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Formaldehyde sensing characteristics of an aluminum-doped zinc oxide (AZO) thin-film-based sensor
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Research Paper
Formaldehyde sensing characteristics of an aluminum-doped zinc oxide (AZO) thin-film-based sensor Cheng-Yu Chi a , Huey-Ing Chen b , Wei-Cheng Chen a , Ching-Hong Chang a , Wen-Chau Liu a,∗ a Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, 1 University Road, Tainan 70101, Taiwan, Republic of China b Department of Chemical Engineering, National Cheng-Kung University, No.1, University Road, Tainan, 70101, Taiwan, Republic of China
a r t i c l e
i n f o
Article history: Received 1 May 2017 Received in revised form 15 September 2017 Accepted 19 September 2017 Available online xxx Keywords: Aluminum-doped zinc oxide (AZO) Formaldehyde Sensing response Radio-frequency (RF) sputtering
a b s t r a c t An aluminum-doped zinc oxide (AZO) thin-film-based formaldehyde sensor prepared using radiofrequency sputtering (RF) is studied. The proposed device has a very high formaldehyde sensing response of 95.5 (@20 ppm HCHO/air, 350 ◦ C), an extremely low detection limit (≤40 ppb HCHO/air), a relatively low operating temperature (≤350 ◦ C), and a wide sensing range. Moreover, the device has a simple structure and is easily fabricated, low cost, and environmentally friendly. Therefore, the proposed AZO thin-film-based sensor is promising for high-performance formaldehyde sensing applications. © 2017 Published by Elsevier B.V.
1. Introduction Volatile organic compounds (VOCs) are mainly generated through the production and use of everyday products. Generally, VOCs are a threat to human health once concentrations exceed specific threshold levels [1]. For example, formaldehyde (HCHO), a hazardous VOC, is usually considered as the most important and major indoor air pollutant and is listed as a human carcinogen by the International Agency for Research on Cancer [2–4]. The safety limit for formaldehyde, suggested by the World Health Organization, is an exposure of 2 ppm over an 8 h period, and the indoor level should not exceed 80 ppb over a 30 min period [2,5,6]. Therefore, the development of high-performance formaldehyde sensors is important for environmental safety and human health. Numerous semiconducting metal oxides (MOs), e.g., TiO2 [7], SnO2 [8], In2 O3 [9], and NiO [10], have been used to fabricate gas sensors. The n-type MO zinc oxide (ZnO) has attracted considerable attention for gas sensing applications due to its inherent properties of environmental friendliness, low cost, good compatibility,
∗ Corresponding author. E-mail address: [email protected] (W.-C. Liu).
and multiple morphologies [11–14]. Aluminum-doped zinc oxide (AZO) has been shown to have higher conductivity and improved sensing properties compared to those of ZnO [15–18]. Surfacemodified AZO material systems, such as AZO nanoparticles [2] and an Ag-AZO composite structure [4], have been used to fabricate high-performance formaldehyde sensors. However, formaldehyde sensors based on a pristine AZO layer have rarely been reported. The present work demonstrates an AZO thin-film-based formaldehyde sensor prepared using radio-frequency (RF) sputtering. The AZO thin-film-based sensor fabricated using optimal sputtering conditions shows a significantly high sensing response of 95.5 under 20 ppm HCHO/air gas at 350 ◦ C. The sensor has the advantages of low cost, a simple structure, and easy fabrication. The proposed sensor is thus a promising candidate for high-performance formaldehyde sensing applications. 2. Experiment AZO thin film was prepared on a sapphire substrate using RF sputtering. An AZO target with a composition of ZnO:Al2 O3 = 98:2 wt% was used. First, the sapphire substrate was cleaned with acetone and hydrochloric acid for 15 min. Afterwards, 10- and 15 nm-thick chromium (Cr) and platinum (Pt) metals,
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Fig. 1. (a) Schematic cross-section diagram of the studied devices. (b) The geometric designs of the studied device and interdigitated electrodes.
Table 1 A comparison between this work and previous reports. Structure
Sensing/Response(Ra/Rg)
Detect time
Detect limitation
Reference
AZO thin film ZnO nanorods Ga-doped ZnO NPs ZnO NPs MnO2 -doped ZnO NPs SnO2 -ZnO hollow nanofibers 3D center-hollow ZnO Bulbous ZnO NRs&NFs Ag-AZO composite powders
95.5(@20 ppm, 350 ◦ C) 16(@200 ppm, 300 ◦ C) 13(@205 ppm, 400 ◦ C) 43(@205 ppm, 400 ◦ C) 25(@100 ppm, 320 ◦ C) 28.5(@20 ppm, 260 ◦ C) 672%(@20 ppm, RT) 33&40(@25 ppm, 200 ◦ C) 87.66(@100 ppm, 240 ◦ C)
60 s 3s 30 s ∼60 s 27 s 4–6 s 33 s@1 ppm 5s 47 s
40 ppb – – – – 0.1 ppm 1 ppm 5 ppm –
This work [27] [28] [29] [30] [31] [32] [33] [4]
respectively, were sequentially deposited using thermal evaporation and lift-off techniques to form interdigitated electrodes. Finally, a 60 nm-thick AZO sensing layer was deposited using RF sputtering with a power of 100 W under a working pressure of 3 mTorr. The introduced gas was O2 and the flow rate was 30 sccm. After the RF sputtering process, the samples were annealed using a rapid thermal annealing process at 400 ◦ C in an O2 ambient for 30 min. A schematic cross-sectional diagram of the studied device is depicted in Fig. 1(a). The geometric designs of the studied device and interdigitated electrodes are shown in Fig. 1(b). The active area of the studied device was 1500 × 800 m2 . The width of a metal line and the spacing between metal lines in the interdigitated electrodes were both w = 25 m and s = 25 m, respectively. The produced device was bounded onto a TO-8 metal can with Al wires and subsequently placed in a sealed stainless-steel test chamber. Synthetic dry gases with different formaldehyde concentrations were introduced into the chamber with a continuous and stable total flow rate of 500 sccm by a mass flow control system at ambient temperature. Experimental current-voltage (I–V) characteristics and transient response curves were measured using an Agilent B1500 semiconductor parameter analyzer. Typical scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of the obtained AZO thin film are shown in Fig. 2(a) and (b), respectively. Fig. 2(a) clearly shows that a relatively uniform surface morphology was obtained. The cor-
responding root-mean-square (RMS) roughness, obtained from Fig. 2(b), of the studied AZO thin film is 7.61 ± 1.67 nm. The energydispersive X-ray spectroscopy (EDS) analysis results are shown in Fig. 3. Al, Zn, and O peaks can be seen. It is believed that the significant Al peak is mainly from the sapphire (Al2 O3 ) substrate. This spectrum indicates a clean and pure AZO structure based on the absence of other impurities. The X-ray diffraction (XRD), measured by a Rigaku D/MAX 2500 system with a Cu-K␣ line ( = 1.54184 Å), pattern of the studied AZO thin film is shown in Fig. 4. All the diffraction peaks are consistent with the hexagonal structure of ZnO (JCPDS card no. 89-1397) [2,19]. In addition, the relatively sharp diffraction peaks indicate the high crystalline quality of the studied AZO thin film. 3. Results and discussion The formaldehyde sensing mechanism on an AZO surface is illustrated in Fig. 5. In an air ambient, oxygen molecules can capture − 2− electrons from the conduction band of AZO to form O(ads) , O(ads) ,
− adsorbed ions on the AZO thin film surface. This extends and O2(ads)
the surface depletion layer (with depletion width d) of AZO. When a reducing gas, e.g., formaldehyde, is introduced, electrons can be released back to the AZO material due to a reaction between formaldehyde and adsorbed oxygen ions. This decreases (increases) the depletion layer width (conductivity) of n-type AZO, as shown
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Fig. 2. (a) Typical scanning electron microscopy (SEM) and (b) atomic force microscopy (AFM) images of the studied AZO thin film.
in Fig. 5. The reduced depletion width (d ) is also indicated in Fig. 5. The related reactions can be expressed as [20,21]: − HCHO + 2O(ads) → CO2 + H2 O + 2e−
(1)
2− HCHO + 2O(ads) → CO2 + H2 O + 4e−
(2)
− HCHO + O2(ads) → CO2 + H2 O + e−
(3)
Fig. 6 shows the experimental I–V characteristics measured under 40 ppb, 400 ppb, 1 ppm, 4 ppm, and 20 ppm HCHO/air gas at 350 ◦ C. Good Ohmic (linear I–V curves) behavior under the applied negative and positive voltages are observed. The current (resistance) is increased (decreased) with increasing formaldehyde concentration. For example, under an applied voltage of 1 V, the current (resistance) is increased (decreased) from 5.6 × 10−6 A (178.6 k) to 9.6 × 10−4 A (1.04 k) when the formaldehyde con-
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Fig. 3. The energy-dispersive X-ray spectroscopy (EDS) analysis results of the studied AZO thin film.
Fig. 6. I-V characteristics of studied device with different HCHO/air concentrations. Fig. 4. The X-ray diffraction (XRD) pattern of the studied AZO thin film.
centration is increased from 40 ppb to 20 ppm HCHO/air. Generally, the sensing response SR of MOS can be defined as [22]: SR =
Rair − Rgas Rgas
(4)
Fig. 5. Schematic diagram of formaldehyde sensing mechanism on the AZO surface.
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Fig. 7. Sensing response (SR ) versus temperatures under 40 ppb, 400 ppb, 1 ppm, 4 ppm and 20 ppm HCHO/air gases. A magnified sensing diagram under 40 ppb HCHO/air gas is shown in the inset.
where Rair and Rgas are the resistances measured in air and target gas ambients, respectively. SR as a function of temperature is shown in Fig. 7. A magnified diagram of the corresponding performance under 40 ppb HCHO/air gas is shown in the inset. SR is increased with increasing temperature in the range of 300–350 ◦ C and then decreased with a further increase in temperature. Thus, the optimal operating temperature for formaldehyde detection is 300–350 ◦ C. Experimentally, a very high SR of 79.5 (95.5) was obtained under 20 ppm HCHO/air gas at 300 ◦ C (350 ◦ C). This result is superior to those previously reported for MO-based formaldehyde sensors [23–30]. Furthermore, an SR of 1.84 (0.55) is obtained under an extremely low concentration of 40 ppb HCHO/air at 300 ◦ C (350 ◦ C). In general, the SR of MO-based gas sensors can be empirically expressed as [31,32]: SR = A(C)ˇ
(5)
Where A is a prefactor, C is the concentration of the target gas, and ˇ is the exponent [33]. A logarithmic plot between sensing response SR and formaldehyde concentration obtained at 350 ◦ C is shown in Fig. 8. An approximate linearship between log(SR ) and log(CHCHO ) under 40 ppb, 400 ppb, 1 ppm, 4 ppm, and 20 ppm HCHO/air gases is obtained. A ˇ value of 0.848, determined by linear fitting, is obtained for the studied device. Ideal ˇ values of 0.5 and 1, due to surface reactions between reducing gases (e.g., H2 , NH3 , and 2− HCHO) and chemisorbed oxygen species, are observed for O(ads)
− and O(ads) ions, respectively [34–36]. In this work, the ˇ value of
0.848 shows that the surface reaction of the studied device may be predominated by O− ions. (ads) The transient behaviors of the studied device under 20 ppm HCHO/air gas at various temperatures are shown in Fig. 9(a). Magnified diagrams of the corresponding performance at 200 and 250 ◦ C are shown in the inset. The applied voltage is kept at VA = 1 V. The response current is increased (decreased) when the formaldehyde gas is introduced (removed). In addition, the response speed is significantly increased when the temperature is increased from 200 to 350 ◦ C. Fig. 9(b) shows three repetitive dynamic responses measured upon the introduction and removal
Fig. 8. Logarithmic plot between sensing response SR and formaldehyde concentration at 350 ◦ C.
of 20 ppm HCHO/air gas at 350 ◦ C. The studied device demonstrates stable and repeatable formaldehyde sensing performance. The response (recovery) time a ( b ) is defined as the time needed for 90% of a full response (recovery) [32]. The response and recovery times ( a and b ) as functions of temperature under 4 and 20 ppm HCHO/air gases are shown in Fig. 10. Magnified diagrams of the corresponding performance at 300 and 350 ◦ C are shown in the inset. The applied voltage is fixed at VA = 1 V. Both a and b are rapidly decreased with increasing temperature. For example, under 20 ppm HCHO/air gas, a ( b ) is decreased from 8426 s (1402 s) to 60 s (26 s) when the temperature is increased from 200 to 350 ◦ C. The selectivity of the studied AZO device is investigated by exposing the device to hydrogen, ammonia, ethanol, methane, carbon monoxide, and formaldehyde gases with concentrations of 20 ppm H2 /air, 35 ppm NH3 /air, 100 ppm CH4 /air, 100 ppm C2 H5 OH/air, 200 ppm CO/air, and 20 ppm HCHO/air, respectively. The related histogram analysis on sensing response at 350 ◦ C is illustrated
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Fig. 9. (a)Transient response curves upon exposure to 20 ppm HCHO/air gas at 200, 250, 300 and 350 ◦ C. Magnified diagrams of the corresponding performance at 200 and 250 ◦ C are shown in the inset. (b) Repeatable transient response curves under 20 ppm HCHO/air gas at 350 ◦ C.
in Fig. 11. Clearly, the sensing response of the studied AZO device to 20 ppm HCHO/air is 95.53, which is much higher than 0.56–20 ppm H2 /air, 8.34–35 ppm NH3 /air, 5.3–100 ppm CH4 /air, 2.9–100 ppm C2 H5 OH/air, and 0.24–200 ppm CO/air gases. This demonstrates the good selectivity of the studied AZO-based formaldehyde sensor. A comparison of formaldehyde sensing performance between the proposed sensor and existing sensors is presented in Table 1. The studied device shows a remarkably high sensing response of 95.5 under 20 ppm HCHO/air gas at 350 ◦ C. This performance is superior to that of other sensors.
4. Conclusion An AZO thin-film-based formaldehyde sensor prepared using RF sputtering is studied. Experimentally, the studied sensor shows good formaldehyde sensing performance, including a very high sensing response of 95.5 (@20 ppm HCHO/air, 350 ◦ C) and an extremely low detection limit (≤40 ppb HCHO/air). The optimal operating temperature of the studied device is 350 ◦ C. Furthermore, the device exhibits the advantages of a simple structure, easy fabrication, low cost, relatively low operating temperature (≤350 ◦ C), and wide sensing range. The proposed AZO thin-film-based sensor device is thus promising for high-performance formaldehyde sensing applications.
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Fig. 10. Response and recovery time constants a and b versus temperature with 4 and 20 ppm HCHO/air gases. Magnified diagrams of the corresponding performance at 300 and 350 ◦ C are shown in the inset.
Fig. 11. Histogram analysis sensing responses of different gases at 350 ◦ C.
Acknowledgements Part of this work was supported by the Ministry of Science and Technology of the Republic of China under grant NSC-100-2221E-006-244-MY3 and the Advanced Optoelectronic Technology Center, National Cheng Kung University.
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Biographies Cheng-Yu Chi is currently pursuing the M.S. degree with the Institute of Microelectronics and Department of Electrical Engineering, National Cheng-Kung University, Tainan, Taiwan. His research has focused on semiconductor gas sensors. Huey-Ing Chen received the B.S., M.S., and Ph.D. degrees from Cheng Kung University (NCKU), Tainan, Taiwan, in 1979, 1981, and 1994, respectively, all in chemical engineering. She joined the faculty at NCKU as an Instructor, an Associate Professor, and a Professor in the Department of Chemical Engineering in 1981, 1994, and 2003, respectively. She is currently a professor in the same department. Her research presently focuses on chemical sensors, nanomaterials, gas membrane separation and, adsorption and catalysis. Wei-Cheng Chen is currently pursuing the Ph.D. degree with the Institute of Microelectronics and Department of Electrical Engineering, National Cheng-Kung University, Tainan, Taiwan. His research has focused on the III–V compound lightemitting diodes. Ching-Hong Chang is currently working toward the Ph.D. degree in the Institute of Microelectronics and the Department of Electrical Engineering, NCKU, Tainan, Taiwan. His research has focused on the III–V heterostructure field-effecttransistors and field of semiconductor gas sensors. Wen-Chau Liu received the B.S., M.S., and Ph.D. degrees in electrical engineering from the National Cheng-Kung University (NCKU), Tainan, Taiwan, in 1979, 1981, and 1986, respectively. He was with the faculty at National Cheng-Kung University, as an Instructor and an Associate Professor with the Department of Electrical Engineering in 1983, 1986, and 1992, respectively. Since 2002, he has been a Distinguished Professor in the same department. He has published more than 300 journal papers. He is the holder of 54 patents in the semiconductor field.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Research Paper
Formaldehyde sensing characteristics of an aluminum-doped zinc oxide (AZO) thin-film-based sensor Cheng-Yu Chi a , Huey-Ing Chen b , Wei-Cheng Chen a , Ching-Hong Chang a , Wen-Chau Liu a,∗ a Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, 1 University Road, Tainan 70101, Taiwan, Republic of China b Department of Chemical Engineering, National Cheng-Kung University, No.1, University Road, Tainan, 70101, Taiwan, Republic of China
a r t i c l e
i n f o
Article history: Received 1 May 2017 Received in revised form 15 September 2017 Accepted 19 September 2017 Available online xxx Keywords: Aluminum-doped zinc oxide (AZO) Formaldehyde Sensing response Radio-frequency (RF) sputtering
a b s t r a c t An aluminum-doped zinc oxide (AZO) thin-film-based formaldehyde sensor prepared using radiofrequency sputtering (RF) is studied. The proposed device has a very high formaldehyde sensing response of 95.5 (@20 ppm HCHO/air, 350 ◦ C), an extremely low detection limit (≤40 ppb HCHO/air), a relatively low operating temperature (≤350 ◦ C), and a wide sensing range. Moreover, the device has a simple structure and is easily fabricated, low cost, and environmentally friendly. Therefore, the proposed AZO thin-film-based sensor is promising for high-performance formaldehyde sensing applications. © 2017 Published by Elsevier B.V.
1. Introduction Volatile organic compounds (VOCs) are mainly generated through the production and use of everyday products. Generally, VOCs are a threat to human health once concentrations exceed specific threshold levels [1]. For example, formaldehyde (HCHO), a hazardous VOC, is usually considered as the most important and major indoor air pollutant and is listed as a human carcinogen by the International Agency for Research on Cancer [2–4]. The safety limit for formaldehyde, suggested by the World Health Organization, is an exposure of 2 ppm over an 8 h period, and the indoor level should not exceed 80 ppb over a 30 min period [2,5,6]. Therefore, the development of high-performance formaldehyde sensors is important for environmental safety and human health. Numerous semiconducting metal oxides (MOs), e.g., TiO2 [7], SnO2 [8], In2 O3 [9], and NiO [10], have been used to fabricate gas sensors. The n-type MO zinc oxide (ZnO) has attracted considerable attention for gas sensing applications due to its inherent properties of environmental friendliness, low cost, good compatibility,
∗ Corresponding author. E-mail address: [email protected] (W.-C. Liu).
and multiple morphologies [11–14]. Aluminum-doped zinc oxide (AZO) has been shown to have higher conductivity and improved sensing properties compared to those of ZnO [15–18]. Surfacemodified AZO material systems, such as AZO nanoparticles [2] and an Ag-AZO composite structure [4], have been used to fabricate high-performance formaldehyde sensors. However, formaldehyde sensors based on a pristine AZO layer have rarely been reported. The present work demonstrates an AZO thin-film-based formaldehyde sensor prepared using radio-frequency (RF) sputtering. The AZO thin-film-based sensor fabricated using optimal sputtering conditions shows a significantly high sensing response of 95.5 under 20 ppm HCHO/air gas at 350 ◦ C. The sensor has the advantages of low cost, a simple structure, and easy fabrication. The proposed sensor is thus a promising candidate for high-performance formaldehyde sensing applications. 2. Experiment AZO thin film was prepared on a sapphire substrate using RF sputtering. An AZO target with a composition of ZnO:Al2 O3 = 98:2 wt% was used. First, the sapphire substrate was cleaned with acetone and hydrochloric acid for 15 min. Afterwards, 10- and 15 nm-thick chromium (Cr) and platinum (Pt) metals,
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Fig. 1. (a) Schematic cross-section diagram of the studied devices. (b) The geometric designs of the studied device and interdigitated electrodes.
Table 1 A comparison between this work and previous reports. Structure
Sensing/Response(Ra/Rg)
Detect time
Detect limitation
Reference
AZO thin film ZnO nanorods Ga-doped ZnO NPs ZnO NPs MnO2 -doped ZnO NPs SnO2 -ZnO hollow nanofibers 3D center-hollow ZnO Bulbous ZnO NRs&NFs Ag-AZO composite powders
95.5(@20 ppm, 350 ◦ C) 16(@200 ppm, 300 ◦ C) 13(@205 ppm, 400 ◦ C) 43(@205 ppm, 400 ◦ C) 25(@100 ppm, 320 ◦ C) 28.5(@20 ppm, 260 ◦ C) 672%(@20 ppm, RT) 33&40(@25 ppm, 200 ◦ C) 87.66(@100 ppm, 240 ◦ C)
60 s 3s 30 s ∼60 s 27 s 4–6 s 33 s@1 ppm 5s 47 s
40 ppb – – – – 0.1 ppm 1 ppm 5 ppm –
This work [27] [28] [29] [30] [31] [32] [33] [4]
respectively, were sequentially deposited using thermal evaporation and lift-off techniques to form interdigitated electrodes. Finally, a 60 nm-thick AZO sensing layer was deposited using RF sputtering with a power of 100 W under a working pressure of 3 mTorr. The introduced gas was O2 and the flow rate was 30 sccm. After the RF sputtering process, the samples were annealed using a rapid thermal annealing process at 400 ◦ C in an O2 ambient for 30 min. A schematic cross-sectional diagram of the studied device is depicted in Fig. 1(a). The geometric designs of the studied device and interdigitated electrodes are shown in Fig. 1(b). The active area of the studied device was 1500 × 800 m2 . The width of a metal line and the spacing between metal lines in the interdigitated electrodes were both w = 25 m and s = 25 m, respectively. The produced device was bounded onto a TO-8 metal can with Al wires and subsequently placed in a sealed stainless-steel test chamber. Synthetic dry gases with different formaldehyde concentrations were introduced into the chamber with a continuous and stable total flow rate of 500 sccm by a mass flow control system at ambient temperature. Experimental current-voltage (I–V) characteristics and transient response curves were measured using an Agilent B1500 semiconductor parameter analyzer. Typical scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of the obtained AZO thin film are shown in Fig. 2(a) and (b), respectively. Fig. 2(a) clearly shows that a relatively uniform surface morphology was obtained. The cor-
responding root-mean-square (RMS) roughness, obtained from Fig. 2(b), of the studied AZO thin film is 7.61 ± 1.67 nm. The energydispersive X-ray spectroscopy (EDS) analysis results are shown in Fig. 3. Al, Zn, and O peaks can be seen. It is believed that the significant Al peak is mainly from the sapphire (Al2 O3 ) substrate. This spectrum indicates a clean and pure AZO structure based on the absence of other impurities. The X-ray diffraction (XRD), measured by a Rigaku D/MAX 2500 system with a Cu-K␣ line ( = 1.54184 Å), pattern of the studied AZO thin film is shown in Fig. 4. All the diffraction peaks are consistent with the hexagonal structure of ZnO (JCPDS card no. 89-1397) [2,19]. In addition, the relatively sharp diffraction peaks indicate the high crystalline quality of the studied AZO thin film. 3. Results and discussion The formaldehyde sensing mechanism on an AZO surface is illustrated in Fig. 5. In an air ambient, oxygen molecules can capture − 2− electrons from the conduction band of AZO to form O(ads) , O(ads) ,
− adsorbed ions on the AZO thin film surface. This extends and O2(ads)
the surface depletion layer (with depletion width d) of AZO. When a reducing gas, e.g., formaldehyde, is introduced, electrons can be released back to the AZO material due to a reaction between formaldehyde and adsorbed oxygen ions. This decreases (increases) the depletion layer width (conductivity) of n-type AZO, as shown
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Fig. 2. (a) Typical scanning electron microscopy (SEM) and (b) atomic force microscopy (AFM) images of the studied AZO thin film.
in Fig. 5. The reduced depletion width (d ) is also indicated in Fig. 5. The related reactions can be expressed as [20,21]: − HCHO + 2O(ads) → CO2 + H2 O + 2e−
(1)
2− HCHO + 2O(ads) → CO2 + H2 O + 4e−
(2)
− HCHO + O2(ads) → CO2 + H2 O + e−
(3)
Fig. 6 shows the experimental I–V characteristics measured under 40 ppb, 400 ppb, 1 ppm, 4 ppm, and 20 ppm HCHO/air gas at 350 ◦ C. Good Ohmic (linear I–V curves) behavior under the applied negative and positive voltages are observed. The current (resistance) is increased (decreased) with increasing formaldehyde concentration. For example, under an applied voltage of 1 V, the current (resistance) is increased (decreased) from 5.6 × 10−6 A (178.6 k) to 9.6 × 10−4 A (1.04 k) when the formaldehyde con-
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Fig. 3. The energy-dispersive X-ray spectroscopy (EDS) analysis results of the studied AZO thin film.
Fig. 6. I-V characteristics of studied device with different HCHO/air concentrations. Fig. 4. The X-ray diffraction (XRD) pattern of the studied AZO thin film.
centration is increased from 40 ppb to 20 ppm HCHO/air. Generally, the sensing response SR of MOS can be defined as [22]: SR =
Rair − Rgas Rgas
(4)
Fig. 5. Schematic diagram of formaldehyde sensing mechanism on the AZO surface.
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Fig. 7. Sensing response (SR ) versus temperatures under 40 ppb, 400 ppb, 1 ppm, 4 ppm and 20 ppm HCHO/air gases. A magnified sensing diagram under 40 ppb HCHO/air gas is shown in the inset.
where Rair and Rgas are the resistances measured in air and target gas ambients, respectively. SR as a function of temperature is shown in Fig. 7. A magnified diagram of the corresponding performance under 40 ppb HCHO/air gas is shown in the inset. SR is increased with increasing temperature in the range of 300–350 ◦ C and then decreased with a further increase in temperature. Thus, the optimal operating temperature for formaldehyde detection is 300–350 ◦ C. Experimentally, a very high SR of 79.5 (95.5) was obtained under 20 ppm HCHO/air gas at 300 ◦ C (350 ◦ C). This result is superior to those previously reported for MO-based formaldehyde sensors [23–30]. Furthermore, an SR of 1.84 (0.55) is obtained under an extremely low concentration of 40 ppb HCHO/air at 300 ◦ C (350 ◦ C). In general, the SR of MO-based gas sensors can be empirically expressed as [31,32]: SR = A(C)ˇ
(5)
Where A is a prefactor, C is the concentration of the target gas, and ˇ is the exponent [33]. A logarithmic plot between sensing response SR and formaldehyde concentration obtained at 350 ◦ C is shown in Fig. 8. An approximate linearship between log(SR ) and log(CHCHO ) under 40 ppb, 400 ppb, 1 ppm, 4 ppm, and 20 ppm HCHO/air gases is obtained. A ˇ value of 0.848, determined by linear fitting, is obtained for the studied device. Ideal ˇ values of 0.5 and 1, due to surface reactions between reducing gases (e.g., H2 , NH3 , and 2− HCHO) and chemisorbed oxygen species, are observed for O(ads)
− and O(ads) ions, respectively [34–36]. In this work, the ˇ value of
0.848 shows that the surface reaction of the studied device may be predominated by O− ions. (ads) The transient behaviors of the studied device under 20 ppm HCHO/air gas at various temperatures are shown in Fig. 9(a). Magnified diagrams of the corresponding performance at 200 and 250 ◦ C are shown in the inset. The applied voltage is kept at VA = 1 V. The response current is increased (decreased) when the formaldehyde gas is introduced (removed). In addition, the response speed is significantly increased when the temperature is increased from 200 to 350 ◦ C. Fig. 9(b) shows three repetitive dynamic responses measured upon the introduction and removal
Fig. 8. Logarithmic plot between sensing response SR and formaldehyde concentration at 350 ◦ C.
of 20 ppm HCHO/air gas at 350 ◦ C. The studied device demonstrates stable and repeatable formaldehyde sensing performance. The response (recovery) time a ( b ) is defined as the time needed for 90% of a full response (recovery) [32]. The response and recovery times ( a and b ) as functions of temperature under 4 and 20 ppm HCHO/air gases are shown in Fig. 10. Magnified diagrams of the corresponding performance at 300 and 350 ◦ C are shown in the inset. The applied voltage is fixed at VA = 1 V. Both a and b are rapidly decreased with increasing temperature. For example, under 20 ppm HCHO/air gas, a ( b ) is decreased from 8426 s (1402 s) to 60 s (26 s) when the temperature is increased from 200 to 350 ◦ C. The selectivity of the studied AZO device is investigated by exposing the device to hydrogen, ammonia, ethanol, methane, carbon monoxide, and formaldehyde gases with concentrations of 20 ppm H2 /air, 35 ppm NH3 /air, 100 ppm CH4 /air, 100 ppm C2 H5 OH/air, 200 ppm CO/air, and 20 ppm HCHO/air, respectively. The related histogram analysis on sensing response at 350 ◦ C is illustrated
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Fig. 9. (a)Transient response curves upon exposure to 20 ppm HCHO/air gas at 200, 250, 300 and 350 ◦ C. Magnified diagrams of the corresponding performance at 200 and 250 ◦ C are shown in the inset. (b) Repeatable transient response curves under 20 ppm HCHO/air gas at 350 ◦ C.
in Fig. 11. Clearly, the sensing response of the studied AZO device to 20 ppm HCHO/air is 95.53, which is much higher than 0.56–20 ppm H2 /air, 8.34–35 ppm NH3 /air, 5.3–100 ppm CH4 /air, 2.9–100 ppm C2 H5 OH/air, and 0.24–200 ppm CO/air gases. This demonstrates the good selectivity of the studied AZO-based formaldehyde sensor. A comparison of formaldehyde sensing performance between the proposed sensor and existing sensors is presented in Table 1. The studied device shows a remarkably high sensing response of 95.5 under 20 ppm HCHO/air gas at 350 ◦ C. This performance is superior to that of other sensors.
4. Conclusion An AZO thin-film-based formaldehyde sensor prepared using RF sputtering is studied. Experimentally, the studied sensor shows good formaldehyde sensing performance, including a very high sensing response of 95.5 (@20 ppm HCHO/air, 350 ◦ C) and an extremely low detection limit (≤40 ppb HCHO/air). The optimal operating temperature of the studied device is 350 ◦ C. Furthermore, the device exhibits the advantages of a simple structure, easy fabrication, low cost, relatively low operating temperature (≤350 ◦ C), and wide sensing range. The proposed AZO thin-film-based sensor device is thus promising for high-performance formaldehyde sensing applications.
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Fig. 10. Response and recovery time constants a and b versus temperature with 4 and 20 ppm HCHO/air gases. Magnified diagrams of the corresponding performance at 300 and 350 ◦ C are shown in the inset.
Fig. 11. Histogram analysis sensing responses of different gases at 350 ◦ C.
Acknowledgements Part of this work was supported by the Ministry of Science and Technology of the Republic of China under grant NSC-100-2221E-006-244-MY3 and the Advanced Optoelectronic Technology Center, National Cheng Kung University.
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Biographies Cheng-Yu Chi is currently pursuing the M.S. degree with the Institute of Microelectronics and Department of Electrical Engineering, National Cheng-Kung University, Tainan, Taiwan. His research has focused on semiconductor gas sensors. Huey-Ing Chen received the B.S., M.S., and Ph.D. degrees from Cheng Kung University (NCKU), Tainan, Taiwan, in 1979, 1981, and 1994, respectively, all in chemical engineering. She joined the faculty at NCKU as an Instructor, an Associate Professor, and a Professor in the Department of Chemical Engineering in 1981, 1994, and 2003, respectively. She is currently a professor in the same department. Her research presently focuses on chemical sensors, nanomaterials, gas membrane separation and, adsorption and catalysis. Wei-Cheng Chen is currently pursuing the Ph.D. degree with the Institute of Microelectronics and Department of Electrical Engineering, National Cheng-Kung University, Tainan, Taiwan. His research has focused on the III–V compound lightemitting diodes. Ching-Hong Chang is currently working toward the Ph.D. degree in the Institute of Microelectronics and the Department of Electrical Engineering, NCKU, Tainan, Taiwan. His research has focused on the III–V heterostructure field-effecttransistors and field of semiconductor gas sensors. Wen-Chau Liu received the B.S., M.S., and Ph.D. degrees in electrical engineering from the National Cheng-Kung University (NCKU), Tainan, Taiwan, in 1979, 1981, and 1986, respectively. He was with the faculty at National Cheng-Kung University, as an Instructor and an Associate Professor with the Department of Electrical Engineering in 1983, 1986, and 1992, respectively. Since 2002, he has been a Distinguished Professor in the same department. He has published more than 300 journal papers. He is the holder of 54 patents in the semiconductor field.
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