Gas Sensor

Abstract

To provide a gas sensor having the high detection sensitivity to ammonia, and an ammonia gas detection method using the same.SOLUTION: A gas sensor according to an embodiment of the present invention includes: a first electrode; an insulating layer; a pair of second electrodes, and a channel layer. The insulating layer is provided on the first electrode. The pair of second electrodes is provided on the insulating layer. The channel layer includes: a graphene film; and an aluminum oxide film. The graphene film is provided on the insulating layer and electrically connects between the pair of second electrodes. The aluminum oxide film covers the graphene film.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a gas sensor for detecting ammonia. [Background technology]

[0002] Conventionally, it has been difficult to achieve both high sensitivity and gas selectivity in highly sensitive gas sensors. To achieve gas selectivity, a common approach is to introduce a material that selectively adsorbs specific gas molecules onto the gas detection structure, and measure the gas concentration from the amount of adsorption.

[0003] A method of doping impurities into a sensor using an oxide semiconductor as a base material has been commonly tried, and a graphene sensor that measures and identifies changes in work function due to molecular adsorption on the graphene surface is also known. For example, Patent Document 1 discloses a gas sensor including a semiconductor layer, a graphene film provided above the semiconductor layer, a barrier film between the semiconductor layer and the graphene film, a first electrode in contact with and electrically connected to the graphene film, and a pair of second electrodes in contact with and electrically connected to the semiconductor layer.

[0004] Furthermore, as a sensor capable of detecting a target substance with high sensitivity, for example, Patent Document 2 discloses a sensor structure including a graphene film and at least two electrodes that are in electrical contact with the graphene film and that read out changes in the electrical properties of the graphene film due to interaction with the target substance. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6687862 [Patent Document 2] Japanese Patent Application Laid-Open No. 2018-163146 Summary of the Invention [Problem to be solved by the invention]

[0006] When sensing a specific gas, it is desirable for the sensor to respond only to the target gas and not to other gases. However, it is generally considered difficult to achieve high selectivity with oxide semiconductor gas sensors. Furthermore, the gas sensor described in Patent Document 1 utilizes graphene for interaction with gas molecules, and gas selectivity depends on the work function. In this case, gases that can act as donors for graphene may be erroneously detected, and there are issues with ammonia selectivity. Furthermore, a method of chemically modifying graphene itself is also mentioned, but there are issues with its simplicity.

[0007] In view of the above circumstances, an object of the present invention is to provide a gas sensor having high detection sensitivity for ammonia. [Means for solving the problem]

[0008] A gas sensor according to one aspect of the present invention comprises: In a gas sensor for detecting ammonia, The device includes a first electrode, an insulating layer, a pair of second electrodes, and a channel layer. The insulating layer is provided on the first electrode. The pair of second electrodes is provided on the insulating layer. The channel layer includes a graphene film and an aluminum oxide film. The graphene film is provided on the insulating layer and electrically connects the pair of second electrodes. The aluminum oxide film covers the graphene film. It is a porous membrane .

[0009] According to the gas sensor, the graphene film has a channel layer covered with an aluminum oxide film, and therefore selective adsorption of ammonia gas is achieved, thereby improving the detection sensitivity for ammonia gas.

[0010] The aluminum oxide film may be an aggregate of nanoparticle-sized particles, which can further improve the adsorption ability for ammonia gas and increase the detection sensitivity. The thickness of the aluminum oxide film is, for example, not less than 2 nm and not more than 40 nm. The coverage of the aluminum oxide film on the surface of the graphene film is, for example, 60% or more.

[0011] A gas sensor according to another aspect of the present invention comprises: In a gas sensor for detecting ammonia, a semiconductor substrate of one conductivity type that functions as a gate electrode; an insulating layer provided on the semiconductor substrate; a rectangular graphene film provided on the insulating layer; a source electrode and a drain electrode formed on the insulating layer, the source electrode and the drain electrode covering the opposing sides of the graphene film and their vicinities; A graphene film is provided on the graphene film between the source electrode and the drain electrode. It is a porous membrane an aluminum oxide film; Equipped with 。 [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a gas sensor having high detection sensitivity for ammonia. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a side cross-sectional view schematically showing the configuration of a gas sensor according to one embodiment of the present invention. [Figure 2] FIG. 2 is a plan view of the gas sensor. [Figure 3] FIG. 2 is a cross-sectional view schematically illustrating the structure of a channel layer in the gas sensor. [Figure 4] 3A to 3C are schematic diagrams illustrating a method for detecting ammonia gas using the gas sensor. [Figure 5] FIG. 3 is a schematic diagram showing the relationship between the gate voltage and the drain current of the gas sensor. [Figure 6] FIG. 2 is a schematic diagram showing a gas detection system including the gas sensor. [Figure 7]4 is a flowchart showing an operation procedure of the gas detection system. [Figure 8] The following is an experimental result illustrating the operation of the gas sensor. [Figure 9] 10 shows other experimental results illustrating the operation of the gas sensor. [Figure 10] 10 is a result of an experiment illustrating the operation of a gas sensor according to a comparative example. [Figure 11] 10 is another experimental result illustrating the operation of the gas sensor according to the embodiment of the present invention. [Figure 12] 10 is a diagram illustrating the relationship between the particle size of fine particles constituting a silicon oxide film and the detection sensitivity of ammonia gas. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0015] [Gas sensor] FIG. 1 is a side cross-sectional view that schematically shows the configuration of a gas sensor according to one embodiment of the present invention, and FIG. 2 is a plan view thereof. In FIG. 2, the X-axis direction corresponds to the vertical direction of the paper, the Y-axis direction corresponds to the horizontal direction of the paper, and the Z-axis direction corresponds to the height of the paper (thickness of the device).

[0016] The gas sensor 10 of this embodiment is a three-electrode semiconductor device, and in this embodiment has a field effect transistor (FET) structure.

[0017] The gas sensor 10 includes a semiconductor substrate 11 , an insulating layer 12 , a pair of metal electrodes 13 and 14 , and a channel layer 15 .

[0018] The semiconductor substrate 11 is, for example, a silicon (Si) substrate doped with a P-type or N-type impurity element. The semiconductor substrate 11 functions as a gate electrode (first electrode) and is connected to a first DC power supply (power supply for gate voltage Vg (see FIG. 4)). The thickness of the semiconductor substrate 11 is not particularly limited and is, for example, about 500 μm.

[0019] The insulating layer 12 is provided on the semiconductor substrate 11. The insulating layer 12 is made of, for example, silicon oxide and functions as a gate insulating film. The insulating layer 12 may typically be a thermally oxidized film of the semiconductor substrate 11, or may be a film formed on the semiconductor substrate 11 by a CVD method, a sputtering method, or the like. The thickness of the insulating layer 12 is not particularly limited and is, for example, 285 nm.

[0020] The pair of metal electrodes 13, 14 are provided on the insulating layer 12 and are arranged opposite each other with a gap in the Y-axis direction. The pair of metal electrodes 13, 14 function as a drain electrode and a source electrode (a pair of second electrodes), and the metal electrode 13 as the drain electrode is connected in series to a second DC power supply (a power supply for drain voltage Vd (see FIG. 4)) as shown in FIG. 4. The metal electrode 14 as the source electrode is grounded. The metal material constituting the pair of metal electrodes 13, 14 is not particularly limited, and may be, for example, a film (Au / Cr) in which gold (Au) is laminated on chromium (Cr).

[0021] The pair of metal electrodes 13, 14 has contact portions 13a, 14a and pad portions 13b, 14b. The contact portions 13a, 13b are provided at positions where the pair of metal electrodes 13, 14 face each other, and are connected to the channel layer 15. The pad portions 13b, 14b correspond to electrode pads for drawing out wiring.

[0022] The device structure will now be described in more detail. As shown in FIG. 2, the Si substrate 11, whose surface is covered with an insulating layer 12, is rectangular in plan view, with a channel layer 15 located at its center. This channel layer 15 is very thin and rectangular in plan view, a rectangle in this example. Contact portions 13a and 14a are provided to cover opposing sides of the channel layer 15 and their vicinity. Because the contact portions 13a and 14a are too small for external electrodes such as those used for bonding or soldering, extended rectangular pad portions 13b and 14b are provided adjacent to the contact portions 13a and 14a or via wiring. As will be described later, because the channel layer 15 is extremely thin and its planar size is small, the contact portions 13a and 14a are formed of a film that is thicker than the channel layer 15 but thinner than the pad portions 13b and 14b. The thin contact portions 13a and 14a reduce stress on the channel layer 15 and improve pattern accuracy. Pad portions 13b and 14b need to be thick because they are connected to the outside by soldering, wire bonding, etc. For this reason, the cross sections of metal electrodes 13 and 14 are formed in a stepped shape in FIG.

[0023] The thickness of each portion of the metal electrodes 13, 14 is not particularly limited; for example, the contact portions 13a, 14a are 40 nm thick, and the main portions 13b, 14b are 80 nm thick. The method for forming the metal electrodes 13, 14 is also not particularly limited; any appropriate film formation method such as sputtering or vacuum deposition can be applied. Note that the formation of the contact portions 13a, 14a may be omitted; in this case, the metal electrodes 13, 14 are formed to a uniform thickness.

[0024] 3 is a cross-sectional view schematically showing the structure of the channel layer 15. As shown in FIG. 3, the channel layer 15 has a graphene film 151 and an aluminum oxide film 152 that covers the graphene film 151.

[0025] The graphene film 151 is provided on the insulating layer 12 and disposed between the pair of metal electrodes 13 and 14. The graphene film 151 is a conductive layer that electrically connects the metal electrodes 13 and 14. When the pair of metal electrodes 13 and 14 have contact portions 13a and 14a, the graphene film 151 is sandwiched between the respective contact portions 13a and 14a and the insulating layer 12, thereby establishing electrical continuity with the metal electrodes 13 and 14. When the pair of metal electrodes 13 and 14 do not have the contact portions 13a and 14a, the graphene film 151 is sandwiched between the respective pad portions 13b and 14b and the insulating layer 12, thereby establishing electrical continuity with the metal electrodes 13 and 14. The gas sensor 10 detects the adsorption of a specific gas in the channel layer 15 based on a change in the electrical conduction characteristics of the channel layer 15. An example of the specific gas is ammonia gas. The graphene film 151 functions as a main conductor in the channel layer 15.

[0026] The graphene film 151 is a single-layer film, and in this embodiment, the length and width are 200 nm, and the thickness is 0.34 nm (corresponding to one graphene layer). The graphene film 151 is not limited to a single-layer film, and may be a multi-layer film having several layers. There are no particular limitations on the method for forming the graphene film 151, and here, the graphene film 151 is provided on the insulating layer 12 by a transfer method.

[0027] The aluminum oxide film 152 is formed on the surface of the graphene film 151 to provide high selectivity for ammonia. The aluminum oxide film 152 undergoes an oxidation reaction upon contact with ammonia gas, and excess electrons thus generated flow into the graphene film 151. By utilizing this phenomenon, the concentration of ammonia gas can be detected according to the amount of electrons flowing from the aluminum oxide film 152 to the graphene film 151.

[0028] Furthermore, because aluminum oxide is a porous material, it selectively adsorbs ammonia gas onto the graphene film 151. By supporting the aluminum oxide film 152 on the surface of the graphene film 151 in this manner, the adsorption of ammonia gas is improved, thereby providing a gas sensor 10 with high detection sensitivity for ammonia.

[0029] The composition ratio of oxygen to aluminum constituting the aluminum oxide film 152 is typically Al2O3, which is a stoichiometric ratio. The aluminum oxide film 152 is preferably an aggregate of nano-sized particles. By forming the aluminum oxide film 152 from an aggregate of nano-sized particles, the specific surface area of ​​the aluminum oxide film 152 can be increased. This further improves the adsorption of ammonia gas on the surface of the channel layer 15 compared to when the channel layer 15 is formed only from the graphene film 151, thereby further increasing the detection sensitivity for ammonia gas.

[0030] The average particle size of the fine particles is, for example, 10 nm or more and 50 nm or less, and more preferably 20 nm or more and 40 nm or less. The coverage of the surface of the graphene film 151 with the aluminum oxide film 152 is preferably high, typically 60% or more, more preferably 80% or more, and even more preferably 90% or more.

[0031] The aluminum oxide film 152 is formed by forming a metal aluminum film on the graphene film 151 by vacuum deposition or the like, and then oxidizing the metal aluminum film. In this embodiment, since the thickness of the aluminum is thin, aluminum oxide is generated by natural oxidation. The method for oxidizing the aluminum metal film is not particularly limited, and thermal oxidation in an oxygen atmosphere is also possible. The aluminum oxide film 152 may be formed directly on the graphene film 151 by, for example, an ALD method without undergoing oxidation treatment of the aluminum metal film.

[0032] The thickness of the aluminum oxide film 152 is, for example, 2 nm to 40 nm, more preferably 5 nm to 20 nm. If the thickness is less than 2 nm, the amount of aluminum oxide supported cannot be ensured sufficiently, and it is difficult to stably form the aluminum oxide film 152 on the graphene film 151 with a desired coverage. This increases the chances of the graphene film 151 coming into contact with gas species other than ammonia, which inevitably reduces the selectivity and sensitivity to ammonia gas, which is the detection target. On the other hand, if the thickness exceeds 40 nm, it becomes difficult for excess electrons generated by the reaction with ammonia gas to flow into the graphene film 151, which may adversely affect the detection sensitivity for ammonia gas.

[0033] Since aluminum expands in volume upon oxidation, when forming the aluminum oxide film 152 by oxidizing an aluminum film, the thickness of the aluminum oxide film 152 can be kept within the above range by adjusting the film thickness of the metallic aluminum formed on the graphene film 151. For example, it has been confirmed that an aluminum oxide film with a thickness of 2.5 nm to 10 nm can be formed by adjusting the thickness of the metallic aluminum film to a range of 0.5 nm to 5 nm. Furthermore, when the aluminum oxide film 152 was formed from a metallic aluminum film with a thickness of 0.5 nm, the average particle size of the aluminum oxide nanoparticles was approximately 30 nm, and the coverage of the graphene film 151 was approximately 80%.

[0034] The degree of agglomeration of the nanoparticle film that constitutes the aluminum oxide film 152 varies depending on the film-forming conditions of the metallic aluminum, and the faster the film-forming speed, the stronger the agglomeration tends to be. Strong film agglomeration is likely to cause an increase in particle size and uneven film thickness. For this reason, the film-forming speed of the metallic aluminum is preferably 1.0 Å / sec or less, more preferably 0.5 Å / sec or less, and even more preferably 0.1 Å / sec or less.

[0035] Next, a method for manufacturing the gas sensor 10 configured as above will be briefly described. A Si substrate (semiconductor substrate 11) doped with an impurity element of one conductivity type is prepared, and the substrate is exposed to an oxidizing atmosphere to form an insulating layer 12 on the substrate. Subsequently, a graphene film 151 that has been separately prepared is transferred onto the insulating layer 12. Next, a pair of metal electrodes 13 and 14 are formed as source / drain electrodes by using lithography and vapor deposition so as to cover both ends of the graphene film 151. Next, the graphene film 151 between the contact portions 13a, 14a of the pair of metal electrodes 13, 14 is protected with a resist of a predetermined size (e.g., 200 nm x 200 nm), and then the excess graphene that is not protected is removed by oxygen plasma etching, thereby processing the graphene film 151 into a rectangular ribbon shape. After removing the resist used as the protective layer, the area other than the graphene film 151 is again protected with resist by lithography, and aluminum is then deposited by evaporation. The deposited aluminum naturally oxidizes to aluminum oxide when removed from the vacuum chamber. By removing the remaining resist, the gas sensor 10 is fabricated, which has a channel layer 15 in which aluminum oxide nanoparticles are supported on the graphene film 151.

[0036] Due to this manufacturing process, aluminum oxide may be formed on part of the upper surfaces of the contact portions 13a and 14a. A small amount of aluminum oxide may also be formed on the side surfaces of the contact portions 13a and 14a. The above explanation also applies to cases where the metal electrodes 13 and 14 do not have the contact portions 13a and 14a. In that case, aluminum oxide is formed on the upper surfaces or side surfaces of the pad portions 13b and 14b. Aluminum may be formed by vapor deposition without using a resist, in which case aluminum oxide is also formed on the upper or side surfaces of the main body portions 13b, 14b and the contact portions 13a, 14a of the metal electrodes 13, 14.

[0037] [Gas detection principle] Next, a method for detecting ammonia gas using the gas sensor 10 configured as above will be described.

[0038] FIG. 4 is a schematic diagram illustrating a method for detecting ammonia gas using a gas sensor 10. A power supply for gate voltage Vg is connected to a semiconductor substrate 11 serving as a gate electrode, and a power supply for drain voltage Vd is connected to a metal electrode 13 serving as a drain electrode. A metal electrode 14 serving as a source electrode is connected to ground potential. The power supply for gate voltage Vg is a variable power supply capable of applying a sweep voltage to the gate electrode, for example, from -40 V to +40 V (or from -80 V to +80 V). The power supply for drain voltage Vd is a fixed power supply capable of applying a constant voltage of, for example, 2 mV to the drain electrode. Then, with a drain voltage applied between the source and drain, changes in the current (drain current Id) flowing between the source and drain are detected when the gate electrode is swept within the voltage range. The sweep conditions are not particularly limited; for example, the sweep interval is 0.5 V and the sweep time is 1 minute.

[0039] FIG. 5 is a diagram showing the relationship between the gate voltage Vg and the drain current Id (Id-Vg characteristics). In the figure, Reference numeral 51 denotes a drain current curve before the introduction of the gas to be detected; Reference numeral 52 denotes the drain current curve after the introduction of the target gas. Both curves show a downwardly convex parabolic shape. The minimum values ​​31 and 32 of the drain current curves 51 and 52 are also called charge neutral points (CNP).

[0040] The charge neutral point indicates the point at which the total amount of positive and negative carriers in the carrier layer (graphene film 151 in this embodiment) is minimal, and when an electron-donating or electron-accepting gas is adsorbed, the gate voltage that gives the charge neutral point changes. Electron-donating property is also called donor property, and refers to the property of giving an electric charge. A gas with this property is called a donor. Ammonia gas functions as a donor. Conversely, electron-donating property is also called acceptor property, and refers to the property of receiving an electric charge. By tracking this change in gate voltage, gas adsorption can be detected. Hereinafter, when the charge neutral point is mentioned, it refers to the gate voltage that gives this minimal point. Furthermore, the amount of change in the charge neutral point before and after gas introduction is referred to as the charge neutral point shift amount (ΔV CNP (absolute value)) The shift amount (ΔV CNP ) tends to increase in proportion to the concentration of the gas to be detected. In this embodiment, this detection principle is used to detect the concentration of ammonia gas. Hereinafter, this detection principle will also be referred to as the CNP method.

[0041] 6 is a schematic diagram showing a gas detection system 1 including a gas sensor 10. The gas detection system 1 includes a detection chamber 2, an information processing device 4, a display device 5, and a storage unit 6.

[0042] The detection chamber 2 includes a gas sensor 10 and a heating unit 26 in a storage chamber 20 . The storage chamber 20 has an intake port 21 for drawing in gas from the outside, and an exhaust port 22 for exhausting gas from the storage chamber 20 to the outside. A valve 24 for regulating the inflow of gas into the storage chamber 20 is provided at the intake port 21, and a valve 25 for regulating the outflow of gas from the storage chamber 20 to the outside is provided at the exhaust port 22. A vacuum pump is connected to the exhaust port 22 beyond the valve 25. The heating unit 26 is, for example, a heater, and heats the gas sensor 10 to a predetermined temperature to clean the surface of the graphene film 151. Cleaning means desorbing gas and moisture adsorbed on the graphene film 151.

[0043] The information processing device 4 includes an acquisition unit 41, a calculation unit 42, and a control unit 43. The acquisition unit 41 acquires current information relating to the drain current Id (see FIG. 4) of the gas sensor 10 and its changes. The calculation unit 42 calculates the charge neutral points of the gas sensor 10 before and after the introduction of the gas based on the current information acquired by the acquisition unit 41, and outputs the calculation results to the display device 5. The control unit 43 comprehensively controls the operation of the gas detection system 1, such as the power supply for the gate voltage Vg and the power supply for the drain voltage Vd shown in FIG.

[0044] The display device 5 has a display unit, and displays the type and concentration of gas output from the information processing device 4 on the display unit.

[0045] The storage unit 6 stores software programs executed by the control unit 43. The storage unit 6 also stores acquired data by the acquisition unit 41, calculation parameters and calculation results by the calculation unit 42, and control data in the control unit 43, all of which are transmitted from the information processing device 4. The storage unit 6 may be located on a cloud server with which the information processing device 4 can communicate, or may be built into the information processing device 4.

[0046] FIG. 7 is a flowchart showing the operation procedure of the gas detection system 1.

[0047] First, the inside of the accommodation chamber 20 is evacuated to a vacuum (step S101). Subsequently, under the conditions of the CNP method described above, the charge neutral point of the channel layer 15 is measured based on the drain current characteristics of the gas sensor 10 in the vacuum atmosphere (step S102).

[0048] Subsequently, ammonia gas is introduced into the accommodation chamber 20 (step S103). The introduction pressure of the ammonia gas here was set to 700 Torr (93.3 kPa) in gauge pressure. Subsequently, under the conditions of the CNP method described above, the charge neutral point of the channel layer 15 is measured based on the drain current characteristics of the gas sensor 10 in the ammonia gas atmosphere (step S104).

[0049] Next, the shift amount of the charge neutral point after the introduction of ammonia gas relative to the charge neutral point before the introduction of ammonia gas is calculated, and the concentration of ammonia gas corresponding to this shift amount is determined (steps S105, S106). The ammonia gas concentration is determined, for example, using a table showing the relationship between the shift amount of the charge neutral point and the ammonia gas concentration, which is stored in advance in the storage unit 6.

[0050] [Operation of this embodiment] In the gas sensor 10 of this embodiment, the channel layer 15 includes a graphene film 151 and an aluminum oxide film 152 covering the graphene film 151. Aluminum oxide is believed to have the ability to promote the oxidation of ammonia, as evidenced by the ammonia response of oxide semiconductor gas sensors based on aluminum-doped zinc oxide. Oxide ions present on the aluminum oxide layer oxidize ammonia gas, generating excess electrons. These excess electrons flow from the aluminum oxide film 152 into the graphene film 151, changing the charge neutral point of the graphene film 151.

[0051] When considering the aluminum oxide film 152, its thickness is limited as follows. First, it is considered that a relatively thin aluminum oxide film 152 is better. This is because the oxidation reaction of ammonia occurs on the surface of the aluminum oxide film 152, and the excess electrons generated must flow to the graphene film 151. Therefore, if the thickness of the aluminum oxide film 152 is too thick, the excess electrons generated on the surface cannot reach the graphene film 151, which adversely affects sensitivity. On the other hand, the aluminum oxide film 152 needs to have a certain thickness or more. This is because if the aluminum oxide film 152 is thin, that is, if the amount of aluminum oxide carried is small, the graphene film 151 will be exposed, which will allow gas to directly interact with the graphene film 151 and will also mean a decrease in the surface area of ​​the aluminum oxide film 151, which is thought to result in a decrease in selectivity and sensitivity. Therefore, the aluminum oxide film 152 needs to have a certain thickness. From the above, it is considered that the optimum thickness of the aluminum oxide film 152 is in the range of approximately 2 nm to 40 nm. Note that, since higher selectivity is expected as the exposed graphene film 151 becomes smaller, it is preferable that the coverage of the aluminum oxide film 152 covering the graphene film 151 is high. Specifically, 80% or more is preferable. Furthermore, supporting aluminum oxide in the nanoparticle form on the graphene film 151 is important in the sense that it increases the surface area that contributes to gas adsorption. By making the aluminum oxide film 151 into a nanoparticle-sized fine particle film, it is possible to increase the number of adsorption reaction sites for ammonia without increasing the film thickness, and it is thought that this will lead to improved sensitivity.

[0052] The characteristics are explained using Figure 8. The thickness of the graphene film 151 is 0.34 nm. The thickness of the metal aluminum film is 5 nm, and this was oxidized. A gas sensor 10 (hereinafter also referred to as sample 1) having an aluminum oxide film 152 formed under these conditions was fabricated, and the Id-Vg characteristics of this gas sensor 10 are shown in FIG. In the same figure, Curve C1 is the result of measurement under a vacuum atmosphere before the introduction of ammonia gas. Curve C2 is the result of measurement in an atmosphere containing 4 ppm ammonia gas. Curve C3 shows the results of measurements taken after the measurement shown by curve C2, stopping the introduction of additional gas, leaving the gas sensor 10 standing for five minutes, and then measuring again. The measurement shown by curve C3 shows that there is little change in the Id-Vg characteristics of the gas sensor 10, even after five minutes. The measurement shown by curve C3 was performed to confirm the Id-Vg characteristics of the gas sensor 10, and is not essential for the measurements of the present invention.

[0053] As shown by curves C1 and C2 in Figure 8, the charge neutral point (CNP), which is the minimum point, shifts by 2 V in the negative direction, i.e., -2 V, due to the introduction of gas. This indicates that ammonia functions as a donor for the graphene film 151 due to ammonia adsorption.

[0054] 9 shows the experimental results when measuring the relationship between the charge neutral point (CNP) and the ammonia concentration in the channel layer 15 of the gas sensor 10 corresponding to Sample 1. The measurement temperature was room temperature. As shown in Figure 9, there is a strong correlation between the ammonia gas concentration and the charge neutral point shift, with the higher the concentration, the larger the shift. Note that the charge neutral point shift was constant in the concentration ranges of 2 ppm to 500 ppb and 250 ppb to 25 ppb. This is presumably due to the manufacturing precision of Sample 1. For example, in an atmosphere of ammonia gas with a concentration of 3 ppm, a charge shift of approximately -1.5 V or more would be measured.

[0055] For comparison, a gas sensor (hereinafter also referred to as Sample 2) having a channel layer consisting of a single graphene film 151 without a silicon oxide film 152 was used to measure the shift of the charge neutral point in a vacuum atmosphere and in an atmosphere containing ammonia gas at a concentration of 3 ppm. The measurement temperature was room temperature. The experimental results are shown in FIG. 10. As shown in FIG. 10, the shift of the charge neutral point was −1 V.

[0056] 9 and 10, it is confirmed that the gas sensor 10 of this embodiment, which has a channel layer in which the graphene film 151 is covered with the aluminum oxide film 152, has higher sensitivity to ammonia gas than a sensor without the aluminum oxide film 152.

[0057] Next, as another example, a gas sensor 10 (hereinafter also referred to as Sample 3) was fabricated having an aluminum oxide film 152 formed by oxidizing a 2-nm-thick metallic aluminum film covering a 0.34-nm-thick graphene film 151. The gas selectivity of this gas sensor 10 was evaluated. Five samples (Devices 1 to 5) fabricated under the same conditions as Sample 3 were used in the experiment. Five types of test gases were used: ammonia (NH), acetone (CHO), ethanol (CHO), formaldehyde (CHO), and hydrogen (H). Measurements were performed in a mixed gas atmosphere of each individual gas and nitrogen (N). The concentration of each gas was 3 ppm, and the measurement temperature was room temperature. The experimental results are shown in Figure 11.

[0058] As shown in Figure 11, among the five types of gases used for evaluation, the highest sensitivity (shift of the charge neutral point ΔV CNP ) was obtained. On the other hand, for the other gases, acetone, ethanol, formaldehyde, and hydrogen, the fluctuation was at most -1 V, which was relatively small and equivalent to noise. In other words, the sensor responded only to ammonia, and almost no response was observed for the other gases. This confirms that the gas sensor 10 of this embodiment has high selectivity for ammonia gas. Furthermore, similar results were obtained for the five samples 3 (Devices 1 to 5) used in the measurement, confirming that reproducibility can be achieved.

[0059] Figure 12 shows the results of a simulation based on the experimental results of Sample 1 (see Figure 8) that shows the relationship between the particle size of the fine particle film that makes up the aluminum oxide film 152 and the shift amount of the charge neutral point. Here, it is assumed that each fine particle in the aluminum oxide film is ideally spherical. If the amount of aluminum oxide supported is the same, the smaller the particle diameter is, based on the surface volume ratio, the larger the specific surface area, and therefore it is predicted that the sensitivity to ammonia gas will tend to improve accordingly. [Explanation of symbols]

[0060] 1...Gas detection system 10...Gas sensor 11...Semiconductor substrate (first electrode) 12...Insulating layer 13, 14...Metal electrodes (second electrodes) 15...Channel layer 151...Graphene film 152...Aluminum oxide film

Claims

1. A gas sensor for detecting ammonia, a first electrode; an insulating layer provided on the first electrode; a pair of second electrodes provided on the insulating layer; a channel layer including a graphene film provided on the insulating layer and electrically connecting the pair of second electrodes, and an aluminum oxide film that is a porous film covering the graphene film; A gas sensor comprising:

2. 2. The gas sensor according to claim 1, The aluminum oxide film is an aggregate of nano-sized particles. Gas sensor.

3. 3. The gas sensor according to claim 1, The thickness of the aluminum oxide film is 2 nm or more and 40 nm or less. Gas sensor.

4. The gas sensor according to any one of claims 1 to 3, The coverage of the surface of the graphene film with the aluminum oxide film is 60% or more. Gas sensor.

5. A gas sensor for detecting ammonia, a semiconductor substrate of one conductivity type that functions as a gate electrode; an insulating layer provided on the semiconductor substrate; a rectangular graphene film provided on the insulating layer; a source electrode and a drain electrode formed on the insulating layer, the source electrode and the drain electrode covering the opposing sides of the graphene film and their vicinities; an aluminum oxide film that is a porous film provided on the graphene film between the source electrode and the drain electrode; A gas sensor comprising:

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