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SnInO-based chlorine gas sensor
Sensors and Actuators
SnInO-based A. Galdikas, Semiconductor
633
B, 7 (1992) 633-636
chlorine gas sensor
Z. MartUnas
Physics Institute
and A. Setkus
of Lithuanian
Academy
of Sciences,
A. GoStauto
I I, Vilnius 232600 (Lithuania)
Abstract The dependence of the resistance of InSnO (ITO) layers on a small amount of chlorine gas in air is studied here. The chlorine-sensitive IT0 films have been fabricated by reactive sputtering of In and Sn from a metal alloy target (In and Sn in the proportion 0.94:0.06 by weight) on silicon substrates. It is found that the chlorine gas in the ambient atmosphere makes the IT0 resistance increase. The dependence of the resistance on chlorine concentration tends to saturate at concentrations above 3 ppm. The sensitivity maximum is observed at 300 “C. The addition of Pt significantly increases the sensitivity and reduces the temperature of the sensitivity maximum. The response time is x 150 s at 90 “C and decreases exponentially with increasing temperature. The results obtained are typical for gas sensors based on tin oxide or IT0 and can be explained in terms of electron localization at surface adsorption centres dominated by chlorine molecules and the electric transport features at the polycrystalline film grain boundaries.
1. Introduction
Chemisorption of reducing or oxidizing gases on the surface of semiconducting metal oxides leads to a change of the semiconductor resistance, so that these materials are useful in gas-detection and concentration-measurement systems. The high sensitivity, small size and the possibility of low-cost production are the advantages of such semiconductor gas detectors. The electrical response of metal oxides (generally SnOz, SnInO, ZnO) to the adsorption of gases such as 02, HI, CO, NO,, SO1 and C, H, has been studied in many papers [l-4]. Less attention has been paid to chlorine gas. In this paper the resistance response of IT0 thin film to chlorine in air is investigated. 2. Experimental IT0 thin films were prepared by reactive d.c. sputtering from an In-Sn alloy target with the composition 94% In and 6% Sn by weight. A mixture of 40% Ar and 60% O2 was used as the reactive gas. The Ar and O2 gas-flow rates and the total pressure of 5 x lop4 Torr were kept constant during the sputtering. IT0 thin films were deposited on Si substrates with an SiOz insulating layer. The substrate was heated up to approximately 400 “C before deposition, but the temperature was not controlled during the film growth, which lasted 1 0925-4005/92/$5.00
min. Some IT0 films were surface activated by vacuum deposition of Pt or Pd. The above-mentioned sputtering and annealing parameters were shown to be close to the optimum for the fabrication of highly chlorine-sensitive samples. Ohmic contacts were formed by vacuum heat treatment of the IT0 thin film at 400 “C for 30 min with subsequent evaporation of Al on the free surface of the film. In order to obtain a stable resistance and to improve the sensitivity to chlorine gas, the films with Al contacts were annealed at temperatures ranging from 200 to 400 “C for 2-3 h in air. The dependence of the d.c. conductivity on the reciprocal temperature of an annealed IT0 film is shown in Fig. 1. It is similar to results obtained elsewhere (see, for example, [3,4]). The developed sensor consists of a silicon substrate with insulating SiOz layer, a resistive heating element, a metallic thin-film thermometer around the sensitive IT0 layer.
3. Results Figure 2 shows the dependence of the resistance change AR at 300 “C on the chlorine gas concentration. The resistance rises steeply up to a chlorine concentration of P z 3 ppm, then a tendency to saturation is observed. Similar resistance versus P characteristics have been observed in the temperature range 92-430 “C. @ 1992- Elsevier Sequoia. All rights reserved
634
10
1
0.1
0.01
0.001
Fig. 1. The dependence of the sheet conductivity temperature without chlorine gas.
of the IT0 thin film on
Fig. 3. The dependences of the sheet resistance of the IT0 thin film on film temperature at IO ppm concentration of chlorine gas in the atmosphere; experimental data are: (0), without additives, ( x ), with Pt, ( 0). with Pd; (-) is the calculation according to eqns. (5) and (6).
1
0.1
0.01
11 1
1.6
1
I
2
2.6
lO'/T,K-'
Fig. 2. The dependence of the sheet resistance change of the IT0 thin film on the concentration of chlorine gas in the atmosphere when the film temperature is 300 “C: ( 0) are experimental data, (-) is the calculation according to eqn. (5) (C = 1.76, b = 0.32).
Fig. 4. The dependence sensor on temperature.
The maximum sensitivity AR/R0 of undoped and Pd-activated samples occurs at the temperature TM z 300 “C (Fig. 3). Pt activation increases the sensitivity and reduces TM. The influence of Pd activation on AR/R is not so significant as that of
Pt. It should be noted that further investigations of IT0 layer activation by catalyst metals are necessary. The response time z to chlorine for an unactivated film decreases exponentially when the tem-
of the response
time of the IT0 chlorine
gas
635
perature rises (Fig. 4). This result has been obtained for all the investigated chlorine concentrations and it is usual for a metal-oxide gas sensor. The opposite signal response of the IT0 film (i.e., a resistance decrease) induced by the gases Hz, CO and C2HSOH has also been established. 4. Discussion It is well known that IT0 thin films are polycrystalline n-type semiconductors with approximately equal electron and donor concentrations [ 51. The electrical conductivity of such films arises from the potential barriers cp of electrically active grain boundaries. Assuming the same height for all the barriers, the conductivity CJmay be expressed as G = Doexp( - q/kBT)
(1)
where ks is the Boltzmann constant and co is the bulk conductivity. Many authors [l-3] have suggested that the potential barrier in polycrystalline metal oxides such as SnO, or IT0 is caused by oxygen ions. The chemisorption of oxygen molecules gives rise to electron localization at the grain surface adsorption centre, therefore the chemisorbed oxygen molecule acts as an acceptor. Assuming the depletion region has a density Nd of completely ionized shallow donors, the maximum band bending cpat the grain boundary can be expressed by [6] cp = eN:/8aaONd
(2)
where Nt is the surface density of the trapped molecules, e is the electron charge, aa0is the permittivity of the semiconductor film and Nd is the bulk donor concentration. We propose that the action of oxidizing chlorine gas is similar to that of oxygen, i.e., the adsorption of chlorine molecules leads to an increase of cp according to eqn. (2). This explains the experimentally observed chlorine-induced resistance increase of an IT0 film (Fig. 2). The saturation of the sensitivity versus concentration dependence (Fig. 2) may be explained by a phenomenological adsorption-desorption rate equation [ 71:
dNt -= dt
PS ’ (2mkBT)“*
(N - NJ - voN, exp( - E,lk,
T) (3)
where N and s are the surface concentration
and
effective cross section of the identical non-interacting adsorption centres, respectively, m is the molecular mass, P is the gas pressure, 5 is the probability that the incident molecule will be trapped by the centre, v. exp( - E,/k, T) is the desorption probability and Eq is the desorption energy. For the steady-state eqn. (3) leads to the following expression for N,:
-’ where b
=
O&T)” b
\,0 exp
I
(4)
According to eqns. ( l), (2) and (3) we get
0
1,; =- C 0
.
I,;
with C = e2N2/8aaoNdks T As we can see from Fig. 2, the results of the calculations based on eqn. (5) show a similar dependence of AR/R, on concentration to the experimental data. In order to obtain the temperature dependence of the sensitivity from eqn. (3), it is necessary to define the dependence of t on temperature. We assume 5 = lo exp( - EAlkB T)
(6)
where EA is the activation energy. Equation (6) predicts the main experimental feature of the dependence of chlorine sensitivity on temperature, i.e., a maximum of AR/R0 at a certain temperature (see Fig. 3). Investigations of the oxygen adsorption on the Sn02 surface [ 1,3] indicate that the concentration N and/or the cross section s of the adsorption centres in eqn. (3) may also depend on the temperature. However, the dependence of N and s on T has not been taken into account here. 5. Conclusions
The main properties of IT0 films prepared reactive sputtering from an In-Sn alloy target summarized as follows: (1) In order to obtain chlorine-sensitive resistance-stable films, the growth parameters annealing conditions must be optimized.
by are and and
636
(2) The resistance change due to chlorine gas can be explained in terms of a simple model of a polycrystalline layer with an intergrain potential barrier modified by the adsorbed oxidizing chlorine molecules. (3) Chlorine gas sensors based on IT0 films are suitable for the design of a chlorine alarm detector.
References I D. Kohl, Surface processes in the detection of reducing gases with SnO,-based devices, Sensors and Actuators, 18 (1989) 71113.
2 S. Roy Morrison, Selectivity in semiconductor gas sensors, Sensors and Actuators, 12 (1987) 425-440. 3 V. Lantto, P. Romppainen and S. Lepplvuori, A study of the temperature dependence of the barrier energy in porous tin dioxide, Sensors and Actuators, 14 (1988) 149-163. 4 Cl. Sberveglieri, S. Groppeli and G. Coccoli, Radio frequency magnetron sputtering growth and characterization of indium-tin oxide (ITO) thin films for NO, gas sensors, Sensors and Actuators, 1.5 (1988) 235-242. 5 Prem Nath, R. F. Bunshah, B. M. Basol and 0. M. Staffsud, Electrical and optical properties of In,O,:Sn films prepared by activated reactive evaporation, Thin Solid Films, 72 (1980) 463-468. 6 F. Greuter and Cl. Blatter, Electrical properties of grain boundaries in polycrystalline compound semiconductors, Semicond. Sci. Technol., 5(1990) 111-137. 7 H. J. Kreuzer, Theory of surface processes, Appl. Phys., A51 ( 1980) 491-497.
SnInO-based A. Galdikas, Semiconductor
633
B, 7 (1992) 633-636
chlorine gas sensor
Z. MartUnas
Physics Institute
and A. Setkus
of Lithuanian
Academy
of Sciences,
A. GoStauto
I I, Vilnius 232600 (Lithuania)
Abstract The dependence of the resistance of InSnO (ITO) layers on a small amount of chlorine gas in air is studied here. The chlorine-sensitive IT0 films have been fabricated by reactive sputtering of In and Sn from a metal alloy target (In and Sn in the proportion 0.94:0.06 by weight) on silicon substrates. It is found that the chlorine gas in the ambient atmosphere makes the IT0 resistance increase. The dependence of the resistance on chlorine concentration tends to saturate at concentrations above 3 ppm. The sensitivity maximum is observed at 300 “C. The addition of Pt significantly increases the sensitivity and reduces the temperature of the sensitivity maximum. The response time is x 150 s at 90 “C and decreases exponentially with increasing temperature. The results obtained are typical for gas sensors based on tin oxide or IT0 and can be explained in terms of electron localization at surface adsorption centres dominated by chlorine molecules and the electric transport features at the polycrystalline film grain boundaries.
1. Introduction
Chemisorption of reducing or oxidizing gases on the surface of semiconducting metal oxides leads to a change of the semiconductor resistance, so that these materials are useful in gas-detection and concentration-measurement systems. The high sensitivity, small size and the possibility of low-cost production are the advantages of such semiconductor gas detectors. The electrical response of metal oxides (generally SnOz, SnInO, ZnO) to the adsorption of gases such as 02, HI, CO, NO,, SO1 and C, H, has been studied in many papers [l-4]. Less attention has been paid to chlorine gas. In this paper the resistance response of IT0 thin film to chlorine in air is investigated. 2. Experimental IT0 thin films were prepared by reactive d.c. sputtering from an In-Sn alloy target with the composition 94% In and 6% Sn by weight. A mixture of 40% Ar and 60% O2 was used as the reactive gas. The Ar and O2 gas-flow rates and the total pressure of 5 x lop4 Torr were kept constant during the sputtering. IT0 thin films were deposited on Si substrates with an SiOz insulating layer. The substrate was heated up to approximately 400 “C before deposition, but the temperature was not controlled during the film growth, which lasted 1 0925-4005/92/$5.00
min. Some IT0 films were surface activated by vacuum deposition of Pt or Pd. The above-mentioned sputtering and annealing parameters were shown to be close to the optimum for the fabrication of highly chlorine-sensitive samples. Ohmic contacts were formed by vacuum heat treatment of the IT0 thin film at 400 “C for 30 min with subsequent evaporation of Al on the free surface of the film. In order to obtain a stable resistance and to improve the sensitivity to chlorine gas, the films with Al contacts were annealed at temperatures ranging from 200 to 400 “C for 2-3 h in air. The dependence of the d.c. conductivity on the reciprocal temperature of an annealed IT0 film is shown in Fig. 1. It is similar to results obtained elsewhere (see, for example, [3,4]). The developed sensor consists of a silicon substrate with insulating SiOz layer, a resistive heating element, a metallic thin-film thermometer around the sensitive IT0 layer.
3. Results Figure 2 shows the dependence of the resistance change AR at 300 “C on the chlorine gas concentration. The resistance rises steeply up to a chlorine concentration of P z 3 ppm, then a tendency to saturation is observed. Similar resistance versus P characteristics have been observed in the temperature range 92-430 “C. @ 1992- Elsevier Sequoia. All rights reserved
634
10
1
0.1
0.01
0.001
Fig. 1. The dependence of the sheet conductivity temperature without chlorine gas.
of the IT0 thin film on
Fig. 3. The dependences of the sheet resistance of the IT0 thin film on film temperature at IO ppm concentration of chlorine gas in the atmosphere; experimental data are: (0), without additives, ( x ), with Pt, ( 0). with Pd; (-) is the calculation according to eqns. (5) and (6).
1
0.1
0.01
11 1
1.6
1
I
2
2.6
lO'/T,K-'
Fig. 2. The dependence of the sheet resistance change of the IT0 thin film on the concentration of chlorine gas in the atmosphere when the film temperature is 300 “C: ( 0) are experimental data, (-) is the calculation according to eqn. (5) (C = 1.76, b = 0.32).
Fig. 4. The dependence sensor on temperature.
The maximum sensitivity AR/R0 of undoped and Pd-activated samples occurs at the temperature TM z 300 “C (Fig. 3). Pt activation increases the sensitivity and reduces TM. The influence of Pd activation on AR/R is not so significant as that of
Pt. It should be noted that further investigations of IT0 layer activation by catalyst metals are necessary. The response time z to chlorine for an unactivated film decreases exponentially when the tem-
of the response
time of the IT0 chlorine
gas
635
perature rises (Fig. 4). This result has been obtained for all the investigated chlorine concentrations and it is usual for a metal-oxide gas sensor. The opposite signal response of the IT0 film (i.e., a resistance decrease) induced by the gases Hz, CO and C2HSOH has also been established. 4. Discussion It is well known that IT0 thin films are polycrystalline n-type semiconductors with approximately equal electron and donor concentrations [ 51. The electrical conductivity of such films arises from the potential barriers cp of electrically active grain boundaries. Assuming the same height for all the barriers, the conductivity CJmay be expressed as G = Doexp( - q/kBT)
(1)
where ks is the Boltzmann constant and co is the bulk conductivity. Many authors [l-3] have suggested that the potential barrier in polycrystalline metal oxides such as SnO, or IT0 is caused by oxygen ions. The chemisorption of oxygen molecules gives rise to electron localization at the grain surface adsorption centre, therefore the chemisorbed oxygen molecule acts as an acceptor. Assuming the depletion region has a density Nd of completely ionized shallow donors, the maximum band bending cpat the grain boundary can be expressed by [6] cp = eN:/8aaONd
(2)
where Nt is the surface density of the trapped molecules, e is the electron charge, aa0is the permittivity of the semiconductor film and Nd is the bulk donor concentration. We propose that the action of oxidizing chlorine gas is similar to that of oxygen, i.e., the adsorption of chlorine molecules leads to an increase of cp according to eqn. (2). This explains the experimentally observed chlorine-induced resistance increase of an IT0 film (Fig. 2). The saturation of the sensitivity versus concentration dependence (Fig. 2) may be explained by a phenomenological adsorption-desorption rate equation [ 71:
dNt -= dt
PS ’ (2mkBT)“*
(N - NJ - voN, exp( - E,lk,
T) (3)
where N and s are the surface concentration
and
effective cross section of the identical non-interacting adsorption centres, respectively, m is the molecular mass, P is the gas pressure, 5 is the probability that the incident molecule will be trapped by the centre, v. exp( - E,/k, T) is the desorption probability and Eq is the desorption energy. For the steady-state eqn. (3) leads to the following expression for N,:
-’ where b
=
O&T)” b
\,0 exp
I
(4)
According to eqns. ( l), (2) and (3) we get
0
1,; =- C 0
.
I,;
with C = e2N2/8aaoNdks T As we can see from Fig. 2, the results of the calculations based on eqn. (5) show a similar dependence of AR/R, on concentration to the experimental data. In order to obtain the temperature dependence of the sensitivity from eqn. (3), it is necessary to define the dependence of t on temperature. We assume 5 = lo exp( - EAlkB T)
(6)
where EA is the activation energy. Equation (6) predicts the main experimental feature of the dependence of chlorine sensitivity on temperature, i.e., a maximum of AR/R0 at a certain temperature (see Fig. 3). Investigations of the oxygen adsorption on the Sn02 surface [ 1,3] indicate that the concentration N and/or the cross section s of the adsorption centres in eqn. (3) may also depend on the temperature. However, the dependence of N and s on T has not been taken into account here. 5. Conclusions
The main properties of IT0 films prepared reactive sputtering from an In-Sn alloy target summarized as follows: (1) In order to obtain chlorine-sensitive resistance-stable films, the growth parameters annealing conditions must be optimized.
by are and and
636
(2) The resistance change due to chlorine gas can be explained in terms of a simple model of a polycrystalline layer with an intergrain potential barrier modified by the adsorbed oxidizing chlorine molecules. (3) Chlorine gas sensors based on IT0 films are suitable for the design of a chlorine alarm detector.
References I D. Kohl, Surface processes in the detection of reducing gases with SnO,-based devices, Sensors and Actuators, 18 (1989) 71113.
2 S. Roy Morrison, Selectivity in semiconductor gas sensors, Sensors and Actuators, 12 (1987) 425-440. 3 V. Lantto, P. Romppainen and S. Lepplvuori, A study of the temperature dependence of the barrier energy in porous tin dioxide, Sensors and Actuators, 14 (1988) 149-163. 4 Cl. Sberveglieri, S. Groppeli and G. Coccoli, Radio frequency magnetron sputtering growth and characterization of indium-tin oxide (ITO) thin films for NO, gas sensors, Sensors and Actuators, 1.5 (1988) 235-242. 5 Prem Nath, R. F. Bunshah, B. M. Basol and 0. M. Staffsud, Electrical and optical properties of In,O,:Sn films prepared by activated reactive evaporation, Thin Solid Films, 72 (1980) 463-468. 6 F. Greuter and Cl. Blatter, Electrical properties of grain boundaries in polycrystalline compound semiconductors, Semicond. Sci. Technol., 5(1990) 111-137. 7 H. J. Kreuzer, Theory of surface processes, Appl. Phys., A51 ( 1980) 491-497.