JP4433746B2 - Organic field effect transistor and manufacturing method thereof - Google Patents

Description

  The present invention relates to an organic field effect transistor (organic FET) and a method for manufacturing the same.

  Organic semiconductors have attracted attention as semiconductor materials used for semiconductor elements such as transistors and diodes. An organic semiconductor element using an organic semiconductor can be manufactured at a much lower cost than an element using an inorganic semiconductor because a semiconductor layer can be formed by a simple process such as a printing method, a spray method, or an ink jet method. it can. In addition, there is a possibility that a light-weight and thin integrated circuit having a large area can be easily manufactured.

  In recent years, the carrier mobility of organic semiconductors has been improved, and those capable of expressing mobility comparable to amorphous silicon have been found. As such organic semiconductor materials, pentacene, polyalkylthiophene, and the like are known.

  Among these organic semiconductor elements using organic semiconductors, organic FETs are one of interesting research objects, and are expected to be applied to lightweight and thin liquid crystal displays, organic EL displays, IC cards and the like.

  In these organic FETs, electrodes such as a source electrode and a drain electrode are generally composed of an inorganic material such as a metal. However, the affinity between these electrodes and the organic semiconductor layer made of an organic material is low, and as a result, the contact resistance between the electrodes and the organic semiconductor layer tends to increase. Therefore, the organic FET is likely to cause a loss of the applied gate voltage, and an excessive gate voltage is often required to obtain a current value equivalent to that of the FET using the inorganic semiconductor. Thus, conventional organic FETs have been difficult to use within a practical voltage range, despite the high mobility inherent in organic semiconductors.

  As an attempt to reduce the contact resistance between an electrode made of an inorganic material such as a metal and an organic semiconductor, for example, the following method has been studied. That is, first, the following Patent Document 1 describes a method in which a dopant having a property of being paired with an organic semiconductor is doped in a region of a semiconductor layer in contact with a source electrode and a drain electrode in an organic FET. In the organic FET in which the electrode and the organic semiconductor are joined in this way, a charge transfer complex is formed in the region doped with the dopant, and the carrier density is increased by the charge transfer complex, thereby the electrode and the organic semiconductor layer. The contact resistance with is reduced.

Further, in Non-Patent Document 1 below, in an organic semiconductor element, an alkanethiol having a plurality of functional groups having different dipole sizes is grafted onto an electrode, and the monomolecular film composed of the alkanethiol is formed on the electrode. A method for forming an organic semiconductor layer is described. In an organic semiconductor element in which an electrode and an organic semiconductor are bonded in this way, the work function of the surface is shifted by the monomolecular film on the surface of the electrode, and the potential barrier due to the contact between the electrode and the organic semiconductor layer is lowered. The contact resistance is reduced.
JP 2002-204012 A IH Campbell et al, “Applied Physics Letters”, vol 71, pp3528 (1997).

  However, even a method as described in the above prior art tends to undesirably increase the contact resistance between the electrode and the organic semiconductor. For this reason, in order to obtain a practical organic FET, it was necessary to further improve the contact between the two.

  The present invention has been made in view of such circumstances, and an object thereof is to provide an organic FET in which contact resistance between a source or drain electrode and an organic semiconductor layer is sufficiently reduced, and a method for manufacturing the same. .

  When the inventor further examined the organic semiconductor element to which the method for reducing the contact resistance according to the above-described prior art is applied, the contact between the electrode and the organic semiconductor becomes unfavorable. It became clear that it was due to the cause as shown in. That is, first, the organic FET described in Patent Document 1 easily causes electrical interaction between the electrode and the organic semiconductor layer due to carrier movement by increasing the carrier density in the vicinity of the source and drain electrodes in the organic semiconductor layer. is doing. However, even with such a technique, the affinity between the source and drain electrodes and the organic semiconductor layer remained low, so that the contact resistance could not be reduced sufficiently.

  In a so-called top contact type organic FET in which a source electrode and a drain electrode are formed on an organic semiconductor layer, these electrodes are generally formed by vapor deposition or the like. However, like the organic FET described in Patent Document 1, when the electrode and the organic semiconductor are in direct contact with each other, the organic semiconductor layer is often subjected to thermal damage due to the high temperature during the vapor deposition, thereby joining the electrode. In some cases, the organic semiconductor layer deteriorates near the portion. Such deterioration of the organic semiconductor layer has also contributed to an increase in contact resistance between the electrode and the organic semiconductor layer.

  Furthermore, in the method described in Non-Patent Document 1, since an organic semiconductor layer is formed on the electrode after a monomolecular film is formed on the electrode surface in advance, the alkanethiol constituting the monomolecular film is formed. Tends to be always oriented in a certain direction with respect to the electrode. For this reason, in the manufactured organic FET, the monomolecular film may be polarized in a direction opposite to the direction in which carriers flow. If it becomes like this, the movement of the carrier between an electrode and an organic-semiconductor layer will be suppressed by the reverse polarization state which a monomolecular film has, and it exists in the tendency for both contact resistance to become large by this.

  As a result of further research based on such knowledge, the present inventor introduced a buffer layer capable of generating an arbitrary polarization state between the source electrode or the drain electrode and the organic semiconductor layer, so that The present inventors have found that the contact resistance can be reduced and the carrier is likely to move.

That is, the organic FET of the present invention includes a source electrode and a drain electrode made of a metal material , an organic semiconductor layer serving as a channel between the source electrode and the drain electrode, and a gate electrode for controlling the amount of current passing through the channel. are formed between the at least one electrode of the organic semiconductor layer and the source and drain electrodes, Bei example a buffer layer comprising a compound having a permanent dipole moment, the permanent dipole moment contained in the buffer layer In the path constituted by the source electrode, the channel and the drain electrode, the compound having a charge having a different polarity from the carrier passing through the channel on the side close to the source electrode and the charge having the same polarity as the carrier on the side close to the drain electrode It is characterized by having an orientation .

  The buffer layer containing a compound having a permanent dipole moment in the organic FET having such a configuration can easily change its polarization state. Then, by polarizing the buffer layer in a predetermined direction, the contact resistance between the source or drain electrode and the organic semiconductor layer can be further reduced. Specifically, in the path composed of the source electrode, the channel, and the drain electrode, the carrier has a charge of a polarity different from that of the carrier passing through the channel on the side close to the source electrode, and the charge of the same polarity as the carrier on the side close to the drain electrode. Can be polarized. In an organic FET, when a buffer layer having such a polarization state is formed between a source or drain electrode and an organic semiconductor layer, carrier movement between them is extremely likely to occur, and as a result, the source or drain The resistance between the electrode and the organic semiconductor layer is significantly reduced.

  Although the cause of the tendency of carrier movement by polarization of the buffer layer in this way is not yet clear, the present inventor considers as follows. In the operation of the organic FET having the above configuration, for example, when the organic semiconductor layer is a p-type semiconductor, a negative voltage is applied to the gate electrode while applying a voltage so that the source electrode side is positive and the drain electrode side is negative. By application, holes serving as carriers in this case move through the paths of the source electrode, the channel, and the drain electrode. When the organic semiconductor layer is an n-type semiconductor, a positive voltage is applied to the gate electrode while applying a voltage so that the source electrode side is negative and the drain electrode side is positive. The resulting electrons move through the source electrode, channel and drain electrode paths.

  When carrier movement occurs in such a path, the above-described polarized buffer layer has a charge having a polarity different from that of the carrier on the carrier injection side. As a result, for example, the buffer layer provided between the source electrode and the organic semiconductor layer has a function of strongly extracting carriers from the source electrode, and the buffer layer provided between the drain electrode and the organic semiconductor layer is an organic semiconductor layer. It has the effect of pulling out the carrier strongly. Since the buffer layer has such an action, the movement of carriers from the source electrode to the channel or the movement of carriers from the organic semiconductor layer to the drain electrode occurs very easily. Thus, by forming the buffer layer between the source or drain electrode and the organic semiconductor layer, the electrical interaction between the two increases, thereby making it easier for carrier tunnel injection to occur. It is considered that carrier movement between the electrode and the organic semiconductor layer is facilitated. However, the action is not limited to these.

  Further, when the above-described top contact type organic FET is manufactured, the source or drain electrode is formed on the buffer layer in the organic FET of the present invention. For this reason, the organic FET of the present invention reduces the heat damage of the organic semiconductor layer received when the electrode is deposited, as compared with the conventional organic FET in which the source or drain electrode is directly formed on the organic semiconductor layer. Therefore, the organic semiconductor layer is less deteriorated by this.

  Furthermore, when the present inventors examined, when the organic FET of the said patent document 1 was exposed to air | atmosphere for a long time, or the frequency | count of use increased, an organic-semiconductor layer may produce deterioration with time. It turns out that there is sex. This is because in this organic FET, the dopant doped only in the organic semiconductor layer in the vicinity of the source and drain electrodes gradually diffuses or migrates to other regions of the organic semiconductor layer. In this case, an undesirably large current flows even when the transistor is off, and it becomes difficult to use it as a transistor.

  On the other hand, according to the organic FET of the present invention, it is possible to improve the contact state between the source and drain electrodes even in an organic semiconductor layer substantially composed of a single organic semiconductor. It is. Therefore, it is not necessary to introduce a dopant or the like into the organic semiconductor layer as in the conventional organic FET, and therefore, the deterioration of the organic semiconductor layer over time due to the diffusion of the dopant or the like cannot occur.

  As a compound having a permanent dipole moment that mainly constitutes the buffer layer, a functional group that exhibits electron-withdrawing property by a resonance effect or a functional group that exhibits electron-donating property by a resonance effect is bonded to a π-conjugated structure, and is asymmetric. A structure in which a π-conjugated molecule having a charge distribution of, or a functional group that exhibits an electron-withdrawing property by an induced effect or a functional group that exhibits an electron-donating property by an induced effect is bonded to a linear hydrocarbon structure Polymers having repeating units and having an asymmetric charge distribution are preferred. These compounds are organic compounds having a permanent dipole moment, and are extremely excellent in affinity with an organic semiconductor layer that is an organic material. For this reason, compared with the case where a source electrode or a drain electrode and an organic-semiconductor layer are made to contact directly, the increase in contact resistance resulting from the low affinity of both can be decreased.

  More specifically, the functional group that exhibits electron-withdrawing property due to the resonance effect is preferably at least one of an aldehyde group, an alkylcarbonyl group, a carboxyl group, an alkoxycarbonyl group, a cyano group, and a nitro group, The functional group that exhibits an electron donating property by the resonance effect is preferably at least one group selected from a halogen atom, a hydroxyl group, an alkoxy group, an amino group, and an alkylamino group.

  Examples of the π-conjugated structure to which these functional groups are bonded include a benzene ring structure, more preferably a π-conjugated structure having one or a plurality of benzene rings. The above-mentioned functional group is bonded to such a π-conjugated structure, and a π-conjugated molecule having an asymmetric charge distribution has a larger dipole moment and is composed of such a π-conjugated molecule. The buffer layer can form a polarization state more easily. As such a π-conjugated molecule, paranitroaniline is particularly preferable.

  In addition, functional groups that exhibit electron-withdrawing properties by inducing effects include halogen atoms, hydroxyl groups, alkoxy groups, amino groups, alkylamino groups, aldehyde groups, alkylcarbonyl groups, carboxyl groups, alkoxycarbonyl groups, cyano groups, and nitro groups. Of these, at least one group is preferred. A polymer having a repeating unit in which these functional groups are bonded to a linear hydrocarbon structure also has a large dipole moment, like the π-conjugated molecule. As such a polymer, polyvinylidene fluoride is particularly preferable.

  The compound having the permanent dipole moment constituting the buffer layer has a charge different in polarity from the carrier passing through the channel on the side close to the source electrode in the path constituted by the source electrode, the channel and the drain electrode. And having the same polarity as the carriers on the side close to the drain electrode, the above-described polarization state is formed in the buffer layer.

  Another form of the organic FET according to the present invention includes a source electrode and a drain electrode, an organic semiconductor layer serving as a channel between the source electrode and the drain electrode, and a gate electrode for controlling an amount of current passing through the channel. In a path formed by at least one of the source electrode and the drain electrode and the organic semiconductor layer and configured by the source electrode, the channel, and the drain electrode, a polarity different from that of the carrier passing through the channel on the side closer to the source electrode And a buffer layer having a charge of the same polarity as the carrier on the side close to the drain electrode. In the organic FET having such a configuration, the buffer layer preferably contains a compound having a permanent dipole moment.

  In this organic FET, since the buffer layer is in the polarization state as described above, it has a function of strongly attracting carriers. For this reason, it is very easy for carriers to move between the source or drain electrode and the organic semiconductor layer, thereby greatly reducing the resistance between the electrode and the organic semiconductor layer.

In addition, the organic FET manufacturing method according to the present invention includes a source electrode and a drain electrode made of a metal material , an organic semiconductor layer serving as a channel between the source electrode and the drain electrode, and an amount of current passing through the channel. A method for easily producing the organic FET of the present invention comprising a gate electrode, mainly from a compound having a permanent dipole moment between at least one of the source electrode and the drain electrode and the organic semiconductor layer. have a step of forming a configured buffer layer, after the step of forming a buffer layer, in the path constituted by the source electrode, the channel and the drain electrode, different from the carrier through the channel closer to the source electrode Buffer so that it has a charge of polarity and the same polarity as the carrier on the side close to the drain electrode Characterized by further comprising the step of polarizing the.

  In the organic FET manufactured in this way, in the path constituted by the source electrode, the channel, and the drain electrode, it has a charge of a polarity different from the carrier passing through the channel on the side close to the source electrode, and on the side close to the drain electrode. By further performing the step of polarizing the buffer layer so as to have the same polarity of charge as the carrier, the polarization state as described above can be generated in the buffer layer in the organic FET.

More specifically, the method of manufacturing the organic FET according to the present invention comprises the steps that form an insulating layer on the gate electrode and the gate electrode, forming an organic semiconductor layer on the insulation layer, on the organic semiconductor layer step process, the source over source electrode and the drain electrode at least one electrode among the individual electrodes formed so as to be positioned on the buffer layer forming a buffer layer, and constituted by the source electrode, the channel and the drain electrode In the path, each step of polarizing the buffer layer so as to have a charge of a different polarity from the carrier passing through the channel on the side close to the source electrode and to have a charge of the same polarity as the carrier on the side close to the drain electrode It is characterized by. By such a manufacturing method, a so-called top contact type organic FET in which a source electrode and a drain electrode are formed on an organic semiconductor layer is manufactured.

The method for manufacturing an organic FET according to the present invention also provides a method for easily manufacturing a bottom contact type organic FET having a source and a gate electrode under the organic semiconductor layer. Such manufacturing methods, i.e., step that form an insulating layer on the gate electrode and the gate electrode, forming a source electrode and a drain electrode on the insulation layer, on at least one electrode of the source electrode and the drain electrode Forming a buffer layer; forming an organic semiconductor layer such that the buffer layer is sandwiched between at least one of the source electrode and the drain electrode; and a path formed by the source electrode, the channel, and the drain electrode. Each step of polarizing the buffer layer so as to have a charge different in polarity from the carrier passing through the channel on the side close to the electrode and to have the same polarity as the carrier on the side close to the drain electrode .

  In these organic FET manufacturing methods, the buffer layer is formed between at least one of the source electrode and the drain electrode and the organic semiconductor layer. Accordingly, the step of polarizing the buffer layer is performed for each buffer layer when the buffer layer is provided between the source electrode and the organic semiconductor layer and between the drain electrode and the organic semiconductor layer. When a layer is formed on only one of the layers, it is performed only on the buffer layer.

  ADVANTAGE OF THE INVENTION According to this invention, organic FET by which the contact resistance between a source electrode or a drain electrode and an organic-semiconductor layer was reduced, and its manufacturing method can be provided.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described in detail with reference to the drawings. In addition, the same code | symbol is attached | subjected to the same element and the overlapping description is abbreviate | omitted. Also, the positional relationship such as up / down / left / right is based on the positional relationship of the drawings.

  FIG. 1 is a cross-sectional view schematically showing the main part of an organic FET according to the first embodiment of the present invention. The organic FET 1 includes a gate electrode 2 provided on one side (lower side in the figure) of the gate insulating film 4 (insulating layer), an organic semiconductor layer 6 provided on the other side (upper side in the figure), and a gate. On the other side of the insulating film 4 (upper side in the figure), between the source electrode 10 and the drain electrode 12 provided on the organic semiconductor layer 6, and between the organic semiconductor layer 6 and the source electrode 10 and drain electrode 12. Each of them has a buffer layer 8 provided, and is an organic FET having a so-called top contact type configuration.

  The gate electrode 2 in the organic FET 1 having such a configuration has a function of controlling the amount of drain current passing through the organic semiconductor layer 6 described later functioning as a channel. The gate electrode 2 is made of a conductive member such as polysilicon, doped Si, metal, or conductive polymer, and also serves as a substrate. Note that an insulating substrate such as a glass material, a ceramic material, or a plastic material can be provided separately from the gate electrode 2 as a substrate, and in that case, the gate electrode 2 made of the above-described material or the like is formed on these substrates.

The gate insulating film 4 formed on the gate electrode 2 is made of a material that can exhibit appropriate dielectric properties. Specific examples include inorganic dielectrics such as SiO ₂ , Al ₂ O ₃ , Si ₃ N ₄ , and TiO ₂ , and organic polymers such as polyimide, mylar, polyvinylidene fluoride, and polymethyl methacrylate. When the gate electrode 2 is made of a material that is oxidized to exhibit dielectric properties, the gate insulating film 4 can be an oxide film formed by oxidizing the surface of the gate electrode 2. Examples of the gate electrode 2 and the gate insulating film 4 include Si and SiO ₂ .

The organic semiconductor layer 6 formed on the gate insulating film 4 has a function as a so-called channel that becomes a current path between a source electrode 10 and a drain electrode 12 described later. The organic semiconductor layer 6 can be applied without distinction between p-type and n-type as long as it is an organic substance having such semiconductor characteristics as to realize this channel structure. For example, the p-type semiconductor is arranged in series such as pentacene and tetracene. Examples thereof include polycycles (acene) composed of 4 or 5 or more ortho-fused benzene rings, polyalkylthiophenes, thiophene oligomers, and the like. Examples of n-type semiconductors include C ₆₀ and fluorinated phthalocyanines.

  The buffer layer 8 is formed between the organic semiconductor layer 6 and the source electrode 10 and the drain electrode 12 so as not to contact each other. In the organic FET 1, the buffer layer 8 is formed in a polarized state.

2A and 2B are schematic enlarged views of the regions R ₁ and R ₂ around the buffer layer 8 when a p-type semiconductor is used for the organic semiconductor layer 6. As shown in the figure, each of the pair of buffer layers 8 has a charge different in polarity from holes that are carriers passing through the channel on the side close to the source electrode in the path constituted by the source electrode, the channel, and the drain electrode. The carrier has a charge of the same polarity as this carrier on the side close to the drain electrode. More specifically, the buffer layer 8 (FIG. 2A, R ₁ ) formed between the organic semiconductor layer 6 and the source electrode 10 has a negative charge on the side close to the source electrode 10, Moreover, it has a positive charge on the side close to the organic semiconductor layer 6. Further, the buffer layer 8 (FIG. 2B, R ₂ ) formed between the organic semiconductor layer 6 and the drain electrode 12 has a positive charge on the side close to the drain electrode 12, and the organic semiconductor It has a negative charge on the side close to the layer 6. In the organic FET 1, when an n-type semiconductor is used for the organic semiconductor layer 6 and carriers passing through the channel are electrons, the buffer layer 8 is opposite to the polarization state shown in FIGS. 2 (a) and 2 (b). It becomes a state polarized in the direction of.

  Here, the path constituted by the source electrode, the channel, and the drain electrode means a path through which a current flows when the organic FET 1 is operated. Such a path is indicated by a dotted line L in FIG. Is.

  The buffer layer 8 is mainly composed of a compound having a permanent dipole moment, that is, a polarity (hereinafter abbreviated as “polar compound”). In order to improve the contact state between the source electrode 10 and the drain electrode 12 and the organic semiconductor layer 6, the dipole moment of the polar compound is preferably 2D or more, and is 3 to 20D. More preferably, it is more preferable in it being 3-12D. As the compound having a permanent dipole moment, a compound that can form a film stably and is excellent in stability after film formation is preferable.

  As the polar compound, an organic polar compound and an inorganic polar compound can be used. Specific examples of the organic polar compound include a π-conjugated molecule in which a functional group that exhibits an electron-withdrawing property due to a resonance effect, or a functional group that exhibits an electron-donating property due to a resonance effect is bonded to a π-conjugated structure. Examples thereof include a polymer having, as a monomer unit, a functional group that exhibits an electron-withdrawing effect due to an effect or a structure in which a functional group that exhibits an electron-donating property due to an inducing effect is bonded to a linear hydrocarbon. In these compounds, the above functional groups are arranged so as not to have steric symmetry. This creates an asymmetric charge distribution in the polar compound, so that the polar compound has a permanent dipole moment.

A functional group that exhibits an electron-withdrawing property by a resonance effect refers to a functional group that has an effect of binding to a π-conjugated structure such as benzene and reducing the electron density of the π-conjugated structure by resonance. Specifically, for example, an aldehyde group (—CHO), an alkylcarbonyl group (—COR; R represents an alkyl group, the same shall apply hereinafter), a carboxyl group (—COOH), an alkoxycarbonyl group (—COOR), a cyano group. (-CN), include a nitro group (-NO ₂₎ and the like. In addition, the functional group exhibiting an electron donating property due to the resonance effect means a functional group having an effect of increasing the electron density of the π-conjugated structure in the above-described example, and is a halogen atom (—F, —Cl, —Br, —I). Etc.), hydroxyl group (—OH), alkoxy group (—OR), amino group (—NH ₂ ), alkylamino group (—NR ₂ ), alkyl group (—R) and the like.

As a π-conjugated structure to which a functional group that exhibits electron-withdrawing property or electron-donating name is bonded by these resonance effects, a π-conjugated structure having one benzene ring represented by the following general formula (1), or the following general formula ( A π-conjugated structure having a plurality of benzene rings represented by 2a) or (2b) can be exemplified.

  In the above formula, X is the functional group described above showing electron withdrawing property or electron donating property by resonance effect, n is 1 to 6, and m is an integer of 0 to 3. In the compounds represented by the above formulas (1), (2a) and (2b), when having a plurality of functional groups represented by X, the plurality of X may be the same or different from each other. , Selected such that the molecules are not symmetric.

  Specific examples of such a compound include paranitroaniline, ethylbenzoic acid, aminobenzonitrile, aminobenzophenone, 5-acetyl-2-methoxybenzaldehyde, aminobenzoic acid, cyanophenol, p-hydroxybenzaldehyde, 4 ′. -Nitro-4-biphenylamine, 4'-hydroxy-4-biphenylcarboxylic acid, 4'-hydroxy-4-biphenylcarbonitrile, 4'-ethyl-4-biphenylcarboxylic acid, 6-amino-2-naphthalenecarboxylic acid , 6-fluoro-2-naphthaldehyde, and 6-methoxy-2-naphthonitrile. Among these, paranitroaniline is preferable because it has a property of being easily oriented in a predetermined direction when used in the buffer layer.

  Further, the functional group that exhibits electron-withdrawing property due to the inducing effect refers to a functional group that has an effect of reducing the electron density of the structure to which the functional group is bonded through a σ bond. Examples of such functional groups include halogen atoms, hydroxyl groups, alkoxy groups, amino groups, alkylamino groups, aldehyde groups, alkylcarbonyl groups, carboxyl groups, alkoxycarbonyl groups, cyano groups, nitro groups, and the like.

The linear hydrocarbon structure to which these functional groups are bonded is preferably a hydrocarbon structure having 1 to 3 carbon atoms, and more preferably a monomer unit based on ethylene. As the repeating unit having such a structure, for example, a structure represented by the following formula (3) is preferable.

  In the formula, Y is a hydrogen atom or a functional group that exhibits an electron-withdrawing property or an electron-donating property due to the inductive effect described above. A plurality of Ys may be the same or different from each other, but are selected so that the formed polymer has no symmetry. The polymer to be a polar compound has such a repeating unit, and its weight average molecular weight is preferably about 100,000 to 3,000,000.

  Examples of such polymers include polyvinylidene fluoride, poly (vinylidene cyanide), polyacrylonitrile, polyvinyl chloride, polyvinyl fluoride, polyvinyl alcohol, polyacrylic acid, and polyacrylic acid esters. Among these, polyvinylidene fluoride is preferable.

  In addition, as the inorganic polar compound, an inorganic compound having a dipole moment in the above-described range and excellent in film formability can be appropriately selected and used. As such an inorganic compound, for example, BaO (7.954D), LiO (6.84D) and the like are preferable.

  Further, the source electrode 10 and the drain electrode 12 formed on the buffer layer 8 are made of a known conductive material, and at least one of the electrodes 10 and 12 is made of a metal material. It is preferable. As the metal material, those usually used as materials for electrodes, such as Au, Ag, Cu, and Pt, can be applied without particular limitation.

Next, an example of a procedure for manufacturing the organic FET 1 will be described with reference to FIG. FIG. 3 is a process diagram schematically showing a method of manufacturing the organic FET 1. First, after preparing a gate electrode 2 made of n-type silicon or the like and also serving as a substrate (FIG. 3A), the gate electrode 2 is subjected to an appropriate heat treatment, and a thermal oxide film having a thickness of about 30 to 500 nm ( A gate insulating film 4 made of (SiO ₂ film) is formed, and a laminated body 5 made of the gate electrode 2 and the gate insulating film 4 is formed (FIG. 3B). If the gate electrode 2 is not composed of a material that also serves as a substrate, a separate substrate is prepared, and then a layer made of metal or the like is laminated thereon by a known method to form the gate electrode 2. Furthermore, the gate insulating film 4 can be formed by vapor-depositing an insulating material or the like thereon.

  Next, the organic semiconductor material as described above is stacked on the gate insulating film 4 in the stacked body 5 by vapor deposition or the like to form the organic semiconductor layer 6 having a thickness of about 10 to 100 nm (FIG. 3C). ). Further, when it is difficult to form a uniform film by vapor deposition, for example, when a low molecular weight monomer having a molecular weight of 300 or less is used as the organic semiconductor material, the monomer is put into a matrix material made of an organic polymer material together with a solvent. The organic semiconductor layer 6 can also be formed by using a dispersed solution and applying it to the gate insulating film 4 by a spin coating method or the like. As this matrix material, for example, polyimide, mylar, polyvinylidene fluoride, polymethyl methacrylate, or the like can be used.

  Thereafter, the buffer layer 8 having a thickness of preferably 0.5 to 10 nm is formed on the organic semiconductor layer 6, and the portion on the organic semiconductor layer 6 corresponding to the position where the source electrode 10 and the drain electrode 12 are formed in a later step. (FIG. 3D). The buffer layer 8 is mainly composed of a compound having a permanent dipole moment. As a method for forming the buffer layer 8, after vapor deposition, sputtering, or the like, a material containing a polar compound is dissolved in a solvent, and this solution is applied onto the organic semiconductor layer 6 by a known method such as a spin coating method. Examples include a method of drying the solvent. In the case of the vapor deposition method, the buffer layer 8 can be formed at the above-described desired position by performing vapor deposition through a shadow mask having an opening at a portion where the buffer layer 8 is to be formed.

  Further, by depositing an electrode material made of a metal such as Au on the buffer layer 8 thus formed, the source electrode 10 and the drain electrode 12 each having a thickness of about 50 to 200 nm are formed, and the organic FET 1 is obtained. (FIG. 3 (e)). In this case, the channel length is preferably about 0.1 to 100 μm, and the channel width is preferably about 0.1 to 10 mm.

  In the manufacture of the organic FET according to the present invention, it is preferable that after the organic FET 1 is manufactured by the above-described steps, a step of causing a predetermined polarization in the buffer layer 8 in the organic FET 1 is further performed. Specifically, for example, when the organic semiconductor layer 6 is made of a p-type semiconductor material and carriers are holes, the buffer layer 8 formed between the organic semiconductor layer 6 and the source electrode 10 is used. Thus, a negative charge is polarized on the side close to the source electrode 10 and a positive charge is polarized on the side close to the organic semiconductor layer 6, and is formed between the drain electrode 12 and the organic semiconductor layer 6. The buffer layer 8 is polarized so as to have a positive charge on the side close to the drain electrode and a negative charge on the side close to the organic semiconductor layer 6.

  A specific method for generating such polarization includes a method of applying a voltage to the buffer layer 8. For example, when the organic semiconductor layer 6 is composed of a p-type semiconductor as described above, the side close to the source electrode 10 in the buffer layer 8 is positive and organic with respect to the buffer layer between the source electrode 10 and the organic semiconductor layer 6. A voltage is applied so that the side close to the semiconductor layer 6 is negative. On the other hand, a voltage is applied to the buffer layer 8 between the drain electrode 12 and the organic semiconductor layer 6 so that the side close to the drain electrode 12 in the buffer layer 8 is negative and the side close to the organic semiconductor layer 6 is positive. . The voltage can be applied to the buffer layer 8 by applying a voltage to the source and drain electrodes 10 and 12 and the gate electrode 2 formed in the stacked body so as to satisfy the above conditions. In the case where the organic semiconductor layer 6 of the organic FET 1 is composed of an n-type semiconductor, a voltage is applied in the direction opposite to the above in the polarization step.

  When a voltage is applied to the buffer layer 8 under these conditions, since the polar compound constituting the buffer layer 8 has a dipole moment, the molecular compound corresponds to the potential applied to each electrode. The orientation is changed so that the charge is oriented in the above-described direction, and thus a polarization state is formed in the buffer layer 8.

  In this case, it is preferable to apply a voltage while heating the organic FET 1 from the viewpoint of easily causing a change in the molecular orientation state. For example, when the buffer layer is mainly composed of a polymer, the heating is desirably performed at a temperature equal to or higher than the glass transition temperature of the polymer. By doing so, the molecular structure of the polymer can be changed relatively freely, and the change in the orientation state of the polymer due to the application of voltage is more likely to occur. Further, when the organic FET 1 is cooled to the glass transition temperature or lower after the orientation, the orientation state of the polymer is fixed and it becomes easy to maintain a suitable polarization state.

  In the organic FET 1, if the buffer layer 8 is polarized as described above, carrier movement easily occurs between the source and drain electrodes 10 and 12 and the organic semiconductor layer 6. For this reason, the contact resistance between the electrodes 10 and 12 and the organic semiconductor layer 6 is extremely smaller than that of a conventional organic FET in which the electrodes and the organic semiconductor are directly joined.

  Further, since the buffer layer 8 is composed of an organic compound having a permanent dipole moment, the affinity with the organic semiconductor layer 6 is good. By interposing the buffer layer 8 between the source and drain electrodes 10 and 12 and the organic semiconductor layer 6, the electrodes 10 and 12 having low affinity with the organic semiconductor layer 6 are brought into direct contact with each other. Compared with the case, it also has the effect | action that the resistance between these can be reduced significantly.

  Furthermore, since the source electrode 10 and the drain electrode 12 are formed on the organic semiconductor layer 6 via the buffer layer 8, thermal damage to the organic semiconductor layer 6 when the electrodes 10 and 12 are formed by vapor deposition or the like. The organic semiconductor layer hardly deteriorates during the production of the organic FET, which has been a problem in the past.

  The first embodiment of the organic FET of the present invention has been described above. However, such a top contact type organic FET is not necessarily limited to the above-described configuration, and does not depart from the spirit of the present invention. Various modifications are possible within the range.

  For example, the buffer layer 8 is not necessarily formed between both the source electrode 10 and the drain electrode 12 and the organic semiconductor layer 6, and at least one of the electrodes 10 and 12 and the organic semiconductor layer 6 If formed in between, the effect of improving the contact state can be exhibited. When only one of the electrodes 10 and 12 is made of a metal material, it is desirable that the buffer layer 8 be formed on the side of the electrode made of metal.

  Moreover, the buffer layer 8 should just be provided so that at least the source electrode 10 and the drain electrode 12 and the organic-semiconductor layer 6 may not contact. For this reason, for example, the size of the buffer layer 8 may be larger than the electrodes 10 and 12 formed on the upper portion.

  Next, a second embodiment of the organic FET of the present invention will be described. FIG. 4 is a cross-sectional view schematically showing a main part of an organic FET according to the second embodiment of the present invention. The organic FET 11 is an organic FET having a so-called bottom contact type structure, and a gate electrode 2 for controlling the amount of drain current passing through the channel is provided on one side (lower side in the figure) of the gate insulating film 4 (insulating layer). The source electrode 10 and the drain electrode 12 are formed and arranged on the other side (upper side in the drawing) of the gate insulating film 4 with a certain interval. Further, a buffer layer 18 mainly composed of a polar compound is formed on the other side (the upper side in the drawing) of the gate insulating film 4 and on the source electrode 10 and the drain electrode 12. An organic semiconductor layer 6 serving as a channel between the source electrode 10 and the drain electrode 12 is formed above and between them. Each component of the organic FET 11 can be formed of the same material as the organic FET 1 described above. The buffer layer 18 has a charge different in polarity from the carrier passing through the channel on the side close to the source electrode in the path constituted by the source electrode, the channel and the drain electrode, and has the same polarity as the carrier on the side close to the drain electrode. Have a charge of.

  This organic FET 11 can be manufactured as follows, for example. That is, first, the gate electrode 2 is prepared, and the gate insulating film 4 is formed thereon by the same method as in the organic FET 1 to obtain a stacked body. Next, the source electrode 10 and the drain electrode 12 arranged at a predetermined interval are formed on the gate insulating film 4 of the stacked body. In this case, the method for forming both electrodes 10 and 12 is not particularly limited. For example, after the electrode material is formed on the gate insulating film 4 by sputtering, patterning is performed by photolithography or the like.

  Next, a buffer layer 18 made of a polar compound is formed on the formed source electrode 10 and drain electrode 12 by the same means as in the organic FET 1. At this time, the buffer layer 18 is preferably formed so as to cover both the electrodes 10 and 12 in order to prevent the electrodes 10 and 12 from contacting the organic semiconductor layer 6 formed thereon. .

  After the buffer layer 18 is formed, the organic semiconductor layer 6 is formed so as to provide a current path for the source electrode 10 and the drain electrode 12 above and between the buffer layers 18 formed on the respective electrodes, thereby obtaining the organic FET 11. . As a method for forming the organic semiconductor layer 6, a vapor deposition method is preferable.

  After manufacturing the organic FET 11 in this manner, a step of causing the buffer layer 18 to generate a predetermined polarization is further performed. In this polarization step, for example, when a p-type semiconductor is used for the organic semiconductor layer 6 and the carriers are holes, the organic FET 11 obtained is formed between the organic semiconductor layer 6 and the source electrode 10. The buffer layer has a negative charge on the side close to the source electrode 10 and a positive charge on the side close to the organic semiconductor layer 6, and is formed between the drain electrode 12 and the organic semiconductor layer 6. The buffer layer 8 is polarized so as to have a positive charge on the side close to the drain electrode 12 and a negative charge on the side close to the organic semiconductor layer 6. On the other hand, when an n-type semiconductor is used for the organic semiconductor layer 6, the buffer layer 8 is polarized so as to be in the opposite state. Such a polarization step can be performed in the same manner as in the manufacture of the organic FET 1 described above.

  The method for causing polarization in the buffer layer 18 is not necessarily limited to such a method, and various methods can be applied. For example, a method of forming an electrode for polarization for causing polarization on the organic semiconductor layer 6 in the organic FET 11 can be mentioned. FIG. 5 is a cross-sectional view schematically showing a main part of an organic FET further having a polarization electrode. As illustrated, the organic FET 21 has a configuration in which a polarization electrode 20 is further provided on the organic semiconductor layer 6 in the organic FET 11.

  In the organic FET 21, a predetermined polarization can be generated by applying a voltage between the source electrode 10 or the drain electrode 12 and the polarization electrode 20. Specifically, for example, when the organic semiconductor layer 6 is made of a p-type semiconductor, the polarization electrode 20 is negative with respect to the buffer layer 18 formed between the source electrode 10 and the organic semiconductor layer 6. A voltage is applied so that the electrode is positive, and the voltage is applied to the buffer layer 8 between the drain electrode 12 and the organic semiconductor layer 6 so that the polarization electrode 20 is positive and the drain electrode 12 is negative. Apply. After the polarization step, the polarization electrode 20 is preferably removed from the organic FET 21, but the polarization electrode 20 may remain installed in the organic FET 21 as long as the function as the FET is not impaired. Absent.

  The bottom contact type organic FET 11 and the organic FET 21 configured as described above can be variously modified in the same manner as the organic FET 1 described above. For example, the buffer layer 18 does not necessarily have to be formed on both the source electrode 10 and the drain electrode 12, and if it is formed on either one, the source electrode 10 or the drain electrode 12 and the organic semiconductor The effect of improving the contact state with the layer 6 can be exhibited.

  In the organic FET 11 and the organic FET 21, the organic semiconductor layer 6, the source electrode 10 and the drain electrode 12 are in contact with each other with a buffer layer 18 mainly composed of a polar compound interposed therebetween. For this reason, as in the case of the organic FET 1, the resistance due to contact between the electrodes 10, 12 and the organic semiconductor layer 6 becomes extremely small due to the buffer layer 18.

  The organic FET having the top contact type and the bottom contact type structure according to the present invention has been described above, but the organic FET of the present invention is not necessarily limited to these structures. Specifically, for example, a top-and-bottom contact type organic FET in which one of a source electrode and a drain electrode is formed on one side of the organic semiconductor layer and the other electrode is formed on the other side, or a source electrode A vertical organic FET in which a gate electrode is formed between the gate electrode and the drain electrode and an organic semiconductor layer serving as a channel between the source electrode and the drain electrode is formed around the gate electrode is also an organic FET of the present invention. included. In any organic FET, a buffer layer mainly composed of a polar compound is formed between at least one of the source electrode and the drain electrode and the organic semiconductor layer, whereby the source electrode or the drain electrode And the contact resistance between the organic semiconductor layer is low.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these.

[Manufacture of organic semiconductor elements]
(Reference Example 1)
Gold was vacuum-deposited on a glass substrate (# 7059, manufactured by Corning) using a resistance heating evaporation source to form a lower electrode having a thickness of 100 nm. On this lower electrode, a cyclohexanone solution of polyvinylidene fluoride was applied by spin coating, and then dried under reduced pressure to form a buffer layer having a thickness of 10 nm. Next, pentacene was vacuum-deposited on the buffer layer under a vacuum condition of 1 × 10 ⁻⁶ Torr (1.33 × 10 ⁻⁴ Pa) to form an organic semiconductor layer having a thickness of 100 nm. Furthermore, gold was vacuum-deposited on the organic semiconductor layer to form an upper electrode having a thickness of 100 nm to obtain a laminate. And while heating the obtained laminated body to 50 degreeC, the voltage of the lower electrode was set to 0V, the voltage of + 10V was applied to the upper electrode, and it returned to room temperature, and the organic-semiconductor element was obtained.

(Comparative Reference Example 1)
An organic semiconductor element was obtained in the same manner as in Reference Example 1 except that the buffer layer was not formed.

[Characteristic evaluation]
For the organic semiconductor elements obtained in Reference Example 1 and Comparative Reference Example 1, a semiconductor parameter analyzer (4155C, manufactured by Agilent Technologies) was used, and the voltage applied to the upper electrode was continuously changed in the range of −15 to + 15V. Each current value obtained was measured. FIG. 6 is a graph showing a current value with respect to an applied voltage obtained by the organic semiconductor elements of Reference Example 1 and Comparative Reference Example 1 (the curve showing the current value with respect to the applied voltage is hereinafter abbreviated as “voltage-current curve”). It is. In addition, the value of the applied voltage in the figure indicates the potential on the upper electrode side when the lower electrode side is set to 0V. In FIG. 6, the curve indicated by the solid line is the voltage-current curve obtained by the organic semiconductor element of Reference Example 1, and the curve indicated by the dotted line is the voltage-current curve obtained by the organic semiconductor element of Comparative Reference Example 1. Each is shown.

  From FIG. 6, the organic semiconductor element of Reference Example 1 in which the buffer layer is formed has a positive potential on the upper electrode with respect to the lower electrode as compared with the organic semiconductor element of Comparative Reference Example 1 in which the buffer layer is not formed. In other words, it was found that a remarkably large current value can be obtained when a voltage is applied in the same direction as when the element was formed.

[Manufacture of organic FET]
Example 1
First, a highly doped n-type silicon substrate (bulk resistivity: 1 Ωcm) also serving as a gate electrode on which a thermal oxide film of about 200 nm was formed as a gate insulating film was prepared, and this was cut into a 9 × 25 mm rectangular plate shape. . Next, pentacene was vacuum-deposited on the gate insulating film of the substrate to form an organic semiconductor layer having a thickness of about 50 nm.

  On this organic semiconductor layer, paranitroaniline was vacuum-deposited through a shadow mask in which a portion to be a channel portion was separated to form a buffer layer having a thickness of about 2 nm. Further, a gold film having a thickness of about 80 nm was vacuum deposited on the buffer layer to form a source electrode and a drain electrode, thereby obtaining a top contact type organic FET. The channel length was 20 μm and the channel width was 5 mm.

(Example 2)
An organic FET was produced in the same manner as in Example 1, except that a cyclohexanone solution of polyvinylidene fluoride was spin-coated on the organic semiconductor layer and then dried under reduced pressure to form a buffer layer.

  Next, in a state where the organic FET is heated to 50 ° C. under reduced pressure, a voltage of +50 V is applied to the source electrode and a voltage of −50 V to the gate electrode is independently applied to the drain electrode. Then, it was cooled to room temperature to obtain an organic FET for characteristic evaluation.

(Comparative Example 1)
Organic FET was obtained like Example 1 except not having formed the buffer layer.

(Characteristic evaluation)
For the organic FETs obtained in Examples 1 and 2 and Comparative Example 1, the change in drain current with respect to the gate voltage was measured. This measurement is performed using a semiconductor parameter analyzer (4155C, manufactured by Agilent Technologies), and the voltage between the gate and the source electrode (gate voltage) with respect to the voltage between the drain and source electrodes (drain voltage) of various values. ) Was continuously monitored to monitor the value of the current (drain current) flowing between the source electrode and the drain electrode.

Next, the field effect mobility and the gate voltage threshold value (Vth) in the FET structure were calculated from the plot of the change in drain current against the obtained gate voltage. Specifically, the square root of the drain current value versus the gate voltage value when the drain voltage is set to −100 V is plotted. The mobility was calculated from the slope of the tangent line at a gate voltage of −50 V, which is a condition for obtaining a sufficiently saturated region in this plot. Vth was derived by reading the X-axis intercept at this tangent. The obtained mobility and Vth results are shown in Table 1.

  From Table 1, the organic FETs of Example 1 and Example 2 having a buffer layer have significantly higher mobility and threshold voltage than the organic FET of Comparative Example 1 having no buffer layer. It turned out to be low. In particular, the organic FET of Example 2 in which the buffer layer was polarized by applying a voltage at the time of manufacture showed a remarkably good result, so when the buffer layer is polarized along the carrier movement direction, It has been found that the best results are obtained.

It is sectional drawing which shows typically the principal part of organic FET which concerns on 1st Embodiment of this invention. (A) and (b) is a schematic enlarged view of the region near the buffer layer 8 R _1, R ₂ in the case of using a p-type semiconductor in the organic semiconductor layer 6. It is process drawing which shows the manufacturing method of organic FET1 typically. It is sectional drawing which shows typically the principal part of organic FET which concerns on 2nd Embodiment of this invention. It is sectional drawing which shows typically the principal part of organic FET which further has an electrode for polarization. It is a graph which shows the electric current value with respect to the applied voltage obtained with the organic-semiconductor element of the reference example 1 and the reference example 2. FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Organic FET, 2 ... Gate electrode, 4 ... Gate insulating film, 5 ... Laminated body, 6 ... Organic-semiconductor layer, 8 ... Buffer layer, 10 ... Source electrode, 11 ... Organic FET, 12 ... Drain electrode, 18 ... Buffer Layer, 20 ... electrode for polarization, 21 ... organic FET.

Claims (9)

A source electrode and a drain electrode made of a metal material ;
An organic semiconductor layer serving as a channel between the source electrode and the drain electrode;
A gate electrode for controlling the amount of current through the channel;
A buffer layer that is formed between the organic semiconductor layer and at least one of the source electrode and the drain electrode and includes a compound having a permanent dipole moment ;
The compound having the permanent dipole moment contained in the buffer layer has a charge different in polarity from carriers passing through the channel on a side close to the source electrode in a path constituted by the source electrode, the channel, and the drain electrode. the a, are oriented so as to have the carrier to the same polarity of charge on the side closer to the drain electrode, organic electromechanical field effect transistor.   The compound having a permanent dipole moment has an asymmetric charge distribution in which a functional group that exhibits electron-withdrawing property by a resonance effect or a functional group that exhibits electron-donating property by a resonance effect is bonded to a π-conjugated structure. a π-conjugated molecule, or a functional group that exhibits an electron-withdrawing property by an inductive effect or a structure in which a functional group that exhibits an electron-donating property by an inductive effect is bonded to a linear hydrocarbon structure as a repeating unit; 2. The organic field effect transistor according to claim 1, which is a polymer having an asymmetric charge distribution.   The functional group exhibiting electron withdrawing property due to the resonance effect is at least one group of aldehyde group, alkylcarbonyl group, carboxyl group, alkoxycarbonyl group, cyano group and nitro group, and exhibits electron donating property due to the resonance effect. 3. The organic field effect transistor according to claim 2, wherein the functional group is at least one of a halogen atom, a hydroxyl group, an alkoxy group, an amino group, and an alkylamino group.   The organic field effect transistor according to claim 2 or 3, wherein the π-conjugated structure is a π-conjugated structure having one or a plurality of benzene rings.   The functional group that exhibits electron withdrawing due to the inducing effect is a halogen atom, a hydroxyl group, an alkoxy group, an amino group, an alkylamino group, an aldehyde group, an alkylcarbonyl group, a carboxyl group, an alkoxycarbonyl group, a cyano group, or a nitro group. 3. The organic field effect transistor according to claim 2, which is at least one group.   2. The organic field effect transistor according to claim 1, wherein the compound having a permanent dipole moment is polyvinylidene fluoride or paranitroaniline. Organic field effect comprising: a source electrode and a drain electrode made of a metal material ; an organic semiconductor layer serving as a channel between the source electrode and the drain electrode; and a gate electrode for controlling the amount of current passing through the channel. A method of manufacturing a transistor, comprising:
Between at least one electrode and the organic semiconductor layer of the source electrode and the drain electrode, it has a step of forming a buffer layer comprising a compound having a permanent dipole moment,
After performing the step of forming the buffer layer, in the path constituted by the source electrode, the channel, and the drain electrode, has a charge of a polarity different from the carrier passing through the channel on the side close to the source electrode, further comprising, chromatic electromechanical field effect method for producing a transistor of the step of polarizing the buffer layer so as to have a charge of the same polarity as the carrier closer to the drain electrode. A step that form an insulating layer on the gate electrode and the gate electrode,
Before Symbol insulating layer, and forming the organic semiconductor layer,
Before SL organic semiconductor layer, and forming the buffer layer,
A step of at least one of the electrodes is formed so as to be positioned on the buffer layer of the prior SL source electrode and the said drain electrode each electrode,
In the path constituted by the source electrode, the channel, and the drain electrode, it has a charge of a polarity different from that of the carrier passing through the channel on the side close to the source electrode, and the same polarity as the carrier on the side close to the drain electrode Polarizing the buffer layer to have a charge of
The manufacturing method of the organic field effect transistor of Claim 7 which has these. A step that form an insulating layer on the gate electrode and the gate electrode,
Before Symbol insulating layer, and forming the source electrode and the drain electrode,
Forming the buffer layer on at least one of the source electrode and the drain electrode;
Forming the organic semiconductor layer so as to sandwich the buffer layer between at least one of the source electrode and the drain electrode;
In the path constituted by the source electrode, the channel, and the drain electrode, it has a charge of a polarity different from that of the carrier passing through the channel on the side close to the source electrode, and the same polarity as the carrier on the side close to the drain electrode Polarizing the buffer layer to have a charge of
The manufacturing method of the organic field effect transistor of Claim 7 which has these.

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