JP4810650B2 - Carbon nanotube FET - Google Patents

Description

  The present invention relates to a carbon nanotube FET, and more particularly to a technique for realizing an n-type carbon nanotube FET by using a new electrode material.

  Carbon nanotubes have semiconductor characteristics due to chirality, can achieve a high current density, and can form very thin lines that can be regarded as almost one-dimensional conduction. Suitable for quantum device applications that can operate. Among these, carbon nanotubes as quantum wires are promising candidates for next-generation electronic devices, and research on carbon nanotube field effect transistors (hereinafter referred to as “CN-FET”) is being actively conducted.

R. Martel et al, Phys. Rev. Lett. 87, 256805 (2001).

  By the way, a CN-FET tends to be a p-type transistor, and there are few examples of making an n-type CN-FET. There is an example in which an n-type CN-FET is manufactured by doping potassium on the surface of a carbon nanotube, but in order to deposit potassium on the surface of a carbon nanotube channel, an ultra-high vacuum apparatus is required and the process is complicated. If the carbon nanotube channel is doped with potassium, the ionized impurity becomes a carrier scattering center, which causes a problem of low mobility.

  An object of the present invention is to produce an n-type CN-FET without reducing mobility. It is another object of the present invention to create other electronic devices using this creation technique.

  According to one aspect of the present invention, a channel layer made of carbon nanotubes, a gate electrode formed for the channel layer made of carbon nanotubes, and an alkali metal or alkaline earth formed in a region in contact with the channel layer An n-type CN-FET structure having a source electrode and a drain electrode including at least one of metals is provided. When an alkali metal or alkaline earth metal and a carbon nanotube are joined, the energy discontinuity between the Fermi energy of the metal and the conduction band bottom of the carbon nanotube is small, and electrons easily move from the source or drain electrode to the carbon nanotube. It is easy to form an electrode capable of electron injection. Electrons injected from the electron injection electrode move through the channel of the carbon nanotube and reach the other electrode.

  Also, a channel layer made of carbon nanotubes, a gate electrode formed for the channel layer made of carbon nanotubes, and a material having a work function smaller than the work function of carbon nanotubes formed in a region in contact with the channel layer An n-type CN-FET structure having a source electrode and a drain electrode is provided. The work function of the material forming the source electrode and the drain electrode is preferably 4 eV or less, and more preferably 3 eV or less. In the case of carbon nanotubes, since the crystal structure is a closed system, it is not easily affected by band pinning due to surface states, and when a metal is bonded to the carbon nanotube CNT, the work depends on the work function of the metal An electron injection electrode can be formed between the metal having a small function.

  The channel layer is not doped with impurities. Here, doping is an operation of adding impurities to the carbon nanotubes for the purpose of inducing electrons or holes in the carbon nanotubes, and includes the inclusion of impurities in the process of manufacturing the carbon nanotubes or impurities that do not contribute to conduction. I can't. Therefore, “impurities are not doped” means that impurities that are naturally mixed in the process of manufacturing the carbon nanotube are excluded, and impurities that do not contribute to conduction are excluded. In addition, in the vicinity of the interface of the Schottky junction formed when the material defined by the work function as described above is used as an electrode, for example, atoms slightly diffused into the carbon nanotubes are included in the impurities. Does not enter.

  Thus, even when no impurity is added to the carbon nanotube corresponding to the channel layer, an n-type FET can be formed by injecting electrons from the electrode.

  According to another aspect of the present invention, a carbon nanotube, the electron injection electrode structure in contact with the carbon nanotube, and a hole injection having a work function in contact with the carbon nanotube at a position different from the electron injection electrode structure of 5 eV or more. An element comprising an electrode is provided. When a voltage is applied to this element, electrons and holes are injected in a direction approaching each other in the carbon nanotube.

  According to the present invention, an n-type junction can be formed with respect to a carbon nanotube, and an n-type CN-FET can be formed without adding impurities by using this.

  In this specification, an electron injection electrode is an electrode having a function of injecting electrons as a main function, and a hole injection electrode is an electrode having a function of injecting holes as a main function. In this specification, it defines as an electron or a hole based on the carrier which can be inject | poured with a smaller applied voltage.

  As an n-type CN-FET manufacturing technique according to the present invention, the carbon nanotube corresponding to the channel layer is not doped with impurities, but the electrode functions as an injection source of carriers such as electrons or holes, and this injection electrode is configured. A method has been devised for controlling the conductivity type of a CN-FET by selecting the material to be used.

  In general semiconductor (such as Si) and metal bonding, the relationship between the work function and the energy band barrier during heterogeneous bonding due to the influence of dangling bonds (non-bonding hands) formed on the Si surface. Is thin. In other words, in general bonding of semiconductors (such as Si) and metals, band pinning occurs due to surface states due to the influence of dangling bonds formed on the Si surface, and the band structure reflects each work function. It is hard to become.

  On the other hand, in the case of carbon nanotubes, since the crystal structure is a closed system, it is not easily affected by band pinning due to surface states. It is considered that an energy barrier reflecting a work function difference from the carbon nanotube CNT is formed.

  That is, focusing on the fact that the current of the CN-FET is controlled by a Schottky barrier formed at the interface between the contact electrode and the semiconducting carbon nanotube, by using a material having a low work function, for example, Ca for the contact electrode, It was considered that an n-type transistor could be manufactured.

  Here, the work function is the minimum energy required to extract one electron from the surface in vacuum to infinity, and is a value specific to the electronic state of the material surface. The work function can usually be measured by the Kelvin method (vibration capacity method), thermionic emission, photoelectron emission experiments, or the like.

  The work function of the metal Metal referred to here is the work function of the metal in contact with the carbon nanotube CNT. For example, when the electrode has a multilayer structure, the work function of the metal in contact with the carbon nanotube CNT is Point to. That is, a combination having a work function of the electrode material smaller than that of the carbon nanotube is selected at the interface between the carbon nanotube and the electrode material. The work function of the electrode material is preferably 4 eV or less, and particularly preferably 3 eV or less.

  If this idea is developed, injection carriers (electrons or holes) into the carbon nanotube can be selected only by selecting an electrode material, and a rectifier diode, a light-emitting diode, or the like can be manufactured as described later.

The joining of metal (Metal) and carbon nanotube (CNT) will be described with reference to FIGS. 1 and 2 in more detail. FIG. 1 is a view showing a state of bonding between a carbon nanotube and a metal such as Au or Pd which is a general electrode material for a semiconductor. In FIG. 1, the upper diagram is an energy band diagram of the metal Metal and the carbon nanotube CNT based on the vacuum level E _VAC , and the middle diagram shows the metal Metal such as Au and Pd and the carbon nanotube CNT. It is an energy band figure at the time of actually joining, and the following figure is an energy band figure at the time of applying a positive voltage to metal Metal.

As shown in FIG. 1, in a metal such as Au and Pd are metals for common electrode, the work function phi _m is about 5 eV. On the other hand, the work function φ _CNT of the carbon nanotube CNT is about 4 eV. Accordingly, as shown in Figure in Figure 1, when bonding the metal Metal and carbon nanotubes CNT, a state where the Fermi energy E _F of the metal and carbon nanotubes CNT coincide, in the valence band of the carbon nanotubes CNT-side Band discontinuity (ΔE _V ) can be made extremely small. The band discontinuity value ΔE _C of the conduction band is a large value of about 1 eV. Therefore, when a positive voltage is applied to the metal metal with respect to the carbon nanotube CNT as shown in the figure below, holes near the valence band of the metal metal can be easily transported to the carbon nanotube CNT, and the hole current is very easy. Flowing into.

On the other hand, when a negative voltage is applied to the metal Metal with respect to the carbon nanotube CNT, electrons near the conduction band of the metal Metal are hardly transported to the carbon nanotube CNT due to a large energy barrier (ΔE _C ), which is very high. As long as no voltage is applied, the electron current hardly flows. Therefore, if a general metal (Au) is used with a structure in which impurities are not doped in the carbon nanotube, it is likely to become a p-type carbon nanotube FET, and the n-type carbon nanotube FET tends to be difficult to realize.

The inventor designs the conductivity type of the CN-FET by devising a material that constitutes at least the carrier injection electrode of the source or drain instead of the conventional method of doping the carbon nanotube itself. I thought. As described above, unlike the bonding of a general semiconductor (such as Si) and a metal, when the metal Metal is bonded to the carbon nanotube CNT, as shown in the diagram in FIG. since a state where the Fermi energy E _F match, the magnitude of the work function (more specifically, phi about _CNT) barrier occurs.

On the other hand, as shown in FIG. 2, selecting a material with a low work function phi _m metal Metal an electrode material (specifically, phi _m is the order of phi _{CNT / 2),} as shown in Figure in Figure 2 a, can be made extremely small Delta] E _C is the discontinuity value of the conduction band at a joint interface between the metal metal and carbon nanotubes CNT, as shown in the lower part of FIG. 2, positive carbon nanotubes CNT to metals metal When the voltage is applied, electrons in the conduction band of the metal Metal are easily injected into the conduction band of the carbon nanotube CNT even when the applied voltage is small. Therefore, it is possible to easily form an n-type CN-FET. Examples of the metal Metal suitable for the n-type CN-FET include Cs (work function φs = 2.14 eV), K (work function φs = 2.3 eV), Li (work function φs = 2), which are alkali metals. Alkaline earth metals such as Ca (work function φs = 2.87 eV), Mg (work function φs = 3.66 eV) and the like are metals in the range of 4 eV or 3 eV. Although not within the above range, transition metals or metals such as Mn, Al, and Ti can also be candidates for the electron injection electrode.

Hereinafter, an n-type CN-FET according to an embodiment of the present invention will be described with reference to the drawings. FIG. 3 is a cross-sectional view showing the structure of the CN-FET according to the present embodiment. As shown in FIG. 3, the CN-FET 1 according to the present embodiment is separated from the p ⁺ type Si substrate 3, the SiO ₂ oxide film 5 having a thickness of 100 nm formed on the surface thereof, and a distance above the p ⁺ type Si substrate 3. Co / Pt catalysts 7a and 7b formed in this manner, carbon nanotubes 11 formed between the Co / Pt catalysts 7a and 7b, and Ca metal formed so as to be in contact with the respective ends of the carbon nanotubes 11. A source electrode (15a / 17a) and a drain electrode (15b / 17b) having 15a and 15b and Al electrodes 17a and 17b formed on the Ca metal 15a and 15b, and the back surface of the p ⁺ type Si substrate 3 And a back gate electrode 21 made of Ti / Au. Work small function φ _m (2.87eV) Ca metal 15a, 15b are in contact at both ends of the carbon nanotubes CNT11, to form the energy band structure as shown in FIG. 2, to create an n-type CN-FET be able to. An antioxidant film such as a photoresist 23 for preventing the oxidation of the Ca metals 15a and 15b may be formed on the carbon nanotube CNT11. Ca and Al are preferably deposited continuously. This is because Ca is an alkaline earth metal and is unstable by itself (when the surface is exposed). That is, Al has a function of a protective film of Ca and a function of reducing resistance. Further, the photoresist 23 also has a function of covering the whole and preventing Ca from oxidation. In the present embodiment, the thickness of each Ca / Al is 3 nm / 250 nm.

With reference to FIG. 3, the manufacturing process of CN-FET is demonstrated. First, a p ⁺ type Si substrate 3 is prepared, a SiO ₂ oxide film 5 is formed on the front surface, and a back gate electrode 21 is formed on the back surface. Next, a Co / Pt catalyst layer is formed on the surface, and Co / Pt catalysts 7a and 7b are formed by patterning. Further, SWNTs (single-walled (carbon) nanotubes) 11 whose positions are controlled between the Co / Pt catalysts 7a and 7b are grown by an alcohol CCVD method. The growth conditions are, for example, a growth temperature of 900 ° C., a gas flow rate of Ar / C ₂ H ₅ OH (100/50 cm ³ / min), a gas pressure of 200 Pa, and a growth time of 1 hour. Next, a source electrode 17a and a drain electrode 17b made of Ca / Al are formed by, for example, a known lift-off method so as to be in contact with the SWNT11. Finally, the entire surface is covered with an organic film such as a photoresist 23, for example. Alternatively, a method of arranging already grown nanotubes on the substrate 3 without growing nanotubes such as SWNTs on the substrate 3 may be used. Alternatively, a structure in which the carbon nanotube is supported by some support member may be used.

Next, operations of the p-type CN-FET and the n-type CN-FET will be described with reference to FIGS. 4A, 4B, 5A, and 5B are diagrams showing energy band structures in an equilibrium state and an operating state of the p-type CN-FET and the n-type CN-FET. As shown in FIG. 4 (A), p-type CN-FET is on top of the energy E _V position near the valence band of the Fermi level E _F is carbon nanotubes CNT is at equilibrium, from the electrode to the carbon nanotube The barrier height φ _Bn related to electrons is large, and the barrier height φ _Bp related to holes from the electrode to the carbon nanotube is small. Therefore, as shown in FIG. 4B, when a voltage is applied between the source and the drain while an appropriate voltage is applied to the gate, holes are injected from the source into the channel region and transported to the drain through the channel region. Is done. A so-called drain current caused by holes flows. FIG. 7 shows the gate voltage V _GS dependence of the drain current _ID of the p-type CN-FET using Pd as an electrode. As shown in FIG. 7, it can be seen that transfer characteristics similar to those of a general p-type FET are obtained.

On the other hand, as shown in FIG. 5 (A), n-type CN-FET is in the energy E _C and close the Fermi level E _F is the conduction band of the carbon nanotubes CNT bottom at equilibrium, from the electrode to the carbon nanotube The barrier height φ _Bp related to the holes of the electrode is large, and the barrier height φ _Bn related to electrons from the electrode to the carbon nanotube is small. Therefore, as shown in FIG. 5B, when a voltage is applied between the source and the drain while an appropriate voltage is applied to the gate, electrons are injected from the source into the channel region and transported to the drain through the channel region. The A so-called drain current caused by electrons flows. FIG. 6 shows the gate voltage V _GS dependence of the drain current _ID of an n-type CN-FET using Ca / Al as an electrode. As shown in FIG. 6, it can be seen that transfer characteristics similar to those of a general n-type FET are obtained. It can be seen that a drain current starts to flow at a gate voltage of about -5 V, and good characteristics can be obtained as an n-type CN-FET.

  As described above, according to the present embodiment, an n-type junction can be formed, and an n-type CN-FET can be formed by using this without adding impurities.

  Since the p-type CN-FET and the n-type CN-FET can be formed on the same substrate, both the gate electrode and the drain electrode can be wired in common, for example, so-called complementary type ( It is also possible to manufacture complementary FETs. In addition, a general electronic device such as a pn junction diode can be easily manufactured.

  In order to form an n-type electrode, for example, the work function of the metal is preferably 4.0 eV or less, and preferably 3.0 eV or less. Further, since the band gap can be adjusted by the diameter of the carbon nanotube, the threshold value of the CN-FET is adjusted by the diameter of the carbon nanotube instead of the adjustment of the work function of the metal or the adjustment of the work function. Can also be done. The energy band structure of the carbon nanotube can also be adjusted by a so-called peapod structure (see B.W.Smith et al, Nature 396, 323 (1998)) in which fullerene is included in the carbon nanotube.

  In the first embodiment of the present invention, an n-type CN-FET has been described as an example of an element using an electron injection electrode. However, the carbon nanotube element using an electron injection electrode according to an embodiment of the present invention is described. Is applicable to many elements such as many electronic devices and optical devices using so-called semiconductor materials. In the following, an example applied to a bipolar transistor (second embodiment) and an example applied to an LED (third embodiment) will be described. Of course, the present invention is not limited to these.

  Next, a carbon nanotube device according to a second embodiment of the present invention will be described with reference to the drawings. FIG. 8 is a structural example of the carbon nanotube device according to the present embodiment, FIG. 8A is a structural sectional view of the carbon nanotube bipolar transistor according to the present embodiment, and FIG. It is an energy band figure corresponding to).

  As shown in FIG. 8A, the carbon nanotube bipolar transistor 41 according to the present embodiment is composed of carbon nanotubes CNT45 and electron injection electrodes formed in order in contact with the carbon nanotubes CNT45 and in the extending direction of the carbon nanotubes CNT. A Ca emitter electrode 47, a Pd base electrode 51 as a hole injection electrode, and a Ca emitter electrode 53 as an electron injection electrode are included. As shown in FIG. 8B, it can be seen that the energy band diagram of the carbon nanotube CNT45 has the same configuration as that of a general npn bipolar transistor. In the npn bipolar transistor 41 having the above configuration, electrons injected from the Ca collector electrode 47 into the carbon nanotube CNT 45 reach the collector C beyond the base B to generate a collector current. The collector current can be controlled by the emitter-base voltage. Of course, a pnp configuration can be realized instead of the npn configuration. That is, an emitter and a collector are formed by forming either an electron injection electrode or a hole injection electrode for the carbon nanotube, and the electron injection electrode or the hole injection electrode that is not used for the emitter and collector is used as a base. By doing so, a bipolar transistor using carbon nanotubes can be realized.

  As described above, according to the present embodiment, it is possible to form a bipolar transistor by arranging the electrode material in contact with the carbon nanotube so as to be pnp or npn.

  Next, a carbon nanotube optical device according to a third embodiment of the present invention will be described with reference to the drawings. FIG. 9 shows a configuration example of the carbon nanotube optical device according to the present embodiment. In the device 61 shown in FIG. 9, a hole injection electrode is provided at one end (source side) of the carbon nanotube 62 and the other end (drain side). The structure in which the electron injection electrode is arranged. In this structure, when a negative voltage is applied to the drain 67 with respect to the source 65, holes are injected into the carbon nanotube from the source side and electrons are injected into the carbon nanotube from the drain side. The light emission resulting from is obtained, and so-called LEDs can be formed. That is, since the p-type and n-type can be formed by the electrodes, application to optical devices is also possible.

  That is, by forming an electron injection electrode and a hole injection electrode with respect to a carbon nanotube, an electron and a hole are generated in the carbon nanotube, and by recombining the electron and the hole, a light emitting device, etc. An optical element can be formed.

  As described above in the second and third embodiments, the electron injection electrode can be easily formed, and both p-type and n-type can be obtained in electronic and optical devices. A device group using a typical semiconductor can be replaced with a device group using a carbon nanotube.

  The present invention can be applied to electronic devices and optical devices using general semiconductor materials. In addition to CN-FETs, the present invention can be applied when various devices using carbon nanotubes are realized.

It is a figure which shows the energy band structure regarding this metal and carbon nanotube CNT using a metal with a comparatively large work function as an electrode material, and the upper figure is each of a metal and a carbon nanotube on the basis of the vacuum level _EVAC. The middle diagram is an energy band diagram when a metal and a carbon nanotube are actually joined, and the lower diagram is an energy band diagram when a positive voltage is applied to the metal. It is a figure at the time of selecting a material with a comparatively small work function as an electrode material, and is a figure corresponding to FIG. It is sectional drawing which shows the structural example of n-type CN-FET by the 1st Embodiment of this invention. 4A and 4B are diagrams showing energy band structures in the equilibrium state and the operating state of the p-type CN-FET. FIGS. 5A and 5B are diagrams showing energy band structures in an equilibrium state and an operating state with the n-type CN-FET. It is a figure which shows the gate voltage _VGS dependence of the drain current _ID of n-type CN-FET which used Ca / Al as an electrode. It is a figure which shows the gate voltage _VGS dependence of the drain current _ID of p-type CN-FET which used Pd as an electrode. FIG. 8A is a structural example of a carbon nanotube device according to a second embodiment of the present invention, FIG. 8A is a structural sectional view of a carbon nanotube bipolar transistor according to the present embodiment, and FIG. It is an energy band figure corresponding to A). It is a structural example of the carbon nanotube optical device by the 3rd Embodiment of this invention.

Explanation of symbols

1 ... CN-FET, 3 ... p + -type Si substrate, 5 ... SiO ₂ oxide film, 7a, 7b ... Co / Pt catalyst, 11 ... carbon nanotube, 15a, 15b ... Ca metal, 17a, 17b ... Al electrode, 21 ... back gate electrode.

Claims (13)

A channel layer made of carbon nanotubes;
A gate electrode formed for the channel layer made of the carbon nanotube;
An n-type CN-FET structure comprising a source electrode and a drain electrode containing at least one of an alkali metal and an alkaline earth metal formed in a region in contact with the channel layer. A channel layer made of carbon nanotubes;
A gate electrode formed for the channel layer made of the carbon nanotube;
An n-type CN-FET structure comprising a source electrode and a drain electrode formed in a region in contact with the channel layer and made of a material having a work function smaller than that of a carbon nanotube.   The n-type CN-FET structure according to claim 2, wherein a work function of a material forming the source electrode and the drain electrode is 4 eV or less.   The n-type CN-FET structure according to claim 2, wherein a work function of a material forming the source electrode and the drain electrode is 3 eV or less.   5. The device according to claim 1, wherein at least one of the source electrode and the drain electrode has a portion in contact with the carbon nanotube in a direction substantially perpendicular to the extending direction thereof. The n-type CN-FET structure described in 1.   6. The n-type CN-FET structure according to claim 1, wherein the channel layer is not doped with impurities. Carbon nanotubes,
And an electrode including an alkali metal or an alkaline earth metal in contact with the carbon nanotube. Carbon nanotubes,
An electron injection electrode structure having an electrode having a work function of 4 eV or less in contact with the carbon nanotube. Carbon nanotubes,
An electron injection electrode structure having an electrode having a work function of 3 eV or less in contact with the carbon nanotube. Carbon nanotubes,
The electron injection electrode structure according to any one of claims 7 to 9, which is in contact with the carbon nanotube,
A device comprising: a hole injection electrode having a work function of 5 eV or more in contact with the carbon nanotube at a position different from the electron injection electrode structure. Forming a source electrode and a drain electrode containing at least one of an alkali metal and an alkaline earth metal so as to be in contact with the channel of the carbon nanotube;
And a step of forming a gate electrode at a position where an electric field can be applied to the channel. Forming a source electrode and a drain electrode containing a material having a work function of 4 eV or less so as to be in contact with the channel of the carbon nanotube;
And a step of forming a gate electrode at a position where an electric field can be applied to the channel. Forming a source electrode and a drain electrode containing a material having a work function of 3 eV or less so as to be in contact with the channel of the carbon nanotube;
And a step of forming a gate electrode at a position where an electric field can be applied to the channel.

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