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FIELD OF THE INVENTION
This invention pertains to thin film transistors (TFTs) that comprise an
organic active layer, and to methods of making such transistors.
BACKGROUND OF THE INVENTION
Thin film transistors (TFTs) are known, and are of considerable commercial
significance. For instance, amorphous silicon-based TFTs are used in a
large fraction of active matrix liquid crystal displays.
TFFs with an organic active layer are also known. See, for instance, F.
Garnier et al., Science, Vol. 265, pp. 1684-1686; H. Koezuka et at.,
Applied Physics Letters, Vol. 62 (15), pp. 1794-1796; H. Fuchigami et al.,
Applied Physics Letters, Vol. 63 (10), pp. 1372-1374; G. Horowitz et al.,
J. Applied Physics, Vol. 70 (1), pp. 469-475, and G. Horowitz et al.,
Synthetic Metals, vol. 41-43, pp. 1127-1130. These devices typically are
field effect transistors (FETs). Such devices potentially have significant
advantages over conventional TFTs, including a potentially simpler (and
consequently cheaper) fabrication process, the possibility for low
temperature processing, and compatibility with non-glass (e.g, plastic)
substrates. Bipolar transistors that utilize both p-type and n-type
organic material are also known. See, for instance, U.S. Pat. No.
5,315,129. S. Miyauchi et al., Synthetic Metals, 41-43 (1991), pp.
1155-1158, disclose a junction FET that comprises a layer of p-type
polythiophene on n-type silicon.
However, despite considerable research and development effort, "organic"
TFTs have not yet reached commercialization, at least in part due to the
relatively poor device characteristics of prior art organic TFTs.
An important device characteristic of a switching transistor is the on/off
ratio of the source/drain current. Prior art organic TFTs typically have
relatively low on/off ratios. For instance, H. Fuchigami et al. (op. cit.)
recently reported a device that had carrier mobility comparable to
amorphous silicon, but had an on/off ratio of only about 20. That paper
also discloses purification of semiconducting material (PTV) to reduce the
carrier scattering by impurities. The material had a conductivity in the
range 10.sup.-5 -10.sup.-6 S/cm.
H. Koezuka et al. (op. cit.) report attainment of an on/off ratio
(modulation ratio) of the channel current of about 10.sup.5 in a device
with doped polypyrole (a highly conducting polymer)-coated source and
drain contacts. According to these authors, this is the highest on/off
ratio achieved in organic FETs. Nevertheless, the reported on/off ratio is
still substantially smaller than on/off ratios typically available in
conventional FETs and demanded for many potential applications of organic
TFTs. Furthermore, the organic TFT had very low carrier mobility
(2.times.10.sup.-4 cm.sup.2 /V.s), and thus would not have been suitable
for high-speed operation.
In view of the potential significance of organic TFTs, it would be
desirable to have available such devices that have improved
characteristics, including improved on/off ratio of the source/drain
current. This application discloses such devices, and a method of making
the devices.
Definitions and Glossary
An "organic semiconductor" herein is a material that contains a substantial
amount of carbon in combination with other elements, or that comprises an
allotrope of elemental carbon (excluding diamond), and exhibits charge
carrier mobility of at least 10.sup.-3 cm.sup.2 /V.s at room temperature
(20.degree. C.). Organic semiconductors of interest for TFTs typically
have conductivity less than about 1 S/cm at 20.degree. C.
A "p-type" ("n-type") organic semiconductor herein is an organic
semiconductor in which the Fermi energy is closer to (farther from) the
energy of the highest occupied orbital of the molecules or aggregates
present in the material than it is to (from) the energy of the lowest
unoccupied orbital. The term is also intended to mean an organic
semiconductor which transports positive charge carriers more (less)
efficiently than negative carriers. Positive (negative) carriers are
generally referred to as "holes" ("electrons").
SUMMARY OF THE INVENTION
In a broad aspect the invention is embodied in an article that comprises a
novel organic TFT that can have substantially improved characteristics
(e.g., on/off ratio), as compared to prior art organic TFTs. A method of
making the article is also disclosed.
Specifically, the organic TFT comprises an organic material layer, spaced
apart first and second contact means (e.g., gold electrodes) in contact
with said layer, and third contact means that are spaced from each of the
first and second contact means and that are adapted for controlling, by
means of a voltage applied to the third contact means, a current between
the first and the second contact means through said layer. Significantly,
the organic material of the layer is selected from the group consisting of
.alpha.-quaterthienylene (.alpha.-4T), .alpha.-hexathienylene
(.alpha.-6T), .alpha.-octathienylene (.alpha.-8T), .alpha.-pentathienylene
(.alpha.-5T), .alpha.-heptathienylene (.alpha.-7T), and
.alpha.-nonathienylene (.alpha.-9T), each with or without substituents on
the 4 or 5 carbon of the terminal rings (these compounds will collectively
be referred to as ".alpha.-nT", with n being an integer from 4 to 9), said
organic material layer having an electrical conductivity of at most
5.times.10.sup.-8 S/cm (preferably less than 1.times.10.sup.-8 S/cm)at
20.degree. C., either in the as-deposited condition or subsequent to a
rapid thermal anneal. In currently preferred embodiments the organic layer
material is .alpha.-6T or .alpha.-8T, with .alpha.-6T being most
preferred.
We have made the surprising discovery that, for example, .alpha.-6T can be
produced and deposited in a manner that results in a layer of extremely
low conductivity, and that a TFT that comprises such a low-conductivity
active layer can have greatly improved properties, including a
substantially improved source/drain current on/off ratio. Indeed, TFTs
that comprise the novel active layer material can have an on/off ratio
that is comparable to those of the novel (2-layer) TFTs described in the
concurrently filed co-assigned patent application by the same inventors of
title "Article Comprising an Organic Thin Film Transistor". Thus, TFTs
according to the, instant invention will typically, but not necessarily,
comprise a single organic layer, the "active" layer (but not excluding the
presence of, e.g., a protective layer over said active layer).
In a further aspect the invention is embodied in a method of making a TFT
that comprises an .alpha.-mT (m=4, 6 or 8) active layer. The method
comprises providing a quantity of .alpha.-mT, and depositing a layer of
the .alpha.-mT on a substrate. The .alpha.-mT is produced by a process
that comprises providing .alpha.-(m/2) thienyl, de-protonated in the
5-position, in an organic solvent. Significantly, the process of producing
said .alpha.-mT still further comprises contacting said .alpha.-(m/2)
thienyl in the organic solvent with a non-halogenating oxidizing agent
such that an .alpha.-mT-containing mixture is formed, and isolating said
.alpha.-mT from the mixture. As will be described later in detail, the
"isolating" step comprises a multiplicity of substeps.
Although the discussion below will be primarily in terms of .alpha.-6T, we
currently believe that many if not all of the other members of the above
defined group .alpha.-nT can also be synthesized/treated to meet the
specified conductivity requirement. A prior art method of making compounds
such as .alpha.-6T is disclosed in Chemical Abstracts, Vol. 114, p. 22,
item 186387g (1991).
As will be discussed in detail below, .alpha.-6T according to the invention
differs from prior art .alpha.-6T not only with regard to carrier
concentration but typically also with regard to such characterizing
properties as melting point, X-ray diffraction pattern and elemental
analysis. Similar differences are expected for .alpha.-4T and .alpha.-8T,
and indeed for all .alpha.-nT. These differences support the conclusion
that the active layer materials according to the invention are essentially
new materials that differ qualitatively from the analogous prior art
materials. However, we will refer to materials according to the invention
by the chemical names that have commonly been used to refer to the
analogous prior art materials.
Exemplarily, a TFT according to the invention, with .alpha.-6T active
layer, has exhibited in the as-deposited condition an on/off ratio of more
than 10.sup.6 at 20.degree. C., substantially higher (e.g., by a factor of
10.sup.2) than the ratios typically exhibited by prior art organic TFTs.
The active layer of the exemplary TFT according to the invention was only
lightly p-type at 20.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically shows an exemplary prior art TFT;
FIGS. 2 and 3 schematically depict exemplary TFTs according to the
invention;
FIGS. 4 and 5 show data on drain current vs. drain voltage for 2 TFTs
according to the invention, of somewhat different conductivities;
FIGS. 6 and 7 show the X-ray diffraction pattern of, respectively,
.alpha.-6T according to the invention, and an exemplary prior art
.alpha.-6T;
FIG. 8 shows exemplary differential scanning calorimetry data for
.alpha.-6T according to the invention; and
FIG. 9 shows an exemplary drive circuit in an active matrix liquid crystal
display that comprises TFTs according to the invention.
DETAILED DESCRIPTION
FIG. 1 schematically depicts a prior art organic TFT 10 of the
metal-insulator-semiconductor field effect transistor (MIS-FET) type,
wherein numerals 11-16 refer, respectively, to substrate, first electrode
(e.g., source electrode), second electrode (e.g., drain electrode), third
electrode (gate electrode), insulator layer, and active layer.
FIG. 2 schematically shows a MIS-FET type device according to the
invention. The transistor 20 also comprises substrate 11 (e.g., a glass,
silicon or plastic body), first and second contacts 12 and 13, third
contact 14, gate dielectric 15 and active layer 16. It will be recognized
that topologically transistor 20 is the same as prior an transistor 10.
However, the former comprises an active layer material that differs
significantly from prior art materials, resulting in substantially
improved performance (e.g., substantially higher on/off ratio of the
source/drain current), as compared to prior art organic TFFs.
FIG. 3 schematically depicts another embodiment of the invention, namely,
an organic TFT of the metal-semiconductor (MES)-FET type. Numerals 31-35
designate, respectively, substrate, active layer, first, second and third
contacts.
By way of example, MIS-FET type TFTs according to the invention, with 12
.mu.m channel length and 250 .mu.m gate length, were fabricated on
thermally oxidized, conductive Si substrates. The oxide, serving as gate
dielectric, was 300 nm thick. The gate region was accessed by a gold ohmic
contact to the Si, and gold source and drain contacts were
lithographically defined on the silicon oxide. The organic active layer
was then formed by evaporation onto the entire assembly at room
temperature and at a pressure of 10.sup.-6 torr. The active layer was 50
nm thick, and was not heat treated. Measurements were made in situ, in
vacuum.
FIG. 4 shows curves of drain current vs. drain voltage obtained from an
exemplary TFT as described above, with the active layer material
(.alpha.-6T) prepared as described in detail below.
The off-current (gate voltage 0 or positive, drain voltage -100 V) was
about 10.sup.-11 A, and the on/off ratio was greater than 10.sup.6. The as
deposited .alpha.-6T at 20.degree. C. had a conductivity below 10.sup.-8
S/cm, (approximately 10.sup.-9 S/cm), and was barely p-type (i.e., it was
substantially intrinsic), as those skilled in the art will recognize from
the substantial overlap of the curves for 0-60 V gate voltage.
FIG. 5 shows analogous results for a TFT as described above, but with
.alpha.-6T of somewhat higher conductivity (approximately
1.4.times.10.sup.-8 S/cm) due to adventitious impurity doping. The carrier
mobility was about 1-2.times.10.sup.-2 cm.sup.2 /V.multidot.s, and the
on/off ratio was greater than 10.sup.6 for gate voltages between 20 V and
-80 V.
As can be seen from the above comparative results, material preparation is
a significant aspect of TFTs according to the invention. We will next
describe a novel method of making .alpha.-6T that can yield material of
very low conductivity, exemplarily .ltoreq.5.times.10.sup.-8 S/cm at
20.degree. C. Use of such material in a TFT can result in very low
off-current, a feature that inter alia can result in high on/off ratio and
good dynamic response of the TFT. The closely related compounds .alpha.-8T
and .alpha.-4T can be made by substantially the same method.
Material Preparation
Reagent grade .alpha.-terthienyl was dissolved in about three times its
weight of toluene and applied to a column of ten times its weight of
silica gel packed with hexane in a chromatography column. The
.alpha.-terthienyl was eluted from the column with hexane and the eluate
was concentrated in vacuum. This purified .alpha.-terthienyl (4.5 g) was
placed in a round-bottom flask equipped with a magnetic stir bar, and 400
ml of tetrahydrofuran (THF), freshly distilled from sodium benzophenone
ketyl, was added. The flask was capped with a rubber stopper. The solution
was then purged with nitrogen, and cooled (with magnetic stirring) under
nitrogen to <-70.degree. C. A syringe containing 7.3 ml of 2.5M
n-butyllithium in hexane was emptied into the flask via a needle through
the stopper over 10 min., followed by stirring for 20 additional minutes
below -70.degree. C., resulting in formation of a substantial quantity of
.alpha.-terthienyl de-protonated in the 5 position, namely,
5-lithio-.alpha.-terthienyl. A precipitate formed. The resulting
suspension was added via a cannula to a second stirred flask containing a
non-halogenating oxidizing agent, e.g., 6.4 g of reagent grade ferric
acetylacetonate, and 150 ml of the THF, cooled to <-70.degree. C. under
nitrogen. The low temperature was maintained for one hour after the
addition; then the mixture was allowed to warm to ambient temperature over
5-20 hours. The remaining part of the procedure is directed towards
isolation of the .dbd.-6T.
The suspended solids were collected by vacuum filtration and washed in
sequence with 100 ml of ethyl ether, 300 ml of 1% HCl in water, 300 ml of
(DI) water, 100 ml of ethanol, and 100 ml of ether, yielding 3.5 g of
crude solid product. The solid was further washed with 100 ml each of 0.1%
Na.sub.2 CO.sub.3 in water, water, water again, ethanol, methyl ethyl
ketone, and toluene, all heated to just below boiling and with vigorous
shaking to prevent bumping. The undissolved solids were extracted with an
inert solvent with boiling point >120.degree. C., e.g., 800 ml of boiling
mesitylene under nitrogen for one hour. The extract was filtered at the
boiling point through a funnel heated with the vapors of the same solvent
(e.g., mesitylene) that was boiling in the receiving flask. The filtrate
was allowed to cool to ambient temperature, and crystals were obtained.
The supernatant was separated from the crystals by suction filtration and
used to further extract the undissolved solids. A total of 1.5 g of
crystals were obtained. Portions of 200-300 mg of the thus produced
crystals were placed at the bottom of a glass sublimator measuring 5 cm in
diameter and 6-12 cm high. The material was sublimed >1 cm along the glass
tube by heating at about 300.degree. C. and 10.sup.-4 torr. After cooling
under vacuum to ambient temperature, the sublimed material was scraped
from the glass tube.
The above described exemplary novel method of making .alpha.-6T comprises
features. that can significantly affect product quality. Among these is
the use of a non-halogenating oxidizing agent, exemplarily ferric
acetylacetonate. We have made the surprising observation that prior art
.alpha.-6T (e.g., .alpha.-6T made with cupric chloride) contains
significant amounts (typically 0.1-1 weight percent) of chlorine. To the
best of our knowledge, the presence of this undesirable constituent was
unknown up to now, and its substantial absence (less than 0.1 weight %
chlorine or other halogens, preferably less than 0.05 weight %) from
.alpha.-6T according to the invention is a significant aspect of the
invention that is considered important for attainment of desirably low
conductivity and/or high mobility. The .alpha.-6T made according to the
above described method thus typically differs significantly from prior art
.alpha.-6T with regard to elemental analysis.
Preferred methods of making .alpha.-6T also comprise solid extraction with
an inert solvent with boiling point greater than 120.degree. C.,
exemplarily mesitylene, and isolation of the .alpha.-6T by crystallization
of the extract. The preferred methods further comprise a multi-step
washing sequence applied to the (pre-washed) crude solid product, and
sublimation of the washed solid product along a significant distance,
typically >1 cm.
We have also made the surprising discovery that .alpha.-6T made according
to the above described method has a melting point that differs
substantially from reported melting points of prior art .alpha.-6T, which
are in the range 280.degree.-307.degree. C. Specifically, using
differential scanning calorimetry under nitrogen, we found the melting
point of the novel .alpha.-6T to be about 313.degree. C., as demonstrated
by FIG. 8.
Furthermore, we have found significant differences between the X-ray
diffraction patterns of .alpha.-6T made according to the novel method and
published diffraction patterns of prior art .alpha.-6T, as demonstrated by
FIGS. 6 and 7. The former pertains to .alpha.-6T according to the
invention, and the latter is the published diffraction pattern of a prior
art .alpha.-6T. See B. Servet et al., Advanced Materials, Vol. 5(6), p.
461 (1993).
Generally speaking, material according to the invention has more numerous
and sharper X-ray peaks. Specifically, in .alpha.-6T according to the
invention the prior art peaks (see the above cited article by B. Servet et
al.) at about 20.degree. and 22.5.degree. each are resolved into at least
two peaks.
The above discussed observations indicate that .alpha.-6T according to the
invention is a material that can give rise to evaporated films that can
exist in a more stable, more dense and better ordered crystalline form
than prior art .alpha.-6T. Indeed, electron microscopy has confirmed that
as-deposited films of .alpha.-6T according to the invention (deposited on
a substrate at room temperature) can consist of large, anisotropic
platelets with lengths of 100-200 nm that are clearly interconnected. This
is to be contrasted with similarly deposited prior art .alpha.-6T films
which are reported to have grain sizes of 50 nm, with the isotropic grains
more or less separated from each other (B. Servet et al., Chemistry of
Materials, Vol. 6, (1994), p. 1809). These structural differences are
reflected in differences in carrier mobility. The .alpha.-6T films
according to the invention can have mobility of about 10.sup.-2 cm.sup.2
/V.multidot.s, whereas the above described prior art material was reported
to have mobility of only 2.times.10.sup.-3 cm.sup.2 /V.multidot.s.
We currently believe that the above discussed improved properties are
important for attainment of the improved device performance, e.g., for
attainment of a high on/off ratio of the source/drain current. In
particular, we believe that halogen content below 0.1 weight % (preferably
below 0.05 weight %) is an important aspect of active layer material
according to the invention, since active layer material with higher
halogen content is likely to have relatively low carrier mobility.
Those skilled in the an will appreciate that the above described method of
making .alpha.-6T is exemplary, and that modifications of the method are
possible. For instance, alternative inert solvents (e.g., xylene) of
similar polarity to that of mesitylene may be used, other metal-alkyl
reagents (e.g., sec-butyllithium) may be used instead of n-butyllithium,
and other non-halogenating oxidizing agents (e.g., ferric
trifluoro-acetylacetonate, manganese (III) acetylacetonate) may be used.
Furthermore, the method is not limited to the synthesis of .alpha.-6T but
applies, with obvious modifications (e.g., starting material, quantities
and temperatures of solvents, sublimation temperature and pressure), also
to the preparation of .alpha.-4T and .alpha.-8T, and to the preparation of
closely related compounds such as the end-substituted derivatives of
.alpha.-6T, .alpha.-4T and .alpha.-8T. Furthermore, at least the
purification aspects of the novel method are expected to be applicable,
with obvious modifications (e.g., use of solvents of appropriate polarity
and boiling point, sublimation temperature and pressure) to other related
compounds such as .alpha.-5T, .alpha.-7T and .alpha.-9T, and their 4 or 5
substituted derivatives.
An appropriate quantity (e.g., 50 mg) of .alpha.-6T produced as described
above was placed into a conventional tungsten boat in a conventional
evaporator system. The base pressure in the evaporation chamber was about
10.sup.-7 torr. The boat was heated to about 300.degree. C., and a 50 nm
thick .alpha.-6T film was deposited uniformly over an appropriate
substrate at room temperature. The as-deposited material typically was
polycrystalline, with average grain size of order 100 nm.
We have found that an appropriate heat treatment of the as-deposited
.alpha.-nT can change the morphology of the layer, and potentially further
improve device characteristics. More specifically, we have found that
rapid thermal annealing (RTA) of as-deposited films of, e.g., .alpha.-6T
can substantially increase the grain size of the material, to the extent
that average grain size can be comparable to or even exceed the channel
length (typically 4-12 .mu.m) of the intended TFT. If this is the case
then the active layer can behave substantially like a single crystal
layer.
Annealing an as-deposited .alpha.-6T film for a short time (typically less
than 10 seconds, e.g., 1 second) at a temperature close to (optionally
even slightly above) the melting point (e.g., 295.degree.-315.degree. C.)
exemplarily has resulted in increase of the average grain size to above 2
.mu.m, e.g., in the range 5-100 .mu.m. Annealing is desirably done in an
inert atmosphere, e.g., N.sub.2. Any suitable heat source (e.g., a bank of
halogen lamps focused to a susceptor, or a graphite strip heater) can be
used. It is anticipated that the morphology of the other .alpha.-nTs can
also be improved by appropriate RTA.
Furthermore, we have discovered that RTA of as-deposited p-type .alpha.-6T
films can result in substantial decrease of the conductivity, with
consequent increase in on/off ratio. For instance, a as-deposited
.alpha.-6T layer exhibited a conductivity of about 10.sup.-6 S/cm. After a
RTA (296.degree. C. for 1 second) in N.sub.2, the layer exhibited a
conductivity of 0.7.times.10.sup.-8 S/cm. It is expected that other
members of the group .alpha.-nT will show similar conductivity decreases.
TFFs according to the invention can be produced in substantially the same
way as analogous prior an TFTs, provided that a .alpha.-nT of the required
low conductivity is used. Exemplary substrates are glass, plastics such as
MYLAR.RTM. or KAPTON.RTM., or Si (coated with SiO.sub.2 or other
insulator). Use of encapsulant that protects the active layer material is
contemplated.
Transistors according to the invention can be used as discrete devices but
will more typically be used in integrated circuits that comprise a
multiplicity of transistors according to the invention, possibly in
conjunction with conventional semiconductor devices, with conductors
interconnecting the devices and providing means for energizing the
devices, providing input signals to the circuit and optionally receiving
output signals therefrom.
By way of example, transistors according to the invention are used as
current switches in liquid crystal displays in functionally the same way
as prior art semiconductor TFTs are currently used. This is schematically
illustrated in FIG. 9, which is based on an illustration at p. 102 of
"Amorphous and Microcrystalline Devices", J. Kanicki, editor, Artech
House, Boston (1991). FIG. 9 depicts relevant aspects of an exemplary
circuit diagram of an active-matrix liquid crystal display, wherein
transistors 101 are TFTs according to the invention, and the remainder of
the circuit is conventional. Numerals 102 refer to liquid crystal, and
numerals 103-105 refer to signal lines, gate lines and common electrode,
respectively. Video signals and gate pulses are also shown schematically.
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