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High temperature Ga2O3-gas sensors and SnO2-gas sensors: a comparison
Sensors and Actuators B 78 (2001) 6±11
High temperature Ga2O3-gas sensors and SnO2-gas sensors: a comparison Ulrich Hoefera,*, Joachim Frankb, Maximilian Fleischerb a
b
STEINEL AG, Allmeindstrasse 10, CH-8840 Einsiedeln, Switzerland Department ZT MS 2, Corporate Research and Development, Siemens AG, Otto-Hahn-Ring 6, D-81739 Munich, Germany
Abstract High temperature Ga2O3-gas sensors show some differences to other sensors based on other metal oxides like SnO2. Among the advantages of Ga2O3-based sensors good long-term stability, fast response and recovery times, good reproducibility, low cross sensitivity to humidity and short pre-ageing times have to be mentioned. The good stability in sulphur-containing atmospheres makes them suitable for use in domestic burner controls. On the other hand, is the high operation temperature of about 600±8008C which means that power consumption is comparably high (<1 W). The sensitivity of Ga2O3-sensors to CO and NO2 is lower compared to other metal oxide-based sensors. In this article measurements are presented in order to compare the performances of high temperature Ga2O3-gas sensors and SnO2sensors. Electronic conductivity models are proposed explaining the above mentioned differences and recommendations for the different application areas are given. # 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The commercial success of most metal oxide sensors is limited by disadvantageous properties like unintended cross sensitivities, drift, changing sensitivities in time and bad reproducibility. Most commercial sensors are based on SnO2. Ga2O3-sensors in comparison show the potential to overcome some of the disadvantages mentioned above. This kind of semiconductor gas sensors is operated at high temperatures of about 600±8008C. Ga2O3-sensors show faster response and recovery times and lower cross sensitivity to humidity than SnO2-based sensors. Additionally, Ga2O3based sensors show stable long-term sensor properties and good reproducibility even in sulphur-containing exhaust gases like it is the case for domestic fuel oil burners. Additionally, no pre-ageing is necessary (compared to SnO2). In this work a comparison of commercially available Ga2O3 and SnO2 sensors concerning parameters like sensitivity, selectivity, long-term stability, response and recovery time will be presented. Different conductivity models explaining the different sensor properties will be proposed. 2. Sensor mechanism In general, the change in resistance of metal oxide gas sensors when exposed to different atmospheres is due to a * Corresponding author. E-mail address: [email protected] (U. Hoefer).
charge carrier exchange of adsorbed gas with the oxide surface. While the material is exposed to oxygen-containing atmospheres, negatively charged oxygen ions are adsorbed on the semiconductor surface. Acting as an acceptor this leads to a depletion region near the surface. The width of the depletion layer which is caused by the adsorbed oxygen ions is determined by the Debye length lD (material parameter). The reaction of molecules with the O -adsorbates results in a change of acceptor concentration at the surface. The change of resistance then is a consequence of the change of free charge carrier concentration in the depletion region (see Fig. 1). A second possible mechanism is the reaction of the gas molecules with lattice oxygen leading to oxygen vacancies acting as donors (n-type semiconductor). 3. Conductivity models of SnO2 Depending on the different deposition techniques and subsequent sintering procedures leading to different morphologies two main conductivity models can be proposed for SnO2-sensors. The resistance of polycrystalline SnO2-sensors is controlled by changing the barrier height of the grain boundaries. Fig. 1 shows the electronic band model for grain boundary controlled conduction as it is the case for grain diameters larger than the Debye length. The sensor properties are strongly dependent on the crystallite size relative to the Debye length and the grade of sintering as discussed by Yamazoe and Miura [1]. Grain boundary
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 7 8 4 - 5
U. Hoefer et al. / Sensors and Actuators B 78 (2001) 6±11
Fig. 1. Conductivity model of SnO2: (a) in air; (b) during exposure to CO (Debye length lD ! grain diameter); and (c) conduction path after strong sintering.
controlled sensors like SnO2, ZnO and TiO2, therefore, are very sensitive to changes in production parameters. For these reasons the preparation parameters have to be kept in extremely narrow tolerances. Additionally to the initial product spread interdiffusion, change of the metal/semiconductor interface, change of crystallite size and irreversible reactions with the gas phase (SO2, Cl2) during operation can lead to long-term drift and poor reproducibility of sensitivity in time. SnO2 is an n-type semiconductor. In many applications the metal oxide is contacted with Pt-electrodes. The work function of SnO2 amounts to about 4.7 eV [2]. Since Pt has a higher work function of about 5.7 eV, the oxide/metal contact interface can show a Schottky-diode like behaviour and, therefore, can in¯uence or even dominate the sensor performance. With the use of a sophisticated asymmetrical contact geometry and well sintered SnO2-®lms which were observed to reveal a quasi monocrystalline behaviour (where electrical potential barriers do not occur), highly sensitive NO2-sensor devices could be realised [3]. Other ®eld effectbased SnO2-gas sensor devices were presented in [4]. 4. Conduction model of Ga2O3 The conductivity mechanism of Ga2O3 had been investigated by in situ high-temperature Hall measurements to
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separate the behaviour of carrier density and carrier mobility, both in¯uencing the overall electronic conductivity which is the sensor output. In this work polycrystalline and single crystalline material had compared [5]. The main result was, that the carrier mobility is almost the same for both types of specimens. This means that the carrier mobility is not in¯uenced by the grain boundaries, which in lowtemperature gas sensing materials are dif®cult to reproduce and always tend to change due to sintering effects in operation of the sensors. The carrier mobility is directly determined by the crystal lattice Ð a property which is highly reproducible. The thermally activated behaviour of the carrier mobility points to a hopping type conduction mechanism (small polaron). As a result, a very reproducible sensor base resistance is observed in the application of the sensors. The gas sensitivity of the electrical conductivity arises due to gas-induced changes of the carrier density by a change of the crystal defect equilibrium above 8008C or by surface effects involving surface reactions or charged chemisorption [6]. The three gas sensitive mechanisms are depicted in Fig. 2. Additionally, the high operation temperature ensures the fast burn out of organic deposits and the desorption of most adsorbates, giving rise to a ``self cleaning'' effect of the sensor surface. It can be assumed that these material properties are responsible for the very good reproducibility of
Fig. 2. Mechanisms governing the reaction between an n-type semiconducting Ga2O3 and the surrounding atmosphere: direct chemisorption of the reducing gas (top), surface reactions which lead to the formation of oxygen defects located at the surface of the metal oxide (center) and change in the crystal defect equilibrium (bottom) (an oxygen vacancy in the inner part of the lattice is formed by an oxygen atom leaving the crystal at the surface and position changes). Me represents a metal atom at its regular lattice site, O0 an oxygen atom at its regular lattice site and V0 an oxygen vacancy.
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ground resistance, sensitivity and selectivity and for good stability compared to SnO2-sensors. Additionally, the cross sensitivity to humidity is small (see Fig. 5). The Debye length of Ga2O3 in the temperature range around 7508C extents over several mm. A possible consequence is the poor sensitivity to electrophilic adsorbates as NO2. As it can be assumed that the Ga2O3-sheet (thickness 2 mm) is completely depleted in air the exposure to electrophilic NO2 acting as additional acceptor has only a small in¯uence on the charge carrier concentration. Additionally, it can be assumed that the above mentioned lack of grain boundary effects will lead to a lower overall sensitivity of Ga2O3 compared to grain boundary controlled sensors [7]. 5. Experimental The investigations of the different sensor properties of Ga2O3 and SnO2 were focused on product spread (with respect to ground resistance and sensitivity); burn-in time (time a sensor has to be operated until a measurement can be started); long-term stability of sensitivity and ground resistance; response and recovery time (settling time and time after gas exposure until the sensor reaches its initial ground resistance). The measurements were performed with commercially available SnO2 Figaro sensors (TGS 800 air quality; TGS 812 CO, propane, butane; TGS 842 CH4) and Ga2O3-sensors which have just been commercialised (STEINEL SGAS 2000, ethanol; SGAS 2100, CO, CH4). The SGAS 2000 is equipped with a 2 mm Ga2O3-®lm, whereas the SGAS 2100 possesses an additional ®lter sheet which is deposited on the Ga2O3. The SGAS 2100 is nearly insensitive to changes in ambient humidity and shows only poor cross sensitivity to ethanol. With respect to their ground resistance SnO2-based sensors can show considerable product spread. Fig. 3 shows a comparison between Ga2O3-sensor and SnO2sensor with respect to their ground resistances and longterm stability in laboratory air. The initial ground resistances of the three tested SnO2-TGS 812-sensors vary from 87±175 kO (100% difference) whereas the maximum difference in ground resistance of the tested Ga2O3-SGAS 2000sensors amounts to 20%. It can be assumed that small differences in grain boundary morphologies of the SnO2 material, which cannot totally be controlled during production process are responsible for comparably huge differences in ground resistance. The conduction mechanism of Ga2O3sensors is not controlled by grain boundaries. In this case the ground resistances are within a comparably narrow tolerance interval.
Fig. 3. Reproducibility and stability of ground resistance. TGS 812-SnO2 (above) and Ga2O3 (SGAS 2000 at 7508C operation temperature, below).
SnO2-sensors are operated at temperatures around 150± 3508C whereas the operation temperature of Ga2O3-sensors lies around 600±8508C. This fact and the bulk-like conduction mechanism of Ga2O3 is responsible for the comparably short pre-ageing and settling times. Fig. 4 shows the result of ageing experiments with two SnO2-sensors and two Ga2O3sensors. One of each sensor type was aged for 5 months and then was compared with new sensors. For the case of Ga2O3 the time until the ground resistance reaches a stable resistance level is not dependent on the pre-ageing time. Both sensors, whether pre-aged or new are stable within 20 min. For the case of SnO2 the time until a measurement can be started is strongly dependent on the pre-ageing time. A new sensor is not stable within several hours. A TGS 812 which was pre-aged for a period of 5 months is stable after 90 min. In this measurement the sensors were operated in a gas measurement chamber which itself is thermal inert. When operated outside the chamber in laboratory air the settling times of both kinds of sensors are shorter by a factor of about 1/7. After the pre-ageing period the sensors were exposed to 5000 ppm methane. Within the testing period all sensors showed reproducible sensitivities. It can be assumed that the comparably long periods until SnO2-sensors are stable are due to interdiffusion near the contact region and/or changes
U. Hoefer et al. / Sensors and Actuators B 78 (2001) 6±11
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of barrier heights between grain boundaries which in our case did not affect the sensitivities. The comparably high operation temperature of Ga2O3sensors is presumably responsible for the low humidity cross sensitivity of Ga2O3-sensors as can be seen in Fig. 5. In this measurement the relative humidity was varied from 30 to 75%. It can be assumed that OH-binding forces are too small at operation temperatures around 7508C and, thus, no pronounced signal change can be observed at the SGAS 2100. This sensor is equipped with a porous Ga2O3 catalyst ®lter which is responsible for the very poor humidity cross sensitivity. Without ®lter (SGAS 2000) the cross sensitivity is slightly higher. For the case of SnO2 where the operation temperature is lower a pronounced resistance increase during humidity change from 530±900 kO (84%) has been observed. In general Ga2O3-sensors show faster recovery times as can be seen in Fig. 6. It can be assumed that the higher operation temperature leads to a faster desorbtion of adsorbates on the sensor surface. The cross sensitivity of the TGS 800 to a change in relative humidity simulates different sensitivities to the target gas.
Fig. 4. Transient behaviour of new and aged Ga2O3 (SGAS 2100; operation temperature 7508C) and SnO2-sensors (TGS 812). Sensitivity to methane.
Fig. 5. Cross sensitivity to humidity of a TGS 800 and of two different types of Ga2O3-sensors (SGAS).
Fig. 6. Ga2O3-sensors operated at 7508C show lower cross sensitivity to humidity and a faster recovery time than SnO2-based sensors.
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U. Hoefer et al. / Sensors and Actuators B 78 (2001) 6±11
Ga2O3-sensors reach stable ground resistances after applying the heater operation voltage within a couple of minutes whether they are new or pre-aged. Ga2O3-sensors show a very small humidity cross sensitivity. Ga2O3-sensors are stable in atmospheres with low oxygen concentrations. This makes them suitable for use in burner control units (exhaust gas). Due to the high operation temperature Ga2O3-sensors show faster recovery times than SnO2-sensors. No cleaning cycles are necessary and smut or other organic residues are burnt-off. Possible advantages of SnO2 are sensor material are comparably low power consumption due to lower operation temperature; SnO2-sensors show higher sensitivity to a number of gases. References
Fig. 7. The TGS 800-sensor shows considerable drift when exposed to higher CO-concentrations (100±1500 ppm), but also a higher sensitivity than Ga2O3-sensors.
When exposed to CO concentrations between 100 and 1500 ppm the TGS 800 SnO2-sensor shows drift effects. Fig. 7 shows the result of an according measurement. This effect can be explained if we assume that the drift is due to different time constants of the reaction of CO not only with surface oxygen but also with bulk oxygen. The reaction with bulk oxygen affects the ground resistance with a longer time constant whereas the surface reaction is responsible for the faster resistance decrease during CO exposure. 6. Conclusions Ga2O3-based sensors show the potential to overcome some of the disadvantages of SnO2-based sensors which were the reason for the limitation of applications. In this work, recently commercialised STEINEL Ga2O3-sensors were compared with well introduced Figaro SnO2-sensors. Ga2O3-sensors show lower product spread with respect to their ground resistance. If the reproducibility of ground resistance (and sensitivity) will be good enough it will not be necessary to perform time consuming calibration procedures in future sensor systems using this kind of gas sensors like for example natural gas detectors.
[1] N. Yamazoe, N. Miura, Some basic aspects of semiconductor gas sensors, in: S. Yamauchi (Ed.), Chemical Sensor Technology, Vol. 4, Kodansha Ltd., Tokyo, 1992. [2] Landolt-BoÈrnstein, in: O. Madelung (Ed.), Zahlenwerte und Funktionen aus Naturwissenschaften und Technik, Gruppe III: Kristallund FestkoÈrperphysik, Band 17 Halbleiter, Teilband f, Physik der nicht-tetraedisch gebundenen Verbindungen II, Springer, Berlin, 1983. [3] U. Hoefer, H. BoÈttner, E. Wagner, D. Kohl, Highly sensitive NO2 sensor device featuring a JFET-like transducer mechanism, Sens. Actuat. B 47 (1/3) (1998) 212±216. [4] J. WoÈllenstein, M. Scheulin, M. Jaegle, H. BoÈttner, W.J. Becker, Metal oxide channel thin film transistor, a novel sensor type featuring gate voltage selectivity control, in: Proceedings of the Eighth International Meeting on Chemical Sensos, Basel, 2000, p. 1999. [5] M. Fleischer, H. Meixner, Electron mobility in single and polycrystalline Ga2O3, J. Appl. Phys. 74 (1) (1993) 300±305. [6] M. Fleischer, H. Meixner, Thin film gas sensors based on high temperature operated metal oxides, J. Vac. Sci. Technol. A 17 (45) (1999) 1866±1872. [7] C. Krummel, Charakterisierung der OberflaÈchenreaktionen von CH4 und H2 auf Ga2O3, Dissertation, University of Giessen, 1998.
Biographies Ulrich Hoefer was born in Darmstadt, Germany, in 1966. In 1991 he joined the Fraunhofer Institute of Physical Measurement Techniques in Freiburg i.Br. Germany, where he has been engaged in the development and application of thin film SnO2-gas sensors. There his special interest included the investigation of the influence of metal electrodes on the performance of metal oxide gas sensors. In 1994 he received the diploma in physics from the Albert-Ludwigs-University of Freiburg i.Br. and in 1997 he received the Dr. rer. nat. from the University of Giessen. He has been engaged in several national and European projects. Since 1999 he is head of the gas sensor department at STEINEL AG in Switzerland where he is currently introducing a Ga2O3-gas sensor production line. Joachim Frank was born in Eisfeld, Germany, in 1968. He studied solid state electronics at the Technische UniversitaÈt Ilmenau and at the ETH ZuÈrich as well as business administration at the FernUniversitaÈt Hagen.
U. Hoefer et al. / Sensors and Actuators B 78 (2001) 6±11 After being with the former Siemens semiconductor group in Villach (Austria) he joined the research laboratories of Siemens AG in Munich in 1995. In 1999 he received the Dr.-Ing. from the Technische UniversitaÈt Ilmenau. His current interests are the development of new gas sensors and sensor systems for household and industrial applications. Maximilian Fleischer was born in Munich on 7 May 1961. He received his doctoral degree in Physics from the Technical University in Munich in 1992. Since 1992 he is in the employee of the Corporate R&D of Siemens AG and is engaged in the development of new type of gas sensors based on high temperature stable semiconducting metal oxides. Since 1996 he is the
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responsible project manager for the gas project group. In 1998 he received the Dr. habil. from the Technical University of Budapest for work about gas/metaloxide interactions. His research interests include new types of semiconducting metal oxides for gas sensors useable in domestic, automotive and industrial applications, usage of thin and thick film technology for their preparation, surface chemistry and modifications, catalytic gas filtering for the realization of selective gas sensors, application of laser diodes for selective gas sensors, application of work function methods for the realisation of low power gas FETs and ultrasonic motors. He has been engaged in National and European research projects and collaborations with German and European Universities.
High temperature Ga2O3-gas sensors and SnO2-gas sensors: a comparison Ulrich Hoefera,*, Joachim Frankb, Maximilian Fleischerb a
b
STEINEL AG, Allmeindstrasse 10, CH-8840 Einsiedeln, Switzerland Department ZT MS 2, Corporate Research and Development, Siemens AG, Otto-Hahn-Ring 6, D-81739 Munich, Germany
Abstract High temperature Ga2O3-gas sensors show some differences to other sensors based on other metal oxides like SnO2. Among the advantages of Ga2O3-based sensors good long-term stability, fast response and recovery times, good reproducibility, low cross sensitivity to humidity and short pre-ageing times have to be mentioned. The good stability in sulphur-containing atmospheres makes them suitable for use in domestic burner controls. On the other hand, is the high operation temperature of about 600±8008C which means that power consumption is comparably high (<1 W). The sensitivity of Ga2O3-sensors to CO and NO2 is lower compared to other metal oxide-based sensors. In this article measurements are presented in order to compare the performances of high temperature Ga2O3-gas sensors and SnO2sensors. Electronic conductivity models are proposed explaining the above mentioned differences and recommendations for the different application areas are given. # 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The commercial success of most metal oxide sensors is limited by disadvantageous properties like unintended cross sensitivities, drift, changing sensitivities in time and bad reproducibility. Most commercial sensors are based on SnO2. Ga2O3-sensors in comparison show the potential to overcome some of the disadvantages mentioned above. This kind of semiconductor gas sensors is operated at high temperatures of about 600±8008C. Ga2O3-sensors show faster response and recovery times and lower cross sensitivity to humidity than SnO2-based sensors. Additionally, Ga2O3based sensors show stable long-term sensor properties and good reproducibility even in sulphur-containing exhaust gases like it is the case for domestic fuel oil burners. Additionally, no pre-ageing is necessary (compared to SnO2). In this work a comparison of commercially available Ga2O3 and SnO2 sensors concerning parameters like sensitivity, selectivity, long-term stability, response and recovery time will be presented. Different conductivity models explaining the different sensor properties will be proposed. 2. Sensor mechanism In general, the change in resistance of metal oxide gas sensors when exposed to different atmospheres is due to a * Corresponding author. E-mail address: [email protected] (U. Hoefer).
charge carrier exchange of adsorbed gas with the oxide surface. While the material is exposed to oxygen-containing atmospheres, negatively charged oxygen ions are adsorbed on the semiconductor surface. Acting as an acceptor this leads to a depletion region near the surface. The width of the depletion layer which is caused by the adsorbed oxygen ions is determined by the Debye length lD (material parameter). The reaction of molecules with the O -adsorbates results in a change of acceptor concentration at the surface. The change of resistance then is a consequence of the change of free charge carrier concentration in the depletion region (see Fig. 1). A second possible mechanism is the reaction of the gas molecules with lattice oxygen leading to oxygen vacancies acting as donors (n-type semiconductor). 3. Conductivity models of SnO2 Depending on the different deposition techniques and subsequent sintering procedures leading to different morphologies two main conductivity models can be proposed for SnO2-sensors. The resistance of polycrystalline SnO2-sensors is controlled by changing the barrier height of the grain boundaries. Fig. 1 shows the electronic band model for grain boundary controlled conduction as it is the case for grain diameters larger than the Debye length. The sensor properties are strongly dependent on the crystallite size relative to the Debye length and the grade of sintering as discussed by Yamazoe and Miura [1]. Grain boundary
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 7 8 4 - 5
U. Hoefer et al. / Sensors and Actuators B 78 (2001) 6±11
Fig. 1. Conductivity model of SnO2: (a) in air; (b) during exposure to CO (Debye length lD ! grain diameter); and (c) conduction path after strong sintering.
controlled sensors like SnO2, ZnO and TiO2, therefore, are very sensitive to changes in production parameters. For these reasons the preparation parameters have to be kept in extremely narrow tolerances. Additionally to the initial product spread interdiffusion, change of the metal/semiconductor interface, change of crystallite size and irreversible reactions with the gas phase (SO2, Cl2) during operation can lead to long-term drift and poor reproducibility of sensitivity in time. SnO2 is an n-type semiconductor. In many applications the metal oxide is contacted with Pt-electrodes. The work function of SnO2 amounts to about 4.7 eV [2]. Since Pt has a higher work function of about 5.7 eV, the oxide/metal contact interface can show a Schottky-diode like behaviour and, therefore, can in¯uence or even dominate the sensor performance. With the use of a sophisticated asymmetrical contact geometry and well sintered SnO2-®lms which were observed to reveal a quasi monocrystalline behaviour (where electrical potential barriers do not occur), highly sensitive NO2-sensor devices could be realised [3]. Other ®eld effectbased SnO2-gas sensor devices were presented in [4]. 4. Conduction model of Ga2O3 The conductivity mechanism of Ga2O3 had been investigated by in situ high-temperature Hall measurements to
7
separate the behaviour of carrier density and carrier mobility, both in¯uencing the overall electronic conductivity which is the sensor output. In this work polycrystalline and single crystalline material had compared [5]. The main result was, that the carrier mobility is almost the same for both types of specimens. This means that the carrier mobility is not in¯uenced by the grain boundaries, which in lowtemperature gas sensing materials are dif®cult to reproduce and always tend to change due to sintering effects in operation of the sensors. The carrier mobility is directly determined by the crystal lattice Ð a property which is highly reproducible. The thermally activated behaviour of the carrier mobility points to a hopping type conduction mechanism (small polaron). As a result, a very reproducible sensor base resistance is observed in the application of the sensors. The gas sensitivity of the electrical conductivity arises due to gas-induced changes of the carrier density by a change of the crystal defect equilibrium above 8008C or by surface effects involving surface reactions or charged chemisorption [6]. The three gas sensitive mechanisms are depicted in Fig. 2. Additionally, the high operation temperature ensures the fast burn out of organic deposits and the desorption of most adsorbates, giving rise to a ``self cleaning'' effect of the sensor surface. It can be assumed that these material properties are responsible for the very good reproducibility of
Fig. 2. Mechanisms governing the reaction between an n-type semiconducting Ga2O3 and the surrounding atmosphere: direct chemisorption of the reducing gas (top), surface reactions which lead to the formation of oxygen defects located at the surface of the metal oxide (center) and change in the crystal defect equilibrium (bottom) (an oxygen vacancy in the inner part of the lattice is formed by an oxygen atom leaving the crystal at the surface and position changes). Me represents a metal atom at its regular lattice site, O0 an oxygen atom at its regular lattice site and V0 an oxygen vacancy.
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ground resistance, sensitivity and selectivity and for good stability compared to SnO2-sensors. Additionally, the cross sensitivity to humidity is small (see Fig. 5). The Debye length of Ga2O3 in the temperature range around 7508C extents over several mm. A possible consequence is the poor sensitivity to electrophilic adsorbates as NO2. As it can be assumed that the Ga2O3-sheet (thickness 2 mm) is completely depleted in air the exposure to electrophilic NO2 acting as additional acceptor has only a small in¯uence on the charge carrier concentration. Additionally, it can be assumed that the above mentioned lack of grain boundary effects will lead to a lower overall sensitivity of Ga2O3 compared to grain boundary controlled sensors [7]. 5. Experimental The investigations of the different sensor properties of Ga2O3 and SnO2 were focused on product spread (with respect to ground resistance and sensitivity); burn-in time (time a sensor has to be operated until a measurement can be started); long-term stability of sensitivity and ground resistance; response and recovery time (settling time and time after gas exposure until the sensor reaches its initial ground resistance). The measurements were performed with commercially available SnO2 Figaro sensors (TGS 800 air quality; TGS 812 CO, propane, butane; TGS 842 CH4) and Ga2O3-sensors which have just been commercialised (STEINEL SGAS 2000, ethanol; SGAS 2100, CO, CH4). The SGAS 2000 is equipped with a 2 mm Ga2O3-®lm, whereas the SGAS 2100 possesses an additional ®lter sheet which is deposited on the Ga2O3. The SGAS 2100 is nearly insensitive to changes in ambient humidity and shows only poor cross sensitivity to ethanol. With respect to their ground resistance SnO2-based sensors can show considerable product spread. Fig. 3 shows a comparison between Ga2O3-sensor and SnO2sensor with respect to their ground resistances and longterm stability in laboratory air. The initial ground resistances of the three tested SnO2-TGS 812-sensors vary from 87±175 kO (100% difference) whereas the maximum difference in ground resistance of the tested Ga2O3-SGAS 2000sensors amounts to 20%. It can be assumed that small differences in grain boundary morphologies of the SnO2 material, which cannot totally be controlled during production process are responsible for comparably huge differences in ground resistance. The conduction mechanism of Ga2O3sensors is not controlled by grain boundaries. In this case the ground resistances are within a comparably narrow tolerance interval.
Fig. 3. Reproducibility and stability of ground resistance. TGS 812-SnO2 (above) and Ga2O3 (SGAS 2000 at 7508C operation temperature, below).
SnO2-sensors are operated at temperatures around 150± 3508C whereas the operation temperature of Ga2O3-sensors lies around 600±8508C. This fact and the bulk-like conduction mechanism of Ga2O3 is responsible for the comparably short pre-ageing and settling times. Fig. 4 shows the result of ageing experiments with two SnO2-sensors and two Ga2O3sensors. One of each sensor type was aged for 5 months and then was compared with new sensors. For the case of Ga2O3 the time until the ground resistance reaches a stable resistance level is not dependent on the pre-ageing time. Both sensors, whether pre-aged or new are stable within 20 min. For the case of SnO2 the time until a measurement can be started is strongly dependent on the pre-ageing time. A new sensor is not stable within several hours. A TGS 812 which was pre-aged for a period of 5 months is stable after 90 min. In this measurement the sensors were operated in a gas measurement chamber which itself is thermal inert. When operated outside the chamber in laboratory air the settling times of both kinds of sensors are shorter by a factor of about 1/7. After the pre-ageing period the sensors were exposed to 5000 ppm methane. Within the testing period all sensors showed reproducible sensitivities. It can be assumed that the comparably long periods until SnO2-sensors are stable are due to interdiffusion near the contact region and/or changes
U. Hoefer et al. / Sensors and Actuators B 78 (2001) 6±11
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of barrier heights between grain boundaries which in our case did not affect the sensitivities. The comparably high operation temperature of Ga2O3sensors is presumably responsible for the low humidity cross sensitivity of Ga2O3-sensors as can be seen in Fig. 5. In this measurement the relative humidity was varied from 30 to 75%. It can be assumed that OH-binding forces are too small at operation temperatures around 7508C and, thus, no pronounced signal change can be observed at the SGAS 2100. This sensor is equipped with a porous Ga2O3 catalyst ®lter which is responsible for the very poor humidity cross sensitivity. Without ®lter (SGAS 2000) the cross sensitivity is slightly higher. For the case of SnO2 where the operation temperature is lower a pronounced resistance increase during humidity change from 530±900 kO (84%) has been observed. In general Ga2O3-sensors show faster recovery times as can be seen in Fig. 6. It can be assumed that the higher operation temperature leads to a faster desorbtion of adsorbates on the sensor surface. The cross sensitivity of the TGS 800 to a change in relative humidity simulates different sensitivities to the target gas.
Fig. 4. Transient behaviour of new and aged Ga2O3 (SGAS 2100; operation temperature 7508C) and SnO2-sensors (TGS 812). Sensitivity to methane.
Fig. 5. Cross sensitivity to humidity of a TGS 800 and of two different types of Ga2O3-sensors (SGAS).
Fig. 6. Ga2O3-sensors operated at 7508C show lower cross sensitivity to humidity and a faster recovery time than SnO2-based sensors.
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U. Hoefer et al. / Sensors and Actuators B 78 (2001) 6±11
Ga2O3-sensors reach stable ground resistances after applying the heater operation voltage within a couple of minutes whether they are new or pre-aged. Ga2O3-sensors show a very small humidity cross sensitivity. Ga2O3-sensors are stable in atmospheres with low oxygen concentrations. This makes them suitable for use in burner control units (exhaust gas). Due to the high operation temperature Ga2O3-sensors show faster recovery times than SnO2-sensors. No cleaning cycles are necessary and smut or other organic residues are burnt-off. Possible advantages of SnO2 are sensor material are comparably low power consumption due to lower operation temperature; SnO2-sensors show higher sensitivity to a number of gases. References
Fig. 7. The TGS 800-sensor shows considerable drift when exposed to higher CO-concentrations (100±1500 ppm), but also a higher sensitivity than Ga2O3-sensors.
When exposed to CO concentrations between 100 and 1500 ppm the TGS 800 SnO2-sensor shows drift effects. Fig. 7 shows the result of an according measurement. This effect can be explained if we assume that the drift is due to different time constants of the reaction of CO not only with surface oxygen but also with bulk oxygen. The reaction with bulk oxygen affects the ground resistance with a longer time constant whereas the surface reaction is responsible for the faster resistance decrease during CO exposure. 6. Conclusions Ga2O3-based sensors show the potential to overcome some of the disadvantages of SnO2-based sensors which were the reason for the limitation of applications. In this work, recently commercialised STEINEL Ga2O3-sensors were compared with well introduced Figaro SnO2-sensors. Ga2O3-sensors show lower product spread with respect to their ground resistance. If the reproducibility of ground resistance (and sensitivity) will be good enough it will not be necessary to perform time consuming calibration procedures in future sensor systems using this kind of gas sensors like for example natural gas detectors.
[1] N. Yamazoe, N. Miura, Some basic aspects of semiconductor gas sensors, in: S. Yamauchi (Ed.), Chemical Sensor Technology, Vol. 4, Kodansha Ltd., Tokyo, 1992. [2] Landolt-BoÈrnstein, in: O. Madelung (Ed.), Zahlenwerte und Funktionen aus Naturwissenschaften und Technik, Gruppe III: Kristallund FestkoÈrperphysik, Band 17 Halbleiter, Teilband f, Physik der nicht-tetraedisch gebundenen Verbindungen II, Springer, Berlin, 1983. [3] U. Hoefer, H. BoÈttner, E. Wagner, D. Kohl, Highly sensitive NO2 sensor device featuring a JFET-like transducer mechanism, Sens. Actuat. B 47 (1/3) (1998) 212±216. [4] J. WoÈllenstein, M. Scheulin, M. Jaegle, H. BoÈttner, W.J. Becker, Metal oxide channel thin film transistor, a novel sensor type featuring gate voltage selectivity control, in: Proceedings of the Eighth International Meeting on Chemical Sensos, Basel, 2000, p. 1999. [5] M. Fleischer, H. Meixner, Electron mobility in single and polycrystalline Ga2O3, J. Appl. Phys. 74 (1) (1993) 300±305. [6] M. Fleischer, H. Meixner, Thin film gas sensors based on high temperature operated metal oxides, J. Vac. Sci. Technol. A 17 (45) (1999) 1866±1872. [7] C. Krummel, Charakterisierung der OberflaÈchenreaktionen von CH4 und H2 auf Ga2O3, Dissertation, University of Giessen, 1998.
Biographies Ulrich Hoefer was born in Darmstadt, Germany, in 1966. In 1991 he joined the Fraunhofer Institute of Physical Measurement Techniques in Freiburg i.Br. Germany, where he has been engaged in the development and application of thin film SnO2-gas sensors. There his special interest included the investigation of the influence of metal electrodes on the performance of metal oxide gas sensors. In 1994 he received the diploma in physics from the Albert-Ludwigs-University of Freiburg i.Br. and in 1997 he received the Dr. rer. nat. from the University of Giessen. He has been engaged in several national and European projects. Since 1999 he is head of the gas sensor department at STEINEL AG in Switzerland where he is currently introducing a Ga2O3-gas sensor production line. Joachim Frank was born in Eisfeld, Germany, in 1968. He studied solid state electronics at the Technische UniversitaÈt Ilmenau and at the ETH ZuÈrich as well as business administration at the FernUniversitaÈt Hagen.
U. Hoefer et al. / Sensors and Actuators B 78 (2001) 6±11 After being with the former Siemens semiconductor group in Villach (Austria) he joined the research laboratories of Siemens AG in Munich in 1995. In 1999 he received the Dr.-Ing. from the Technische UniversitaÈt Ilmenau. His current interests are the development of new gas sensors and sensor systems for household and industrial applications. Maximilian Fleischer was born in Munich on 7 May 1961. He received his doctoral degree in Physics from the Technical University in Munich in 1992. Since 1992 he is in the employee of the Corporate R&D of Siemens AG and is engaged in the development of new type of gas sensors based on high temperature stable semiconducting metal oxides. Since 1996 he is the
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responsible project manager for the gas project group. In 1998 he received the Dr. habil. from the Technical University of Budapest for work about gas/metaloxide interactions. His research interests include new types of semiconducting metal oxides for gas sensors useable in domestic, automotive and industrial applications, usage of thin and thick film technology for their preparation, surface chemistry and modifications, catalytic gas filtering for the realization of selective gas sensors, application of laser diodes for selective gas sensors, application of work function methods for the realisation of low power gas FETs and ultrasonic motors. He has been engaged in National and European research projects and collaborations with German and European Universities.