U.S. patent application number 12/270429 was filed with the patent office on 2009-05-28 for carbon nanotube, method for positioning the same, field-effect transistor made using the carbon nanotube, method for making the field-effect transistor, and semiconductor device.
This patent application is currently assigned to Sony Corporation. Invention is credited to Houjin Huang.
Application Number | 20090136411 12/270429 |
Document ID | / |
Family ID | 36027669 |
Filed Date | 2009-05-28 |
United States Patent
Application |
20090136411 |
Kind Code |
A1 |
Huang; Houjin |
May 28, 2009 |
CARBON NANOTUBE, METHOD FOR POSITIONING THE SAME, FIELD-EFFECT
TRANSISTOR MADE USING THE CARBON NANOTUBE, METHOD FOR MAKING THE
FIELD-EFFECT TRANSISTOR, AND SEMICONDUCTOR DEVICE
Abstract
Carbon nanotube, method for positioning the same, field effect
transistor made using the carbon nanotube, method for making the
field-effect transistor, and a semiconductor device are provided.
The carbon nanotube includes a bare carbon nanotube and a
functional group introduced to at least one end of the bare carbon
nanotube.
Inventors: |
Huang; Houjin; (Kanagawa,
JP) |
Correspondence
Address: |
K&L Gates LLP
P. O. BOX 1135
CHICAGO
IL
60690
US
|
Assignee: |
Sony Corporation
Tokyo
JP
|
Family ID: |
36027669 |
Appl. No.: |
12/270429 |
Filed: |
November 13, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11186298 |
Jul 20, 2005 |
|
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12270429 |
|
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Current U.S.
Class: |
423/414 ;
257/E21.04; 423/415.1; 423/443; 438/597; 562/512; 977/742 |
Current CPC
Class: |
C01B 32/174 20170801;
D01F 11/12 20130101; C01B 2202/34 20130101; C01B 2202/36 20130101;
C01B 2202/02 20130101; H01L 51/0049 20130101; B82Y 10/00 20130101;
B82Y 30/00 20130101; B82Y 40/00 20130101; H01L 51/0508 20130101;
D01F 11/14 20130101; C01B 2202/06 20130101 |
Class at
Publication: |
423/414 ;
423/443; 562/512; 423/415.1; 438/597; 977/742; 257/E21.04 |
International
Class: |
D01F 9/12 20060101
D01F009/12; H01L 21/44 20060101 H01L021/44 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 2, 2004 |
JP |
2004-225834 |
Claims
1. A carbon nanotube comprising: a bare carbon nanotube; and a
functional group introduced to at least one end of the bare carbon
nanotube.
2. The carbon nanotube according to claim 1, wherein the functional
group is introduced to both ends of the bare carbon nanotube.
3. The carbon nanotube according to claim 2, wherein the functional
group introduced to one end is different from another functional
group introduced to the other end.
4. The carbon nanotube according to claim 1, wherein the functional
group selectively interacts with a predetermined conductive
material.
5. The carbon nanotube according to claim 2, wherein the functional
groups selectively interact with predetermined conductive
materials, respectively.
6. The carbon nanotube according to claim 3, wherein the functional
groups selectively interact with predetermined conductive
materials, respectively.
7. A method for positioning a carbon nanotube, the method
comprising: forming a carbon nanotube thin film including
semiconducting bare carbon nanotubes densely aligned on a first
substrate in a direction intersecting a longitudinal direction of
the carbon nanotubes; introducing a first functional group to a
first end of each of the bare carbon nanotubes constituting the
carbon nanotube thin film; attaching a second substrate onto a side
of the carbon nanotube thin film opposite to the first substrate;
removing the first substrate and introducing a second functional
group to a second end of each of the bare carbon nanotubes;
separating the carbon nanotube thin film from the second substrate
and placing the carbon nanotube thin film in a solvent to disperse
the carbon nanotubes having the first and second functional groups
so as to prepare a dispersion; and applying the dispersion onto
electrodes composed of conductive materials that selectively
interact with the first and second functional groups, the
electrodes being formed in advance by patterning, so as to position
the respective carbon nanotubes across the electrodes.
8. A semiconductor device comprising: a semiconducting carbon
nanotube; a first region to which one end of the carbon nanotube is
fixed; and a second region to which the other end of the carbon
nanotube is fixed, wherein the material of the first region is
different from another material of the second region.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] The present application is a divisional of U.S. patent
application Ser. No. 11/186,298, filed on Dec. 21, 2006 and claims
priority to Japanese Patent Application JP 2004-225834, filed in
the Japanese Patent Office on Aug. 2, 2004, the entire contents of
which being incorporated herein by reference.
BACKGROUND
[0002] The present invention relates to a semiconducting carbon
nanotube, a method for positioning the carbon nanotube, a field
effect transistor (FET) made using the carbon nanotube, and a
semiconductor device.
[0003] Ever since the year 1947 when a first semiconductor
transistor was invented, the degree of integration of silicon
microelectronics has grown substantially exponentially. Such
growth, however, is not expected to continue in the near future. In
particular, as the scale of integration approximates the nanometer
order, the structure is reaching a physical limit of reliably
achieving a desired function. With the increasing scale of
integration, the cost of manufacture is also increasing
exponentially, thereby inhibiting realization of higher
integration.
[0004] As the technology that can overcome the limitation imposed
by the principle of the silicon technology, the field of molecular
electronics has drawn much attention. According to the molecular
electronics, a monomolecular device can be fabricated at relatively
low cost by self-alignment technology.
[0005] In the field of molecular electronics, molecular structures
such as fullerenes and carbon nanotubes are increasingly attracting
attentions. In particular, single-walled carbon nanotubes (SWNTs),
which are rolled graphene sheets having diameters on the nanometer
order, have been vigorously investigated as to their properties
desirable in the field of electronics ever since their discovery in
early 1990's.
[0006] SWNTs can show metallic or semiconducting electrical
behavior depending on the angle and/or chirality of the spiral
lattices of carbon molecules constituting the tube. The electrical
performance of SWNTs is expected to surpass that of the best metal
or semiconductor.
[0007] In 1998, a field-emission transistor (FET) incorporating a
single SWNT was realized at room temperature (refer to Trans, S. J.
et al., Nature, 1998, vol. 393, p. 49). An inverter, which is the
simplest logical gate, was realized using a unipolar or
complementary FET incorporating one or two carbon nanotubes. Other
logical gates, such as NOR, AND, and static RAMs (SRAMs), were also
fabricated using a complementary or multi-complementary mode. Ring
oscillators realizing an oscillation frequency of 220 Hz were
fabricated using arrays of p- or n-type carbon nanotube FETs (refer
to Bachtold, A. et al., Science, 2001, vol. 294, p. 1317, and
Derycke, V. et al., Nano Letters, 2002, vol. 2, p. 929).
[0008] Basic logical circuits incorporating transistors including
SWNTs described above are mainly fabricated by two techniques. One
is to disperse SWNTs in a solvent so that the carbon nanotubes can
be positioned by scanning with an atomic force microscope (AFM) at
the corresponding electrodes patterned in advance (refer to Trans,
S. J. et al. and Bachtold, A. et al. above).
[0009] In this technique (first technique), SWNTs having a diameter
of about 1 nm fabricated by laser abrasion are typically suspended
in dichloroethane and this suspension is distributed on a wafer so
that the SWNTs can be placed on gate electrodes using an AFM.
Subsequently, selective deposition of Au is performed by
lithography to form contact electrodes and leads on these
nanotubes. According to an example of this technique disclosed in
Martel, R. et al., Applied Physics Letters, 1998, vol. 73, p. 2447,
a hole mobility of 20 cm.sup.2/(Vs) is achieved in a back-gate
structure.
[0010] Furthermore, a top-gate FET achieving a mutual conductance
as high as 2,321 S/m by incorporating a carbon nanotube (CNT) as
the gate electrode has been reported (Wind, S. J. et al., Applied
Physics Letters, 2002, vol. 80, p. 38).
[0011] The other technique (second technique) is to directly
deposit SWNTs by chemical vapor deposition (CVD) on electrode
patterns formed in advance. Examples thereof are found in Javey, A.
et al., Nature, 2003, vol. 424, p. 654 and in Tseng, Y. et al.,
Nano Letters, 2004, vol. 1, p. 123.
[0012] Transistors fabricated by this technique exhibit a mutual
conductance as high as 6,000 S/m and a carrier mobility as high as
3,000 cm.sup.2/(Vs), which are important properties for
transistors. These values are one digit larger than those of
silicon semiconductors.
[0013] In particular, a transistor prepared by this technique
achieves a carrier mobility as high as 79,000 cm.sup.2/(Vs) by
incorporating a semiconductor CNT 300 .mu.m in length, as reported
in Durkop, T. et al., Nano Letters, 2004, vol. 4, p. 35.
[0014] The first technique that uses an AFM is hardly practicable
since it concerns manual placement of a large number of CNTs on
devices. Its application to semiconductor devices, such as memories
of central processing unit (CPU) chips, is difficult.
[0015] The second technique that employs CVD is a high-temperature
process. Thus, accurate positioning of CNTs on a large number of
electrodes is difficult. The second technique is rarely suitable
for integrated circuit applications. In fact, an actual case of
mounting SWNTs onto part of a silicon metal oxide semiconductor
(MOS) by CVD reported low alignment accuracy, i.e., that only 1% of
about 2,000 CNTs functioned as back gates (refer to Tseng, Y. et
al., Nano Letters, 2004, vol. 4, p. 123).
SUMMARY
[0016] The present invention is directed to a carbon nanotube that
can be highly accurately positioned at a predetermined location in
making an integrated circuit having FETs including carbon
nanotubes. The present invention is also directed to a method for
positioning the carbon nanotube, a FET made using the carbon
nanotube, a method for making the FET, and a semiconductor
device.
[0017] An embodiment of the carbon nanotube of the present
invention includes a bare carbon nanotube and a functional group
introduced to at least one end of the bare carbon nanotube. With
this structure, at least one end can be selectively bonded or
attached to a particular material.
[0018] The functional group may be introduced at both ends of the
bare carbon tube so that both ends can be selectively bonded or
attached to particular materials.
[0019] The functional group introduced to one end may be different
from the functional group introduced to the other end. Preferably,
the functional groups selectively interact with predetermined
conductive materials. In this manner, the ends can be selectively
bonded or attached to a plurality of materials.
[0020] Another embodiment is a method for positioning a carbon
nanotube. The method includes forming a carbon nanotube thin film
including semiconducting bare carbon nanotubes densely aligned on a
first substrate in a direction intersecting the longitudinal
direction of the carbon nanotubes; introducing a first functional
group to a first end of each of the bare carbon nanotubes
constituting the carbon nanotube thin film; attaching a second
substrate onto the side of the carbon nanotube thin film opposite
to the first substrate; removing the first substrate and
introducing a second functional group to a second end of each of
the bare carbon nanotubes; placing the carbon nanotube thin film in
a solvent to disperse the carbon nanotubes having the first and
second functional groups so as to prepare a dispersion; and
applying the dispersion onto electrodes composed of conductive
materials that selectively interact with the first and second
functional groups, the electrodes being formed in advance by
patterning, so as to position the respective carbon nanotubes
across the electrodes.
[0021] According to this method, carbon nanotubes can be easily
positioned at target locations without complicated procedures using
expensive equipment or without high-temperature processes.
[0022] Yet another embodiment is a field-effect transistor that
includes source and drain electrodes and a gate in which current is
controlled by varying the conductivity of a channel functioning as
a current path between the source and drain electrodes. At least
the material of the channel is a semiconducting carbon nanotube.
Since the carbon nanotube is used as the channel, a FET having
excellent properties can be produced.
[0023] Still another embodiment is a method for making a
field-effect transistor having source and drain electrodes and a
gate in which current is controlled by varying the conductivity of
a channel functioning as a current path between the source and
drain electrodes. The method includes forming a carbon nanotube
thin film including semiconducting bare carbon nanotubes densely
aligned on a first substrate in a direction intersecting the
longitudinal direction of the carbon nanotubes; introducing a first
functional group to a first end of each of the bare carbon
nanotubes constituting the carbon nanotube thin film; attaching a
second substrate onto the side of the carbon nanotube thin film
opposite to the first substrate; removing the first substrate and
introducing a second functional group to a second end of each of
the bare carbon nanotubes; placing the carbon nanotube thin film in
a solvent to disperse the carbon nanotubes having the first and
second functional groups so as to prepare a dispersion; and
applying the dispersion onto the source and drain electrodes
composed of conductive materials that selectively interact with the
first and second functional groups, the source and drain electrodes
being formed in advance by patterning, so as to position the
respective carbon nanotubes across the source and drain
electrodes.
[0024] According to this method, the carbon nanotubes can be easily
and accurately positioned on the source and drain electrodes and
the productivity can be increased.
[0025] Another embodiment is a semiconductor device including a
semiconducting carbon nanotube, a first region to which one end of
the carbon nanotube is fixed, and a second region to which the
other end of the carbon nanotube is fixed. The material of the
first region is different from the material of the second region.
The semiconductor device simplifies the positioning of the carbon
nanotubes having excellent semiconducting properties.
[0026] Additional features and advantages are described herein, and
will be apparent from, the following Detailed Description and the
figures.
BRIEF DESCRIPTION OF THE FIGURES
[0027] FIG. 1 is a schematic diagram of an example of a carbon
nanotube.
[0028] FIG. 2 is a schematic diagram of another example of a carbon
nanotube.
[0029] FIG. 3 shows a method for positioning carbon nanotubes and
method for making a FET accordingly to an embodiment of the present
invention.
[0030] FIG. 4 shows a method for positioning carbon nanotubes and
method for making a FET according to another embodiment of the
present invention.
[0031] FIG. 5 shows a method for positioning carbon nanotubes
method for making a FET according to another embodiment of the
present invention.
[0032] FIG. 6 shows a method for positioning carbon nanotubes and
method for making a FET according to another embodiment of the
present invention.
[0033] FIG. 7 shows a method for positioning carbon nanotubes and
method for making a FET according to another embodiment of the
present invention.
[0034] FIG. 8 shows a method for positioning carbon nanotubes and
method for making a FET according to another embodiment of the
present invention.
[0035] FIG. 9 shows a method for making a FET according to another
embodiment of the present invention.
[0036] FIG. 10 shows a method for making a FET according to another
embodiment of the present invention.
[0037] FIG. 11 shows a method for making a FET according to another
embodiment of the present invention.
[0038] FIG. 12 shows a method for making a FET according to another
embodiment of the present invention.
[0039] FIG. 13 shows a method for making a FET according to another
embodiment of the present invention.
DETAILED DESCRIPTION
[0040] The present invention relates to a semiconducting carbon
nanotube, a method for positioning the carbon nanotube, a field
effect transistor (FET) made using the carbon nanotube, and a
semiconductor device.
[0041] Preferred embodiments of the present invention will now be
described without limitation to the scope of the invention.
[0042] An example structure of a carbon nanotube is shown in the
schematic diagram of FIG. 1. A CNT 1 has a functional group in one
end of the bare carbon nanotube. In particular, the CNT 1 includes
a functional group 3 bonded to a first end of the bare CNT by
chemical interaction. Examples of the functional group 3 include
--COOH, --C.dbd.O, and --NH.sub.2.
[0043] Another example of a carbon nanotube is shown in FIG. 2. In
a carbon nanotube 30, the functional group 3 is bonded to the first
end of the bare PCT and a different functional group 6 is bonded to
the other end (second end) of the bare CNT.
[0044] The CNT 1 or 30 may be a single-walled carbon nanotube
(SWNT) or a multi-walled carbon nanotube (MWNT). The diameter of
the CNT may be about 0.4 nm to about 100 nm, and the length of the
CNT may be about 2 nm to about 1 mm. Examples of the functional
groups 3 and 6 include various functional groups containing
elements of Groups I, II, and XIII to XVI in the periodic
table.
[0045] Examples of the method of making the CNT, the method of
positioning the CNT at a predetermined location, and the method of
making a FET using the CNT will now be described with reference to
FIGS. 3 to 13. Each method includes the following:
[0046] forming, on a first substrate, a carbon nanotube thin film
composed of bare carbon nanotubes densely aligned in a direction
intersecting the longitudinal direction of the bare carbon
nanotubes;
[0047] introducing a predetermined functional group into a first
end of each bare carbon nanotube;
[0048] attaching a second substrate to the side of the carbon
nanotube thin film opposite to the first substrate and then
removing the first substrate;
[0049] introducing another functional group into a second end of
each carbon nanotube to prepare a functionalized carbon
nanotube;
[0050] separating the carbon nanotube thin film from the second
substrate and dispersing the functionalized carbon nanotubes in a
solvent; and
[0051] positioning each functionalized carbon nanotube between two
electrodes formed by self-alignment, the two electrodes being
formed on a third substrate in advance.
[0052] In the carbon nanotube thin film, the bare carbon nanotubes
are aligned on the first substrate using radiofrequency (RF) plasma
or the like (e.g., refer to Fan S. et al., Science, 1999, vol. 283,
p. 512 and Murakami Y. et al., Chemical Physics Letters, 2004, vol.
385, p. 298), for example.
[0053] In this embodiment, the first substrate (a substrate 2 in
FIG. 3) may be composed of glass, quartz, silicon, or the like. Any
other materials that can withstand high temperature for depositing
carbon nanotubes may also be used. In forming SWNTs, the first
substrate is preferably composed of a material that does not react
with the catalyst used for deposition at high temperature.
[0054] As shown in FIG. 3, in this embodiment, SWNTs can be densely
formed on the substrate 2, which is composed of glass or the like,
using RF plasma such that the SWNTs extend in a substantially
perpendicular direction with respect to the surface of the
substrate 2. In particular, a carbon nanotube thin film (CNT thin
film) 20' is composed of arrays of bare carbon nanotubes 8' with
spacing substantially equal to or less than the diameter of the
carbon nanotube in a direction intersecting the longitudinal
direction of the carbon nanotubes. By using the RF plasma, arrays
of bare SWNTs can be formed, and the percentage of the SWNTs
exhibiting the semiconducting behavior in all the SWNTs formed can
be increased.
[0055] The bare carbon nanotubes 8' may be aligned substantially
perpendicular to the surface of the substrate, as shown in FIG. 3
or may be aligned with a particular angle while maintaining the
spacing between the carbon nanotubes substantially the same. Even
when the CNTs 8' are inclined with a particular angle, the
functional group can still be introduced to the ends of the CNTs
8'.
[0056] Next, a functional group 3, such as --COOH, --C.dbd.O,
--NH.sub.2, or the like, is introduced to a first end, i.e., the
end not attached to the substrate 2, of each CNT 8'.
[0057] Examples of the method for introducing the functional group
to the first end of each bare CNT include chemical methods and
electrochemical methods that use solvents and plasma.
[0058] In order to introduce a --COOH group or the like as the
functional group 3, the carbon nanotube thin film 20' may be
immersed in an acidic solution and then oxidized by application of
positive voltage, for example.
[0059] The solution here contains a chemical substance, such as an
acid, an alkali, or an oxide. For example, the acidic substance may
be nitric acid, sulfuric acid, or a combination of these. The
alkaline substance may be NaOH, KOH, or a combination of these. The
oxide may be H.sub.2O.sub.2, a bromide, or a combination of
these.
[0060] In order to introduce a functional group such as --C.dbd.O,
plasma treatment in an oxide atmosphere may be employed. In order
to introduce a functional group such as --NH.sub.2, RF plasma
treatment in a NH.sub.3 atmosphere may be employed.
[0061] Next, a flat second substrate 4 composed of glass, quartz,
silicon, or the like is attached to the other side of the carbon
nanotube thin film 20''. In order to efficiently attach the second
substrate 4 onto the carbon nanotube thin film 20'', an adhesive
layer 5 that can adhere onto the functionalized ends of the carbon
nanotubes is formed on the surface of the second substrate 4 in
advance. The adhesive layer 5 may be composed of a material, such
as an adhesive polymer film, that has adhesiveness and that can be
easily removed in the subsequent process. Alternatively, the
adhesive layer 5 may be composed of a material that can physically
attach to the functionalized ends of the carbon nanotubes by
electrostatic interaction.
[0062] It is preferable to avoid chemical interaction between the
functional groups 3 and the adhesive layer 5 in order that the
functional groups 3 introduced to the ends of the carbon nanotubes
are prevented from being modified. That the functional groups 3
maintain their properties is desirable for achieving selective
bonding of the functional groups 3 to a particular electrode
material and for the self alignment of the carbon nanotubes
performed in subsequent process stages. It is possible to use
chemical reaction, such as acid-alkali reaction, as long as the
reaction does not modify the properties of the functional group 3
of reacting to the particular material.
[0063] Subsequently, as shown in FIG. 6, the carbon nanotube thin
film 20 is separated from the first substrate 2.
[0064] In introducing another functional group, as shown in FIG. 7,
functional groups 6 are introduced to the second ends of the carbon
nanotubes of the carbon nanotube thin film 20. The functional
groups 6 may be introduced by the same process described in FIG.
4.
[0065] The functional groups 6 are preferably different from the
functional groups 3 described with reference to FIG. 4. This is
desirable to allow the functional groups 6 to selectively react
with an electrode material different from the electrode material to
which the functional groups 3 selectively react, so that the carbon
nanotubes can be self-aligned.
[0066] Next, as shown in FIG. 8, the carbon nanotube thin film 20
is separated from the second substrate 4 and placed in a solvent 7
to disperse functionalized CNTs 8. Examples of the solvent 7
include dichloroethane (DCE), dimethylformamide (DMF), and
tetrahydrofuran (THF).
[0067] The solvent 7 preferably contains as little contaminants as
possible. Contamination can be prevented by ultrasonic wave
treatment. For example, the adhesive layer 5 can be sufficiently
prevented from entering the solvent 7 by adequately selecting the
power and duration of ultrasonic wave treatment.
[0068] Referring now to FIG. 9, electrodes 10 and 12 for forming
source and drain electrodes of transistors are formed by a typical
semiconductor production process, such as lithography, on a
substrate 9 for forming a semiconductor device including FETs. The
substrate 9 is composed of silicon. In the drawing, the gate
structures 11, such as source/drain regions, in the substrate are
schematically shown. Predetermined interconnections are also formed
but are not depicted in the drawing.
[0069] Next, the solvent 7 containing dispersed carbon nanotubes 8
is applied on the electrodes 10 and 12 by dipping, spin-coating, or
the like, as shown in FIG. 10.
[0070] The electrodes 10 are composed of a material that
selectively interacts with one of the functional group 3 and the
functional group 6 at the ends of the carbon nanotubes 8, whereas
the electrodes 12 are composed of a material that selectively
interacts with the other one of the functional group 3 and the
functional group 6. The possible combinations of the electrode
material and the functional group are provided in Table 1. Note
that the interaction between the electrode material and the
functional group of each combination is achieved by physical
bonding resulting from interatomic force, electronic transition
bonding, chemical bonding, or the like.
TABLE-US-00001 TABLE 1 Functional group Electrode material --SH Au,
Pt, Ag, Pd, Cu --S--S-- Au, Pt, Ag, Pd, Cu --COOH Al, Fe, Co, Ni,
Zn --SO.sub.3H Al, Fe, Co, Ni, Zn --OH Pt
[0071] In this manner, the functional groups 3 and 6 at the ends of
the CNTs 8 selectively bond with the electrodes 10 and 12, as shown
in FIG. 10, and the CNTs 8 are self-aligned. The process does not
involve a complicated procedure such as using an AFM or a
high-temperature process such as one accompanying the CVD but can
position carbon nanotubes on predetermined electrodes with high
accuracy.
[0072] The electrodes may be composed of an element of Groups III
to XIII in the periodic table. A chemical substance including H, C,
N, OP, S, or the like may cover or introduced to the conductive
material of the electrodes so that the connection between the
functional groups at the ends of the carbon nanotubes and
electrodes can be strengthened.
[0073] Alternatively, the chemical substance may be added to the
solvent 7 for dispersing the carbon nanotubes 8 to achieve the same
effect.
[0074] For example, the group --SH of aminoethanethiol
(NH.sub.2CH.sub.2CH.sub.2SH) may be bonded to a Au electrode
material and NH.sub.2 may be reacted with --COOH introduced at the
ends of the carbon nanotube 8 so that the electrode can be more
securely connected to the carbon nanotube.
[0075] As is evident from the above, the range of usable material
that can yield interaction between the carbon nanotubes and the
electrodes can be widened by combining various chemical substances
reactive to particular materials with the electrode material. The
speed of interaction and selectivity can be further increased.
[0076] The CNTs 8 self-aligned on the electrodes 10 and 12 may be
annealed at a temperature in the range of 200.degree. C. to
2,000.degree. C. to substantially remove the materials other than
carbon. Thus, as shown in FIG. 11, carbon nanotubes 13 without
functional groups at the ends can be disposed on the electrodes 10
and 12, and the contact resistance between the carbon nanotubes 13
and the electrodes 10 and 12 can thereby be reduced.
[0077] In this annealing process, the functional groups start to
separate from the carbon nanotubes 8 at about 200.degree. C., and
almost all functional groups completely separate from the carbon
nanotubes 8 at about 400.degree. C. to about 500.degree. C. The
maximum annealing temperature is preferably 2,000.degree. C. or
less to prevent damage on the carbon nanotubes and the substrates
and to avoid high-temperature processing. More preferably, the
maximum annealing temperature is 800.degree. C. or less. Annealing
at 100.degree. C. to 500.degree. C. is a low-temperature process
compared with deposition of CNTs by CVD.
[0078] The annealing process is preferably conducted by introducing
He or Ar gas while maintaining a predetermined degree of
vacuum.
[0079] Subsequently, as shown in FIG. 12, a cover layer 15 composed
of a dielectric material is disposed on the carbon nanotubes 13 and
the electrodes 10 and 12. The cover layer 15 may be composed of a
material having a dielectric constant of 2.0 or more so that
generation of leak current can be securely prevented and the gate
effect is not affected. The thickness of the cover layer 15 is
preferably 1 nm to 1,000 nm.
[0080] As shown in FIG. 13, gate electrodes 16 are formed to obtain
FETs that exhibit desirable properties by having the channel
structure constituted from the carbon nanotubes 13.
[0081] The FETs thus prepared exhibit superior properties compared
to known transistors composed of silicon materials in terms of
mutual conductance and carrier mobility since the FETs include
channels constituted from the carbon nanotubes 13 having
semiconducting behavior.
[0082] According to the carbon nanotube and the method for
positioning the carbon nanotube described above, carbon nanotubes
having excellent semiconducting properties can be accurately
positioned at predetermined locations by self alignment without
complicated procedures or high temperature processes. By making
FETs using the carbon nanotubes, the productivity of FETs having
excellent properties can be increased.
[0083] Moreover, the method of positioning the carbon nanotube
described above can facilitate production of various semiconductor
devices each including a carbon nanotube, a first region to which
one end of the carbon nanotube is fixed, and a second region to
which the other end of the carbon nanotube is fixed, the first and
second regions being composed of different materials, without
expensive equipment or high-temperature processes.
[0084] Examples of such semiconductor devices include switching
elements of various displays, next-generation logic devices, and
optoelectronic memory devices.
[0085] It should be understood by those skilled in the art that the
present invention is not limited to the materials and structures
described in the embodiments above. Various modifications,
combinations, and alterations may occur depending on design
requirements and other factors insofar as they are within the scope
of the appended claims or the equivalents thereof. For example,
MWNTs may be used as the CNTs or the method of positioning may be
applied to making of various other semiconductor devices.
[0086] It should be understood that various changes and
modifications to the presently preferred embodiments described
herein will be apparent to those skilled in the art. Such changes
and modifications can be made without departing from the spirit and
scope of the present subject matter and without diminishing its
intended advantages. It is therefore intended that such changes and
modifications be covered by the appended claims.
* * * * *