JPH04133351A - Electronic element using oligothiophene - Google Patents

Abstract

PURPOSE:To attain excellent performance characteristics without conducting additional treatment such as heat treatment, electrochemical dope-removing, etc., to an oligothiophene film by using specific oligothiophene, a terminal of which is methylated or ethylated, as a channel on a silicon oxide film between a source and a drain. CONSTITUTION:A source electrode 3 and a drain electrode 4 are formed onto a silicon oxide film 2 formed on a surface by oxidizing an n-type silicon substrate 1. The thin-film 6 of oligothiophene shown in formula I is shaped to a channel section on the silicon oxide film 2, thus manufacturing a field-effect transistor. Oligothiophene used at that time can be mounted to a base body such as a silicon wafer easily through a normally conducted film formation technique such as casting, evaporation, etc., and the oligothiophene thin-film is employed as the channel section, thus acquiring and excellent field-effect transistor. Drain currents under gate voltage are made larger than conventional devices, terminals of which are not displaced, by an improvement in the mobility of oligothiophene by methylation or ethylation, thus obtaining superior transistor characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electronic device using oligothiophene, and particularly to a field effect transistor useful as an electronic device.

[Prior Art] Conventionally, electronic devices such as transistors using oligothiophenes have been proposed. J, Pal.

h e i m o et al., Applied Physics
There are descriptions about them in Letters, No. 56, pages 1157 to 1159 (1990).

The field-effect transistor fabricated by J. Paloheimo et al. uses a Langmuir-Prodgett film (LB film) of a type of oligothiophene and a fatty acid in the channel part, and a Langmuir-Prodgett film (LB film) of a kind of oligothiophene and a fatty acid. A Langmuir-Prodgett film, which is an organic semiconductor thin film, is obtained by repeatedly accumulating a mixed monomolecular film of this material on the channel portion.

Furthermore, G. Horowitz, Kabe Solid State Communications, No. 72, pages 381 to 384 (1989), reports on a field effect transistor using a vapor-deposited film of sexithiophene (hexamer) in the channel portion.

In both of these methods, a silicon oxide film is formed on an n-type silicon substrate, a metal electrode pair is arranged on the silicon oxide film, and an n-type silicon oxide film is formed on the silicon oxide film.
The type silicon substrate is used as the gate, the electrodes on the silicon oxide film are used as the source and the drain, and an organic semiconductor film is formed on the channel portion, that is, the silicon oxide film, and between the source and drain electrodes.

[Problems to be Solved by the Invention] However, a transistor using an unsubstituted oligothiophene having hydrogen at both ends is G, Hor.

As seen in the deposited film of sexithiophene reported by Wi tz et al., heat treatment at 120° C. for 3 hours is required to obtain field effect transistor operation.

This is because the sexithiophene used in the experiments of G., Horowitz, et al. had a low purity, so it was necessary to remove impurities by heating (dedoping).

Alternatively, as in J. Paloheimo et al.
The drawback was that normal device operation could only be achieved in a special form incorporated into a Langmuir-Prodgett film together with other support molecules.

The present invention eliminates these difficulties and limitations and provides an electronic device with superior operating characteristics.

[Means for Solving the Problems] The field effect transistor of the present invention uses an oligothiophene represented by the following general formula for a channel on a silicon oxide film between a source and a drain.

However, n is 3.4 or 5, and R represents a methyl group or an ethyl group.

The production of oligothiophenes according to the present invention can be carried out, for example, by
Tamao et al., Tetrahedron, Volume 38, 3347
3345 pages, R6M, Kellogg
et al., The Journal of Organic Chemistry, Volume 34, pages 343 to 346 (19
69) and J. M. Barker et al., Synthetic Communications, Vol. 5, pp. 59 (1975), p. 1-64 (1975).

FIG. 1 shows the structure of a field effect transistor according to the present invention in which an oligothiophene methylated or ethylated at both ends is disposed between a source and a drain. Oligothiophenes can be easily attached to a substrate such as a silicon wafer using commonly used film-forming techniques such as casting or vapor deposition, and good field effect transistors can be obtained by using oligothiophene thin films in the channel region. can.

No additional treatments such as heat treatment or electrochemical dedoping are required for the oligothiophene film to obtain field effect transistor operation. The good operating properties obtained without additional treatments are based on the use of highly pure alkyl (methyl or ethyl) substituted oligothiophenes.

The improved purity leads to a decrease in the drain current at the gate voltage OV in field effect transistor operation,
Improves characteristics as a field effect transistor.

For example, in the diethylquatathiophene of the present invention, J,
Approximately 1 compared to 5 x 10”/cm3t of the casting film of finkethiophene of Paloheimo et al.
A thin film with a 0-3 times lower carrier concentration was obtained. In addition, the carrier density n = calculated from the electrical conductivity σ and mobility μ of the device of G.
The carrier density is approximately 10-2 times smaller than that of a/e p, 2.3X10"7cm3 (before heat treatment)1°. Also, in field effect transistors using oligothiophenes with methylated or ethylated terminals, the terminals are replaced. The drain current increases under the same gate voltage compared to conventional devices that do not have the same gate voltage, and good transistor characteristics can be obtained.

This is due to the increased mobility of oligothiophenes due to methylation or ethylation. Methylation or ethylation increases the length of the conjugated system of π electrons in oligothiophene, lowering the energy of the π-π° transition and I/X, which is also thought to be a factor in increasing the mobility. It will be done.

In the preparation of the device according to the present invention, J.

Paloheimo and other applied physics
A special film forming technique such as the Langmuir-Prodgett method as described in Letters, No. 56, pages 1157 to 1159 (1990) is not necessarily required.

This can be explained by the self-assembly effect derived from the hydrophobic interaction of the methyl or ethyl groups at both ends of the oligothiophene molecule related to the present invention. In other words, such self-assembling action can be considered equivalent to the film-assembling property due to compression of the air-water interface in the Langmuir-Prodgett method. In fact, the oligothiophene molecule related to the present invention can be easily produced into a large crystal by a method such as recrystallization, as compared to one in which both ends are not substituted. It is believed that such good crystallinity contributes to improved purity.

Furthermore, it goes without saying that the methyl and ethyl groups at both ends of the oligothiophene molecule contribute to chemical stabilization of the molecule and ensure stable operation of electronic devices using these compounds.

[Function] The electronic device of the present invention uses oligothiophene between the source and drain, and uses oligothiophene with both ends methylated or ethylated, and is processed by conventional methods such as casting or vapor deposition. It can be easily attached to a substrate such as a silicon wafer using film formation technology, and a good field effect transistor can be obtained by using an oligothiophene thin film in the channel part. There is no need for additional treatments such as dedoping or electrochemical dedoping.
Furthermore, the drain current at the gate voltage OV can also be reduced compared to the conventional one.

The present invention will be explained in more detail with reference to Examples below.

[Example] Consequence of oligothiophene The production of a field effect transistor and the measurement of field effect transistor characteristics will be explained in this order.

Trimeric thiophene with a methyl group at the end 5゜5''-
Dimethyl-2,2':5'2''-terthiophene was synthesized by the following method.

Synthesize 2-methyl-5-bromothiophene according to the method described by R, M, Kel logg et al., The Journal of Organic Chemistry, No. 33, pages 2902 to 2909 (1968), Purified by vacuum distillation. From this 0.01 mole (
1,77 g) was collected, dissolved in 20 ml of diethyl ether, and passed through a separation filtration to obtain diethyl ether (2
0 ml) to prepare Grignard reagent.

To this was added a catalytic amount of 1,3-bis(diphenylphosphino)propane nickel(II) chloride, and then 0.004 mol (0,
A diethyl ether solution in which 97 g of
Dimethyl-2,2': 5',2''-terthiophene was obtained.

This was recrystallized from ethanol to obtain pale yellow crystals.

Furthermore, 5,5''-diethyl-2,2': 5' was prepared in exactly the same manner using 2-ethyl-5-bromothiophene instead of the above 2-methyl-5-bromothiophene.
, 2''-terthiophene was synthesized. This was recrystallized from methanol to obtain pale yellow crystals.

A tetramer of thiophene with a methyl group at the end, 5,5-dimethyl-2.2': 5', 2" = 5", 2"'-quarterthiophene was synthesized and purified by vacuum distillation. From this, 0.01 mol (1,77 g) was collected, dissolved in 20 ml of diethyl ether, passed through a separation filtration, and added to diethyl ether (20 ml) in which 0.01 mol (0,243 g) of magnesium had been dispersed, and the Grignard reagent was added. To this was added a catalytic amount of 1,3-bis(diphenylphosphino)propane nickel (II) chloride and 70.004 mol (1,30 g) of 5,5'-dibromo-2,2'-bithiofe. were sequentially added in solid form, stirred day and night, and then refluxed for 4 hours to form an orange-brown 5,5゛゛-dimethyl-2,2':5',2":5"2.
``゛-Quartathiophene was synthesized.

This was recrystallized from acetone to obtain needle-shaped crystals with golden reflected light.

In addition, the amount of thiophene with a methyl group at the end is 5゜5.
-dimethyl-2,2":5",2":5"2"':5
``゛'' 21+11-finkethiophene was synthesized by the following method.

J. Barker et al., Synthetic Communications, Volume 5, pages 59-64 (
2-Methyl-5-iodothiophene was synthesized according to the method described in (1975) and purified by vacuum distillation.

A Grignard reagent is prepared and a catalytic amount of 1°3-bis(diphenylphosphino)propane nickel (I
I) Chloride and 5,5''-dibromo-2,2':
5',2''-terthiophene 0.004 mol (1,62
g) and were added sequentially in solid form and stirred for a day and night,
5,5" perdimethyl 2,2': 5',2"
- 5'', 2'''': 52°°゛-finkethiophene was obtained. This was recrystallized from chlorobenzene to obtain crystals with golden yellow reflected light.

Furthermore, using 2-ethyl-5-promotifen in place of the above-mentioned 2-methyl-5-iodobromooffene, 5,5 dimethyl-2,2"=5",
2”: 5.

5"°"2°"-finkethiophene was obtained. This was recrystallized from chlorobenzene to obtain crystals.

Example 1 FIG. 1 shows a cross-sectional view of a field effect transistor using a silicon substrate as a gate electrode.

A silicon oxide film 2 with a thickness of 270 nm is formed on the surface by oxidizing an n-type silicon substrate 1 (ρ=0.01Ωcm).
A source electrode 3 and a drain electrode 4 were formed on top. The electrodes were comb-shaped so that the length (L) of the channel 5 was 4 μm and the width (W) was 1.5 mm. For the source and drain electrodes, metallic chromium was deposited to a thickness of 15 nm, and then gold was deposited to a thickness of 150 nm.

Next, the oligothiophene thin film 6 is placed on a silicon oxide film 2.
A field effect transistor was fabricated by forming it in the upper channel portion.

The electrical characteristics of each field effect transistor were measured in a dark box at room temperature and atmospheric pressure. The measurement is performed by applying a voltage to the gate from the gate power supply 7 and measuring the current flowing to the drain power supply 8 with an ammeter 9.

In the obtained field effect transistor, when a negative voltage was applied to the gate, the drain current increased, indicating that the carriers were holes.

The following relational expression ΔID/ΔV o= (p CoxW/L ) V
n and engineering. -V, mobility was determined from the characteristics.

Here, CoX is the electric capacity of the silicon oxide film, W is the channel width, and L is the channel length. Also, the gate voltage is O
The residual carrier surface density was obtained from the current value at v, and the carrier density was determined by dividing it by the film thickness.

The concentration of diethylquatathiophene is 1.5mg/
A chloroform solution of m 1 was cast onto the channel portion in a glove box purged with nitrogen, and air-dried at room temperature. The film thickness is estimated to be about 2 μm based on the mass of cast diethylquathiophene, the film area, and the specific gravity. A field effect transistor with good electrical characteristics was obtained without any post-treatment of the film obtained by such a simple method.

The results are expressed as various gate voltages V. FIG. 2 shows the relationship between the change in the drain current V1 and the change in the drain voltage VD of the platform. The mobility obtained from this result is μ=5×1
-0-5cm2/V-s, carrier surface density 2. 2X1
0"7cm2, and the carrier density was 1.1X10"/cm'.

Example 2 A thin film of oligothiophene was formed in the same manner as in Example 1, except that the thin film of oligothiophene was formed by vacuum deposition.

Vapor deposition of oligothiophene was carried out at a vacuum level of 5 x 10-5 To
Various oligothiophenes provided on a tungsten boat were vapor-deposited on the silicon oxide film at rr. All oligothiophenes were melted on a tungsten boat and deposited for 30 seconds until the glass was colored. on the other hand,
Silver was deposited on an oligothiophene thin film deposited on a fused silica glass substrate under the same conditions, and the film thickness was measured using a multiple reflection interferometer.

The properties of field effect transistors for a series of oligothiophenes are shown in Table 1.

Table 1 [The margins below indicate that it was not possible to confirm the transistor operation in the device fabricated using unsubstituted terthiophene], but it was not possible to confirm the transistor operation in the device fabricated using dimethylterthiophene with methylation at both ends. Electric power is 1.9xlO-
``We have obtained a mobility of cm2/Vs.

In the tetramer, 2゜2X10 of unsubstituted quarterthiophene
-Tcm2/Vs, dimethylquatathiophene was 1.4X 10-' cm2/Vs, diethylquatathiophene was 9X 10-5 cm2/Vs, and two to three orders of magnitude improvement in mobility was observed.

Also, in the pentamer, dimethyl quinketiophene has 2.5×10−5 cm2/Vs compared to 2. 5X10-'cm2/V
S, diethylfinkethiophene is 9.0×10-'
It was confirmed that the mobility was improved by one to one and a half orders of magnitude (cm2/Vs), and it became clear that the improvement in mobility was caused by methylation or ethylation of both ends of the oligothiophene. In addition, oligothiophene has a degree of polymerization (the number of thiophene units in oligothiophene)
It was found that the larger the value, the larger the mobility μ tends to be.

Also, for hexamers or more, the effect of substitution can be expected as in the case of S-mers.

The areal density of carriers in the channel of the field effect transistor was on the order of 101 days to 101'/cm2 as shown in Table 1 for all oligothiophenes.

As described above, it is clear that in the electronic device of the present invention, the oligothiophene thin film can be formed by either casting or vapor deposition.

In other words, when looking at diethylquatathiophene, the difference in mobility is less than 2 times, and the carrier density is I x 1015/cm in the cast film.
3. The thickness of the deposited film is 7×10”/cm3, which is not a big difference.

Furthermore, X-ray diffraction experiments confirmed sharp diffraction peaks in both the cast film and the deposited film. In addition, the distance between the surfaces of the cast film is 19, 4 and 0. 1 angstrom, and both crystals have a diameter of 19°60. It has a structure similar to that of 1 angstrom.

As described above, a good field effect transistor could be obtained using an alkyl-substituted oligothiophene by a simple method such as casting or vapor deposition.

[Effects of the Invention] The present invention provides an electronic device with good operating characteristics by using oligothiophene with a methylated or ethylated terminal. Compared to transistors, field effect transistors with thin films of alkyl-substituted oligothiophenes have increased mobility, resulting in increased drain current.

[Brief explanation of the drawing]

FIG. 1 shows a cross section of a field effect transistor having an alkyl group-substituted oligothiophene thin film of the present invention,
FIG. 2 shows the operating characteristics of a field effect transistor using a cast film of diethylquathiophene as the oligothiophene thin film. 1... Silicon substrate, 2... Silicon oxidation [3.
... Source electrode 4... Drain electrode 5... Channel, 6... Oligothiophene thin WL 7... Gate power supply, 8... Drain electric plate 9... Ammeter

Claims (1)

[Claims] An electronic device in which an oligothiophene represented by the following general formula is continuously arranged between a source and a drain. ▲There are mathematical formulas, chemical formulas, tables, etc.▼ However, n is 3, 4, or 5, and R represents a methyl group or an ethyl group.