JP2006086259A - Method for forming tunnel junction - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a tunnel junction easily at a predetermined part of a carbon nanotube. <P>SOLUTION: A silicon substrate 101 is carried into the processing chamber of a predetermined electron beam exposure system and a desired region of a carbon nanotube 103 is irradiated with an electron beam 110. The electron beam 110 is a focusing electron beam having an acceleration voltage of 1 kV and an exposure of 1×10<SP>17</SP>/cm<SP>2</SP>(1.6×10<SP>-2</SP>C/cm<SP>2</SP>). A defective portion 106 is formed at a part of the carbon nanotube 103 irradiated with the electron beam. The defective portion 106 formed in the carbon nanotube 103 through irradiation with electron beam acts as a tunnel junction and as a Coulomb barrier. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a tunnel junction forming method that can be applied to an element using carbon nanotubes as a part of its structure.

  As is well known, the carbon nanotube is a cylindrical structure having a very fine diameter on the order of nanometers, and has excellent electrical and mechanical properties. Therefore, application to fine electronic devices such as single-electron transistors is expected. A single electron transistor is generally composed of a source and drain electrodes, two micro tunnel junctions, quantum dots sandwiched between two micro tunnel junctions, and a gate electrode joined to the quantum dots with an insulator interposed therebetween. Therefore, in the application of carbon nanotubes to single-electron transistors, a technique for forming a tunnel junction in the element is important.

  Here, a method of manufacturing a single-electron transistor using carbon nanotubes that is mainly used at present will be described. First, an oxide film is formed on a silicon substrate, and a plurality of carbon nanotubes are formed in a state of being dispersedly arranged on the oxide film by a predetermined method. Next, an isolated carbon nanotube suitable for device fabrication is searched for by observation with a scanning electron microscope or an atomic force microscope. Next, a source / drain electrode is formed so as to be connected to the found carbon nanotube by a known lithography technique and etching technique.

  Part of the element thus formed operates as a single electron transistor. In single-electron transistors using carbon nanotubes, tunnel junctions are formed between the source / drain electrodes and the nanotubes, and these tunnel junctions act as Coulomb barriers, and are considered to operate as single-electron transistors. (Refer nonpatent literature 1).

  In addition, a method for manufacturing a single-electron transistor using carbon nanotubes has been proposed as shown below. First, defects are introduced into the nanotubes formed on the substrate by hydrogen etching. Thereafter, an electrode having good ohmic characteristics that does not act as a tunnel junction is formed on the nanotube by lithography. According to this manufacturing method, the yield is improved as compared with the above-described method (see Non-Patent Document 2).

  Since defects formed in the nanotube by hydrogen etching act as a tunnel junction, it is considered that an element manufactured by the above-described method functions as a single electron transistor. According to the method described above, quantum dots smaller than the distance between the source and drain electrodes can be formed in the nanotube, and thus the operating temperature as a single electron transistor is improved.

The applicant has not yet found prior art documents related to the present invention by the time of filing other than the prior art documents specified by the prior art document information described in this specification.
K. Ishibashi et al. "Quantum Dots in Carbon Nonotubes", Japanese Journal of Applied Physics Vol.39, pp.7053-7057, (2000). Torigoe et al., “Introduction of defects into carbon nanotubes by in situ hydrogen etching”, Spring 51st Joint Lecture on Applied Physics in Spring 2004, Proceedings 28a-ZX-5.

  However, in the method of Non-Patent Document 1, the place where the tunnel junction is formed is limited to the junction portion with the electrode part, and the tunnel junction cannot be produced at an arbitrary place. Since the distance between the source and drain electrodes described above is within a range that can be manufactured by the existing lithography technique, the size of the quantum dot sandwiched between the two tunnel junctions is determined by the distance between the two electrodes determined by the existing lithography technique. It cannot be made smaller. Thus, in the single electron transistor manufactured by the method described above, the quantum dots cannot be made very small, and the operating temperature of the element is limited to a low temperature. In addition, in the manufacturing method described above, it is difficult to obtain a high yield.

  Further, according to the method of Non-Patent Document 2, defects are randomly generated at unspecified locations in the nanotube, so that a tunnel junction cannot be formed at an arbitrary location of the nanotube. Therefore, the device design is greatly restricted. In addition, since hydrogen etching uses a reactive gas called hydrogen, there is a problem that safety equipment is expensive.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to allow a tunnel junction to be easily formed at a predetermined location of a carbon nanotube.

  The method for forming a tunnel junction according to the present invention includes a first step in which carbon nanotubes are formed on a substrate, and irradiating the carbon nanotubes with an electron beam having a predetermined acceleration voltage of at most 86 kV. At least a second step in which the is formed. A defect formed by electron beam irradiation becomes a tunnel junction.

  In the tunnel junction forming method, the carbon nanotube may be heated to a predetermined temperature in an atmosphere containing a combustion-supporting gas so that the tunnel junction can be efficiently formed in the region irradiated with electrons. In this treatment, for example, the carbon nanotubes may be heated to 350 to 500 ° C. in an atmosphere containing oxygen. In the tunnel junction forming method, the predetermined acceleration voltage may be at most 30 kV, and particularly preferably the predetermined acceleration voltage is in the range of 500 V to 3 kV.

  As described above, according to the present invention, a defect is generated in a carbon nanotube in a desired region by irradiating an electron beam having a predetermined acceleration voltage with an acceleration voltage of 86 kV or less. An excellent effect is obtained in that a tunnel junction can be easily formed at a predetermined location.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a process diagram showing an example of a method for forming a tunnel junction according to an embodiment of the present invention. FIG. 1 shows an example of a method for manufacturing an electronic element (single electron transistor) using a tunnel junction. First, as shown in FIG. 1A, an insulating layer 102 made of silicon oxide having a predetermined thickness is formed on a silicon substrate 101, and a carbon nanotube 103 is formed on the insulating layer 102. State. Next, as shown in FIG. 1B, the source electrode 104 and the drain electrode 105 are formed on the insulating layer 102 so as to be spaced apart from each other at a predetermined interval and partially connected to the carbon nanotube 103 by ohmic contact. To do.

Next, the silicon substrate 101 is carried into a processing chamber of a predetermined electron beam exposure apparatus, and a desired region of the carbon nanotube 103 is irradiated with the electron beam 110 as shown in FIG. The electron beam 110 is a convergent electron beam having an acceleration voltage of 1 kV and an irradiation amount of 1 × 10 ¹⁷ ions / cm ² (1.6 × 10 ⁻² C / cm ² ). As will be described later, the electron beam 110 to be irradiated may have an acceleration voltage of 86 kV or less. By this electron beam irradiation, as shown in FIG. 1 (d), a state in which the defect portion 106 is formed in the electron beam irradiation portion of the carbon nanotube 103 is obtained.

  The defect 106 formed in the carbon nanotube 103 by the electron beam irradiation described above acts as a tunnel junction (tunnel barrier) and acts as a Coulomb barrier. Accordingly, the electronic element shown in FIG. 1D operates as a single electron transistor using the silicon substrate 101 as the back gate electrode. The defect portion 106 is a plurality of at least two or more defects, two coulomb barriers are formed by the two defects, and quantum dots smaller than the distance between the source and drain electrodes are formed in the carbon nanotube 103. It becomes a state.

  According to the above-described forming method described with reference to FIG. 1, a tunnel junction can be easily formed in a desired region of the carbon nanotube 103, and a state in which two Coulomb barriers (quantum dots) are formed is obtained. It is done. As a result, according to the present method, it becomes easy to easily manufacture a single electron transistor operating at a higher temperature with a high yield. In addition, since the formation location of the quantum dots can be controlled, there is no restriction on the element design.

  Next, defect generation (formation) by electron beam irradiation will be described. It has been known that the structure of carbon nanotubes is destroyed by irradiation with highly accelerated electron beams (BWSmith and DELuzzi, "Electron irradiation effects in single wall carbon nanotubes" Journal of Applied Physics, Vol. .90, No.7, pp.3509-3515, 1 October 2001.). This is due to the carbon atom in the carbon nanotube being repelled by the incident electrons having high kinetic energy, which is said to be knock-on damage. In the above document, the acceleration of the electron beam occurs at 86 kV. .

  The value of 86 kV for causing knock-on damage is larger than the acceleration voltage of an electron beam used in an electron beam drawing apparatus usually used for manufacturing a semiconductor device. The use acceleration voltage of a general electron beam drawing apparatus is 1 to 50 kV. Therefore, knock-on damage cannot be caused using an electron beam drawing apparatus. In contrast, in a transmission electron microscope, the normal use acceleration voltage is 100 to 400 kV, which is sufficient for causing the above-described knock-on damage. However, it is not practical to cause knock-on damage to carbon nanotubes at a predetermined location using an electron microscope. In fact, the technique of causing knock-on damage to the formed carbon nanotubes has not been utilized industrially.

  In response to the phenomenon of knock-on damage described above, the inventors have found a phenomenon in which a defect is generated in a carbon nanotube by irradiating the carbon nanotube with an electron beam of 86 kV or less. This phenomenon has been recently quantitatively clarified by the inventors (Fukuba, Suzuki, Kanzaki, Honma, “damage of carbon nanotubes by low acceleration electron beam”, 2004, 2004) (The 3rd volume of the 28th Applied Physics Related Conference Lecture Proceedings, 28a-ZX-2, page 1665, published on March 28, 2004).

  As described above, knock-on damage occurs when the acceleration voltage exceeds 86 kV. Therefore, the occurrence of defects in carbon nanotubes by electron beam irradiation with an acceleration voltage of 86 kV or less is considered to be due to a mechanism that is essentially different from knock-on damage. . This phenomenon is inferred that defects are induced by electron excitation by electron beam irradiation.

  According to the researches of the inventors, when the acceleration voltage of the electron beam is between 1 kV and 86 kV, the damage to the carbon nanotube is larger as the acceleration voltage is smaller. In particular, the carbon nanotube is efficiently converted into the carbon nanotube at an acceleration voltage of 30 kV or less. A tunnel junction can be formed by giving a defect. Further, according to experiments by the inventors, it has been confirmed that defects occur in carbon nanotubes when the accelerating voltage of the irradiating electron beam is in the range of 100 V to 25 kV, and particularly in the range of 500 V to 3 kV. In particular, defects can be given to the carbon nanotubes. Moreover, the density of defects generated in the electron beam irradiation portion increases with the amount of electron beam irradiation.

  The electron irradiation does not have to be performed using a convergent electron beam in the electron beam drawing apparatus. For example, the electron irradiation may be performed on the entire nanotube in a scanning electron microscope. In addition, by irradiating the electron beam and then heating the carbon nanotubes in an atmosphere containing a combustion-supporting gas or a flammable gas, the electron beam irradiation partially destroys the structure of the defective part and forms a tunnel junction more efficiently. can do.

  By heating a carbon nanotube with defects formed by electron beam irradiation in an atmosphere containing a combustion-supporting gas or a flammable gas, the defect portion quickly binds to a gas component, and the structure of the carbon nanotube is partially Destroyed. On the other hand, carbon nanotubes without defects that are not irradiated with an electron beam are chemically tough, and thus the reaction proceeds more slowly than the irradiated portion. As a result, by heating as described above after irradiating the electron beam, a state in which the tunnel junction is selectively and efficiently formed at the electron beam irradiated portion can be obtained.

  As the above-mentioned combustion-supporting gas, oxygen gas may be used. Therefore, by heating in the atmosphere, a state in which a tunnel junction is efficiently formed by the electron beam irradiation portion of the carbon nanotube can be obtained. When heating in the atmosphere, the heating temperature may be heated to about 350 to 500 ° C. By setting this temperature range, it is possible to increase the difference in the burning rate between the part not irradiated with the electron beam and the part where the defect is generated by the electron beam irradiation, and it is possible to form the tunnel junction more efficiently. It becomes.

  By the way, in the element shown in FIG. 1, the silicon substrate 101 is used as the back gate electrode. However, the present invention is not limited to this, and other forms may be adopted. For example, an electrode disposed close to the carbon nanotube 103 on the insulating layer 102 may be used as the gate electrode. In FIG. 1, the silicon substrate is used. However, the present invention is not limited to this. In the case of an element configuration in which a gate electrode is provided on the carbon nanotube formation surface, another insulating substrate may be used. Needless to say.

It is process drawing which shows the example of the formation method of the tunnel junction in embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 ... Silicon substrate, 102 ... Insulating layer, 103 ... Carbon nanotube, 104 ... Source electrode, 105 ... Drain electrode, 106 ... Defect part, 110 ... Electron beam.

Claims (5)

A first step in which carbon nanotubes are formed on a substrate;
And a second step of irradiating the carbon nanotube with an electron beam having a predetermined acceleration voltage of an acceleration voltage of at most 86 kV to form a defect.   The tunnel junction forming method according to claim 1, further comprising a third step of heating the carbon nanotube to a predetermined temperature in an atmosphere containing a combustion-supporting gas.   3. The tunnel junction forming method according to claim 2, wherein, in the third step, the carbon nanotubes are heated to 350 to 500 ° C. in an oxygen-containing atmosphere.   4. The tunnel junction forming method according to claim 1, wherein the predetermined acceleration voltage is at most 30 kV. 5. 5. The method for forming a tunnel junction according to claim 4, wherein the predetermined acceleration voltage is 500 V to 3 kV.

Images