Gas detection device

Abstract

This eliminates the need for a resistivity recovery mechanism in IGZO thin-film ozone detectors. [Solution] The detection unit 10 consists of a conductive substrate 1, an insulating film 2, and an IGZO thin film 3. The IGZO thin film 3 is provided with a source electrode 4 and a drain electrode 5, and the conductive substrate 1 is connected to a gate voltage source 7. NaOH is added to the precursor solution of the IGZO thin film. In the embodiment and the comparative example without Na, the drain current decreases without ozone exposure, as shown by the solid line and dashed line 11 starting from time 0 minutes. If ozone exposure is for 1 minute and the exposure ends at 2 minutes, the drain current does not recover even after 2 minutes in the comparative example (curve 12), while the drain current recovers rapidly after 2 minutes in the embodiment (curve 13). A heating mechanism and recovery mechanism for sensor operation are not required, enabling the realization of a small, low-power ozone sensor.

Description

Technical Field

[0001] The present invention relates to a gas detection device that measures the concentration of an oxidizing gas such as ozone gas, converts it into an electrical signal, and outputs the signal. More specifically, it is a gas detection device that includes an oxide semiconductor thin film as a means for detecting an oxidizing gas, and measures the concentration of the oxidizing gas by changing the conductivity of the oxide semiconductor thin film due to exposure to the oxidizing gas. After exposure to the oxidizing gas, the changed conductivity is returned to the value before exposure without the need for heat treatment or light irradiation, which was necessary in the past, enabling reuse, thereby providing a small-sized, low-cost, and low-power-consuming oxidizing gas sensor.

Background Art

[0002] As the main conventional methods for detecting ozone gas, there are: (1) a method of measuring the ozone concentration by utilizing the absorption of light with a wavelength of 250 nm in the ultraviolet region by ozone gas and measuring the absorbance of the light with high precision (detection method (1)), and (2) a method of measuring the ozone concentration from the change in conductivity by contacting ozone with a thin film of a conductor or semiconductor having reactivity to ozone (detection method (2)).

[0003] Patent Document 1 discloses an invention related to detection method (1). This invention is composed of means for introducing gas into a cell for a measurement sample, means for irradiating the introduced gas with ultraviolet light, and means for measuring the light transmittance from the intensity of the light transmitted through the gas. However, the cost for constituting these means is generally higher than that of detection method (2) described later.

[0004] Non-Patent Documents 1, 2, and 3 disclose inventions related to detection method (2). The invention disclosed in Non-Patent Document 1 relates to an ozone sensor using a heated indium tin oxide (ITO) conductive film as the ozone reactant. In this example sensor, the ITO conductive film is heated to an active state capable of detecting ozone, and then ozone is detected by measuring the change in the resistivity of the ITO conductive film that occurs only when it comes into contact with ozone. Here, it is necessary to heat the ITO conductive film to a temperature of 300°C or higher in order to activate it.

[0005] Non-patent document 2 is part of a catalog for commercially available ozone sensors using ITO conductive films and is disclosed on the internet. This product also has a heater, and although the temperature is unknown, it is presumed that heating the ITO conductive film activates it to a state where ozone can be detected.

[0006] Non-Patent Document 3 discloses an ozone sensor invention using an indium (In) gallium (Ga) zinc (Zn) oxide (IGZO) thin film deposited by sputtering as the ozone reactant. The IGZO thin film does not require heating, which was necessary in the inventions disclosed in Non-Patent Documents 1 and 2, and can detect ozone even at room temperature. On the other hand, while the sensors of the inventions disclosed in Non-Patent Documents 1 and 2 only experience a change in resistivity when in contact with ozone, in the invention of Non-Patent Document 3, once the resistivity changes after contact with ozone, the change in resistivity remains unchanged for a long time even after contact with ozone is removed. Therefore, before detecting the gas again, an operation is performed to restore the change in resistivity by means of ultraviolet light irradiation or heating. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2012-13573 [Non-patent literature]

[0008] [Non-Patent Document 1] Ozone sensor made by dip coating method, Akiyoshi Hattori, Hirokazu Tachibana, Nobuyuki Yoshiike, Akihiko Yoshida: Sensors and Actuators 77 1999 120 125 [Non-Patent Document 2] https: / / www.fisinc.co.jp / common / pdf / J _A1320301-SP361-xx.pdf [Non-Patent Document 3] Kuen-Lin Chen, Guo-Jhen Jiang, Kai-Wei Chang, Jan-Han Chen, Chiu-Hsien Wu: Gas sensing properties of indium gallium zinc oxide gas sensors in different light intensity, Analytical Chemistry Research 4 (2015) 8 12. [Overview of the project] [Problems that the invention aims to solve]

[0009] In the inventions disclosed in Non-Patent Documents 1 and 2, when ozone detection is desired, the ITO conductive film must be heated to an active state in which ozone can be detected. In other words, the ITO conductive film must be kept constantly heated during ozone detection operation. This heating requires a large amount of power, and when the ozone sensor is configured as a portable device, the battery capacity is limited due to constraints such as size and weight, resulting in a short battery operating time. Furthermore, it is necessary to cover the ITO conductive film, which is heated to a high temperature, with a metal can (housing) to prevent accidental contact with the human body or clothing, making it difficult to miniaturize the entire device.

[0010] In recent years, the invention disclosed in Non-Patent Document 3 has attracted attention as a solution to the aforementioned problems of the inventions disclosed in Non-Patent Documents 1 and 2, and is being diligently researched in various fields. In the invention disclosed in Non-Patent Document 3, an IGZO thin film is used as an ozone detector instead of an ITO conductive film. Since this IGZO thin film is active to ozone without heating, no special heating mechanism is required for ozone detection. Therefore, no power for heating is required, and a considerably longer battery operating time can be expected compared to the inventions in Non-Patent Documents 1 and 2. In addition, in the inventions in Non-Patent Documents 1 and 2, a container was needed to house the ITO conductive film to prevent accidental contact of the human body or clothing with the high-temperature ITO conductive film, but this is unnecessary, which is advantageous for miniaturization.

[0011] However, IGZO thin films have another problem: once their resistivity changes after contact with ozone, it does not return to its original state even after contact with ozone ceases. For this reason, irradiation with ultraviolet light was necessary to restore the altered resistivity (hereinafter referred to as the recovery operation). Although not as significant as the heating method described in Non-Patent Documents 1 and 2, power consumption due to light irradiation is unavoidable, and there are also problems of increased size and cost due to the inclusion of a light irradiation mechanism.

[0012] This invention was made in light of the conventional technology and problems described above, and aims to realize and provide a small, low-power gas detection device (e.g., an ozone sensor) by eliminating the need for light irradiation for recovery operation in an invention using an IGZO thin film that does not require heating for ozone detection, thereby avoiding the increase in size, cost, and power consumption caused by light irradiation. [Means for solving the problem]

[0013] The gas detection device described in claim 1 is characterized by using an oxide semiconductor thin film that contains one or more constituent elements selected from In, Ga, and Zn, and one or more additive elements selected from alkali metals and alkaline earth metals, the resistivity of which changes upon reaction with the gas to be detected.

[0014] The gas detection device described in claim 2 is the gas detection device described in claim 1, The alkali metal element is characterized by being K or Na, and the alkaline earth metal is Ca or Mg.

[0015] The gas detection device described in claim 3 is the gas detection device described in claim 2, The detected gas is characterized by being an oxidizing gas containing a gas selected from ozone, nitric oxide, and nitrogen dioxide.

[0016] The gas detection device described in claim 4 is A conductive substrate and An insulating film formed on the surface of the conductive substrate, An oxide semiconductor thin film comprising one or more constituent elements selected from In, Ga, and Zn, and one or more additive elements selected from alkali metals and alkaline earth metals, formed on the insulating film, wherein its resistivity changes upon reaction with the gas to be detected, A current detector for detecting the current flowing through the oxide semiconductor thin film, It is characterized by having [this feature]. [Effects of the Invention]

[0017] The gas detection device of the present invention uses an oxide semiconductor thin film (e.g., IGZO thin film) containing one or more constituent elements selected from In, Ga, and Zn, and one or more additive elements selected from alkali metals (e.g., Na, K) and alkaline earth metals (e.g., Ca, Mg), as a detection unit for oxidizing gases (e.g., ozone gas, nitric oxide, nitrogen dioxide). According to the present invention, with this configuration, the resistivity of the oxide semiconductor thin film, which increases due to contact with the oxidizing gas, immediately returns to the value before contact after the contact ends. Therefore, it is not necessary to provide special recovery mechanisms such as a heating mechanism required for the operation of the sensor in the inventions of Non-Patent Documents 1 and 2, or an ultraviolet light irradiation mechanism required in the invention of Non-Patent Document 3. As a result, it is possible to realize a small, low-power ozone sensor.

Brief Description of the Drawings

[0018] [Figure 1] It is a schematic perspective view of the gas detector of the embodiment. [Figure 2] It is a schematic cross-sectional view of the gas detector of the embodiment. [Figure 3] It is a graph showing the detection characteristics of ozone gas in the gas detectors of the embodiment and the comparative example.

Embodiments for Carrying Out the Invention

[0019] Embodiments of the invention of the present application will be described with reference to FIGS. 1 to 3. The invention of the present application relates to a gas detection device that uses an oxidizing gas containing a gas selected from ozone gas, nitrogen monoxide, and nitrogen dioxide as the gas to be detected. However, the embodiments described below relate particularly to an ozone detector for detecting ozone gas.

[0020] As shown in Figures 1 and 2, the ozone detector of this embodiment has an ozone detection unit 10. The ozone detection unit 10 consists of a conductive substrate 1 made of a p-type Si single crystal, an insulating film 2 made of SiO2 deposited on the conductive substrate 1, and an IGZO thin film 3 as an oxide semiconductor thin film deposited on the insulating film 2. The conductive substrate 1 needs to have sufficient smoothness in order to accurately and reliably carry out the process of forming the SiO2 insulating film 2 on its surface and then forming the IGZO thin film 3 on top of it, and it also needs to be conductive in order to function as a gate electrode as described later. Therefore, although a p-type Si single crystal substrate, which is a conductive semiconductor, is used in this embodiment, a metal plate with a mirror-finished surface on which the insulating film 2 is formed may also be used. The thickness of the conductive substrate 1 is not particularly limited. The insulating film 2 made of SiO2 only needs to have a thickness that ensures sufficient insulation to insulate the conductive substrate 1 from the IGZO thin film 3 and sufficient strength to stably form the IGZO thin film 3 on top of it, for example, it was about 300 nm in this embodiment. The thickness of the IGZO thin film 3 was, as an example, about 10 to 20 nm. This IGZO thin film 3 may contain Na, K, Ca, Mg, or their ions individually or in any combination, or various compounds of Na, K, Ca, and Mg described later, individually or in any combination. The method for manufacturing the IGZO thin film 3 will be described in detail later.

[0021] As shown in Figures 1 and 2, the surface of the IGZO thin film 3 is provided with a source electrode 4 and a drain electrode 5 for applying a voltage horizontal to the IGZO thin film 3 and causing a current to flow within the IGZO thin film 3. In Figure 2, the current flowing within the IGZO thin film 3 is shown by a dashed line. The source electrode 4 is connected to ground, i.e., a point with a potential of 0V, and the drain electrode 5 is connected to a drain voltage source 6 via a current detector 8 for measuring the drain current. The conductive substrate 1 is connected to a gate voltage source 7 and is given the function of a gate electrode.

[0022] Next, the manufacturing method for IGZO thin film 3 will be described in detail. The oxide semiconductor thin film of the gas detection device of the present invention is composed of one or more metals selected from In, Ga, and Zn as constituent elements. In this embodiment, the oxide semiconductor thin film is an IGZO thin film 3, and its constituent elements are the three elements In, Ga, and Zn.

[0023] First, indium nitrate, gallium nitrate, and zinc nitrate are dissolved in 2-methoxyethanol to a concentration of 0.1 M (mol / L) for each metal ion, and the mixture is thoroughly stirred to obtain three solutions containing the constituent elements In, Ga, and Zn ions. These three solutions are then mixed so that the molar ratio of In, Ga, and Zn ions is 6:1:3 to obtain an IGZO precursor solution.

[0024] In this embodiment, the molar ratio of In, Ga, and Zn ions in the IGZO precursor solution was set to 6:1:3. However, experiments have shown that even if the oxide semiconductor thin film is manufactured using a molar ratio of 6:0:3 or 6:0:0 in the process described later, and an ozone detector is constructed, the result will be equivalent to or better than that of this embodiment. In other words, the content ratio of the constituent elements In, Ga, and Zn in the oxide semiconductor thin film of this embodiment is arbitrary; it is sufficient to include one or more constituent elements selected from the group of these three constituent elements. Furthermore, in this embodiment, the IGZO precursor solution is applied to the insulating layer 3 of the conductive substrate 1 by spin coating as described later, to form the IGZO thin film 3. However, if the sputtering method is used instead of the liquid-phase method, it is preferable to set the molar ratio of In, Ga, and Zn ions to 1:1:1.

[0025] Next, the additive elements are added to the IGZO precursor solution. First, NaOH was dissolved in 2-methoxyethanol to a concentration of 0.1 M (mol / L) and added to the IGZO precursor solution. The amount added was such that the molar ratio of Na ions in the IGZO precursor solution after the addition was 5%. Here, Na ions may be added by adding substances other than NaOH. For example, NaCl (sodium chloride), NaHCO3 (sodium bicarbonate), Na2CO3 (sodium carbonate), or Na2CH3O6 (sodium percarbonate) may also be used.

[0026] Alternatively, K ions may be added instead of Na ions. K ions can be added using KOH, but they can also be added by adding compounds of the same type as those exemplified in the case of Na. For example, KCl (potassium chloride), KHCO3 (potassium bicarbonate), K2CO3 (potassium carbonate), or K2CH3O6 (potassium percarbonate) may be used. When K ions are added, the same results as when Na ions are added will be obtained.

[0027] Furthermore, the same effect as when adding Na or K ions can be obtained by substituting alkaline earth metals such as Ca or Mg ions for alkali metal elements like Na or K ions. When adding Ca or Mg ions, they can be added in the same form as when adding Na or K ions: as hydroxides, chlorides, or carbonate compounds.

[0028] Furthermore, these Na, K, Ca, and Mg ions may be added in any combination, or the Na, K, Ca, and Mg compounds may be added in appropriate combinations. When adding multiple types of ions or compounds to the IGZO precursor solution, it is preferable that the total molar ratio of the added ions be 5%.

[0029] As shown in Figure 1, the IGZO precursor coating solution is applied to the surface of the insulating film 2 of the conductive substrate 1. The coating method can be a spin coating method, but other liquid-phase methods may also be used. Next, the conductive substrate 1 is heated to 250-300°C on a hot plate and held for 60 minutes. Prior to this heating, heating at 90-130°C for 5 minutes or more may be performed to dry the solvent. This allows an IGZO thin film 3 to be formed on the surface of the insulating film 2 of the conductive substrate 1. Furthermore, two Al electrodes are fabricated on the thus formed IGZO thin film 3 by vapor deposition or the like, spaced apart, and these are designated as the source electrode 4 and drain electrode 5, respectively.

[0030] Next, the operation of the ozone detector in this embodiment when detecting ozone will be described. With the output voltage of the drain voltage source 6 and the output voltage of the gate voltage source 7 set to 40V as shown in Figure 1, the magnitude of the current flowing from the drain electrode 5 to the source electrode 4 is measured by a current detector 8 for drain current measurement.

[0031] Figure 3 is a graph showing the time dependence of the drain current flowing from the drain electrode 5 to the source electrode 4 for an ozone detector of an embodiment equipped with an IGZO thin film 3 to which Na ions have been added by the manufacturing method described above, and for an ozone detector of a comparative example equipped with an IGZO thin film without the aforementioned additive elements such as Na ions, both when exposed to ozone gas and when not exposed. In other words, this graph represents the characteristics of both the ozone detector of the embodiment and the ozone detector of the comparative example. The horizontal axis of Figure 3 represents the elapsed time after the start of voltage output from the drain voltage source 6 and the gate voltage source 7, in minutes. The vertical axis represents the current ratio on a logarithmic scale, with the magnitude of the drain current at elapsed time 0 minutes set to 1.

[0032] In Figure 3, the solid line curve starting from the point where elapsed time is 0 minutes and the drain current ratio is 1, and the dashed-dotted line curve 11 smoothly continuing from it, show the change in the drain current ratio common to both the embodiment and the comparative example, which are not exposed to ozone. According to this solid line and the dashed-dotted line curve 11, the drain current gradually decreases with time elapsed since the drain voltage source 6 and the gate voltage source 7 started outputting voltage, even without ozone exposure. This is a property common to both the ozone detector of the embodiment and the ozone detector of the comparative example.

[0033] Next, we will explain the operation when the ozone detector is exposed to ozone gas at a time of 1 minute elapsed in Figure 3, and the exposure to ozone gas is terminated at a time of 2 minutes elapsed. When the ozone detector of the comparative example, which uses an IGZO thin film without added elements such as Na, was exposed to ozone, the drain current decreased sharply from 1 minute after the start of ozone exposure, as shown by the solid line. Furthermore, even when ozone exposure was interrupted at 2 minutes, the drain current remained decreased and did not recover, as shown by the dashed curve 12.

[0034] On the other hand, when the ozone detector of the embodiment equipped with the Na-doped IGZO thin film 3 is exposed to ozone, the drain current decreases sharply from 1 minute after the start of exposure, as shown by the solid line, similar to the comparative example. However, when the ozone exposure is interrupted at 2 minutes, unlike the comparative example, the drain current recovers rapidly, as shown by the solid curve 13, and returns to a value equivalent to the dashed-dotted curve 11 when there is no ozone exposure. The time it takes to return to a value equivalent to when there is no ozone exposure is slightly less than 1 minute in the example in Figure 3, but even when various experimental conditions are changed to take a longer time, it generally remains within 5 minutes.

[0035] From the experimental results above, the ozone detector of this embodiment has a property in which the resistivity of the IGZO thin film 3 increases and the drain current decreases only when exposed to ozone. Therefore, if ozone exposure is interrupted, the resistivity decreases and the drain current recovers immediately without the need for recovery operations such as ultraviolet light irradiation, giving it an advantageous characteristic as an ozone sensor. Accordingly, it is possible to measure the concentration of ozone gas in real time using the value of the drain current as an indicator. Furthermore, since a heating mechanism is not required for the operation of the sensor, and there is no need to provide an ultraviolet light irradiation mechanism or the like as a sensor recovery mechanism, it is possible to realize a small and low-power ozone sensor.

[0036] Thus, the characteristic that the drain current of an IGZO thin film, which decreases due to ozone exposure, immediately recovers upon interruption of exposure is thought to be obtained by adding a specific additive (or a compound thereof) to the IGZO thin film. The detailed mechanism by which Na, K, Ca, Mg, or their ions cause a rapid recovery of resistivity is unknown. However, the inventors of this application estimate that in the case of an IGZO thin film prepared by adding NaOH to the IGZO precursor solution in order to add Na as an additive element, the above characteristic appears through the following mechanism. That is, in the IGZO thin film, the OH contained therein - It is thought that current flows with electrons being supplied by OH, but when the IGZO thin film is exposed to ozone, ozone and OH -Because OH - As the amount of Na decreases, current becomes less likely to flow, the resistivity increases, and the drain current decreases. When ozone exposure is interrupted, the Na in the IGZO thin film mixes with the O2 and H2O (moisture) in the air. + Because the compound binds and NaOH is produced, the resistivity returns to what it was before ozone exposure, and the drain current recovers. The same is thought to be true for compounds other than NaOH.

[0037] Furthermore, ozone detection using the ozone detector of this embodiment is performed as follows. In this embodiment, a different ozone sensor is used as a reference sensor. First, the ozone concentration is measured using the reference sensor. The ozone detector of this embodiment is exposed to the ozone whose concentration is known based on the concentration estimated by the reference sensor, and the rate of decrease in drain current is measured. This measurement is performed similarly for ozone of various concentrations. This provides reference data (or a graph representing this correspondence) showing the relationship between the ozone concentration measured by the reference sensor and the rate of decrease in drain current (rate of decrease from the reference value) detected by the ozone detector of this embodiment. The ozone detector of this embodiment is exposed to ozone of unknown concentration, and measurement results are obtained as shown by the solid line in Figure 3. The rate of decrease in drain current is calculated from the ratio of drain current at the start and end of exposure, and by referring to the reference data, the concentration of the ozone can be determined. The process from calculating the ozone concentration by referring to the reference data from the measurement results can be performed automatically by software.

[0038] In the embodiments described above, ozone was used as the gas to be detected, but other oxidizing gases, such as nitric oxide and nitrogen dioxide, can also be detected in the same way, and it is also possible to detect gases that are mixtures of multiple types of oxidizing gases.

[0039] In the embodiments described above, a gate voltage is applied to the conductive substrate 1 to increase the sensitivity of ozone detection. However, it is possible to detect ozone by measuring the drain current even without applying this voltage. [Explanation of Symbols]

[0040] 1. Conductive substrate 2 insulating film 3 IGZO thin film 4 Source electrodes 5 Drain electrode 6. Drain voltage source 7. Gate voltage source 8. Current detector for measuring drain current. 10 Ozone detection unit 11. Curve when not exposed to ozone 12. Curve of an ozone detector using an IGZO thin film in a comparative example when exposed to ozone. 13. Curve when an ozone detector using the IGZO thin film of the embodiment is exposed to ozone.

Claims

1. A gas detection device characterized by using an oxide semiconductor thin film containing one or more constituent elements selected from In, Ga, and Zn, and one or more additive elements selected from alkali metals and alkaline earth metals, wherein the resistivity changes upon reaction with the gas to be detected.

2. The gas detection device according to claim 1, characterized in that the alkali metal element is K or Na, and the alkaline earth metal is Ca or Mg.

3. The gas detection device according to claim 2, characterized in that the gas to be detected is an oxidizing gas containing a gas selected from ozone gas, nitric oxide, and nitrogen dioxide.

4. A conductive substrate and An insulating film formed on the surface of the conductive substrate, An oxide semiconductor thin film comprising one or more constituent elements selected from In, Ga, and Zn, and one or more additive elements selected from alkali metals and alkaline earth metals, formed on the insulating film, wherein its resistivity changes upon reaction with the gas to be detected, A current detector for detecting the current flowing through the oxide semiconductor thin film, A gas detection device characterized by having the following features.

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