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A model for the gas sensing properties of Al-doped SnO2 thin film after annealing
April 2000
Materials Letters 43 Ž2000. 139–143 www.elsevier.comrlocatermatlet
A model for the gas sensing properties of Al-doped SnO 2 thin film after annealing El-Sayed M. Farag Basic Science of Engineering, Faculty of Engineering-Shebin El-Kom, Menoufia UniÕersity, Egypt Received 20 August 1998; received in revised form 8 October 1999; accepted 11 October 1999
Abstract The effect of quenching in air and vacuum on Al-doped SnO 2 films has been examined. The electrical resistance of tin dioxide films before and after annealing was recorded as a function of operating temperature in air, vacuum, synthetic air, N2 and N2 charged with gases. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrical resistance; Tin dioxide films; Annealing
1. Introduction Tin oxide serves as an important base material in a variety of conductance metric sensors. The wide spread applicability of this semiconducting oxide is related both to its range of conductance variability and to the fact that it responds to both oxidizing and reducing gases w1x. Most gas sensors are operated in air containing the gases of interest only in small concentrations. Therefore, the operating conditions on sensor surfaces are different from those on catalyst surfaces where the reactions are usually offered in a stoichiometeric ratio w2x. The aim of this work, is the study of the temperature dependence of the resistance of Al-doped SnO 2 thin films before and after annealing.
2. Experimental Samples synthesis was performed using a process Rheotaxial Growth Thermal Oxidation ŽRGTO. w3,4x method at Daimler Benz Research Laboratory, Mu-
nich, Germany as shown in Fig. 1. Tin is deposited on a silicon nitride membrane. It is then kept around 2908C, the melting point of the metal which forms microspheres with diameters of 1–6 mm. It is followed by a thermal oxidation step at 5208C for 18 h, transforms there tin microspheres into SnO 2 grains of increased size, thereby providing an electric contact by the grain boundaries. The tin oxide layers have been activated by physical vapor deposition of 10 nm of Al at 1008C. These layers were annealed at 5008C for 1 h in air. The schematic diagram of the measurement setup is shown in Fig. 2. The films were provided with two gold planar electrodes, the heater of the film is made beneath the film from platinum. The sensors are mounted inside a vacuum chamber and can be evacuated to 10y6 mbar through both a rotary and a turbo pump as shown in Fig. 3. The pressure was recorded by a combitron ŽCM 330., and a gas cylinder was connected with the vacuum chamber by a pipe. The gas flows through this pipe to the bell jar by a spiral spring. Also, the pipe of the gas was provided with a pressure reducing valve and a needle valve to control the flow of
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 2 4 6 - 3
140
E.-S.M. Farag r Materials Letters 43 (2000) 139–143
Fig. 1. Schematic diagram of the sensor containing catalytic metal.
gases. The bell jar was connected electrically with three cables. These cables were connected with the sample holder, which carries the samples, and insulated heater, which controls the ambient temperature. The insulated heater was placed inside the vacuum chamber used for created ambient temperature, where the ambient temperature was adjusted at 308C. A load resistor R L was connected in series with the device and dc voltage was applied to the circuit. The voltage applied to the heater must be kept below 11 V to deter the silicon diaphragms from breaking due to temperature-induced stress. Electrical measurements were performed as follows:
electric breakthrough, which destroys the membrane. The virgin samples were pretreated by heating to 680 K and cooling back to 300 K in air. This step
1. Electrical heating of the silicon nitride membrane, 2. Ohmic measurement of the sensitive layer, 3. Electric contact between heater and film to avoid
Fig. 2. Schematic circuit of the device conductivity.
Fig. 3. Schematic design of vacuum pump.
E.-S.M. Farag r Materials Letters 43 (2000) 139–143
was repeated three times before starting the film characterization in order to clean the samples from water vapor and hydroxyl species adsorbed on their surface. The SnO 2rAl film was annealed at 680 K at first for 10 min in air and quenched by switching off the power supply abruptly, and then it was annealed at 1000 K in vacuum ; 10y4 mbar for 10 min, also quenching in vacuum by switching off the power supply. The measurements were performed at an ambient temperature of 308C to study the effect of annealing on conductivity in air, vacuum ; 10y4 mbar, synthetic air, N2 and N2 containing either Ž100 ppm. NH 3 or Ž1000 ppm. H 2 as a function of operating temperature. The electrical resistance of SnO 2rAl was recorded as a function of operating temperature before and after annealing film at pressure 50 mbar.
3. Results and discussions The conductivity of semiconductor thin films depends on both bulk and surface conductivity. The bulk conductivity is expected to be the same as plain tin oxide films to a first degree of approximation, though the presence of catalyst particles could modify film properties. The surface conductivity is due to electron transport mechanisms between the surface
Fig. 4. Temperature dependence of resistance for SnO 2 rAl in air.
141
Fig. 5. Temperature dependence of resistance for SnO 2 rAl in vacuum Ž10y4 mbar..
particles and it is probably due to activated charge carrier creation and tunneling w5x. During the aforementioned measuring cycles, the resistance of the samples was recorded at 10-K intervals as the samples were cooled from 680 to 300 K. The cooling speed was 20 Krmin. The electric resistance of the films in air is shown in Fig. 4. The resistance of the annealed SnO 2rAl film is lower than virgin SnO 2rAl film in the whole temperature range. Accordingly, the conductance of the annealed film is higher than that of the virgin film. The electric resistance decreases with increasing operating temperature to 465 K and then increases with increasing operating temperature. Usually, conductance maximum is observed at around 465 K. Sample conductance increases more precisely in the temperature region from ambient to around 465 K. This conductance increase has been assigned to the loss of physisorbed H 2 O and chemisorbed OHy groups, from arrangement of sites on the oxide surface. In Fig. 5, the resistance of SnO 2rAl films has been recorded as function of operating temperature in vacuum ; 10y4 mbar. The resistance of the virgin film decreases with increasing operating temperature while the resistance of the annealed film increases with increasing operating temperature. A thin layer of Al 2 O 3 may be formed on the surface of semiconductor. After a fast initial oxidation step at the very begin-
142
E.-S.M. Farag r Materials Letters 43 (2000) 139–143
Fig. 6. Temperature dependence of resistance for SnO 2 rAl in synthetic air at pressure 50 mbar.
Fig. 8. Temperature dependence of resistance for SnO 2 rAl in N2 rŽ100 ppm. NH 3 at pressure 50 mbar.
ning, very small oxide growth rate is observed at lower temperature and an enhancement in the rate at higher temperature. Therefore, the electrical conductivity of the annealed film decreases with increasing operating temperature. The resistance of SnO 2rAl films as a function of reciprocal operating temperature is recorded in synthetic air at pressure 50 mbar as shown in Fig. 6. The resistance of the virgin film decreases smoothly with increasing temperature up
to 400 K. Above 400 K, the resistance decreases sharply due to the decreased ionized oxygen on the surface semiconductor leading to increased carrier concentration on the surface. This indicates that a triple ionized aluminum vacancies are the predominant defects at oxygen pressure w6x. The qualitative behavior of annealed film is quite different, but the resistance in synthetic air is higher than in air in the same temperature range. The reason may be the
Fig. 7. Temperature dependence of resistance for SnO 2rAl in N2 gas at pressure 50 mbar.
E.-S.M. Farag r Materials Letters 43 (2000) 139–143
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reasonably brings about the increase of the work function of Al and hence, a barrier formation at the Al–SnO 2 interface w10x. Accordingly, the Schottky barrier will be increased with decreasing temperature.
4. Conclusions
Fig. 9. Temperature dependence of resistance for SnO 2 rAl in N2 rŽ1000 ppm. H 2 at pressure 50 mbar.
effect of the depletion layer between SnO 2 and Al 2 O 3 that increase at higher temperature. Potential barriers exist at the grain boundaries and at the conduct semiconductor interfaces. Fig. 7 shows that the resistance of virgin film decreases with increasing operating temperature in N2 , it may be due to the increase of carrier concentration. But the resistance of the annealed film increases with increasing operating temperature in N2 , which is mainly due to the decrease of carrier concentration w7x. The same trend was observed for films obtained at a higher gas pressure ; 100 mbar. Figs. 8 and 9 illustrate the resistance for SnO 2rAl films during exposure to NH 3 Ž100 ppm. and H 2 in N2 Ž1000 ppm., respectively. The resistance of virgin films decreases with increasing operating temperature. It is clear that there is a difference in apparent resistance in NH 3 and H 2 . It may be due to H 2 gas reacting with oxygen y. species ŽOy ionosorbed on the semiconductor 2 ,O surface decreasing the Schottsky barrier, and the surface oxygen vacancies may diffuse towards the bulk, leading to an increase in the conductance w8,9x. Annealed film resistance increases with increasing temperature as shown in Figs. 8 and 9. Annealing of an SnO 2rAl film increases with the resistance. It is believed that oxygen chemisorbed in the form of y anions like Oy and O 2y on the Al surface then 2, O
The characteristic conductivity of SnO 2 sensors, which are covered with Al before and after annealing, were investigated as a function of operating temperature in air, in vacuum and several gases at pressure of 50 mbar. The results showed that the resistance of the annealed film increases with increasing operating temperature due to the formation of Al 2 O 3 on the semiconductor increasing the Schoty tky barrier. Anions like Oy and O 2y are be2, O lieved to form on the Al surface, leading to increase in the work function of Al and hence a barrier formation at Al–SnO 2 interface. Acknowledgements S.M. Farag would like to thank Prof. Dr. I. Eisele, Federal Armed Forces University, Munich, Germany, and his group for preparing samples and allowing me to perform laboratory measurements.
References w1x S. Semanick, T.B. Frybeyer, Sens. Actuators, B 1 Ž1990. 97–102. w2x D. Kohl, Sens. Actuators, B 1 Ž1990. 158–165. w3x G. Sberveglieri, G. Faglia, S. Groppeli, P. Nelli, A. Faroui, A novel PVD technique for the preparation of SnO 2 thin films as C 2 H 5 OH sensors, SAB 7 Ž1992. . w4x G. Sberveglireri, Classical and Novel techniques for the SAB 6 Ž1992. . w5x C.A. Papadopoulos, J.N. Avaritsioits, Sens. Actuators, B 28 Ž1995. 201–210. w6x Z.M. Jazebski, Oxide Semiconductors Ž1987. . w7x Kikuo Tominagu, Masahiro et al., Thin Solid Films 253 Ž1994. 9–13. w8x V. Lantto, P. Romppainen, Surface Sci. Ž1987. 243–264. w9x Kohl, Sens. Actuators 18 Ž1989. 71–113. w10x N. Yamamoto, S. Tonomura, T. Matsuoka, H. Tsubomura, Surface Sci. 92 Ž1980. 400–406.
Materials Letters 43 Ž2000. 139–143 www.elsevier.comrlocatermatlet
A model for the gas sensing properties of Al-doped SnO 2 thin film after annealing El-Sayed M. Farag Basic Science of Engineering, Faculty of Engineering-Shebin El-Kom, Menoufia UniÕersity, Egypt Received 20 August 1998; received in revised form 8 October 1999; accepted 11 October 1999
Abstract The effect of quenching in air and vacuum on Al-doped SnO 2 films has been examined. The electrical resistance of tin dioxide films before and after annealing was recorded as a function of operating temperature in air, vacuum, synthetic air, N2 and N2 charged with gases. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrical resistance; Tin dioxide films; Annealing
1. Introduction Tin oxide serves as an important base material in a variety of conductance metric sensors. The wide spread applicability of this semiconducting oxide is related both to its range of conductance variability and to the fact that it responds to both oxidizing and reducing gases w1x. Most gas sensors are operated in air containing the gases of interest only in small concentrations. Therefore, the operating conditions on sensor surfaces are different from those on catalyst surfaces where the reactions are usually offered in a stoichiometeric ratio w2x. The aim of this work, is the study of the temperature dependence of the resistance of Al-doped SnO 2 thin films before and after annealing.
2. Experimental Samples synthesis was performed using a process Rheotaxial Growth Thermal Oxidation ŽRGTO. w3,4x method at Daimler Benz Research Laboratory, Mu-
nich, Germany as shown in Fig. 1. Tin is deposited on a silicon nitride membrane. It is then kept around 2908C, the melting point of the metal which forms microspheres with diameters of 1–6 mm. It is followed by a thermal oxidation step at 5208C for 18 h, transforms there tin microspheres into SnO 2 grains of increased size, thereby providing an electric contact by the grain boundaries. The tin oxide layers have been activated by physical vapor deposition of 10 nm of Al at 1008C. These layers were annealed at 5008C for 1 h in air. The schematic diagram of the measurement setup is shown in Fig. 2. The films were provided with two gold planar electrodes, the heater of the film is made beneath the film from platinum. The sensors are mounted inside a vacuum chamber and can be evacuated to 10y6 mbar through both a rotary and a turbo pump as shown in Fig. 3. The pressure was recorded by a combitron ŽCM 330., and a gas cylinder was connected with the vacuum chamber by a pipe. The gas flows through this pipe to the bell jar by a spiral spring. Also, the pipe of the gas was provided with a pressure reducing valve and a needle valve to control the flow of
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 2 4 6 - 3
140
E.-S.M. Farag r Materials Letters 43 (2000) 139–143
Fig. 1. Schematic diagram of the sensor containing catalytic metal.
gases. The bell jar was connected electrically with three cables. These cables were connected with the sample holder, which carries the samples, and insulated heater, which controls the ambient temperature. The insulated heater was placed inside the vacuum chamber used for created ambient temperature, where the ambient temperature was adjusted at 308C. A load resistor R L was connected in series with the device and dc voltage was applied to the circuit. The voltage applied to the heater must be kept below 11 V to deter the silicon diaphragms from breaking due to temperature-induced stress. Electrical measurements were performed as follows:
electric breakthrough, which destroys the membrane. The virgin samples were pretreated by heating to 680 K and cooling back to 300 K in air. This step
1. Electrical heating of the silicon nitride membrane, 2. Ohmic measurement of the sensitive layer, 3. Electric contact between heater and film to avoid
Fig. 2. Schematic circuit of the device conductivity.
Fig. 3. Schematic design of vacuum pump.
E.-S.M. Farag r Materials Letters 43 (2000) 139–143
was repeated three times before starting the film characterization in order to clean the samples from water vapor and hydroxyl species adsorbed on their surface. The SnO 2rAl film was annealed at 680 K at first for 10 min in air and quenched by switching off the power supply abruptly, and then it was annealed at 1000 K in vacuum ; 10y4 mbar for 10 min, also quenching in vacuum by switching off the power supply. The measurements were performed at an ambient temperature of 308C to study the effect of annealing on conductivity in air, vacuum ; 10y4 mbar, synthetic air, N2 and N2 containing either Ž100 ppm. NH 3 or Ž1000 ppm. H 2 as a function of operating temperature. The electrical resistance of SnO 2rAl was recorded as a function of operating temperature before and after annealing film at pressure 50 mbar.
3. Results and discussions The conductivity of semiconductor thin films depends on both bulk and surface conductivity. The bulk conductivity is expected to be the same as plain tin oxide films to a first degree of approximation, though the presence of catalyst particles could modify film properties. The surface conductivity is due to electron transport mechanisms between the surface
Fig. 4. Temperature dependence of resistance for SnO 2 rAl in air.
141
Fig. 5. Temperature dependence of resistance for SnO 2 rAl in vacuum Ž10y4 mbar..
particles and it is probably due to activated charge carrier creation and tunneling w5x. During the aforementioned measuring cycles, the resistance of the samples was recorded at 10-K intervals as the samples were cooled from 680 to 300 K. The cooling speed was 20 Krmin. The electric resistance of the films in air is shown in Fig. 4. The resistance of the annealed SnO 2rAl film is lower than virgin SnO 2rAl film in the whole temperature range. Accordingly, the conductance of the annealed film is higher than that of the virgin film. The electric resistance decreases with increasing operating temperature to 465 K and then increases with increasing operating temperature. Usually, conductance maximum is observed at around 465 K. Sample conductance increases more precisely in the temperature region from ambient to around 465 K. This conductance increase has been assigned to the loss of physisorbed H 2 O and chemisorbed OHy groups, from arrangement of sites on the oxide surface. In Fig. 5, the resistance of SnO 2rAl films has been recorded as function of operating temperature in vacuum ; 10y4 mbar. The resistance of the virgin film decreases with increasing operating temperature while the resistance of the annealed film increases with increasing operating temperature. A thin layer of Al 2 O 3 may be formed on the surface of semiconductor. After a fast initial oxidation step at the very begin-
142
E.-S.M. Farag r Materials Letters 43 (2000) 139–143
Fig. 6. Temperature dependence of resistance for SnO 2 rAl in synthetic air at pressure 50 mbar.
Fig. 8. Temperature dependence of resistance for SnO 2 rAl in N2 rŽ100 ppm. NH 3 at pressure 50 mbar.
ning, very small oxide growth rate is observed at lower temperature and an enhancement in the rate at higher temperature. Therefore, the electrical conductivity of the annealed film decreases with increasing operating temperature. The resistance of SnO 2rAl films as a function of reciprocal operating temperature is recorded in synthetic air at pressure 50 mbar as shown in Fig. 6. The resistance of the virgin film decreases smoothly with increasing temperature up
to 400 K. Above 400 K, the resistance decreases sharply due to the decreased ionized oxygen on the surface semiconductor leading to increased carrier concentration on the surface. This indicates that a triple ionized aluminum vacancies are the predominant defects at oxygen pressure w6x. The qualitative behavior of annealed film is quite different, but the resistance in synthetic air is higher than in air in the same temperature range. The reason may be the
Fig. 7. Temperature dependence of resistance for SnO 2rAl in N2 gas at pressure 50 mbar.
E.-S.M. Farag r Materials Letters 43 (2000) 139–143
143
reasonably brings about the increase of the work function of Al and hence, a barrier formation at the Al–SnO 2 interface w10x. Accordingly, the Schottky barrier will be increased with decreasing temperature.
4. Conclusions
Fig. 9. Temperature dependence of resistance for SnO 2 rAl in N2 rŽ1000 ppm. H 2 at pressure 50 mbar.
effect of the depletion layer between SnO 2 and Al 2 O 3 that increase at higher temperature. Potential barriers exist at the grain boundaries and at the conduct semiconductor interfaces. Fig. 7 shows that the resistance of virgin film decreases with increasing operating temperature in N2 , it may be due to the increase of carrier concentration. But the resistance of the annealed film increases with increasing operating temperature in N2 , which is mainly due to the decrease of carrier concentration w7x. The same trend was observed for films obtained at a higher gas pressure ; 100 mbar. Figs. 8 and 9 illustrate the resistance for SnO 2rAl films during exposure to NH 3 Ž100 ppm. and H 2 in N2 Ž1000 ppm., respectively. The resistance of virgin films decreases with increasing operating temperature. It is clear that there is a difference in apparent resistance in NH 3 and H 2 . It may be due to H 2 gas reacting with oxygen y. species ŽOy ionosorbed on the semiconductor 2 ,O surface decreasing the Schottsky barrier, and the surface oxygen vacancies may diffuse towards the bulk, leading to an increase in the conductance w8,9x. Annealed film resistance increases with increasing temperature as shown in Figs. 8 and 9. Annealing of an SnO 2rAl film increases with the resistance. It is believed that oxygen chemisorbed in the form of y anions like Oy and O 2y on the Al surface then 2, O
The characteristic conductivity of SnO 2 sensors, which are covered with Al before and after annealing, were investigated as a function of operating temperature in air, in vacuum and several gases at pressure of 50 mbar. The results showed that the resistance of the annealed film increases with increasing operating temperature due to the formation of Al 2 O 3 on the semiconductor increasing the Schoty tky barrier. Anions like Oy and O 2y are be2, O lieved to form on the Al surface, leading to increase in the work function of Al and hence a barrier formation at Al–SnO 2 interface. Acknowledgements S.M. Farag would like to thank Prof. Dr. I. Eisele, Federal Armed Forces University, Munich, Germany, and his group for preparing samples and allowing me to perform laboratory measurements.
References w1x S. Semanick, T.B. Frybeyer, Sens. Actuators, B 1 Ž1990. 97–102. w2x D. Kohl, Sens. Actuators, B 1 Ž1990. 158–165. w3x G. Sberveglieri, G. Faglia, S. Groppeli, P. Nelli, A. Faroui, A novel PVD technique for the preparation of SnO 2 thin films as C 2 H 5 OH sensors, SAB 7 Ž1992. . w4x G. Sberveglireri, Classical and Novel techniques for the SAB 6 Ž1992. . w5x C.A. Papadopoulos, J.N. Avaritsioits, Sens. Actuators, B 28 Ž1995. 201–210. w6x Z.M. Jazebski, Oxide Semiconductors Ž1987. . w7x Kikuo Tominagu, Masahiro et al., Thin Solid Films 253 Ž1994. 9–13. w8x V. Lantto, P. Romppainen, Surface Sci. Ž1987. 243–264. w9x Kohl, Sens. Actuators 18 Ž1989. 71–113. w10x N. Yamamoto, S. Tonomura, T. Matsuoka, H. Tsubomura, Surface Sci. 92 Ž1980. 400–406.