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CO-Sensor for domestic use based on high temperature stable Ga2O3 thin films
Sensors and Actuators B 49 (1998) 46 – 51
CO-Sensor for domestic use based on high temperature stable Ga2O3 thin films T. Schwebel a, M. Fleischer b,*, H. Meixner b, C.-D. Kohl a b
a Uni6ersity of Gießen, Heinrich-Buff-Ring 16, 35392 Gießen, Germany Siemens AG, Corporate Technology, Otto-Hahn-Ring 6, D-81739 Munich, Germany
Abstract Gas sensors based on high temperature operated metal oxides, like Ga2O3 thin films show promising properties in terms of reproducibility, long-term stability against interfering gases and low cross sensitivity to humidity. In this paper a surface modification of Ga2O3 is presented which allows the set up of a sensor suitable for indoor CO monitoring. The modification based on Au-clusters on the Ga2O3 surface yields high sensitivity to CO and a distinct reduction of the cross sensitivity towards organic solvents. With the specimens, a resistance change of approx. factor 4 to 100 ppm CO in wet air is attained. By employing catalytic filters of ceramic material, cross sensitivities to organic solvents are virtually completely eliminated. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Gas sensor; CO-detection; Ga2O3
1. Introduction
2. Experimental
In recent times, thin films of n-type semiconducting Ga2O3 have been developed as a new sensor base material for the detection of reducing gases [1]. This sensor base material offers advantages with respect to reproducibility and robustness due to its high operating temperatures and a conduction mechanism which is independent of grain boundaries [2]. The base sensor material Ga2O3 strongly responds to hydrocarbons like CH4, C3H8 and organic solvents. However, sensitivity to CO is one order of magnitude lower [3]. A study using surface modification of the sensor material by addition of other metal oxides was started recently with the intention of broadening the range of applications for Ga2O3 [4]. In contrast to that earlier work, this paper shows results obtained with a modification of the sensor films using a gold dispersion. Similar to the case of ‘low temperature’ operated oxides like SnO2 [5], the gold selectively enhances CO sensitivity.
Polycrystalline Ga2O3 thin films with a thickness of 2 mm were sputter deposited on sensor substrates of Al2O3 which had been covered by a SiO2 anti-diffusion barrier. Afterwards, a heat treatment of 10 h/1050°C was applied, which leads to a polycrystalline structure of the Ga2O3 film with typical crystallite sizes of 50– 100 nm [6]. The sensor chips were equipped with a meander-type heater for thermostatisation at possible operating temperatures of 500–950°C and interdigital electrodes to measure the electrical conductivity of the Ga2O3 thin film (Fig. 1). The chip was suspended by its contact wires between the socket posts. This is to establish a good thermal isolation and optimal gas admission to the sensor chip via convection. The prepared sensor films were modified using two different procedures: In the first, a thin compact layer of gold (typically 30 nm) was deposited using electron beam evaporation. A clustering of the layer was obtained by a subsequent heat treatment at 600–800°C for several hours in laboratory air. In an alternative procedure, a specified volume of a solution of HAuCl4 in methanol was added to the sensor surface with a subsequent heat treatment. The role of methanol as solvent is to ensure a homogeneous distribution of the
* Corresponding author. Tel: + 49 89 63640049; fax: + 49 89 63646881. 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(97)00334-1
T. Schwebel et al. / Sensors and Actuators B 49 (1998) 46–51
47
gold. By this procedure, gold dispersions with cluster sizes between 10 nm and 300 nm and a percentage of covered surface between 10% and 30% were obtained, as imaging in a SEM or AFM reveals in Fig. 2. In general, no burn-in period or temperature cycling was required due to the high operating temperature of the sensor surface, suppressing surface contamination.
Fig. 2. Au-cluster on the Ga2O3 surface (SEM image) deposited by the wet chemical method.
3. Results
3.1. Modification by gold deposition In the first stage, due to the modification, the change of overall conductivity of a Ga2O3 thin film was checked. Both methods were used. The modification by gold deposition led to a slight resistance increase (Fig. 3). Dependent on the preparation parameters, the sensitivity of the modified sensor to CO was strongly enhanced in comparison with the unmodified sensor. An investigation of different operating temperatures showed that the highest sensitivity was obtained at a temperature of 550°C (Fig. 4). An additional increase in CO sensitivity was obtained, when the Ga2O3 thin films donor doped with SnO2 [7] were modified with gold. The maxima of the sensitivity shifted towards lower temperatures.
Fig. 1. Structure of the sensor chip (3 × 3 mm2) (a) first side with contact pads and interdigital electrodes; (b) second side with heater.
Fig. 3. Sensor base resistance in wet air. The open triangles correspond to unmodified sensors, the filled squares to sensors wet chemically modified by gold.
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Fig. 4. Gas sensitivity of pure Ga2O3 thin films, Ga2O3 thin films modified with a Au-dispersion and donor-doped Ga2O3 thin films modified with a gold dispersion to exposure of 5000 ppm CO in wet air for 10 min.
The response behaviour is shown in Fig. 5, with response times (t90) of about 1 min at 550°C. At lower temperatures, longer response times were obtained unfavourable for sensor use. The sensor characteristics of the donor doped specimens modified with gold showed a sensitivity which was high enough to allow detection of CO concentrations in the range of 100 ppm which is a relevant value for homes and garages (Fig. 6). The gold dispersion has been shown to be stable under continuous operation and even at higher temper-
Fig. 5. Transient response of Au-modified sensors operated at 550°C in wet air. The CO concentrations shown in the lower part are 5000, 2000, 1000, 500, 200, 100 ppm.
Fig. 6. Sensor characteristics of gold modified sensors measured in wet air at 600°C. Platinum contact pads not passivated.
atures. The influence of interfering gases relevant for domestic use is shown in Fig. 7. The reproducibility of the sensor characteristics depends on the reproducibility of the structure of the catalyst. Good results have even been obtained using the wet chemical method for the application of the gold (Fig. 8).
3.2. Influence of contact pads With all the results shown previously, the sensitive layer of Ga2O3 covered the interdigital structure shown in Fig. 1a. The contact pads also shown in this figure were covered by a mechanical mask of refractory ce-
Fig. 7. Cross sensitivity of SnO2 doped sensors modified with evaporated gold operated at 550°C towards various gases of interest for CO monitoring in garages and homes; 20/1% O2 denotes a change to 1% O2, 1.2/0.4 an change from 1.2 to 0.4% humidity.
T. Schwebel et al. / Sensors and Actuators B 49 (1998) 46–51
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Fig. 10. Cross-section of a Ga2O3 sensor covered by a gas filter consisting of highly porous Ga2O3. Fig. 8. Reproducibility of two sensors which were modified with gold using wet chemical preparation. The clustering of the gold modification was done for 10 h at 650°C. The operating temperature was 600°C.
ramics during sputtering to ensure an easy welding of the contact wire to the bond pads. However, the open platinum structure displayed strong catalytic activity which may cause a reduction of the concentration of the gases to be detected in the volume near the sensor chip. To eliminate this effect, specimens with contact pads covered by Ga2O3 have been investigated. A comparison of the gas sensitivities of these two types of devices is shown in Fig. 9. A strong increase in CO sensitivity due to the passivation of the pads was attained, which gave a factor 4 resistance change in response to 100 ppm CO.
3.3. Elimination of cross sensiti6ity to sol6ents By covering the sensor structure with a porous layer of a refractory ceramics (especially Al2O3 or even
Fig. 9. Comparison of devices with open and Ga2O3 covered platinum contact pads. The sensor layer is Ga2O3 modified with gold, the operating temperature is 600°C.
Ga2O3; see Fig. 10), the gas diffusing through the ceramics is filtered. Reactive solvents are burnt and only the more stable CO is allowed to reach the sensor surface. Results from an investigation of a Ga2O3 layer covered with a porous filter are shown in Fig. 11. The cross influence of ethanol was nearly completely eliminated with no deterioration of the response behavior. The sensitivity to 5000 ppm CO amounted to about 14 with a very steep transient behaviour. Fig. 10 gives a cross-sectional view of a Ga2O3 thick film layer structure, which may then be modified by Au-clusters. In contrast to an EtOH sensitivity reduction, this led to a strongly enhanced CO sensitivity in comparison with non-gold prepared thick films. To check the role of the Au modification in terms of sensitivity, 5000 ppm CO was applied to both sensor types with gallium oxide filter (gold and non-gold modified). Fig. 12 shows the results for a wide range of temperatures. Non-Au prepared probes yielded a maximum sensitivity of S :6 at 650°C, while Au modified ones markedly increased to a value of S:18 at 600°C. Additional investigations are planned using work func-
Fig. 11. Response of a Au-modified Ga2O3 sensor covered with a porous filter to ethanol and CO (5000, 2000, 1000, 500, 200 ppm) in wet air at 700°C operating temperature.
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Fig. 12. Comparison of CO-sensitivity (to 5000 ppm) as a function of temperature for sensors with gold modified filter and sensors with non-gold modified filter. Error bars were taken from two identically prepared probes.
Fig. 13. Long-term stability of Au modified Ga2O3 filter probes; two identically prepared probes.
tion measurements, which may also be applied as a sensor signal [8].
4. Long-term stability The long-term stability is still under investigation, but measurements after 3 months of constant operation at 700°C gave promising results in terms of resistance stability and sensitivity. Fig. 13 shows the measured signals, with a good correlation of basic resistances and sensitivities (the differences arise from slightly different temperatures).
5. Conclusions The work showed that by using Au-dispersions, a very strong enhancement of the CO sensitivity was attainable, allowing domestic CO detection with robust high-temperature operated metal oxides. Further work will be devoted to detailed investigations of the gold clusters to increase the CO sensitivity further. Using porous ceramic filters, it was possible to suppress crosssensitivities to volatile organic solvents. A temperature of 700°C allowed the fast detection of CO in a matter of seconds. No burn-in time or temperature cycling was necessary to obtain the results shown above.
T. Schwebel et al. / Sensors and Actuators B 49 (1998) 46–51
References [1] M. Fleischer, H. Meixner, Sensing reducing gases at high temperatures using long-term stable Ga22O3 thin films, Sensors and Actuators B 6 – 7 (1992) 257–261. [2] M. Fleischer, H. Meixner, Electron mobility in single and polycrystalline Ga2O3, J. Appl. Phys. 74 (1) (1993) 300–305. [3] M. Fleischer, Fast gas sensors based on metal oxides which are stable at high temperatures, in: Proc. Eurosensors X, Leuven, Belgium, 1996, pp. 25 – 33. [4] M. Seth, M. Fleischer, H. Meixner, C.-D. Kohl, A selective H2-sensor implemented using Ga2O3-thin films which are covered with a gasfiltering SiO2 layer, Sensors and Actuators B 35–36 (1996) 1 – 7.
.
51
[5] K. Fukui, S. Nishida, A selective CO gas sensor based on Au-La2O3 added SnO2 ceramic with siliceous zeolite coat, Sensors and Actuators B 24 – 25 (1996) 486 – 490. [6] M. Fleischer, H. Meixner, Characterization and crystallite growth of semiconducting high-temperature-stable Ga2O3 thin-films, J. Mater. Sci. Lett. 11 (1992) 1728 – 1731. [7] J. Frank, M. Fleischer, H. Meixner, A. Feltz, Enhancement of sensitivity and conductivity of semiconducting Ga2O3 sensors by doping with SnO2, this conference. [8] T. Doll, J. Lechner, I. Eisele, K. Schierbaum, W. Go¨pel, Ozone detection in the ppb-range with workfunction sensors operating at room temperature, Sensors and Actuators B 34 (1996) 506– 510.
CO-Sensor for domestic use based on high temperature stable Ga2O3 thin films T. Schwebel a, M. Fleischer b,*, H. Meixner b, C.-D. Kohl a b
a Uni6ersity of Gießen, Heinrich-Buff-Ring 16, 35392 Gießen, Germany Siemens AG, Corporate Technology, Otto-Hahn-Ring 6, D-81739 Munich, Germany
Abstract Gas sensors based on high temperature operated metal oxides, like Ga2O3 thin films show promising properties in terms of reproducibility, long-term stability against interfering gases and low cross sensitivity to humidity. In this paper a surface modification of Ga2O3 is presented which allows the set up of a sensor suitable for indoor CO monitoring. The modification based on Au-clusters on the Ga2O3 surface yields high sensitivity to CO and a distinct reduction of the cross sensitivity towards organic solvents. With the specimens, a resistance change of approx. factor 4 to 100 ppm CO in wet air is attained. By employing catalytic filters of ceramic material, cross sensitivities to organic solvents are virtually completely eliminated. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Gas sensor; CO-detection; Ga2O3
1. Introduction
2. Experimental
In recent times, thin films of n-type semiconducting Ga2O3 have been developed as a new sensor base material for the detection of reducing gases [1]. This sensor base material offers advantages with respect to reproducibility and robustness due to its high operating temperatures and a conduction mechanism which is independent of grain boundaries [2]. The base sensor material Ga2O3 strongly responds to hydrocarbons like CH4, C3H8 and organic solvents. However, sensitivity to CO is one order of magnitude lower [3]. A study using surface modification of the sensor material by addition of other metal oxides was started recently with the intention of broadening the range of applications for Ga2O3 [4]. In contrast to that earlier work, this paper shows results obtained with a modification of the sensor films using a gold dispersion. Similar to the case of ‘low temperature’ operated oxides like SnO2 [5], the gold selectively enhances CO sensitivity.
Polycrystalline Ga2O3 thin films with a thickness of 2 mm were sputter deposited on sensor substrates of Al2O3 which had been covered by a SiO2 anti-diffusion barrier. Afterwards, a heat treatment of 10 h/1050°C was applied, which leads to a polycrystalline structure of the Ga2O3 film with typical crystallite sizes of 50– 100 nm [6]. The sensor chips were equipped with a meander-type heater for thermostatisation at possible operating temperatures of 500–950°C and interdigital electrodes to measure the electrical conductivity of the Ga2O3 thin film (Fig. 1). The chip was suspended by its contact wires between the socket posts. This is to establish a good thermal isolation and optimal gas admission to the sensor chip via convection. The prepared sensor films were modified using two different procedures: In the first, a thin compact layer of gold (typically 30 nm) was deposited using electron beam evaporation. A clustering of the layer was obtained by a subsequent heat treatment at 600–800°C for several hours in laboratory air. In an alternative procedure, a specified volume of a solution of HAuCl4 in methanol was added to the sensor surface with a subsequent heat treatment. The role of methanol as solvent is to ensure a homogeneous distribution of the
* Corresponding author. Tel: + 49 89 63640049; fax: + 49 89 63646881. 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(97)00334-1
T. Schwebel et al. / Sensors and Actuators B 49 (1998) 46–51
47
gold. By this procedure, gold dispersions with cluster sizes between 10 nm and 300 nm and a percentage of covered surface between 10% and 30% were obtained, as imaging in a SEM or AFM reveals in Fig. 2. In general, no burn-in period or temperature cycling was required due to the high operating temperature of the sensor surface, suppressing surface contamination.
Fig. 2. Au-cluster on the Ga2O3 surface (SEM image) deposited by the wet chemical method.
3. Results
3.1. Modification by gold deposition In the first stage, due to the modification, the change of overall conductivity of a Ga2O3 thin film was checked. Both methods were used. The modification by gold deposition led to a slight resistance increase (Fig. 3). Dependent on the preparation parameters, the sensitivity of the modified sensor to CO was strongly enhanced in comparison with the unmodified sensor. An investigation of different operating temperatures showed that the highest sensitivity was obtained at a temperature of 550°C (Fig. 4). An additional increase in CO sensitivity was obtained, when the Ga2O3 thin films donor doped with SnO2 [7] were modified with gold. The maxima of the sensitivity shifted towards lower temperatures.
Fig. 1. Structure of the sensor chip (3 × 3 mm2) (a) first side with contact pads and interdigital electrodes; (b) second side with heater.
Fig. 3. Sensor base resistance in wet air. The open triangles correspond to unmodified sensors, the filled squares to sensors wet chemically modified by gold.
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Fig. 4. Gas sensitivity of pure Ga2O3 thin films, Ga2O3 thin films modified with a Au-dispersion and donor-doped Ga2O3 thin films modified with a gold dispersion to exposure of 5000 ppm CO in wet air for 10 min.
The response behaviour is shown in Fig. 5, with response times (t90) of about 1 min at 550°C. At lower temperatures, longer response times were obtained unfavourable for sensor use. The sensor characteristics of the donor doped specimens modified with gold showed a sensitivity which was high enough to allow detection of CO concentrations in the range of 100 ppm which is a relevant value for homes and garages (Fig. 6). The gold dispersion has been shown to be stable under continuous operation and even at higher temper-
Fig. 5. Transient response of Au-modified sensors operated at 550°C in wet air. The CO concentrations shown in the lower part are 5000, 2000, 1000, 500, 200, 100 ppm.
Fig. 6. Sensor characteristics of gold modified sensors measured in wet air at 600°C. Platinum contact pads not passivated.
atures. The influence of interfering gases relevant for domestic use is shown in Fig. 7. The reproducibility of the sensor characteristics depends on the reproducibility of the structure of the catalyst. Good results have even been obtained using the wet chemical method for the application of the gold (Fig. 8).
3.2. Influence of contact pads With all the results shown previously, the sensitive layer of Ga2O3 covered the interdigital structure shown in Fig. 1a. The contact pads also shown in this figure were covered by a mechanical mask of refractory ce-
Fig. 7. Cross sensitivity of SnO2 doped sensors modified with evaporated gold operated at 550°C towards various gases of interest for CO monitoring in garages and homes; 20/1% O2 denotes a change to 1% O2, 1.2/0.4 an change from 1.2 to 0.4% humidity.
T. Schwebel et al. / Sensors and Actuators B 49 (1998) 46–51
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Fig. 10. Cross-section of a Ga2O3 sensor covered by a gas filter consisting of highly porous Ga2O3. Fig. 8. Reproducibility of two sensors which were modified with gold using wet chemical preparation. The clustering of the gold modification was done for 10 h at 650°C. The operating temperature was 600°C.
ramics during sputtering to ensure an easy welding of the contact wire to the bond pads. However, the open platinum structure displayed strong catalytic activity which may cause a reduction of the concentration of the gases to be detected in the volume near the sensor chip. To eliminate this effect, specimens with contact pads covered by Ga2O3 have been investigated. A comparison of the gas sensitivities of these two types of devices is shown in Fig. 9. A strong increase in CO sensitivity due to the passivation of the pads was attained, which gave a factor 4 resistance change in response to 100 ppm CO.
3.3. Elimination of cross sensiti6ity to sol6ents By covering the sensor structure with a porous layer of a refractory ceramics (especially Al2O3 or even
Fig. 9. Comparison of devices with open and Ga2O3 covered platinum contact pads. The sensor layer is Ga2O3 modified with gold, the operating temperature is 600°C.
Ga2O3; see Fig. 10), the gas diffusing through the ceramics is filtered. Reactive solvents are burnt and only the more stable CO is allowed to reach the sensor surface. Results from an investigation of a Ga2O3 layer covered with a porous filter are shown in Fig. 11. The cross influence of ethanol was nearly completely eliminated with no deterioration of the response behavior. The sensitivity to 5000 ppm CO amounted to about 14 with a very steep transient behaviour. Fig. 10 gives a cross-sectional view of a Ga2O3 thick film layer structure, which may then be modified by Au-clusters. In contrast to an EtOH sensitivity reduction, this led to a strongly enhanced CO sensitivity in comparison with non-gold prepared thick films. To check the role of the Au modification in terms of sensitivity, 5000 ppm CO was applied to both sensor types with gallium oxide filter (gold and non-gold modified). Fig. 12 shows the results for a wide range of temperatures. Non-Au prepared probes yielded a maximum sensitivity of S :6 at 650°C, while Au modified ones markedly increased to a value of S:18 at 600°C. Additional investigations are planned using work func-
Fig. 11. Response of a Au-modified Ga2O3 sensor covered with a porous filter to ethanol and CO (5000, 2000, 1000, 500, 200 ppm) in wet air at 700°C operating temperature.
T. Schwebel et al. / Sensors and Actuators B 49 (1998) 46–51
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Fig. 12. Comparison of CO-sensitivity (to 5000 ppm) as a function of temperature for sensors with gold modified filter and sensors with non-gold modified filter. Error bars were taken from two identically prepared probes.
Fig. 13. Long-term stability of Au modified Ga2O3 filter probes; two identically prepared probes.
tion measurements, which may also be applied as a sensor signal [8].
4. Long-term stability The long-term stability is still under investigation, but measurements after 3 months of constant operation at 700°C gave promising results in terms of resistance stability and sensitivity. Fig. 13 shows the measured signals, with a good correlation of basic resistances and sensitivities (the differences arise from slightly different temperatures).
5. Conclusions The work showed that by using Au-dispersions, a very strong enhancement of the CO sensitivity was attainable, allowing domestic CO detection with robust high-temperature operated metal oxides. Further work will be devoted to detailed investigations of the gold clusters to increase the CO sensitivity further. Using porous ceramic filters, it was possible to suppress crosssensitivities to volatile organic solvents. A temperature of 700°C allowed the fast detection of CO in a matter of seconds. No burn-in time or temperature cycling was necessary to obtain the results shown above.
T. Schwebel et al. / Sensors and Actuators B 49 (1998) 46–51
References [1] M. Fleischer, H. Meixner, Sensing reducing gases at high temperatures using long-term stable Ga22O3 thin films, Sensors and Actuators B 6 – 7 (1992) 257–261. [2] M. Fleischer, H. Meixner, Electron mobility in single and polycrystalline Ga2O3, J. Appl. Phys. 74 (1) (1993) 300–305. [3] M. Fleischer, Fast gas sensors based on metal oxides which are stable at high temperatures, in: Proc. Eurosensors X, Leuven, Belgium, 1996, pp. 25 – 33. [4] M. Seth, M. Fleischer, H. Meixner, C.-D. Kohl, A selective H2-sensor implemented using Ga2O3-thin films which are covered with a gasfiltering SiO2 layer, Sensors and Actuators B 35–36 (1996) 1 – 7.
.
51
[5] K. Fukui, S. Nishida, A selective CO gas sensor based on Au-La2O3 added SnO2 ceramic with siliceous zeolite coat, Sensors and Actuators B 24 – 25 (1996) 486 – 490. [6] M. Fleischer, H. Meixner, Characterization and crystallite growth of semiconducting high-temperature-stable Ga2O3 thin-films, J. Mater. Sci. Lett. 11 (1992) 1728 – 1731. [7] J. Frank, M. Fleischer, H. Meixner, A. Feltz, Enhancement of sensitivity and conductivity of semiconducting Ga2O3 sensors by doping with SnO2, this conference. [8] T. Doll, J. Lechner, I. Eisele, K. Schierbaum, W. Go¨pel, Ozone detection in the ppb-range with workfunction sensors operating at room temperature, Sensors and Actuators B 34 (1996) 506– 510.