1 to 3 carbon atoms.

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