KR20060050134A - Unipolar nanotube transistor having carrier-trapping material and method of fabricating the same - Google Patents

Abstract

Ambipolar nanotube field effect transistors are used as unipolar nanotube field effect transistors by adsorbing a carrier trapping material such as oxygen molecules on carbon nanotubes or by providing an additional layer containing carrier trapping material adjacent to the carbon nanotubes. Is converted.

Description

Unipolar nanotube transistor having a carrier trapping material and a method of manufacturing the same {Unipolar nanotube transistor having carrier-trapping material and method of fabricating the same}

1 is a side view of a conventional nanotube field effect transistor.

2 is a side view of a nanotube field effect transistor according to an embodiment of the present invention.

Figure 3 is a photograph showing that the oxygen molecules are adsorbed on the nanotubes.

4 is a side view of a nanotube field effect transistor according to another embodiment of the present invention.

5A is a graph showing an energy band gap of a conventional CNT FET.

5b is a graph showing the energy bandgap of a CNT FET according to the present invention with a gate voltage of zero.

5C is a graph showing the energy bandgap of a CNT FET according to the present invention with a gate voltage greater than zero.

6 is a graph showing the energy of electrons and holes adjacent to the source electrode according to the longitudinal position of the carbon nanotubes in the CNT FET according to the present invention.

7 is a graph of LUMO and HOMO of the CNT FET according to an embodiment of the present invention.

The present invention provides a unipolar carbon nanotube field effect transistor (CNT FET) having a carrier trapping material that converts ambipolar nanotube characteristics into unipolar nanotube characteristics. And to a method for producing the same.

Nanotube field effect transistors are widely used in electronic applications because of their excellent electronic properties. Nanotube field effect transistors, however, typically exhibit ambipolar electronic properties, making them undesirable for many device applications.

"Converting ambipolar CNT-FETs to unipolar CNT-FETs" by Yu-Ming Lin, Joerg Appenzeller, Phaedon Avouris NANO LETTERS 2004 Vol. 4, No. 5, PP 947-950, it is well known that the switching characteristics of carbon nanotube field effect transistors are improved by reducing the thickness of the gate oxide. However, reducing the gate oxide thickness is undesirable because it results in significant ambipolar transistor characteristics and high off-currents.

The switching properties of carbon nanotubes have been improved by using high dielectric constant materials. However, short-barrier barrier contacts formed at the interface between metal and nanotubes in transistors cause different scaling behavior in carbon nanotube FETs than in conventional FETs.

In the article, at least one technique is disclosed for converting an ambipolar carbon nanotube transistor into a unipolar carbon nanotube transistor using gate structure engineering. Unipolar CNT FETs were obtained by providing asymmetric gate structures for the source and drain electrodes. By this process, p-type CNT FETs were made from ambipolar CNT FETs.

According to the article, an ambipolar CNT FET is formed of a unipolar CNT FET by forming a gate oxide layer along the longitudinal direction of the drain electrode to form a "V'-shaped trench. In the device, carbon nanotubes are formed between the source and the drain. However, as shown in the above article, relatively long (deep) trenches are required to achieve satisfactory unipolar characteristics.

In the structure disclosed in the article, the trench has a depth extending into the substrate. The trench provides an asymmetry between the source and drain electrostatics, so that only a portion of the nanotubes are electrostatically controlled through the back gate. However, the trench's ability to switch from an ambipolar CNT FET to a unipolar CNT FET is a function of the trench width which makes the device's scale reduction undesirable or problematic. The authors suggest that an n-type CNT FET can be obtained by removing the p-type branch of an ambipolar CNT FET with a similar partial gate structure using a relatively deep trench.

Thus, there is a need for unipolar nanotube field effect transistors that do not require the use of gate structure engineering, such as the use of relatively large trenches.

An object of the present invention is to provide a method for easily converting an ambipolar nanotube field effect transistor into a unipolar nanotube field effect transistor by using a carrier trapping material.

In the present invention, an easy method of converting an ambipolar nanotube field effect transistor into a unipolar nanotube field effect transistor using a carrier trapping material is provided. In an exemplary embodiment of the invention, the carrier trapping material is oxygen (particularly oxygen molecules), and the oxygen is adsorbed onto the nanotubes.

In the method for converting an ambipolar nanotube field effect transistor into a unipolar nanotube field effect transistor according to an embodiment of the present invention, the nanotube field effect transistor includes a source electrode, a drain electrode and a gate, the source electrode and a drain electrode and the And an insulating layer separating the gate. Nanotubes are provided in electrical contact with the source and drain electrodes, and the nanotubes serve as channel regions of the field effect transistor. In the present invention, a carrier trapping material is provided on the nanotube surface.

In one embodiment according to the present invention, providing a carrier trapping material to the nanotubes includes adsorbing a carrier trapping material, which is an oxygen molecule, to the nanotubes.

In another embodiment according to the invention, providing a carrier trapping material to the nanotubes comprises providing a layer of material between the nanotubes and the insulating layer. The material layer comprises the carrier trapping material for the nanotubes, wherein the carrier trapping material is oxygen molecules.

In another embodiment according to the invention, providing a carrier trapping material to the nanotubes comprises adsorbing a carrier trapping material, which is an oxygen molecule, to a surface close to the nanotubes.

A method of manufacturing a field effect transistor according to the present invention includes providing a substrate, forming an insulating layer on the substrate, and forming a source electrode and a drain electrode on the insulating layer. A nanotube is provided between the source electrode and the drain electrode, and the nanotube is disposed in electrical contact with the source electrode and the drain electrode. The nanotubes provided with the carrier trapping material serve as channel regions of the field effect transistor. Preferably, the carrier trapping material is an oxygen molecule and the substrate is doped to act as a back gate for the field effect transistor.

In an embodiment according to the invention, providing a carrier trapping material for the nanotubes comprises adsorbing the carrier trapping material to the nanotubes. In another embodiment, providing a carrier trapping material to the nanotubes may provide the material layer for the nanotubes between the nanotubes and the insulating layer, or provide the carrier trapping material to a surface close to the nanotubes. Adsorbing.

The field effect transistor according to the present invention includes a source electrode and a drain electrode, a gate, an insulating layer spaced apart from the source electrode and the drain electrode, and a nanotube in electrical contact with the source electrode and the drain electrode. The nanotubes serve as channel regions of the field effect transistor, and carrier trapping material is provided on the surface of the nanotubes.

In another embodiment of the present invention, the field effect transistor further includes an additional layer between the insulating layer and the nanotubes. The additional layer includes the carrier trapping material for the nanotubes.

In another embodiment of the invention, the carrier trapping material is adsorbed onto the nanotubes. Preferably, the gate further comprises a substrate for the field effect transistor, wherein the insulating layer is located on the substrate, and the source electrode, drain electrode, and nanotube are located on the insulating layer. Preferably the nanotubes extend between the source and drain electrodes and the substrate is doped to act as a back gate.

Hereinafter, a nanotube field effect transistor having a carrier trapping material and a method of manufacturing the same according to the present invention will be described in detail with reference to the accompanying drawings.

Field effect transistors are provided with carbon nanotubes in conventional FET designs. The use of carbon nanotubes in conjunction with FETs results in devices with characteristics that exceed the current of silicon-based transistors that make CNT FETs.

Referring to FIG. 1, a conventional carbon nanotube field effect transistor includes a p-doped silicon substrate 102 forming a back gate. An insulating layer 104 of silicon oxide SiO 2 is located on the back gate 102, and a source electrode 106 is disposed on the insulating layer 104. Similarly, drain electrode 108 is disposed on insulating layer 104. The carbon nanotubes 110 are positioned between the source 106 and the drain 108, and the carbon nanotubes 110 are in electrical contact with the source electrode 106 and the drain electrode 108.

The operating principle of a CNT FET is generally similar to a conventional silicon field effect transistor. However, the physical device structure of the CNT FET of FIG. 1 is inverted (upper side down) relative to the physical device structure of a typical silicon FET. In other words, the source and drain are above the gate in the CNT FET as compared to the gate over the source and drain in a conventional silicon FET.

In addition, the channel between the source and the drain is provided with carbon nanotubes instead of silicon single crystals. Although in conventional CNT FETs, carbon nanotubes and sources and drains are provided over the gates, the sources and drains may be disposed below the gates or the carbon nanotubes may be buried in the device structure. The present invention is not limited to the use of the CNT FET of FIG. 1 and can be used with any FET with channels provided by nanotubes or nanowires.

In known CNT FETs, the source and drain electrodes may be formed of polysilicon doped to act as a conductor, but the source and drain electrodes are typically made of metal. The substrate forming the backgate 102 is typically made of silicon, although it may be made of any material that provides a functional gate for the CNT FET. The insulating layer is made of a suitable material having a relatively high dielectric constant k. A relatively high dielectric constant k index means that the dielectric constant index of SiO 2 is greater than approximately 4.0. The nanotubes may be made of other materials but are preferably made of carbon.

The nanotube 110 of the device according to the present invention may be a single-wall carbon nanotube (SWCNT) or a double-wall carbon nanotube (DWCNT) or a bundle of such nanotubes. In addition, nanowires may also be used as the nanotubes 110. The nanotubes can comprise any material that forms a sortie barrier, and the energy of the schottky barrier can be varied through the presence of a carrier trapping material, such as adsorbed oxygen molecules. The carrier trapping material preferably has a channel material, i.e., a lower unoccupies molecular orbital (LUMO) present between the bandgaps of the nanotubes. If the LUMO value of the carrier trapping material is below the Fermi level of the channel material, holes are trapped as carriers.

Conventional CNT FETs are ambipolar devices. Although a typical single gate CNT FET device is shown, the invention is applicable to all types of FETs using CNTs as conductors, as well as FETs spanning between source and drain electrodes, as well as multi-gate or multi-wall FETs. Do. CNTs act as channels instead of single crystals of silicon. In the device according to the invention, the CNTs can be embedded in the device.

A junction is typically not present between the CNT and the metal electrode, but instead is provided at the interface between the CNT and the source and drain electrodes. The gate may be provided above or below the CNT.

Referring to FIG. 2, the nanotube field effect transistor according to the present invention is generally equivalent to the conventional device shown in FIG. The nanotube field effect transistor of FIG. 2 includes a p-doped silicon substrate 102 forming a gate of the device. Silicon substrate 102 may be heavily doped to serve as the back gate of the device. An insulating layer 104 of silicon dioxide SiO 2 is provided over the doped substrate 102, and a source electrode 106 is provided over the insulating layer 104. A drain electrode 109 is provided on the insulating layer 104. The source electrode 106 and the drain electrode 109 are preferably made of a metal such as titanium, molybdenum, or gold or an alloy of these elements. The electrodes 106 and 108 may be made of polysilicon doped to act as a conductor. The carbon nanotubes 110 extend between the source electrode 106 and the drain electrode 108 to be in electrical contact with the source electrode 106 and the drain electrode 108.

CNT FET according to the present invention is different from the conventional carbon nanotubes in that the carrier trapping material 112 is provided in the nanotubes. In FIG. 2, the carrier trapping material includes oxygen molecules 112. Oxygen molecules are adsorbed onto the nanotubes 110 during the manufacture of the CNT FET. The presence of carrier trapping material, such as oxygen molecules, suppresses electron injection from the drain electrode, which translates from conventional ambipolar CNT FET devices to unipolar CNT FET devices.

If a carrier trapping material, such as an oxygen molecule, is provided on the CNT (or provided near or above the insulating layer 104 or in or on an additional layer adjacent to the CNT), the presence of the carrier trapping material is approximately LUMO in the middle of the CNT energy gap. (See FIGS. 6A-6C). The LUMO level of the oxygen molecule is given by the Oppπ * orbital level of the oxygen atom. Thus, oxygen molecules (or other carrier trapping materials) trap electrons from the drain electrode. As a result of the trapping of the electrons, the entire band (not only the valence band but also the conductive band) is moved up relative to the metal (or electrode) work function. As a result, the energy barrier for electron injection increases, which causes the CNT FET to be unipolar rather than ambipolar.

In the manufacturing process, the carbon nanotubes are exposed to oxygen molecules, and preferably, the carbon nanotubes are exposed to sufficient oxygen pressure so that the oxygen molecules are easily adsorbed onto the carbon nanotubes. Oxygen molecules are stable up to about 200 ° C., which may be unstable in some semiconductor manufacturing steps. The carrier trapping material may be a molecule other than oxygen (which can endure high temperatures) as long as the LUMO of the material is located within the bandgap of the channel material, ie, approximately coincides with the Fermi level of the CNT.

As shown in Figure 2, the channel between the source and drain is provided with carbon nanotubes instead of silicon single crystals. Although carbon nanotubes and source and drain electrodes are provided on the gate, the present invention is also applicable to a structure in which the source and drain electrodes are provided under the gate, and the carbon nanotubes are embedded in the device structure. As such, the substrate of the CNT FET can be a silicon layer, with a gate spaced apart over the source and drain electrodes or a multi-gate of a well known CNT FET.

The source and drain electrodes are typically made of metal, although they may be made of polysilicon doped sufficiently to act as conductors. The substrate forming the gate 102 is typically made of silicon, although it may be made of any material that provides a gate to the CNT FET. As such, the insulating layer 104 on the gate 102 is preferably made of silicon dioxide, although it may be a suitable insulating material, preferably an oxide. In the same way, the nanotubes may be made of other materials but are preferably made of carbon.

Referring to FIG. 3, oxygen molecules are adsorbed at 3.3 kPa from the surface of the carbon nanotubes. At this time, the binding energy of the oxygen molecules on the nanotube wall is 0.1 eV, and the adsorption state is weak between the oxygen molecules and the nanotubes. Each oxygen atom is supported by adjacent carbon atoms in the nanotubes. As will be described in detail below, oxygen atoms facilitate carrier trapping in nanotubes. Although the carrier trapping material, such as the oxygen molecule, is adsorbed on the carbon nanotube in the embodiment, the carrier trapping material may be adsorbed on the insulating layer 104. Oxygen molecules can be adsorbed on the CNT surface as well as inside the CNT. In addition, carrier trapping materials, such as oxygen molecules, may be adsorbed on insulating layers 104, such as SiO2, in proximity to the CNTs, resulting in unipolar devices. Referring to FIG. 4, the carrier trapping material may be spin coated onto the device to form the additional layer 105, or may be deposited by any suitable method of the semiconductor device manufacturing process.

The carrier trapping material may be provided as the material of the additional layer 105. Or may be adsorbed onto the bed in subsequent steps of the manufacturing process. The additional layer 105 may be made of any suitable material that adsorbs or contains a carrier trapping material, such as an oxygen molecule. The additional layer 105 may be a protective layer of the device, particularly a layer to which a relatively low temperature manufacturing process is applied.

In an exemplary embodiment in which carrier trapping material is included or adsorbed in insulating layer 104 or additional layer 105 rather than nanotube 110, the gap between CNT and the material with carrier trapping material is 1 nm or less (ie, Or less than the quantum length of electron transfer).

Referring back to FIG. 4, an additional layer 105 may be provided on or under the carbon nanotubes or above and below (ie, close to the nanotubes) during the fabrication process. The additional layer 105 may be provided before or after the carbon nanotubes are provided between the source electrode 106 and the drain electrode 108.

Referring to FIG. 5A, an energy-k graph (ie wave vector) at zero gate voltage is shown in a conventional CNT FET (ie without the use of carrier trapping materials such as oxygen molecules). The lower curve represents the highest occupied molecular level (HOMO) of the CNT channel in the conventional CNT FET, and the upper curve represents LUMO.

In FIG. 5B, an energy-k graph (ie wave vector) at zero gate voltage is shown in a CNT FET in accordance with an embodiment of the present invention in which carrier trapping material, such as oxygen molecules adsorbed on the CNT, is shown.

In FIG. 5C, when the gate voltage is increased, the presence of carrier trapping material receives the supply of electrons to raise the curve representing LUMO as well as the curve representing HOMO. Electrons are trapped in the carrier trapping material such that the trap energy level is above the Fermi level.

Referring to FIG. 6, the energy of electron injection at different gate voltages is shown according to the longitudinal position of the carbon nanotubes. As mentioned above, logic gates containing CMOS devices and CNT FETs generally require n or p transistor devices (ie, unipolar devices) and operate at the same gate bias. However, conventional CNT FETs exhibit ambipolar characteristics as a result of carrier injection by lowering the source or drain barrier. In FIG. 6, band energy at different vias conditions of the gate is shown. The energy of hole injection is plotted as a voltage as a function of the longitudinal position of the carbon nanotubes. Hole injection energy is given in electron-voltage (eV) and the location is given in nanometers (nm).

With reference to FIG. 7, the Fermi level 220 of the CNTs is shown by a dashed line between the LUMO level 212 of the CNT and the HUMO level 214 of the CNT. The LUMO level 216 of the oxygen molecule is approximately located between the LUMO level 212 of the CNT and the HOMO level 214 of the CNT.

In the manufacturing method according to the embodiment of the present invention, the nanotube field effect transistor is converted from an ambipolar device to a unipolar device by providing a carrier trapping material such as an oxygen molecule in the nanotube. The carrier trapping material is preferably adsorbed at appropriate pressure and temperature conditions so that the nanotubes are readily adsorbed to the nanotubes.

During the fabrication process of the nanotube field effect transistor, a carrier trapping material may be provided to the insulating layer (such as the SiO 2 layer) of the device. Alternatively (or in addition), other layers may be spin coated or deposited on or under the CNTs during the fabrication of the device. If the carrier trapping material is not provided to the additional layer during the manufacturing process, the device may be exposed to the carrier trapping material (such as oxygen molecules) again during the manufacturing process so that the carrier trapping material may be adsorbed to the additional layer. Preferably, the insulating or additional layer comprising a carrier trapping material is in contact with or in close proximity to the CNT such that the gap between the CNT and the material comprising the carrier trapping material is less than 1 nanometer (or quantum length of electron transfer). Or less).

According to the present invention, the ambipolar transistor can be easily converted into a unipolar transistor by adsorbing a carrier trapping material on the surface of the carbon nanotubes.

While many details are set forth in the foregoing description, they should be construed as illustrative of preferred embodiments, rather than to limit the scope of the invention. Therefore, the scope of the present invention should not be defined by the described embodiments, but should be determined by the technical spirit described in the claims.

Claims (30)

Source and drain electrodes; gate; An insulating layer spaced apart from the source electrode and the drain electrode; A nanotube in electrical contact with the source electrode and the drain electrode and serving as a channel region of a field effect transistor; And And a carrier trapping material on the nanotubes. The method of claim 1, And the carrier trapping material is an oxygen molecule. The method of claim 1, Further provided between the insulating layer and the nanotubes, And the additional layer comprises the carrier trapping material. The method of claim 3, wherein And the carrier trapping material is an oxygen molecule. The method of claim 1, And the carrier trapping material converts the field effect transistor from an ambipolar device to a unipolar device. The method of claim 1, Further comprising a substrate for the field effect transistor, The insulating layer is positioned on the substrate, the source electrode, the drain electrode and the nanotube are positioned on the insulating layer, and the nanotube extends between the source electrode and the drain electrode. . The method of claim 6, And the carrier trapping material is an oxygen molecule. The method of claim 6, Further provided between the insulating layer and the nanotubes, And the additional layer comprises the carrier trapping material. The method of claim 8, And the carrier trapping material is an oxygen molecule. The method of claim 8, And the substrate is doped to act as a back gate. The method of claim 10, And the source and drain electrodes are made of a material selected from the group consisting of at least one of Ti and MO. The method of claim 11, And the carrier trapping material converts the field effect transistor from an ambipolar device to a unipolar device. The method of claim 1, The insulating layer is located on the nanotubes, the gate is a field effect transistor, characterized in that disposed on the insulating layer. The method of claim 13, And the carrier trapping material is an oxygen molecule. The method of claim 13, Further provided between the insulating layer and the nanotubes, And the additional layer comprises the carrier trapping material. The method of claim 15, And the carrier trapping material is an oxygen molecule. The method of claim 16, And the source and drain electrodes are made of a material selected from the group consisting of at least one of Ti and MO. The method of claim 17, And the carrier trapping material converts the field effect transistor from an ambipolar device to a unipolar device. The field effect transistor of claim 1, wherein the carrier trapping material is adsorbed onto the nanotubes. In the method of converting an ambipolar nanotube field effect transistor into a unipolar nanotube field effect transistor, The nanotube field effect transistor may include a source electrode and a drain electrode; gate; An insulating layer spaced apart from the source electrode and the drain electrode; A nanotube in electrical contact with the source electrode and the drain electrode and serving as a channel region of the field effect transistor, Providing an ambipolar nanotube field effect transistor to a unipolar nanotube field effect transistor. The method of claim 20, Providing a carrier trapping material to the nanotubes, adsorbing the carrier trapping material to the nanotubes. The method of claim 21, And the carrier trapping material is an oxygen molecule. The method of claim 20, Providing a carrier trapping material to the nanotubes, providing an additional layer comprising the carrier trapping material for the nanotubes between the insulating layer and the nanotubes. The method of claim 20, Providing a carrier trapping material to the nanotubes, adsorbing the carrier trapping material to a surface near the nanotubes. Preparing a substrate; Forming an insulating layer on the substrate; Forming a source electrode on the insulating layer; Forming a drain electrode on the insulating layer; Providing a nanotube in electrical contact with the source electrode and the drain electrode and serving as a channel region of the field effect transistor, And providing a carrier trapping material to the nanotubes. The method of claim 25, Providing a carrier trapping material to the nanotubes, adsorbing the carrier trapping material to the nanotubes. The method of claim 26, And the carrier trapping material is an oxygen molecule. The method of claim 26, The substrate is doped to act as a back gate in the field effect transistor. The method of claim 25, Providing a carrier trapping material to the nanotubes, providing an additional layer comprising the carrier trapping material for the nanotubes between the insulating layer and the nanotubes. The method of claim 26, Providing a carrier trapping material to the nanotubes, adsorbing the carrier trapping material to a surface near the nanotubes.

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