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CO2 Gas Sensor Based on MIS Structure with LaF3 Layer
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 87 (2014) 1047 – 1050
EUROSENSORS 2014, the XXVIII edition of the conference series
CO2 gas sensor based on MIS structure with LaF3 layer A.E. Varfolomeeva*, A.A. Vasilieva, N. Zaretskiya, W. Moritzb a
Center of Physical and Chemical Technology, National research center Kurchatov Institute, Moscow 123182, Russia b Humboldt University of Berlin, Institute of chemistry, Berlin, Germany
Abstract The aim of this research is to find a CO2 sensor with high value of the response to CO2 concentrations in a range of several hundreds of ppm. The sensor investigated in this work is based on metal/solid electrolyte/insulator/silicon structure with fluorine conducting solid electrolyte LaF3 layer. It is characterized by rather with short response and recovery time of about 1 min (t90) at room temperature. The application of such gas sensor seems to be very promising for the measurement of CO 2 concentration in a range from atmospheric background (300 ppm) to maximum permissible concentration (2000 ppm). © 2014 2014Published The Authors. Published Elsevier Ltd.access article under the CC BY-NC-ND license © by Elsevier Ltd. by This is an open (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014. Peer-review under responsibility of the scientific committee of Eurosensors 2014 Keywords: gas sensor; carbon dioxide; metal-insulator-silicon structure; solid electrolyte; lanthanum fluorocarbonate
1. Introduction The development of inexpensive CO2 gas sensors operating at room temperature remains to be a significant problem, because these sensors are important for the application, for example, in air-conditioners. The sensors, which are used now, are mainly of NDIR type (for example, [1]). In spite of high accuracy and performance of these sensors, their high cost is an obstacle for wide use of such instruments. Earlier, we started the investigations of MIS devices with solid electrolyte layer (that is, MEIS, devices). In [2], we reported the results obtained with proton and CO32- conducting solid electrolyte layer. These MEIS structures are sensitive to CO2, however further improvement of gas sensors is necessary. In our study of MEIS structures with LaF3 layer [3], we found that the aging of the device is due to the formation of lanthanum fluorocarbonate in air. As it was found using MS analysis of gas phase, this layer can be easily decomposed by heating the sensor up to 150–200 oC for several seconds. On the other hand, the results on the application of lanthanum hydroxocarbonate as a material of CO 2 resistive sensor [4, 5] were recently reported. However, the authors of [4, 5] noted that the resistance of sensing layer was too
* Corresponding author E-mail address: [email protected]
1877-7058 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014 doi:10.1016/j.proeng.2014.11.341
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A.E. Varfolomeev et al. / Procedia Engineering 87 (2014) 1047 – 1050
high (of order of 1010 Ohm), and the response time was too long (almost 1 hour) for practical application of such sensors. In addition, the sensor operated at 300 – 350 0C requiring heating power. We used the results of [2 – 4] as a starting point of our research. The idea is the application of lanthanum oxyfluoride layer, which adsorbs easily carbon dioxide with the formation of lanthanum fluorocarbonate, as a sensing layer of MEIS structure. In this case, high resistance of sensing material is not so important, because the measured parameter is capacitance of MEIS structure. 2. Experimental The sensor structure Al/Si/SiO2/LaF3/Pt was fabricated by thermal deposition of LaF3 (200 nm) in high vacuum (Fig.1). Platinum gate (30 nm) was deposited by diode sputtering system in Ar atmosphere [3]. Measurements of capacitance and capacitance-voltage (C-V) characteristics were performed by use of precision LCR meter Agilent E4980A at AC voltage frequency of 10 kHz and amplitude of 10 mV. Measurement of the sensor response was performed in humid carrier air with relative humidity of 40, 60, and 80 % with CO2 concentrations lying in a range from 300 to 3000 ppm; this range is most interesting for practical application. Gas mixtures were prepared using “Microgas-1F” installation equipped with built-in humidifier; gas flows in this computer driven installation were set up with Bronkhorst mass-flow controllers.
Fig. 1. Scheme of MEIS gas sensor. (1) Pt gate electrode; (2) LaF3 solid electrolyte layer; (3-4) Si3N4 layer; (4) SiO2 insulation; (5) Si substrate; (6) Al contact.
3. Results and Discussion The response of MEIS structure to CO2 concentrations was measured in two cases: (1) the virgin sensor “as prepared”, without any additional treatment; (2) after activation, consisting in heating the sensor at 150 – 3000C for 10 min. This last procedure leads to partial decomposition of LaF3 and to the formation of lanthanum oxyfluoride on the surface. C-V characteristics of the structure before and after heating at T=300 0C are shown in Fig.2.
Capacitance (pF)
1500
1000
2 500 1 0 -4
-3
-2
-1
0
1
2
3
4
Bias (V) Fig. 2. C-V characteristics: 1 – before annealing; 2 - after annealing at T=300 oC
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A.E. Varfolomeev et al. / Procedia Engineering 87 (2014) 1047 – 1050
We measured capacitance-voltage (C-V) curves at different humidity (RH0, RH20, RH40, RH60, RH80) and different CO2 concentrations (0 ppm, 500 ppm, 1000 ppm). The C-V curves are shifted weakly at different humidities. It confirms the weak dependence of sensor response on humidity. The left side of C-V curves changes for different CO2 concentrations at negative bias voltage after annealing. It is possible to explain by increasing of charge carrier density in LaF3 electrolyte layer or by decreasing of relaxation time of charge carriers. It confirms that the mechanism of gas sensitivity modifies in this case. Fig.3a shows the response of virgin sensor to different CO2 concentrations in humid air at constant relative humidity of 60 % at room temperature. An increase in air humidity makes shorter the response time down to 50 s (Fig.3b). The response of CO2 MEIS sensors based of PVA-electrolyte, which we investigated earlier in [2], is more affected by humidity compared to the sensor based on LaF3, because PVA-based layer absorbs water much stronger compared to the water adsorption by LaF3 solid electrolyte film. The capacitance changes by ~ 1 % at RH60, i.e. much weaker in comparison with PVA-based sensor. Water adsorption and desorption times were of about 3-4 minutes at room temperature. 7
12 10
2000 ppm
8 6 1000 ppm
4 500 ppm
2 0 2000
3000
4000
5000
6000
1000 ppm
6 Change in capacitance (pF)
Change in capacitance (pF)
3000 ppm
5 4 500 ppm
3 2 1 0
7000
600
900
Time (s)
1200
1500
1800
2100
2400
Time (s)
a
b Fig. 3. The responses to different CO2 concentrations before sensor activation: (a) at RH60; (b) at RH80.
(1) before annealing RH40 (2) before annealing RH60 (3) before annealing RH80 (4) after annealing RH60
Change in capacitance (pF)
25 20 15 10 5 0 0
500
1000 1500 2000 2500 3000
3500
CO 2 concentration (ppm) Fig. 4. Change in capacitance as a function of CO2 concentration: 1 – RH40, 2 – RH60, 3 – RH80. The measurements were performed with sensor without activation. 4 – sensor after activation (heating at 3000C for 10 min) at RH60.
Changes in capacitance as a function of CO2 concentration at different humidities were fitted according to a power law y=axb (Fig.4). This figure confirms the rather weak dependence of sensor response on air humidity. In
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A.E. Varfolomeev et al. / Procedia Engineering 87 (2014) 1047 – 1050
accordance with the suggested mechanism, heating at temperature of 3000C for 10 min increases significantly, by a factor of 2, the response (Fig.4, curve 4) compared to the virgin sensor. These measurements were performed approximately 2 hours after the activation. Much higher sensitivity was obtained, if the measurements were carried out immediately after the activation of the MEIS capacitor. The sensor response increases in this case approximately by a factor of 10. Further development of CO2 gas sensors based on MIS structures with a layer of lanthanum trifluoride is related with the application of fast activation of the sensor. It was shown that the heating of the surface during several milliseconds is sufficient for this [6]. In this paper, it was suggested to use MIS structure located on silicon membrane and equipped with built-in platinum heater (Fig. 5). In this case heating impulse with energy of about 0.2 J is sufficient for the activation of gas sensor; this makes possible the application of such sensors in battery powered instruments.
1
2
Fig. 5. (1) The structure of MIS structure gas sensor with a layer of fluorine conducting solid electrolyte located on this silicon membrane. Pd is a gate layer, Pt – circular heater used for the sensor activation [6]. (2) Simulating heat pulse to the sensor (3 x 3 mm Si-chip with 390 mm / 30 mm membrane) for 1 ms pulse after (a) 1 ms and (b) 10 ms [6].
4. Conclusions The application of “activated” MEIS gas sensor with a layer of fluorine conducting solid electrolyte LaF3 seems to be very promising for the measurement of CO2 concentrations in a range from atmospheric background (300 ppm) to maximum permissible concentration (2000 ppm). The sensing mechanism based on the reversible formation of lanthanum fluorocarbonate on the surface of solid electrolyte layer is suggested. References [1] www.dynament.com [2] A.E.Varfolomeev, I.R.Shandova, A.S.Lagutin, A.A.Vasiliev, A.V.Pisareva, A.V.Levchenko, Yu.A.Dobrovolskiy, Sensor of Carbon Dioxide Based on MIS Structure with Solid Electrolyte Layer, Procedia Engineering, 47 (2012) 170-173. [3] L.Bartolomaus, A.A.Vasiliev, W.Moritz. Semiconductor sensors for fluorine detection - Optimization for low and high concentrations, Sensors and Actuators B, 65 (2000) 270-272. [4] A.Haensch, D.Koziej, M.Niederberger, N.Barsan, U.Weimar, Rare earth oxycarbonates as a material class for chemoresistive CO2 gas sensors, Procedia Engineering, 5 (2010) 139-142. [5] A.Haensch, D.Borowski, N.Barsan, D.Koziej, M.Niederberger, U.Weimar, Faster response times of rare-earth oxycarbonate based CO2 sensors and another readout strategy for real-world applications, Procedia Engineering, 25 (2011) 1429-1432. [6] S. Linke, M. Dallmer, R. Werner, W. Moritz, Low energy hydrogen sensor, International Journal of Hydrogen Energy 37 (2012) 1752317528.
ScienceDirect Procedia Engineering 87 (2014) 1047 – 1050
EUROSENSORS 2014, the XXVIII edition of the conference series
CO2 gas sensor based on MIS structure with LaF3 layer A.E. Varfolomeeva*, A.A. Vasilieva, N. Zaretskiya, W. Moritzb a
Center of Physical and Chemical Technology, National research center Kurchatov Institute, Moscow 123182, Russia b Humboldt University of Berlin, Institute of chemistry, Berlin, Germany
Abstract The aim of this research is to find a CO2 sensor with high value of the response to CO2 concentrations in a range of several hundreds of ppm. The sensor investigated in this work is based on metal/solid electrolyte/insulator/silicon structure with fluorine conducting solid electrolyte LaF3 layer. It is characterized by rather with short response and recovery time of about 1 min (t90) at room temperature. The application of such gas sensor seems to be very promising for the measurement of CO 2 concentration in a range from atmospheric background (300 ppm) to maximum permissible concentration (2000 ppm). © 2014 2014Published The Authors. Published Elsevier Ltd.access article under the CC BY-NC-ND license © by Elsevier Ltd. by This is an open (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014. Peer-review under responsibility of the scientific committee of Eurosensors 2014 Keywords: gas sensor; carbon dioxide; metal-insulator-silicon structure; solid electrolyte; lanthanum fluorocarbonate
1. Introduction The development of inexpensive CO2 gas sensors operating at room temperature remains to be a significant problem, because these sensors are important for the application, for example, in air-conditioners. The sensors, which are used now, are mainly of NDIR type (for example, [1]). In spite of high accuracy and performance of these sensors, their high cost is an obstacle for wide use of such instruments. Earlier, we started the investigations of MIS devices with solid electrolyte layer (that is, MEIS, devices). In [2], we reported the results obtained with proton and CO32- conducting solid electrolyte layer. These MEIS structures are sensitive to CO2, however further improvement of gas sensors is necessary. In our study of MEIS structures with LaF3 layer [3], we found that the aging of the device is due to the formation of lanthanum fluorocarbonate in air. As it was found using MS analysis of gas phase, this layer can be easily decomposed by heating the sensor up to 150–200 oC for several seconds. On the other hand, the results on the application of lanthanum hydroxocarbonate as a material of CO 2 resistive sensor [4, 5] were recently reported. However, the authors of [4, 5] noted that the resistance of sensing layer was too
* Corresponding author E-mail address: [email protected]
1877-7058 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014 doi:10.1016/j.proeng.2014.11.341
1048
A.E. Varfolomeev et al. / Procedia Engineering 87 (2014) 1047 – 1050
high (of order of 1010 Ohm), and the response time was too long (almost 1 hour) for practical application of such sensors. In addition, the sensor operated at 300 – 350 0C requiring heating power. We used the results of [2 – 4] as a starting point of our research. The idea is the application of lanthanum oxyfluoride layer, which adsorbs easily carbon dioxide with the formation of lanthanum fluorocarbonate, as a sensing layer of MEIS structure. In this case, high resistance of sensing material is not so important, because the measured parameter is capacitance of MEIS structure. 2. Experimental The sensor structure Al/Si/SiO2/LaF3/Pt was fabricated by thermal deposition of LaF3 (200 nm) in high vacuum (Fig.1). Platinum gate (30 nm) was deposited by diode sputtering system in Ar atmosphere [3]. Measurements of capacitance and capacitance-voltage (C-V) characteristics were performed by use of precision LCR meter Agilent E4980A at AC voltage frequency of 10 kHz and amplitude of 10 mV. Measurement of the sensor response was performed in humid carrier air with relative humidity of 40, 60, and 80 % with CO2 concentrations lying in a range from 300 to 3000 ppm; this range is most interesting for practical application. Gas mixtures were prepared using “Microgas-1F” installation equipped with built-in humidifier; gas flows in this computer driven installation were set up with Bronkhorst mass-flow controllers.
Fig. 1. Scheme of MEIS gas sensor. (1) Pt gate electrode; (2) LaF3 solid electrolyte layer; (3-4) Si3N4 layer; (4) SiO2 insulation; (5) Si substrate; (6) Al contact.
3. Results and Discussion The response of MEIS structure to CO2 concentrations was measured in two cases: (1) the virgin sensor “as prepared”, without any additional treatment; (2) after activation, consisting in heating the sensor at 150 – 3000C for 10 min. This last procedure leads to partial decomposition of LaF3 and to the formation of lanthanum oxyfluoride on the surface. C-V characteristics of the structure before and after heating at T=300 0C are shown in Fig.2.
Capacitance (pF)
1500
1000
2 500 1 0 -4
-3
-2
-1
0
1
2
3
4
Bias (V) Fig. 2. C-V characteristics: 1 – before annealing; 2 - after annealing at T=300 oC
1049
A.E. Varfolomeev et al. / Procedia Engineering 87 (2014) 1047 – 1050
We measured capacitance-voltage (C-V) curves at different humidity (RH0, RH20, RH40, RH60, RH80) and different CO2 concentrations (0 ppm, 500 ppm, 1000 ppm). The C-V curves are shifted weakly at different humidities. It confirms the weak dependence of sensor response on humidity. The left side of C-V curves changes for different CO2 concentrations at negative bias voltage after annealing. It is possible to explain by increasing of charge carrier density in LaF3 electrolyte layer or by decreasing of relaxation time of charge carriers. It confirms that the mechanism of gas sensitivity modifies in this case. Fig.3a shows the response of virgin sensor to different CO2 concentrations in humid air at constant relative humidity of 60 % at room temperature. An increase in air humidity makes shorter the response time down to 50 s (Fig.3b). The response of CO2 MEIS sensors based of PVA-electrolyte, which we investigated earlier in [2], is more affected by humidity compared to the sensor based on LaF3, because PVA-based layer absorbs water much stronger compared to the water adsorption by LaF3 solid electrolyte film. The capacitance changes by ~ 1 % at RH60, i.e. much weaker in comparison with PVA-based sensor. Water adsorption and desorption times were of about 3-4 minutes at room temperature. 7
12 10
2000 ppm
8 6 1000 ppm
4 500 ppm
2 0 2000
3000
4000
5000
6000
1000 ppm
6 Change in capacitance (pF)
Change in capacitance (pF)
3000 ppm
5 4 500 ppm
3 2 1 0
7000
600
900
Time (s)
1200
1500
1800
2100
2400
Time (s)
a
b Fig. 3. The responses to different CO2 concentrations before sensor activation: (a) at RH60; (b) at RH80.
(1) before annealing RH40 (2) before annealing RH60 (3) before annealing RH80 (4) after annealing RH60
Change in capacitance (pF)
25 20 15 10 5 0 0
500
1000 1500 2000 2500 3000
3500
CO 2 concentration (ppm) Fig. 4. Change in capacitance as a function of CO2 concentration: 1 – RH40, 2 – RH60, 3 – RH80. The measurements were performed with sensor without activation. 4 – sensor after activation (heating at 3000C for 10 min) at RH60.
Changes in capacitance as a function of CO2 concentration at different humidities were fitted according to a power law y=axb (Fig.4). This figure confirms the rather weak dependence of sensor response on air humidity. In
1050
A.E. Varfolomeev et al. / Procedia Engineering 87 (2014) 1047 – 1050
accordance with the suggested mechanism, heating at temperature of 3000C for 10 min increases significantly, by a factor of 2, the response (Fig.4, curve 4) compared to the virgin sensor. These measurements were performed approximately 2 hours after the activation. Much higher sensitivity was obtained, if the measurements were carried out immediately after the activation of the MEIS capacitor. The sensor response increases in this case approximately by a factor of 10. Further development of CO2 gas sensors based on MIS structures with a layer of lanthanum trifluoride is related with the application of fast activation of the sensor. It was shown that the heating of the surface during several milliseconds is sufficient for this [6]. In this paper, it was suggested to use MIS structure located on silicon membrane and equipped with built-in platinum heater (Fig. 5). In this case heating impulse with energy of about 0.2 J is sufficient for the activation of gas sensor; this makes possible the application of such sensors in battery powered instruments.
1
2
Fig. 5. (1) The structure of MIS structure gas sensor with a layer of fluorine conducting solid electrolyte located on this silicon membrane. Pd is a gate layer, Pt – circular heater used for the sensor activation [6]. (2) Simulating heat pulse to the sensor (3 x 3 mm Si-chip with 390 mm / 30 mm membrane) for 1 ms pulse after (a) 1 ms and (b) 10 ms [6].
4. Conclusions The application of “activated” MEIS gas sensor with a layer of fluorine conducting solid electrolyte LaF3 seems to be very promising for the measurement of CO2 concentrations in a range from atmospheric background (300 ppm) to maximum permissible concentration (2000 ppm). The sensing mechanism based on the reversible formation of lanthanum fluorocarbonate on the surface of solid electrolyte layer is suggested. References [1] www.dynament.com [2] A.E.Varfolomeev, I.R.Shandova, A.S.Lagutin, A.A.Vasiliev, A.V.Pisareva, A.V.Levchenko, Yu.A.Dobrovolskiy, Sensor of Carbon Dioxide Based on MIS Structure with Solid Electrolyte Layer, Procedia Engineering, 47 (2012) 170-173. [3] L.Bartolomaus, A.A.Vasiliev, W.Moritz. Semiconductor sensors for fluorine detection - Optimization for low and high concentrations, Sensors and Actuators B, 65 (2000) 270-272. [4] A.Haensch, D.Koziej, M.Niederberger, N.Barsan, U.Weimar, Rare earth oxycarbonates as a material class for chemoresistive CO2 gas sensors, Procedia Engineering, 5 (2010) 139-142. [5] A.Haensch, D.Borowski, N.Barsan, D.Koziej, M.Niederberger, U.Weimar, Faster response times of rare-earth oxycarbonate based CO2 sensors and another readout strategy for real-world applications, Procedia Engineering, 25 (2011) 1429-1432. [6] S. Linke, M. Dallmer, R. Werner, W. Moritz, Low energy hydrogen sensor, International Journal of Hydrogen Energy 37 (2012) 1752317528.