JP2023111672A - gas sensor - Google Patents

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

The present invention relates to a gas sensor that detects ammonia.

Conventionally, it has been difficult for high-sensitivity gas sensors to achieve both high sensitivity and gas selectivity. In order to realize a sensor with gas selectivity, it is common to introduce a material that selectively adsorbs specific gas molecules onto the structure that detects the gas, and measure the gas concentration from the amount of adsorption. be.

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

In addition, as a sensor capable of detecting a detection target with high sensitivity, for example, Patent Document 2 describes a graphene film and a graphene film in electrical contact with each other to read out changes in electrical properties of the graphene film due to interaction with the detection target. A sensor structure is disclosed comprising at least two electrodes of .

Japanese Patent No. 6687862 JP 2018-163146 A

When the purpose is to sense a specific gas as a target, it is preferable to respond only to the target gas and not to other gases. In oxide semiconductor type gas sensors, it is generally considered difficult to obtain high selectivity.
Further, the gas sensor described in Patent Document 1 uses graphene for interaction with gas molecules, and depends on the work function for gas selectivity. In this case, a gas that can be a donor to graphene may be misdetected, and ammonia selectivity is an issue. It also mentions a method of chemically modifying graphene itself, but there is a problem in terms of simplicity.

In view of the circumstances as described above, an object of the present invention is to provide a gas sensor having high detection sensitivity for ammonia.

A gas sensor according to one aspect 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 are provided on the insulating layer.
The channel layer has 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.

According to the above gas sensor, since the graphene film is provided with the channel layer covered with the aluminum oxide film, a selective adsorption action for ammonia gas is obtained, whereby the detection sensitivity for ammonia gas can be improved.

The aluminum oxide film may be an aggregate of nanoparticle-sized fine particles. As a result, adsorption to ammonia gas can be further improved, and detection sensitivity can be enhanced.
The thickness of the aluminum oxide film is, for example, 2 nm or more and 40 nm or less.
A coverage of the aluminum oxide film on the surface of the graphene film is, for example, 60% or more.

A gas sensor according to another aspect of the present invention comprises
a semiconductor substrate having one conductivity type and functioning as a gate electrode;
an insulating layer provided on the semiconductor substrate;
a rectangular graphene film provided on the insulating layer;
an aluminum oxide film provided on the graphene film between the source electrode and the drain electrode formed on the insulating layer and the source electrode and the drain electrode, the aluminum oxide film covering opposite sides and the vicinity thereof of the graphene film;
comprises

ADVANTAGE OF THE INVENTION According to this invention, the gas sensor which has high detection sensitivity with respect to ammonia can be provided.

1 is a side sectional view schematically showing the configuration of a gas sensor according to one embodiment of the present invention; FIG. It is a top view of the said gas sensor. 4 is a cross-sectional view schematically showing the structure of a channel layer in the gas sensor; FIG. It is a schematic diagram explaining the detection method of ammonia gas using the said gas sensor. It is a schematic diagram which shows the relationship between the gate voltage of the said gas sensor, and a drain current. It is a schematic diagram which shows the gas detection system provided with the said gas sensor. 4 is a flow chart showing an operation procedure of the gas detection system; It is one experimental result explaining the effect|action of the said gas sensor. It is another experimental result explaining the effect|action of the said gas sensor. It is one experimental result explaining the effect|action of the gas sensor which concerns on a comparative example. It is another experimental result explaining the effect|action of the gas sensor which concerns on one Embodiment of this invention. FIG. 3 is a diagram for explaining the relationship between the particle size of fine particles forming a silicon oxide film and the detection sensitivity of ammonia gas;

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Gas sensor]
FIG. 1 is a side cross-sectional view schematically showing 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 (the thickness of the device) direction of the paper.

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

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 .

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 (gate voltage Vg power supply (see FIG. 4)). The thickness of the semiconductor substrate 11 is not particularly limited, and is approximately 500 μm, for example.

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

A pair of metal electrodes 13 and 14 are provided on the insulating layer 12 and arranged to face each other with a gap in the Y-axis direction. A pair of metal electrodes 13 and 14 function as a drain electrode and a source electrode (a pair of second electrodes). Vd power supply (see FIG. 4)). A metal electrode 14 as a source electrode is grounded. The metal material forming the pair of metal electrodes 13 and 14 is not particularly limited, and is, for example, a film (Au/Cr) in which gold (Au) is laminated on chromium (Cr).

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

Here, the device structure will be described in a little more detail. As shown in FIG. 2, the Si substrate 11 covered with the insulating layer 12 has a rectangular shape in plan view, and the channel layer 15 is provided in the center thereof. The channel layer 15 is extremely thin and rectangular in plan view, here rectangular. Contact portions 13a and 14a are provided to cover opposite sides of channel layer 15 and the vicinity thereof. Since the contact portions 13a and 14a are small as external electrodes for bonding and soldering, rectangular extended pad portions 13b and 14b are provided adjacent to the contact portions 13a and 14a or via wiring. As will be described later, since the channel layer 15 is extremely thin and has a small planar size, the contact portions 13a and 14a are made of a film that is thicker than the channel layer 15 but thinner than the pad portions 13b and 14b. be. Since the contact portions 13a and 14a are thin, the stress load on the channel layer 15 can be suppressed, and the pattern accuracy is also improved. The pads 13b and 14b need to be thick because they are connected to the outside by soldering or wire bonding. Therefore, in FIG. 1, the cross sections of the metal electrodes 13 and 14 are formed stepwise.

The thickness of each portion of the metal electrodes 13 and 14 is not particularly limited. For example, the thickness of the contact portions 13a and 14a is 40 nm, and the thickness of the body portions 13b and 14b is 80 nm. The method of forming the metal electrodes 13 and 14 is also not particularly limited, and an appropriate film forming method such as a sputtering method or a vacuum vapor deposition method can be applied. Formation of the contact portions 13a and 14a may be omitted, in which case the metal electrodes 13 and 14 are formed with a uniform thickness.

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

A graphene film 151 is provided on the insulating layer 12 and arranged between the pair of metal electrodes 13 and 14 . The graphene film 151 is a conductive layer that electrically connects between 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 connecting the metal electrodes 13 and 14 to each other. conduct. 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 connecting the metal electrodes 13 and 14 to each other. and conduct.
The gas sensor 10 detects adsorption of a specific gas in the channel layer 15 based on changes in electrical conductivity properties of the channel layer 15 . Examples of specific gases include ammonia gas. The graphene film 151 functions as the main conductor in the channel layer 15 .

The graphene film 151 is a single layer film, and in this embodiment, it has a length and a width of 200 nm and a thickness of 0.34 nm (corresponding to one layer of graphene). The graphene film 151 is not limited to a single layer film, and may be a multilayer film of several layers. A method for forming the graphene film 151 is not particularly limited, and here, the graphene film 151 is provided on the insulating layer 12 by a transfer method.

The aluminum oxide film 152 is formed on the surface of the graphene film 151 in order to have high selectivity to ammonia. Since the aluminum oxide film 152 undergoes an oxidation reaction upon contact with ammonia gas, the excess electrons generated flow to the graphene film 151 .
Using 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 .

Further, since aluminum oxide is a porous material, ammonia gas is selectively adsorbed on the graphene film 151 . By supporting the aluminum oxide film 152 on the surface of the graphene film 151 in this way, the adsorption of ammonia gas is improved, and thereby the gas sensor 10 with high detection sensitivity for ammonia can be obtained.

The composition ratio of oxygen and aluminum forming the aluminum oxide film 152 is typically Al ₂ O ₃ which is a stoichiometric ratio. The aluminum oxide film 152 is preferably an aggregate of nano-sized fine particles. The specific surface area of the aluminum oxide film 152 can be increased by forming the aluminum oxide film 152 from aggregates of nano-sized fine particles. As a result, compared to the case where the channel layer 15 is composed only of the graphene film 151, the adsorption of ammonia gas on the surface of the channel layer 15 is further improved, and the detection sensitivity for ammonia gas can be further increased.

The average particle diameter of the fine particles is, for example, 10 nm or more and 50 nm or less, 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 still more preferably 90% or more.

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

The thickness of the aluminum oxide film 152 is, for example, 2 nm or more and 40 nm or less, more preferably 5 nm or more and 20 nm or less. If the thickness is less than 2 nm, a sufficient amount of aluminum oxide to be supported cannot be ensured, and it is difficult to stably form the aluminum oxide film 152 on the graphene film 151 with a desired coverage.
Therefore, the chances of contact between the graphene film 151 and gas species other than ammonia increase, which inevitably lowers the selectivity and sensitivity of the ammonia gas to be detected.
On the other hand, if the thickness exceeds 40 nm, it becomes difficult for surplus 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.

Note that since aluminum expands in volume when oxidized, in the case of forming the aluminum oxide film 152 by oxidizing the aluminum film, the thickness of the aluminum oxide film 152 is adjusted by adjusting the thickness of the metal aluminum film formed on the graphene film 151 . The thickness of 152 can be within the above range. For example, it was confirmed that an aluminum oxide film having a thickness of 2.5 nm to 10 nm can be formed by adjusting the thickness of the metal aluminum film to a range of 0.5 nm to 5 nm. When the aluminum oxide film 152 was formed from a metal aluminum film with a thickness of 0.5 nm, the average particle size of the aluminum oxide nanoparticles was about 30 nm, and the coverage of the graphene film 151 was about 80%.

The degree of agglomeration of the nanoparticle film forming the aluminum oxide film 152 varies depending on the deposition conditions of the metal aluminum, and there is a tendency that the higher the deposition rate, the stronger the agglomeration. Strong aggregation of the film tends to increase the particle size and make the film thickness non-uniform. Therefore, the deposition rate of metallic aluminum is preferably 1.0 Å/sec or less, more preferably 0.5 Å/sec or less, and still more preferably 0.1 Å/sec or less.

Next, a method for manufacturing the gas sensor 10 configured as described above will be briefly described.
A Si substrate (semiconductor substrate 11) doped with an impurity element of one conductivity type is prepared. The substrate is then exposed to an oxidizing atmosphere to form an insulating layer 12 on the substrate.
Subsequently, a separately produced graphene film 151 is transferred onto the insulating layer 12 .
Next, a pair of metal electrodes 13 and 14 are formed as source/drain electrodes so as to cover both ends of the graphene film 151 using a lithography technique and a vapor deposition method.
Subsequently, after protecting the graphene film 151 between the contact portions 13a and 14a of the pair of metal electrodes 13 and 14 with a resist of a predetermined size (for example, 200 nm×200 nm), an unprotected excess portion is removed by oxygen plasma etching. By removing the graphene, the graphene film 151 is processed into a rectangular ribbon shape.
After removing the resist used for the protective layer, portions other than the graphene film 151 are again protected with a resist by lithography, and aluminum is vapor-deposited by vapor deposition. The deposited aluminum is spontaneously oxidized to aluminum oxide upon removal from the vacuum chamber. By removing the remaining resist, the gas sensor 10 having the channel layer 15 in which the graphene film 151 supports the aluminum oxide nanoparticles is fabricated.

Due to this manufacturing process, aluminum oxide may be formed on part of the upper surface of the contact portions 13a and 14a. A slight amount of aluminum oxide may also be formed on the side surfaces of the contact portions 13a and 14a. The above description is the same even when the metal electrodes 13 and 14 do not have the contact portions 13a and 14a. In that case, an 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 or the like without using a resist. At that time, aluminum oxide is also formed on the upper surfaces or side surfaces of the body portions 13b and 14b of the metal electrodes 13 and 14 and the contact portions 13a and 14a.

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

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

FIG. 5 is a diagram showing the relationship (Id-Vg characteristic) between the gate voltage Vg and the drain current Id.
In the figure,
Reference numeral 51 denotes a drain current curve before introduction of the gas to be detected;
Reference numeral 52 is a drain current curve after introduction of the gas to be detected. Both curves exhibit a downward convex parabolic shape. A minimum value 31, 32 of each drain current curve 51, 52 is also called a charge neutral point (CNP).

The charge neutral point indicates the point where the total amount of positive and negative carriers in the carrier layer (the graphene film 151 in this embodiment) is minimal. The gate voltage that gives the neutral point changes. The electron-donating property is also called a donating property, and is a property of giving an electric charge. A gas with this property is called a donor. Ammonia gas acts as a donor. On the contrary, the electron-donating property, also called acceptor property, is the property of receiving an electric charge. By tracking the change in this gate voltage, gas adsorption can be detected. Hereinafter, the term "charge neutral point" refers to the gate voltage that gives this minimum point. The amount of change in the charge neutral point before and after gas introduction is also referred to as the charge neutral point shift amount (ΔV _CNP (absolute value)). The amount of shift (ΔV _CNP ) tends to increase in proportion to the concentration of the gas to be detected. In this embodiment, such a detection principle is used to detect the concentration of ammonia gas. Henceforth, this detection principle is also called CNP method.

FIG. 6 is a schematic diagram showing a gas detection system 1 having a gas sensor 10. As shown in FIG. The gas detection system 1 includes a detection chamber 2 , an information processing device 4 , a display device 5 and a storage section 6 .

The detection chamber 2 includes a gas sensor 10 and a heating section 26 inside the housing chamber 20 .
The housing chamber 20 has an intake port 21 for sucking gas from the outside and an exhaust port 22 for discharging gas from the housing chamber 20 to the outside. The intake port 21 is provided with a valve 24 for adjusting the inflow of gas into the storage chamber 20, and the exhaust port 22 is provided with a valve 25 for adjusting the outflow of the gas in the storage chamber 20 to the outside. A vacuum pump is connected to the exhaust port 22 ahead of the valve 25 .
The heating unit 26 is composed of, for example, a heater, and heats the gas sensor 10 to a predetermined temperature to clean the surface of the graphene film 151 . To purify means to desorb gas and moisture adsorbed on the graphene film 151 .

The information processing device 4 includes an acquisition unit 41 , a calculation unit 42 and a control unit 43 .
The acquisition unit 41 acquires the drain current Id (see FIG. 4) of the gas sensor 10 and current information about its change.
The calculation unit 42 calculates the charge neutral point of the gas sensor 10 before and after the gas introduction based on the current information acquired by the acquisition unit 41 and outputs the calculation result to the display device 5 .
The control unit 43 centrally 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.

The display device 5 has a display section, and displays the type, concentration, etc. of the gas output from the information processing device 4 on the display section.

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

FIG. 7 is a flow chart showing the operation procedure of the gas detection system 1. As shown in FIG.

First, the storage chamber 20 is evacuated (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).

Subsequently, ammonia gas is introduced into the storage chamber 20 (step S103). The introduction pressure of the ammonia gas was 700 Torr (93.3 kPa) in gauge pressure here.
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).

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

[Action of this embodiment]
In the gas sensor 10 of this embodiment, the channel layer 15 has a graphene film 151 and an aluminum oxide film 152 covering it.
Aluminum oxide is believed to have the function of promoting the oxidation of ammonia. This is known from the result of the ammonia response by an oxide semiconductor gas sensor based on zinc oxide doped with aluminum.
Oxide ions present on the aluminum oxide layer oxidize the ammonia gas to generate surplus electrons. The surplus electrons flow from the aluminum oxide film 152 to the graphene film 151 and change the charge neutral point of the graphene film 151 .

When considering the aluminum oxide film 152, its thickness is limited as follows.
First, it is considered that the relatively thin aluminum oxide film 152 is superior. This is because an oxidation reaction of ammonia occurs on the surface of the aluminum oxide film 152 , so the excess electrons generated need to flow to the graphene film 151 . Therefore, if the thickness of the aluminum oxide film 152 is large, surplus electrons generated on the surface cannot reach the graphene film 151, which adversely affects the sensitivity.
On the other hand, the aluminum oxide film 152 needs to have a certain thickness or more. This means that the graphene film 151 is exposed when the thickness of the aluminum oxide film 152 is small, that is, when the amount of supported aluminum oxide is small. It is believed that this means a decrease in surface area, resulting in a decrease in selectivity and sensitivity. Therefore, the aluminum oxide film 152 is required to have a constant 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 the less exposed graphene film 151 is, the higher the selectivity is expected. Specifically, 80% or more is preferable.
In addition, the support on the graphene film 151 in the form of nanoparticles, which is the form of aluminum oxide, is important in terms of increasing the surface area that contributes to gas adsorption. By forming 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 considered that sensitivity can be improved.

Characteristics will be described with reference to FIG. The thickness of the graphene film 151 is 0.34 nm. The metal aluminum film had a thickness of 5 nm and 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 introducing ammonia gas,
Curve C2 is the measurement result in an atmosphere in which ammonia gas with a concentration of 4 ppm is present,
Curve C3 is the result of measuring again after stopping the introduction of additional gas after the measurement shown by curve C2 and allowing the sample to stand still for 5 minutes. The measurement shown by curve C3 showed that the Id-Vg characteristics of the gas sensor 10 did not change much even when measured after 5 minutes. The measurement indicated by curve C3 was performed to confirm the Id-Vg characteristics of the gas sensor 10, and is not essential for the measurement of the present invention.

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

FIG. 9 shows experimental results when measuring the relationship between the charge neutral point (CNP) of the channel layer 15 of the gas sensor 10 corresponding to the sample 1 and the ammonia concentration. The measurement temperature was room temperature.
As shown in FIG. 9, there is a strong correlation between the concentration of ammonia gas and the shift amount of the charge neutral point, and the higher the concentration, the greater the shift amount. The shift amount of the charge neutral point was constant when the concentration was in the range of 2 ppm to 500 ppb and in the range of 250 ppb to 25 ppb. For example, in an atmosphere of ammonia gas with a concentration of 3 ppm, a charge shift amount of about -1.5 V or more is considered to be measured.

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

9 and 10, according to the gas sensor 10 of the present embodiment having the channel layer in which the graphene film 151 is covered with the aluminum oxide film 152, ammonia gas It is confirmed that it has high sensitivity to

Subsequently, as another example, a gas sensor 10 (hereinafter also referred to as sample 3) having an aluminum oxide film 152 formed by oxidizing a metal aluminum film with a thickness of 2 nm covering a graphene film 151 with a thickness of 0.34 nm. was produced, and the gas selectivity of this gas sensor 10 was evaluated. In the experiment, five samples (Devices 1 to 5) manufactured under the same conditions as Sample 3 were used, and ammonia (NH ₃ ), acetone (C ₃ H ₆ O), ethanol (C ₂ H ₆ O), formaldehyde (CH ₂ O) and hydrogen (H ₂ ) were used, and measurements were made under a mixed gas atmosphere of a single gas species and nitrogen (N ₂ ). The concentration of each gas was 3 ppm, and the measurement temperature was room temperature. The experimental results are shown in FIG.

As shown in FIG. 11, among the five gases used as evaluation gases, high sensitivity (shift amount ΔV _CNP of charge neutral point) was obtained for ammonia. On the other hand, the other acetone, ethanol, formaldehyde, and hydrogen showed only −1 V variation at most, showing only relatively small variations corresponding to noise. In other words, it responded only to ammonia, and almost no response was observed for other gases.
From this, it was confirmed that the gas sensor 10 of the present embodiment has high selectivity to ammonia gas. Similar results were obtained for five samples 3 (Devices 1 to 5) used for measurement, confirming that reproducibility was obtained.

FIG. 12 is a simulation result showing the relationship between the particle size of the fine particle film forming the aluminum oxide film 152 and the amount of shift of the charge neutral point, based on the experimental results of sample 1 (see FIG. 8). Here, it is assumed that each fine particle of the aluminum oxide film has an ideal spherical shape. If the amount of aluminum oxide film supported is the same, it is predicted from the surface volume ratio that the smaller the particle size, the larger the specific surface area.

DESCRIPTION OF SYMBOLS 1... Gas detection system 10... Gas sensor 11... Semiconductor substrate (1st electrode)
12... Insulating layer 13, 14... Metal electrode (second electrode)
DESCRIPTION OF SYMBOLS 15... Channel layer 151... Graphene film 152... Aluminum oxide film

Claims (5)

a first electrode;
an insulating layer provided on the first electrode;
a pair of second electrodes provided on the insulating layer;
A gas sensor comprising: a channel layer having a graphene film provided on the insulating layer and electrically connecting the pair of second electrodes; and an aluminum oxide film covering the graphene film. The gas sensor according to claim 1,
The gas sensor, wherein the aluminum oxide film is an aggregate of nano-sized fine particles. The gas sensor according to claim 1 or 2,
The gas sensor, wherein the aluminum oxide film has a thickness of 2 nm or more and 40 nm or less. The gas sensor according to any one of claims 1 to 3,
The gas sensor, wherein a coverage of the aluminum oxide film on the surface of the graphene film is 60% or more. a semiconductor substrate having one conductivity type and functioning as a gate electrode;
an insulating layer provided on the semiconductor substrate;
a rectangular graphene film provided on the insulating layer;
an aluminum oxide film provided on the graphene film between the source electrode and the drain electrode formed on the insulating layer and the source electrode and the drain electrode, the aluminum oxide film covering opposite sides and the vicinity thereof of the graphene film;
A gas sensor comprising:

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