JP4224578B2 - Organic thin film transistor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thin film transistor, and more particularly to a technique for improving a thin film transistor (TFT) using an organic semiconductor in a semiconductor layer.
[0002]
[Prior art]
Thin film transistors are used as display driving transistors and the like. Today, displays and the like have a tendency to increase in size and to become flexible. Along with this, the display drive electronic circuit is also required to have a technology for dealing with a large area and a technology for making it flexible, and the thin film transistor that plays a central role in the drive electronic circuit is also required to support a large area and make it flexible. ing. Furthermore, as portable displays are becoming more and more widespread, there is a growing demand for lower costs.
[0003]
At present, amorphous silicon thin film transistors or polycrystalline silicon thin film transistors are used for such purposes. Polycrystalline silicon thin film transistors have good performance, but have a problem that it is difficult to increase the area or the manufacturing cost is extremely high. On the other hand, even with an amorphous silicon thin film transistor, there is a limit to increasing the area, and there is a problem that the cost increases in order to realize flexibility. In recent years, development of a thin film transistor using an organic semiconductor that facilitates such elements has been studied.
[0004]
The development of thin film transistors using organic semiconductors has been gradually active since the late 1980s, and in recent years, the characteristics of amorphous silicon thin film transistors have been exceeded as basic performance. As a typical example, the performance of a thin film transistor created by Shane et al. Using pentacene as an organic semiconductor is reported in Science, 287, 1022 (2000).
[0005]
An organic thin film transistor exhibits relatively high TFT characteristics such as field effect mobility when the crystallinity of the organic semiconductor constituting the semiconductor thin film layer is extremely high. However, when the organic semiconductor has a low crystallinity and is in a polycrystalline state, carrier trap sites are generated at the crystal grain boundaries, thereby reducing the TFT characteristics such as mobility. In particular, it has been reported that the mobility decreases as the temperature decreases. (Applied Physics Letters, Vol. 74, No. 2, pp. 260-262, 1999)
[0006]
In general, in order for the organic semiconductor layer to be configured with high crystallinity, it is necessary to take a technique such as single crystal growth, but in order to obtain a size capable of forming a thin film transistor, an extremely difficult technique must be taken, The element manufacturing process has a problem that it becomes extremely difficult. On the other hand, when a simple thin film manufacturing technique such as a vacuum deposition method is applied, the crystallinity in the thin film is lowered, and there is a problem that the thin film is formed in a polycrystalline state or an amorphous state. In consideration of a realistic element manufacturing process, the organic semiconductor layer must be formed by a general-purpose thin film process that can easily manufacture elements. For this reason, various techniques for improving the quality such as the size and crystallinity of the polycrystal in the organic semiconductor thin film have been studied.
[0007]
In the literature (Synthetic Metals, 122, 185-189, 2001), an organic semiconductor layer is formed in a polycrystalline form in an organic FET, and the grain size is determined from the source-drain electrode width (channel length). If the crystal grain size of pentacene formed in a thin film is larger, that is, the smaller the grain boundary, the better FET characteristics can be obtained.
[0008]
Japanese Patent Application Laid-Open No. 2000-174277 discloses a technique of using phthalocyanine for a semiconductor layer in a thin film transistor and performing a solvent treatment in order to improve the crystallinity thereof.
[0009]
Japanese Patent Laid-Open No. 2001-94107 discloses a surface treatment technique for realizing an improvement in crystallinity under the problem that an improvement in crystallinity of an organic semiconductor thin film in a thin film transistor is essential for improving TFT performance. It is disclosed that it has been successful in greatly modulating the drain current.
[0010]
Y. Lin et al. Disclosed the results of reducing the trap of the pentacene layer by forming two layers of the pentacene deposited film on the surface of the gate insulating film by different techniques (IEEE Electron Devices Letters, Vol. 18, No. 12, 606-608, 1997). In this case, −80 V is used as the drain voltage and −100 V is used as the gate voltage, which is too high for a practical transistor.
[0011]
[Problems to be solved by the invention]
In organic thin-film transistor technology created by general-purpose thin film processes such as vacuum evaporation, the semiconductor is deposited in a polycrystalline state, so that a sufficiently large mobility cannot be obtained due to an energy barrier at the grain boundary. There is a problem that the source-drain current cannot be sufficiently large. An object of the present invention is to provide a technique for reducing the energy barrier at the crystal grain boundary and increasing the efficiency of extraction of the source-drain current.
[0012]
[Means for Solving the Problems]
In the present invention, in order to introduce another substance that reduces the energy barrier into the crystal grain boundary part of the organic semiconductor thin film, as shown in FIGS. 1 and 2, the insulating layer 60 in the organic semiconductor layer 50 and the counter electrode side, The organic layer 20 having a polarity opposite to that of the semiconductor layer 50 is attached. As a result, carrier traps were reduced, and the efficiency of the source-drain current was successfully increased.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
The organic thin film transistor in the present invention is generally composed of a combination of a p-type semiconductor layer and an insulating layer, and in this case, an electron-accepting compound layer on the insulating layer side and the counter electrode of the p-type semiconductor layer. As a result, the field effect mobility is improved, but a field effect transistor is formed by a combination of an n-type semiconductor layer and an insulating layer, and the n-type semiconductor layer is formed on the opposite side of the insulating layer. A transistor which realizes an improvement in field effect mobility may be formed by forming an electron donating compound layer.
[0014]
As a material of the source electrode 10 and the drain electrode 30 used in the present invention, when a carrier for transporting a charge is a p-type semiconductor, a metal having a large work function is used to make ohmic contact with the semiconductor layer Is desirable. Examples thereof include gold and platinum, but are not limited thereto. Indium, palladium, silver, or the like can also be used. In the case of an n-type semiconductor in which carriers for transporting charges are electrons, it is desirable to use a metal having a low work function, such as aluminum, magnesium, or calcium. There are no particular limitations on the method of producing them, and any method may be used. In general, a method suitably used is a vacuum deposition method or a sputtering method.
[0015]
The gate electrode 40 used in the present invention is often made of gold, but is not limited thereto. The production method is not particularly limited, and any method may be used. In general, a method suitably used is a vacuum deposition method or a sputtering method.
[0016]
The gate dielectric 60 used in the present invention is preferably a material having a large dielectric constant in order to obtain a more effective electric field effect. Examples thereof include SiO ₂ and Al ₂ O ₃ , but are not limited thereto, and polymer dielectrics such as PMMA and polyimide can also be used in order to impart flexibility. It is also possible to coat the gate dielectric to increase the crystal grains of the semiconductor thin film.
[0017]
The organic thin film transistor according to the present invention often has a top contact type element structure in which the source electrode 10 and the drain electrode 30 are configured to be opposed to the insulating layer with the semiconductor layer 50 interposed therebetween, but the element structure is limited to this. It is not something.
[0018]
In the organic thin film transistor according to the present invention, an organic semiconductor material is used for the semiconductor layer 50. The following are known as organic semiconductor materials exhibiting excellent characteristics so far. Anthracene, tetracene, pentacene or derivatives thereof substituted at the terminal. α-sexual thiophene. Copper phthalocyanine and derivatives whose ends are substituted with fluorine or the like. Derivatives in which copper of copper phthalocyanine is substituted with nickel, titanium oxide, fluorinated aluminum or the like, and derivatives in which each terminal is substituted with fluorine or the like. Rubrene, coronene, anthradithiophene and derivatives substituted at their ends.
[0019]
The semiconductor layer in the present invention is prepared by a vacuum deposition method, and the deposition rate at this time is 0.6 nm to 200 nm, preferably 6 nm to 60 nm per minute. Moreover, the substrate temperature at the time of vapor deposition is 0 degreeC or more and 100 degrees C or less, Preferably they are 20 degreeC or more and 70 degrees C or less. The thickness of the layer is 10 nm to 200 nm, preferably 20 nm to 100 nm.
[0020]
In the organic thin film transistor according to the present invention, an electron-accepting organic compound is used for the semiconductor coating layer 20. In order to obtain a higher coating effect, a compound having a higher electron accepting property is desirable. For example, N, N′-diphenyl-N, N′-di (m-tolyl) benzidine (TPD), naphthalenetetracarboxylic anhydride, 3,4,9,10-perylenetetracarboxylic acid-3,4,9 , 10-dianhydride, N, N′-bis (phenylethyl) -perylene-3,4,9-bis (dicarboximide), fullerene, tetracyanoethylene, tetracyanoquinodimethane, triphenylmethane, oxadi Examples thereof include, but are not limited to, azoles, 8-quinolinol aluminum complexes, trinitrofluorene, bisstyrylanthracene, distyrylbenzene, and derivatives of these compounds in which a part of the ends are substituted. Any material may be used as long as it has a higher electron-accepting property than the organic semiconductor material used for the semiconductor layer 50, that is, a material having a higher electron affinity than the organic semiconductor material.
[0021]
The semiconductor coating layer 20 in the present invention is prepared by a vacuum deposition method, but the substance to be deposited at this time needs to be deposited in an amorphous state. As a condition for this, the deposition rate is desirably a relatively high speed, and is 6 nm or more per minute, preferably 10 nm or more and 60 nm or less per minute. Moreover, the substrate temperature at the time of vapor deposition should be as low as possible, and is preferably −100 ° C. or lower. The thickness of the layer to be deposited is not more than the thickness of the organic semiconductor layer, and preferably not less than 1 nm and not more than 50 nm.
[0022]
In the present invention, in the organic thin film transistor shown in FIGS. 1 and 2, the semiconductor coating layer 20 is formed between the source electrode 10 and the drain electrode 30 and on the organic semiconductor layer 50. The semiconductor coating layer 20 may or may not be in contact with the source electrode 10 or the drain electrode 30. Further, a part of the semiconductor coating layer 20 may be in a state of covering the source electrode 10 or the drain electrode 30.
[0023]
In the present invention, the degree of vacuum of the container, the deposition source, and the distance between the deposition source and the substrate during vacuum deposition are not particularly limited. Generally, the degree of vacuum that is used simply is about 10 ⁻⁷ to 10 ⁻⁵ Torr, but a higher vacuum is desirable.
[0024]
The substrate used in the present invention is not particularly limited, and any substrate may be used. In general, a glass substrate such as quartz or a silicon wafer is preferably used, but a flexible transparent plastic substrate such as polycarbonate, polyimide, or PET can also be used.
[0025]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.
[0026]
This will be described with reference to FIG. The n-type silicon substrate 70 grown using the silicon thermal oxide film 300 nm as the insulating layer 60 is subjected to ultrasonic cleaning with a neutral detergent diluted with pure water (Inoue Seieido Co., Ltd .: Pure Soft). The detergent was removed by ultrasonic cleaning. After that, UV irradiation cleaning was performed for 20 minutes with an UV-ozone cleaner. A pentacene thin film 50 was formed on the thus cleaned substrate by vacuum deposition. Pentacene was purified by sublimation purification 10 times. The vacuum deposition conditions were such that the substrate was fixed above the deposition boat, the substrate temperature was adjusted to about −130 ° C., and the degree of vacuum was reduced to 2 × 10 ⁻⁶ Torr. Thereafter, vacuum deposition was performed to a thickness of 50 nm at a rate of 10 nm per minute. Subsequently, as the source electrode 10 and the drain electrode 30, a gold electrode was vacuum-deposited to a thickness of 50 nm at a deposition rate of 6 nm per minute at 30 ° C. of the substrate. Subsequently, N-methyl-3,4,9,10 tetracarboxylic acid diimide was vacuum deposited at a rate of 6 nm per minute to a thickness of 50 nm. In the above embodiment, the gate electrode 40 is formed by doping the substrate 70 with n-type impurities, but the gate electrode may be separately formed on the substrate 70.
[0027]
FIG. 3 shows the source-drain voltage dependence of the source-drain current at the gate voltage of 25 V of the element thus fabricated. The solid line shows the effect when the pentacene layer is covered with the perylene layer, and the wavy line shows the effect when the pentacene layer is not covered with the perylene layer as a reference. When the pentacene layer was coated with a perylene layer, the source-drain current was improved by about 30% compared to the case where the perylene layer was not coated.
[0028]
【The invention's effect】
In the organic thin film transistor of the present invention, since the energy barrier for carrier movement in the semiconductor layer is reduced, low voltage driving can be realized. In addition, since the structure covers the semiconductor layer that causes carrier movement, it also functions as a sealing effect and reduces deterioration of the element due to oxygen and moisture. In addition, since the semiconductor layer, which is the main component layer for carrier movement, is composed of an organic semiconductor, it is easy to manufacture and can be made into a film element, a large-area element, and a flexible element, and the impact resistance is also improved. .
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of an example of an organic thin film transistor in the present invention.
FIG. 2 is a schematic plan view of an example of an organic thin film transistor according to the present invention as viewed from the source and drain electrodes.
FIG. 3 shows the source-drain voltage dependence of the source-drain current when the gate voltage is 25 V in Example 1. Solid line: effect when perylene is adhered to the pentacene layer, wavy line: effect when perylene is not adhered to the pentacene layer.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Source electrode 20 ... Electron-accepting compound layer 30 ... Drain electrode 40 ... Gate electrode 50 ... P-type semiconductor layer 60 ... Insulating layer 70 ... N-type substrate

Claims (8)

A gate electrode is formed in or on the substrate, an insulating film is formed to cover the substrate and the gate electrode , a p-type organic semiconductor layer is formed on the insulating film, and a source is formed on the p-type organic semiconductor layer In the organic semiconductor transistor in which the electrode and the drain electrode are formed, an electron-accepting compound layer made of a single compound is formed on the p-type organic semiconductor layer between the source electrode and the drain electrode. An organic thin film transistor characterized by comprising: A gate electrode is formed in or on the substrate, an insulating film is formed to cover the substrate and the gate electrode , an n-type organic semiconductor layer is formed on the insulating film, and a source is formed on the n-type organic semiconductor layer An organic thin film transistor in which an electrode and a drain electrode are formed, wherein an electron donating compound layer is formed on the n-type organic semiconductor layer between the source electrode and the drain electrode. Thin film transistor. 2. The organic thin film transistor according to claim 1 , wherein the p-type organic semiconductor layer is pentacene, tetracene, anthracene, or a derivative compound having these as a skeleton . 2. The organic thin film transistor according to claim 1, wherein the electron accepting compound is N, N′-diphenyl-N, N′-di (m-tolyl) benzidine (TPD), naphthalenetetracarboxylic acid anhydride, 3,4, It is 9,10-perylenetetracarboxylic acid-3,4,9,10-dianhydride or N, N′-bis (phenylethyl) -perylene-3,4,9-bis (dicarboximide) An organic thin film transistor. In the organic thin film transistor according to claim 1, the organic semiconductor thin film transistor, wherein the electron-accepting compound are deposited in an amorphous state. A gate electrode is formed in or on the substrate, an insulating film is formed to cover the substrate and the gate electrode , a p-type organic semiconductor layer is formed on the insulating film, and a source electrode and a gate electrode are formed on the organic semiconductor layer. In the manufacturing method of the organic thin-film transistor in which the drain electrode is formed, the electron-accepting compound layer which consists of a single compound is formed on the said source electrode, the said drain electrode, and the said organic-semiconductor layer, The organic thin-film transistor characterized by the above-mentioned Manufacturing method. A gate electrode is formed in or on the substrate, an insulating film is formed to cover the substrate and the gate electrode, an n-type organic semiconductor layer is formed on the insulating film, and a source electrode and a gate electrode are formed on the organic semiconductor layer. In the manufacturing method of the organic thin-film transistor in which the drain electrode is formed, the electron-donating compound layer which consists of a single compound is formed on the said source electrode, the said drain electrode, and the said organic-semiconductor layer, The organic thin-film transistor characterized by the above-mentioned Manufacturing method . 7. The method of manufacturing an organic thin film transistor according to claim 6, wherein the formation of the electron-accepting compound layer is performed by cooling the substrate to 0 ° C. or less and vacuuming the substrate to a thickness less than the thickness of the organic semiconductor layer at a rate of 1 nm or more. A method for producing an organic thin film transistor, characterized by being deposited by vapor deposition.

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