- Email: [email protected]
A study of the sensing characteristics of Fe2O3 gas-sensing thin film
591
Sensors and Actuators B, 13-14 (1993) 591-593
A study of the sensing characteristics of Fe,O, gas-sensing thin film J. Peng and C.
C.Chai
Zbfhvelecln9nics Znstitute, Xi&n
Universify, Wan, Shea&
71lM71 (China)
1. Intraluctlon Because the gas-sensitive characteristicsof haematite material are excellent and it needs no noble-metal catalyst, the material has been of great interest since the end of the 197Os, and sintered Fe,O, gas sensors have been commercialized [l, 21. In order to further improve the properties of the sensor, decrease the heating power and make the sensor microminiaturized and integrated, many authors have been concerned with thin-film materials. In this paper, the experimental results of Fe,O, thin films sensitive to smog made by normal-pressure chemical vapour deposition (NPCVD) and Fe,O, thin films sensitive to acetone and alcohol made by plasma-enhanced chemical vapour deposition (PECVD) are reported. Experiments show that doping makes the gas-sensitive characteristics of these thin fihns better.
2. Rxperimental
The schematicdiagramof the NPCVD reaction system is shown in Fig. 1. The metal-organic compound pentacarbonyl iron [Fe(CO),], which has high saturation vapour pressure and is easy to decompose, is used as the reactant. Fe(CO), and impurities dissolved by an organic solvent are carried by argon and react chemically with oxygen in the reaction chamber. The FeO, thin films are deposited on substrates which are heated between 90 and 180 “C. Fe(O), reactant is still used as a source when Fe,O, thin films are prepared by PECVD. The radio frequency
is 10.56 MHz with 100-150 W power. The substrate temperature is 80-150 “C; n-type silicon wafers and ceramics are used as substrates. In order to make the measurements of the gas-sensitive characteristics easy, thick-film Pd-Ag electrodes are provided on a ceramics substrate and a thin flhn of Fe,O, is deposited on them. The temperature of the sample is controlled by applying avoltage across the heater so the measurements of the gas-sensitive characteristics can be carried out.
3. Results and discussion Figure 2 shows the sensitivity versus working temperature of Fe,O, thin films made by NPCVD. It can be seen that the films are highly sensitive to smog. Tin doping increases the sensitivity to’smog by a big margin and decreases the sensitivity to alcohol; as a result, the sensitivity and selectivity are improved. The doping makes the resistance of the lihn in clean air, R,, versus temperature tend to flatten (Fig. 3). This property is useful for us to set the working-temperature point in practice. Table 1 shows the gas-sensitive characteristics of pure and Sn, Ti, Zrdoped films made by NPCVD. X-ray diffraction measurements have been carried out on F%Oo thin Sims made by NPCVD. Theresults show that they are a-Fe,O,. Doping makes the height
30
0 2.1 2.3 2.5 2.1 2.9 T( x 100t) (a)
IAr
Impurity
Fig. 1. NPCVD reaction system.
0925-4005/93/$6.00
0 1.5 1.9 2.3 2.7 3.1 T( x 1OOC) @I
Fig. 2. Sensitivity vs. temperature (NPCVD): (a) undoped 6lm; (b) h-doped film. Grade I smog concentration is that which makes incident light attenuate by 10% at 1 m from the light source.
Q 1993 - Elsevier Sequoia. AU rights reserved
592
‘O?z-x-xT&lOOC) Fig. 3. R. vs. temperature
(NPCVD).
TABLE 1. Gas-sensitive characteristics of FQ03 made by NPCVD’
ol
I
I
I.5 2.0 2.5 3.0351.5 2.0 ti
&doped
S-doped
Tidoped
3.0 3.5
T(r IOK)
Zr-doped
Fig. 5. l’m content modulates the sensitivity (PECVD). Temperature CC) Sensitivity (RJR3
260 26.7
270 55.0
280 28.0
260 7.5
*With grade 1 smog concentration.
I
5
.
.
10 IS 20 25 30 35 40
Fig. 6. The response-restoring
o-L 1.5
(3) I
/I
characteristics
(PECVD).
2.0 2.5 3.a 3.5 T(x 10013)
Fig. 4. Sensitivity vs. temperature
(PECVD).
of the diffraction peak decrease and widen, which shows that the doping makes the gram size small. A small grain size is of much help in increasing the sensitivity. The Fe,OJ thin films made by PECVD are very sensitive to alcohol and acetone, but are not sensitive to other interfering gases. When a certain amount of tin is doped by a suitable method, the sensitivity to acetone increases further and the selectivity improves (Fig. 4). Experiments show that the reason why tin doping can improve the gas-sensitive characteristics is that the tin content can modulate the material’s sensitivity to acetone. As shown in Fig. 5, there is no obvious effect on the sensitivity to alcohol when the tin content varies, but the effect on the sensitivity to acetone is clear. When the tin content is adequate, for example, the sample with 1.6% of Sn4+ ions, it is very sensitive to acetone and the selectivity is excellent. But for the sample with about 20% Sn4+ ions the sensitivity to acetone is about 50% of that to alcohol.
40 50 20 / degree
60
70
Fig. 7. X-ray diffraction pattern (PJZCVD).
The response-restoring characteristics of a tin-doped Fe,O, thin film made by PECVD are shown in Fig. 6. The response time to acetone with 1000 ppm concentration is within 1.5 s and the restoring time is within 6.5 s. So it can be seen that the responserestoring characteristics of this film are good. SEM morphology determination and X-ray diffraction show that the Fe,O, thin films made by PECVD are amorphous bodies with 0.2 w size. Figure 7 shows the X-ray diffraction pattern of a tin-doped F&O, thin film crystallized after heat treatment. The diffractive peak value d is a little larger than the standard value. The radius of Sn’+ is 0.74 A, a little larger than that of Fe3+ (0.67 A) [3], which shows that the Sn4+ might have entered the cells of a-Fe,O, and partly replaced Fe3+, so that a solid solution of tin-doped &Fe,O, is formed.
593
4. Conclusions Fe,O, thin films made by NPCVD and PECVJI are sensitive to smog and acetone, respectively.When tin is doped adequately, a solid solution of LX-F~O,is formed, which is amorphous with fine grain size. The sensitivity, selectivity and response-restoring characteristics of the films are all excellent, and they should have a bright future as a gas-sensor material.
References
1 Y. Nakatani and M. Matsuoka, Some ekctrical properties of yFe& ceramics, ,‘pn. 1 A&. P&S.., 22 (1983) X33-239. 2 Y. Nakatani and M. Matsuoka, Effects of sulfate ion on gas sensitive properties of a-FQO, ceramics, +n. J. Appf. Phys.., 21 (1982) L758-760. 3 R. C Weast, CRC Handbook of Chembby and Physics,CRC Press, Boca Raton, FL, 66th edn., 1985-1986, p. F-164.
Sensors and Actuators B, 13-14 (1993) 591-593
A study of the sensing characteristics of Fe,O, gas-sensing thin film J. Peng and C.
C.Chai
Zbfhvelecln9nics Znstitute, Xi&n
Universify, Wan, Shea&
71lM71 (China)
1. Intraluctlon Because the gas-sensitive characteristicsof haematite material are excellent and it needs no noble-metal catalyst, the material has been of great interest since the end of the 197Os, and sintered Fe,O, gas sensors have been commercialized [l, 21. In order to further improve the properties of the sensor, decrease the heating power and make the sensor microminiaturized and integrated, many authors have been concerned with thin-film materials. In this paper, the experimental results of Fe,O, thin films sensitive to smog made by normal-pressure chemical vapour deposition (NPCVD) and Fe,O, thin films sensitive to acetone and alcohol made by plasma-enhanced chemical vapour deposition (PECVD) are reported. Experiments show that doping makes the gas-sensitive characteristics of these thin fihns better.
2. Rxperimental
The schematicdiagramof the NPCVD reaction system is shown in Fig. 1. The metal-organic compound pentacarbonyl iron [Fe(CO),], which has high saturation vapour pressure and is easy to decompose, is used as the reactant. Fe(CO), and impurities dissolved by an organic solvent are carried by argon and react chemically with oxygen in the reaction chamber. The FeO, thin films are deposited on substrates which are heated between 90 and 180 “C. Fe(O), reactant is still used as a source when Fe,O, thin films are prepared by PECVD. The radio frequency
is 10.56 MHz with 100-150 W power. The substrate temperature is 80-150 “C; n-type silicon wafers and ceramics are used as substrates. In order to make the measurements of the gas-sensitive characteristics easy, thick-film Pd-Ag electrodes are provided on a ceramics substrate and a thin flhn of Fe,O, is deposited on them. The temperature of the sample is controlled by applying avoltage across the heater so the measurements of the gas-sensitive characteristics can be carried out.
3. Results and discussion Figure 2 shows the sensitivity versus working temperature of Fe,O, thin films made by NPCVD. It can be seen that the films are highly sensitive to smog. Tin doping increases the sensitivity to’smog by a big margin and decreases the sensitivity to alcohol; as a result, the sensitivity and selectivity are improved. The doping makes the resistance of the lihn in clean air, R,, versus temperature tend to flatten (Fig. 3). This property is useful for us to set the working-temperature point in practice. Table 1 shows the gas-sensitive characteristics of pure and Sn, Ti, Zrdoped films made by NPCVD. X-ray diffraction measurements have been carried out on F%Oo thin Sims made by NPCVD. Theresults show that they are a-Fe,O,. Doping makes the height
30
0 2.1 2.3 2.5 2.1 2.9 T( x 100t) (a)
IAr
Impurity
Fig. 1. NPCVD reaction system.
0925-4005/93/$6.00
0 1.5 1.9 2.3 2.7 3.1 T( x 1OOC) @I
Fig. 2. Sensitivity vs. temperature (NPCVD): (a) undoped 6lm; (b) h-doped film. Grade I smog concentration is that which makes incident light attenuate by 10% at 1 m from the light source.
Q 1993 - Elsevier Sequoia. AU rights reserved
592
‘O?z-x-xT&lOOC) Fig. 3. R. vs. temperature
(NPCVD).
TABLE 1. Gas-sensitive characteristics of FQ03 made by NPCVD’
ol
I
I
I.5 2.0 2.5 3.0351.5 2.0 ti
&doped
S-doped
Tidoped
3.0 3.5
T(r IOK)
Zr-doped
Fig. 5. l’m content modulates the sensitivity (PECVD). Temperature CC) Sensitivity (RJR3
260 26.7
270 55.0
280 28.0
260 7.5
*With grade 1 smog concentration.
I
5
.
.
10 IS 20 25 30 35 40
Fig. 6. The response-restoring
o-L 1.5
(3) I
/I
characteristics
(PECVD).
2.0 2.5 3.a 3.5 T(x 10013)
Fig. 4. Sensitivity vs. temperature
(PECVD).
of the diffraction peak decrease and widen, which shows that the doping makes the gram size small. A small grain size is of much help in increasing the sensitivity. The Fe,OJ thin films made by PECVD are very sensitive to alcohol and acetone, but are not sensitive to other interfering gases. When a certain amount of tin is doped by a suitable method, the sensitivity to acetone increases further and the selectivity improves (Fig. 4). Experiments show that the reason why tin doping can improve the gas-sensitive characteristics is that the tin content can modulate the material’s sensitivity to acetone. As shown in Fig. 5, there is no obvious effect on the sensitivity to alcohol when the tin content varies, but the effect on the sensitivity to acetone is clear. When the tin content is adequate, for example, the sample with 1.6% of Sn4+ ions, it is very sensitive to acetone and the selectivity is excellent. But for the sample with about 20% Sn4+ ions the sensitivity to acetone is about 50% of that to alcohol.
40 50 20 / degree
60
70
Fig. 7. X-ray diffraction pattern (PJZCVD).
The response-restoring characteristics of a tin-doped Fe,O, thin film made by PECVD are shown in Fig. 6. The response time to acetone with 1000 ppm concentration is within 1.5 s and the restoring time is within 6.5 s. So it can be seen that the responserestoring characteristics of this film are good. SEM morphology determination and X-ray diffraction show that the Fe,O, thin films made by PECVD are amorphous bodies with 0.2 w size. Figure 7 shows the X-ray diffraction pattern of a tin-doped F&O, thin film crystallized after heat treatment. The diffractive peak value d is a little larger than the standard value. The radius of Sn’+ is 0.74 A, a little larger than that of Fe3+ (0.67 A) [3], which shows that the Sn4+ might have entered the cells of a-Fe,O, and partly replaced Fe3+, so that a solid solution of tin-doped &Fe,O, is formed.
593
4. Conclusions Fe,O, thin films made by NPCVD and PECVJI are sensitive to smog and acetone, respectively.When tin is doped adequately, a solid solution of LX-F~O,is formed, which is amorphous with fine grain size. The sensitivity, selectivity and response-restoring characteristics of the films are all excellent, and they should have a bright future as a gas-sensor material.
References
1 Y. Nakatani and M. Matsuoka, Some ekctrical properties of yFe& ceramics, ,‘pn. 1 A&. P&S.., 22 (1983) X33-239. 2 Y. Nakatani and M. Matsuoka, Effects of sulfate ion on gas sensitive properties of a-FQO, ceramics, +n. J. Appf. Phys.., 21 (1982) L758-760. 3 R. C Weast, CRC Handbook of Chembby and Physics,CRC Press, Boca Raton, FL, 66th edn., 1985-1986, p. F-164.