JP6777846B2 - Gas sensor, gas sensor array and gas sensor device - Google Patents

Description

The present invention relates to a gas sensor, a gas sensor array and a gas sensor device.

A gas sensor is a type of chemical substance sensor that detects chemical substances contained in a gas. Gas sensors are used, for example, in medical and diagnostic equipment. It is known that when a person suffers from a specific disease, the content of a specific chemical substance contained in the exhaled breath changes, and if the amount of the change can be detected, a simple and quick diagnosis is possible. It becomes. Simple and quick diagnosis can contribute to maintaining health and controlling medical expenses in an aging society. For example, it is known that when suffering from lung cancer, the concentration of volatile organic compounds (VOC) in exhaled breath increases, and follow-up of the VOC concentration is effective in determining the onset and progression of lung cancer. , The diagnostic threshold is considered to be about several ppb to several hundred ppb. Therefore, a gas sensor capable of detecting ppb-level VOCs is effective for diagnosing lung cancer. VOCs effective in diagnosing lung cancer include isobutane, methanol, ethanol, acetone, pentane, isoprene, isopropanol, dimethyl sulfide, carbon disulfide, benzene and toluene.

The above VOCs can be analyzed using, for example, gas chromatography and mass spectrometry, but this analysis is time consuming. In addition, gas chromatography and mass spectrometry equipment are large-scale and expensive, so that they are not widely used. Therefore, a gas sensor capable of detecting VOC with high sensitivity at the ppb level with a simple configuration is desired.

However, so far, a gas sensor capable of detecting VOC with high sensitivity at the ppb level has not been developed.

Japanese Unexamined Patent Publication No. 5-26832 Japanese Unexamined Patent Publication No. 11-1761

G. Pong, U. Tisch, O. Adams, M. Hakim, N. Shehada, Y. Broza, S. Billan, R. Abdah-Bortnyak, A. Kuten, and H. Haick, nature nanotechnology 4 (2009) 669 RF Machado, D. Laskowski, O. Deffenderfer, T. Burch, S. Zheng, PJ Mazzone, T. Mekhail, C. Jennings, JK Stoller, J. Pyle, J. Duncan, RA Dweik, and C. Erzurum, American J. of Respiratory and Critical Care Medicine 171 (2005) 1286 B. Liu, L. Chen, G. Liu, A. N. Abbas, M. Fathi, and C. Zhou, ACS NANO 8 (2014) 5304 Kea-Tiong Tang, Shih-Wen Chiu, Chung-Hung Shih, Chia-Ling Chang, Chia-Min Yang, Da-Jeng Yao, Jen-Huo Wang, Chien-Ming Huang, Hsin Chen, Kwuang-Han Chang, Chih- Cheng Hsieh, Ting-Hau Chang, Meng-Fan Chang, Chia-Min Wang, Yi-Wen Liu, Tsan-Jieh Chen, Chia-Hsiang Yang, Herming Chiueh, Jyuo-Min Shyu, ISSCC Dig. Tech. Papers, pp. 420 (2014) J. -H. Ahn, M. -J. Lee, H. Heo, J. H. Sung, K. Kim, H. Hwang, and M. -H. Jo, NANO Lett. 9 (2015) 3703

An object of the present invention is to provide a gas sensor, a gas sensor array, and a gas sensor device capable of detecting VOCs with high sensitivity with a simple configuration.

One aspect of the gas sensor includes a gas detection layer in which at least a part is in contact with a gas, and a first electrode and a second electrode connected to the gas detection layer. The gas detection layer is made of one of ZrS ₂ , HfS ₂ , HfSe ₂ , PbS ₂ and PbSe ₂ or any combination thereof, or one of these or any combination thereof and SnS _2. , TiS ₂ and SnSe ₂ or in combination with any combination thereof.

One aspect of the gas sensor array includes the gas sensor described above and a second gas sensor. The second gas sensor includes a second gas detection layer which is at least partially in contact with a gas, and a fourth electrode and a fifth electrode connected to the second gas detection layer. The second gas detection layer contains a Group 6 metal chalcogenide or graphene.

One aspect of the gas sensor device includes a gas sensor and a current detecting means for detecting a current flowing between the first electrode and the second electrode.

According to the above gas sensor and the like, since the gas detection layer contains an appropriate chalcogenide, VOC can be detected with high sensitivity with a simple configuration.

It is sectional drawing which shows the structure of the gas sensor which concerns on 1st Embodiment. It is a figure which shows the electronic state of the monoatomic layer SnS ₂ which adsorbed an acetone molecule. It is a figure which shows the electronic state of the monatomic layer MoS ₂ which adsorbed an acetone molecule. It is a figure which shows the relationship between the electron level of an adsorbed molecule and the electron affinity of chalcogenide. It is a figure which shows the electronic state of the monoatomic layer PbS ₂ which adsorbed a methanol molecule. It is a figure which shows the usage method of the gas sensor which concerns on 1st Embodiment. It is sectional drawing which shows the manufacturing method of the gas sensor which concerns on 1st Embodiment in process order. It is a figure which shows the structure of the gas sensor array which concerns on 2nd Embodiment. It is sectional drawing which shows the manufacturing method of the gas sensor array which concerns on 2nd Embodiment in process order. It is sectional drawing which shows the structure of the gas sensor array which concerns on 3rd Embodiment.

Hereinafter, embodiments will be specifically described with reference to the accompanying drawings.

(First Embodiment)
First, the first embodiment will be described. FIG. 1 is a cross-sectional view showing the structure of the gas sensor according to the first embodiment.

As shown in FIG. 1, the gas sensor 100 according to the first embodiment includes a p-type layer 101, an insulating film 102 on the p-type layer 101, and a gas detection layer 103 on the insulating film 102. The gas sensor 100 includes a gate electrode 104 on the back surface of the p-type layer 101, and a source electrode 107 and a drain electrode 108 on the gas detection layer 103. At least part of the gas detection layer 103 is exposed to gas. The gas detection layer 103 contains a group 4 or group 14 metal chalcogenide having a bandgap of 0.05 eV or more. The source electrode 107 is an example of the first electrode, the drain electrode 108 is an example of the second electrode, and the gate electrode 104 is an example of the third electrode. Group 4 metals include Ti, Zr and Hf, and Group 14 metals include Sn and Pb.

Here, the properties of metallic chalcogenides will be described. When a VOC molecule is adsorbed on a metal chalcogenide, this molecule acts as a donor to the metal chalcogenide, and the metal chalcogenide is doped into n-type. The electronic state of the metal chalcogenide adsorbed with VOC atoms can be obtained by the first-principles calculation method. FIG. 2 is a diagram showing the electronic state of the monoatomic layer SnS ₂ adsorbed with acetone molecules. Here, it is assumed that the gas detection layer 103 contains the monatomic layer SnS ₂ as a metal chalcogenide, and one acetone molecule is adsorbed on a total of 16 unit cells arranged in 4 × 4, and the energy in the two-dimensional plane direction. The band was calculated. As shown in FIG. 2, the energy of the bottom 13 of the conduction band based on the Fermi level 12 is 0.14 eV, and the energy of the top 14 of the valence band based on the Fermi level 12 is −1.32 eV. .. This means that SnS ₂ adsorbing acetone molecules is strongly n-type conductive and exhibits high sensitivity to acetone. The energy level 11 caused by acetone is −0.20 eV.

For reference, FIG. 3 shows a diagram showing the electronic state of the monoatomic layer MoS ₂ adsorbed with acetone molecules. Mo is a Group 6 metal. As shown in FIG. 3, the energy of the bottom 33 of the conduction band based on the Fermi level 32 is 0.68 eV, and the energy of the top 34 of the valence band based on the Fermi level 32 is -1.16 eV. .. This means that MoS ₂ adsorbing acetone molecules has weak conductivity and low sensitivity to acetone. The energy level 31 caused by acetone is −0.70 eV.

Next, the difference in characteristics between SnS ₂ and MoS ₂ as described above will be described by focusing on the electron level of the adsorbed molecule and the electron affinity of chalcogenide.

The inventor of the present application uses the first-principles calculation method to determine the energy of the highest occupied molecular orbital (HOMO) of acetone (hereinafter sometimes referred to as "HOMO level"), the electron affinity of MoS ₂ , and SnS. The electron affinity of ₂ was calculated. As a result, it was clarified that the HOMO level of acetone is -5.72 eV, the electron affinity of MoS ₂ is 4.27 eV, and the electron affinity of SnS ₂ is 5.24 eV. These relationships when the vacuum level is used as the energy reference are shown in FIG. As shown in FIG. 4, the difference between the energy at the bottom of the conduction band of SnS ₂ and the HOMO level of acetone is only 0.48 eV, and some of the electrons are thermally excited to move to SnS ₂ . Can be done. On the other hand, the energy at the bottom of the conduction band of MoS ₂ is 1.45 eV higher than the HOMO level of acetone, and even if it is thermally excited, it is difficult for electrons to move from acetone to MoS ₂ . It is considered that such an electron affinity difference causes a difference in the response to acetone between MoS ₂ and SnS ₂ .

The inventor of the present application calculated the electron affinity of the chalcogenides of various group 6 metals, group 4 metals or group 14 metals by a first-principles calculation method. The results are shown in Table 1. As shown in Table 1, the electron affinity of the group 6 metal chalcogenide is as small as less than 5.00 eV, and the electron affinity of the group 4 or group 14 metal chalcogenide is as large as 5.00 eV or more.

The inventor of the present application has also calculated HOMO levels of various VOCs that are effective in diagnosing lung cancer. The results are shown in Table 2. As shown in Table 2, the HOMO level of these VOCs is about -5 eV or less, which is significantly lower than the energy at the bottom of the conduction band of the Group 6 element chalcogenide based on the vacuum level. On the other hand, the difference between the HOMO level of these VOCs and the energy at the bottom of the conduction band of the group 4 or 14 element chalcogenides based on the vacuum level is small. From this, it can be said that not only SnS ₂ but also group 4 element or group 14 element chalcogenide is suitable for detecting VOC effective for diagnosis of lung cancer.

FIG. 5 shows a diagram showing the electronic state of the monoatomic layer PbS ₂ adsorbed with methanol molecules. As shown in FIG. 5, the energy of the bottom 23 of the conduction band based on the Fermi level 22 is 0.08 eV, and the energy of the top 24 of the valence band based on the Fermi level 22 is −0.64 eV. .. This means that PbS ₂ adsorbed with methanol molecules is strongly n-type conductive and exhibits high sensitivity to methanol. The energy level 21 caused by methanol is −0.13 eV.

Therefore, when VOC molecules are adsorbed on the gas detection layer 103 in the first embodiment, the electric conductivity of the gas detection layer 103 changes significantly even if the amount is small. Therefore, by detecting the amount of this change, the amount of VOC molecules adsorbed on the gas detection layer 103 can be detected with high accuracy, and the concentration of VOC in the environment can be specified with high accuracy.

Some chalcogenides of group 4 or group 14 metals have a bandgap of less than 0.05 eV, but when the bandgap is less than 0.05 eV, the electrical conductivity is high regardless of the presence or absence of adsorption of VOC molecules. It is difficult to detect changes in electrical conductivity due to adsorption of VOC molecules. Therefore, the band gap of the chalcogenide of the group 4 metal or the group 14 metal contained in the gas detection layer 103 is set to 0.05 eV or more. ZrSe ₂ is exemplified as a group 4 metal chalcogenide having a bandgap of less than 0.05 eV. The gas detection layer 103 includes, for example, TiS ₂ , ZrS ₂ , HfS ₂ , HfSe ₂ , SnS ₂ , SnSe ₂ , PbS ₂ or PbSe _2, or any combination thereof.

Next, a method of using the gas sensor 100 will be described. The gas sensor 100 is used, for example, in a gas sensor device. FIG. 6 is a diagram showing a method of using the gas sensor according to the first embodiment. FIG. 6 is also a diagram showing an example of a gas sensor device.

As shown in FIG. 6, the gas sensor 100 is used, for example, by connecting a current monitoring device 111 that detects a current flowing between the source electrode 107 and the drain electrode 108. The source electrode 107 is grounded, a bias voltage is applied to the gate electrode 104 by the bias power supply 112, and a bias voltage is applied to the drain electrode 108 by the bias power supply 113. The current monitoring device 111 may include, for example, various power supplies, an amplifier circuit, a sampling circuit, an analog-to-digital (AD) converter, a data processing computer, and the like. The current monitoring device 111 is an example of the current detecting means.

When the gas detection layer 103 adsorbs the VOC molecule 114, which is a test molecule, the VOC molecule 114 acts as a donor to the chalcogenide contained in the gas detection layer 103, and the chalcogenide is doped into n-type. As a result, the electrical conductivity of the VOC molecule 114 changes, and the drain current also changes. According to the first embodiment, VOC gas effective for diagnosing lung cancer such as ppb level acetone can be easily detected.

The number of unit layers of chalcogenide contained in the gas detection layer 103 is not limited, but it is preferably 1 to 100 layers in consideration of the ease of the fabrication process and the (parasitic) resistance of the gas detection layer 103 itself. Layers are particularly preferred. Further, in order to obtain higher sensitivity, it is preferable that the area of the portion of the gas detection layer 103 in contact with the gas is larger than the area of the portion covered with the electrode or the like.

Next, a method of manufacturing the gas sensor according to the first embodiment will be described. FIG. 7 is a cross-sectional view showing the manufacturing method of the gas sensor according to the first embodiment in the order of processes.

First, as shown in FIG. 7A, the insulating film 102 is formed on one surface (upper surface) of the p-type layer 101, and the gas detection layer 103 is formed on the insulating film 102. For example, the p-type layer 101 can be formed by ion implantation of p-type impurities on the surface of a silicon substrate, and the insulating film 102 can be formed by thermal oxidation of the surface of the p-type layer 101. The insulating film 102 is, for example, a silicon oxide film having a thickness of about 100 nm. In the formation of the gas detection layer 103, for example, a tin oxide film is formed on the insulating film 102, and the insulating film 102 and the p-type layer 101 on which the tin oxide film is formed are arranged in the center of the quartz furnace and reacted in the periphery thereof. Sulfur powder is placed in an alumina boat as a seed. Then, while heating the sulfur powder to 120 ° C. and the tin oxide film to 620 ° C. to 680 ° C., nitrogen gas is flowed from the end to the center of the quartz furnace. As a result, the sulfur atoms sublimated by the alumina boat flow toward the center by the nitrogen gas, and the tin oxide film on the insulating film 102 is reduced. As a result, the gas detection layer 103 including the monatomic layer SnS ₂ is obtained. Such a method is also described in Non-Patent Document 5.

Next, as shown in FIG. 7B, the gate electrode 104 is formed on the other surface (lower surface) of the p-type layer 101. In the formation of the gate electrode 104, for example, a Ti film having a thickness of 5 nm is formed by a vacuum vapor deposition method, and an Au film having a thickness of 300 nm is formed on the Ti film.

After that, as shown in FIG. 7C, the source electrode 107 and the drain electrode 108 are formed on the gas detection layer 103. In the formation of the source electrode 107 and the drain electrode 108, for example, a mask that exposes the region where they are to be formed is formed, a metal film is formed by a vacuum vapor deposition method, and the mask is removed together with the metal film on the mask. That is, the source electrode 107 and the drain electrode 108 can be formed by the lift-off method. In the formation of the metal film, for example, a Ti film having a thickness of 5 nm is formed, and an Au film having a thickness of 100 nm is formed on the Ti film.

In this way, the gas sensor according to the first embodiment can be manufactured.

(Second Embodiment)
Next, the second embodiment will be described. FIG. 8 is a diagram showing the structure of the gas sensor array according to the second embodiment. 8 (a) is a plan view, and FIG. 8 (b) is a cross-sectional view taken along the line I-I in FIG. 8 (a).

As shown in FIG. 8, the gas sensor array 200 according to the second embodiment includes 16 gas sensors 100, and these gas sensors 100 form an array of 4 rows and 4 columns. The gas sensor array 200 includes a protective film 201 having a grid-like planar shape, and while each source electrode 107 and drain electrode 108 are covered with the protective film 201, at least a part of the gas detection layer 103 is protected in each gas sensor 100. Exposed from membrane 201 and exposed to gas. Adjacent gas sensors 100 are separated from each other by an element separation region 202 formed in the gas detection layer 103, for example, an opening.

Also in the second embodiment, a VOC gas effective for diagnosing lung cancer such as acetone at ppb level can be easily detected.

Next, a method of manufacturing the gas sensor array according to the second embodiment will be described. FIG. 9 is a cross-sectional view showing the manufacturing method of the gas sensor array according to the second embodiment in the order of processes.

First, as shown in FIG. 9A, the process up to the formation of the gate electrode 104 is performed in the same manner as in the first embodiment. Next, as shown in FIG. 9B, an element separation region 202 is formed in the gas detection layer 103. In the formation of the element separation region 202, an opening is formed in the gas detection layer 103 by, for example, photolithography and Ar ion etching. After that, as shown in FIG. 9C, the source electrode 107 and the drain electrode 108 are formed in the same manner as in the first embodiment. Subsequently, a protective film 201 is formed which covers the source electrode 107 and the drain electrode 108 and exposes at least a part of the gas detection layer 103.

In this way, the gas sensor according to the second embodiment can be manufactured.

(Third Embodiment)
Next, a third embodiment will be described. FIG. 10 is a cross-sectional view showing the structure of the gas sensor array according to the third embodiment.

As shown in FIG. 10, the gas sensor array 300 according to the third embodiment includes three gas sensors 311 and 312 and 313. The gas sensor 311 includes a SnS ₂ gas detection layer 301, the gas sensor 312 includes a HfS ₂ gas detection layer 302, and the gas sensor 313 includes a MoS ₂ or graphene gas detection layer 303. Other configurations of the gas sensors 311, 312 and 313 are similar to those of the gas sensor 100.

In the third embodiment, the gas sensors 311 and 312 detect VOCs with high sensitivity, and the gas sensors 311 detect ammonia and nitrogen dioxide with high sensitivity. Further, the type of VOC detected with particularly high sensitivity is also different between the gas sensor 311 and the gas sensor 312. Therefore, according to the third embodiment, a wide variety of gases can be detected with high sensitivity, which is suitable for an electronic nose. For example, MoS ₂ and graphene show high sensitivity to substances with low energy (LUMO level) of the lowest unoccupied molecular orbital (LUMO), that is, substances having the property of accepting electrons.

In the second embodiment and the third embodiment, the gate electrode 104 is shared among the plurality of gas sensors, but the gate electrode 104 is divided between the gas sensors, and an independent gate electrode 104 is provided for each gas sensor. You may. In this case, an independent bias voltage can be applied to the gate electrode 104 for each gas sensor.

Hereinafter, various aspects of the present invention will be collectively described as appendices.

(Appendix 1)
A gas detection layer that is at least partially in contact with gas,
The first electrode and the second electrode connected to the gas detection layer,
Have,
The gas detection layer is a gas sensor comprising a group 4 or group 14 metal chalcogenide having a bandgap of 0.05 eV or more.

(Appendix 2)
An insulating film in contact with the gas detection layer between the first electrode and the second electrode,
A third electrode that sandwiches the insulating film with the gas detection layer,
The gas sensor according to Appendix 1, wherein the gas sensor has.

(Appendix 3)
The gas sensor according to Appendix 1 or 2, wherein the gas detection layer contains TiS ₂ , ZrS ₂ , HfS ₂ , HfSe ₂ , SnS ₂ , SnSe ₂ , PbS ₂ or PbSe _2, or any combination thereof. ..

(Appendix 4)
The gas sensor according to any one of Appendix 1 to 3, wherein the gas detection layer is composed of the monoatomic layer of chalcogenide.

(Appendix 5)
The gas sensor according to any one of Appendix 1 to 4 and
With the second gas sensor
Have,
The second gas sensor is
A second gas detection layer that is at least partially in contact with the gas,
A fourth electrode and a fifth electrode connected to the second gas detection layer,
Have,
The gas sensor array, wherein the second gas detection layer contains a Group 6 metal chalcogenide or graphene.

(Appendix 6)
The gas sensor according to any one of Appendix 1 to 4 and
A current detecting means for detecting a current flowing between the first electrode and the second electrode, and
A gas sensor device characterized by having.

100, 311, 312, 313: Gas sensor 103, 301, 302, 303: Gas detection layer 107: Source electrode 108: Drain electrode 111: Current monitoring device 114: VOC molecule 200, 300: Gas sensor array

Claims (4)

A gas detection layer that is at least partially in contact with gas,
The first electrode and the second electrode connected to the gas detection layer,
Have,
The gas detection layer may be one of ZrS ₂ , HfS ₂ , HfSe ₂ , PbS ₂ and PbSe ₂ or any combination thereof, or one of these or any combination thereof and SnS _2. , A gas sensor comprising one of TiS ₂ and SnSe ₂ or a combination thereof with any combination thereof . An insulating film in contact with the gas detection layer between the first electrode and the second electrode,
A third electrode that sandwiches the insulating film with the gas detection layer,
The gas sensor according to claim 1, wherein the gas sensor has. The gas sensor according to claim 1 or 2,
With a second gas sensor,
The second gas sensor is
A second gas detection layer that is at least partially in contact with the gas,
A fourth electrode and a fifth electrode connected to the second gas detection layer,
Have,
The gas sensor array, wherein the second gas detection layer contains a Group 6 metal chalcogenide or graphene. The gas sensor according to claim 1 or 2,
A current detecting means for detecting a current flowing between the first electrode and the second electrode, and
A gas sensor device characterized by having.

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