Japanese Journal of Applied Physics Vol. 47, No. 4, 2008, pp. 3272–3276 #2008 The Japan Society of Applied Physics
Development of Gas Sensors Based on Tungsten Oxide Nanowires in Metal/SiO2 /Metal Structure and Their Sensing Responses to NO2 Rong-Ming K O, Shui-Jinn W ANG, Zhi-Fu W EN, Jun-Ku LIN, Ga-Hong FAN, Wen-I SHU, and Bor-Wen L IOU1 Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, Republic of China 1 Department of Computer Science and Information Engineering, Wu-Feng Institute of Technology, Chiayi Country 621, Taiwan, Republic of China (Received October 2, 2007; accepted December 20, 2007; published online April 25, 2008)
In this work, the fabrication of gas sensors utilizing self-synthesized tungsten oxide nanowires (TONWs) and their response to NO2 are reported. The gas sensor is based on a sputter-deposited WCx /SiO2 /WCx triple-layer structure with the periphery of the SiO2 layer etched chemically. Self-synthesized TONWs with crystalline W18 O49 (010) were grown by simple thermal annealing in nitrogen ambient, which linked, in parallel, the upper and lower WCx electrodes for gas sensing. The TONW-based sensors increased in resistance in NO2 because the surface of TONWs comprised oxygen adsorbates and the adjacent space charge region was electron-depleted. The amount of enlargement in resistance increased with increasing temperature. To improve the detectability of the parallel-connected TONW-based sensor, a connection of several individual sensors in series was proposed to enlarge the number of TONWs for gas sensing. For the 8-series-connected sensor, a sensitivity as high as 9.3, a response time as low as about 9 s, and a detectability as low as 2 ppm for NO2 were obtained. [DOI: 10.1143/JJAP.47.3272] KEYWORDS: gas sensor, tungsten carbide, tungsten oxide nanowires, nitrogen dioxide, sensitivity, detectability
1.
Introduction
Because the electrical properties of metal oxides (MOs) depend strongly on the composition of the surrounding gas atmosphere, compact solid-state (SS) chemical sensors have been widely used in chemical process control, pollutant monitoring, personal safety and medical diagnosis, for example.1) Among them, sensors employing MOs, such as SnO2 ,2) TiO2 ,3) ZnO,4) MoO3 ,5) and WO3 ,6–11) with the advantages of high sensitivity, low cost, simplicity of fabrication, and process compatible with very large scale integration (VLSI) technology, have been demonstrated. Tungsten oxides (TOs) have been shown to be a highly promising candidate for SS sensor fabrication because they have a superior sensitivity to gases including NH3 , H2 S, NO2 , NO, CO, and H2 . However, conventional sensors based on thin films of nanocrystalline or polycrystalline TOs usually exhibit poor performance because carrier transport in these sensors essentially relies upon the adsorption and desorption of gas moleculars near or at grain boundaries on the surface of the film only. The limited surface-to-volume ratio (SVR) of film sensors is one of the obstacles to the further enhancement of both sensitivity and response in gas detection. In contrast, nanosized materials such as carbon nanotubes (CNTs),12) SnO2 nanoribbons,13) SnO2 nanowires,14) In2 O3 nanowires,15) and tungsten-based nanowires,16–21) have attracted much attention in the fabrication of SS sensors because they have very high SVRs and good response for chemical sensing. In recent years, nanosensors with superior characteristics, such as high sensitivity, high response speed, and applicability to room-temperature operation, have been demonstrated. Nevertheless, the utilization of the appropriate nanomaterials having low cost, ease of fabrication, and a process compatible with VLSI technology is still an open problem. Recently, the authors’ group has developed a simple method for the self-synthesis of tungsten oxide nanowires (TONWs) and tungsten carbide nanowires for volume fabrication using sputtering deposition and thermal anneal
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ing.16–21) TONWs with typical diameters of 17 – 23 nm, typical lengths of 0.2 – 0.3 mm, and typical densities of 150 – 720 mm2 have been successfully grown. Essentially, the use of TONWs as gas sensing materials is very attractive because they exhibit physical and chemical properties that are very different from those of their bulk or film counterparts. In addition, the sensitivity of TONW sensors to gases can be substantially improved and the working temperature can also be reduced because of the large SVR as well as the enormous number of surface atoms and/or dimensional confinement of electrons. In this paper, a MO gas sensor utilizing self-synthesized TONWs in a metal/SiO2 /metal sandwich structure is proposed. The material properties and electrical characteristics of TONWs and their advantages, including easy wire length control in linking electrodes of the sensor, high SVR, and low working temperature, were demonstrated. The response of the prepared TONW-based gas sensors to NO2 of different partial pressures at different working temperatures is presented and discussed. Explanations for the sensing of NO2 and improving the detectability of sensors by increasing the number of sensors in a series connection were given and discussed. 2.
Experiments
A schematic diagram of the proposed three-layer sensor structure utilizing TONWs as gas sensing elements is shown in Fig. 1. The TONWs grown in the space confined by the top and bottom electrodes with a distance determined by the thickness of the SiO2 layer were used as gas sensing nanomaterials. Figure 2 shows the key fabrication processes of the proposed TONW-based gas sensor of a metal/SiO2 / metal sandwich structure. N-type 0.1 cm h100i Si wafers were used as substrates. After wafer cleaning, RF sputtering and plasma-enhanced chemical vapor deposition (PECVD) were employed for the deposition of electrodes and the SiO2 layer, respectively. A WCx (W : C ¼ 70 : 30 wt %) target with a purity of 99.5% was used. Electrode deposition at a ˚ /s was conducted at a power deposition rate of around 0.2 A 3 of 100 W under 2 10 Torr. The thicknesses of the bottom and top electrodes were 60 and 120 nm, respectively. The middle 300-nm-thick SiO2 layer was deposited using
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Metal (120 nm)
WCx
SiO2 (300 nm)
SiO2
Metal (60 nm)
WCx
n-type Si
n-type Si Fig. 1. (Color online) Schematic diagram of proposed three-layer sensor structure utilizing TONWs as gas sensing elements.
(a)
120 nm 300 nm 60 nm (b) Fig. 3. (Color online) SEM images of proposed gas sensor at key fabrication stages: (a) after edge etching of middle SiO2 layer and (b) after self-synthesis of TONWs. Fig. 2. (Color online) Key fabrication processes of proposed TONWbased gas sensor.
SiH4 : N2 O ¼ 250 : 50 sccm at 300 C at a working pressure of 0.5 Torr and an RF power of 100 W. Note that the thickness of the SiO2 layer determines the appropriate distance between the two electrodes for the linkage of TONWs grown upward or downward by the following thermal annealing, without the use of submicron photolithography technology. The devices were patterned by reactive ion etching (RIE). After that, the samples were subjected to buffer oxide wet etching for 90 s to remove the SiO2 layer from the periphery. Finally, thermal annealing at 750 C in N2 (60 sccm) ambient for 30 min was conducted for the selfgrowth of TONWs in the window area created by the side etching of the SiO2 layer. The surface morphology of the samples before and after oxidation was examined by scanning electron microscopy (SEM). The diameter and nanostructure of the oxidized nanowires were inspected by transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) pattern analysis. X-ray diffraction (XRD) patterns of Cu K ˚ ) were used to identify the radiation (wavelength of 1.5418 A phase and microstructure of the films. 3.
Results and Discussion
Figures 3(a) and 3(b) show SEM images of the proposed device structure immediately after edge etching of the middle SiO2 layer and the self-synthesis of TONWs, respec-
tively. It is seen that some TONWs with lengths 0.3 mm link the top and bottom electrodes [Fig. 3(b)]. Note that TONWs had good length controllability (wire length 0.15 – 0.4 mm) within a wide range of thermal budgets used for the thermal annealing.17,19) According to the current– voltage (I–V) measurement, the proposed three-layer structure with an area of 1 1 mm2 conducts currents of 2 1010 and 4 107 A at 2 V before and after thermal annealing, respectively, indicating the firm linking of TONWs between the top and bottom electrodes. Essentially, a single TONW with typical lengths of 0.15 – 0.4 mm and diameters of 17 – 25 nm has SVRs of around ð1:6 { 2:35Þ 106 cm1 , which are equivalent to specific surface areas (SSAs) of around 20.8 – 30.8 m2 /g, when a density of 7.71 g/ cm3 was used for the TONW.22) Both the SVRs and SSAs were found to be about 7.5 –11.6 times higher than those of the film WO3 sensors prepared at a firing temperature of 700 C.8) Figure 4 shows XRD patterns of the WCx electrodes after thermal annealing in nitrogen for 30 min at different temperatures. Our experimental results showed that no nanowires were observed in samples annealed at temperatures below 700 C. For the 750 C-annealed samples, dense nanowires were observed and clear (010) peaks at around 23 – 24 were seen, which reflect that the main phase of the TONWs may be monoclinic W18 O49 . The present results are different from those of our previous work.16,17) in annealing temperature. It is suspected that, because the present sample was subjected to PECVD and BOE, it might
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WO3
WO2/W2C
W5Si3
W2C
α-W
WO3-x
750°C 550°C 350°C as-deposited
20
30
40
50
60
70
80
1.05 1.04
Normalized Resistance
W18O49
Sensitivity ( RNO2 /RAIR )
Intensity (arb. unit)
1.08
1.06
AIR
200°C
AIR
NO2 100ppm
1.06
180°C
1.04
160°C
1.02 1.00 0.98
0
3 6 Time ( × 100 s )
1.03
9
NO2 100ppm
1.02 1.01
2 theta
160
180
200
Temperature ( °C )
Fig. 4. (Color online) XRD patterns of WCx electrodes after thermal annealing in nitrogen for 30 min at different temperatures.
Fig. 6. (Color online) Influence of working temperature on gas sensing performance of proposed TONW-based gas sensors.
Fig. 5. TEM images of single nanowire obtained from WCx film after thermal annealing at 750 C for 30 min. The inset shows the corresponding SAED pattern.
contain extra oxygen-related particles inside the device structure. In addition, a SiO2 layer was sandwiched between the two sputter-deposited WCx films during thermal annealing; as a result, the self-growth of nanowires, even in the same nitrogen ambient, might be strongly affected by additional oxygen. In general, the self-catalytic growth of TONWs should be attributed mainly to the formation of -W2 C structures initially caused by carbon depletion in the WCx films and the subsequently accompanying oxidation during thermal annealing.16,17) The nanocrystalline structure of TONWs was also examined by TEM and SAED analysis. Figure 5 presents typical TEM images of a single nanowire obtained from the WCx film after thermal annealing at 750 C for 30 min. The inset shows the corresponding SAED pattern. Clear stripes of the lattice plane and the associated SAED pattern indicate that
the nanowire is well crystallized. On the basis of the present results, the interplane distance of d space was determined to ˚ . The crystallization phase of the TONW could be 3.78 A be identified as nonstoichiometric monoclinic W18 O49 (010) according to JCPDS card no. 36-0101. The nonstoichiometric W18 O49 films should have more favorable absorption sites, because of their oxygen-deficient defect structure, than the stoichiometric WO3 with several active sites.11) Since the monoclinic W18 O49 behaves as an n-type semiconductor with oxygen vacancies,10) the adsorption of oxygen-gas related molecules reduces oxide vacancies in the wires and results in a reduced electron concentration in the conduction band; it thus causes the resistance of TONWs to increase with the amount of oxygen adsorption. Figure 6 shows the typical measured responses of the TONW-based sensors to NO2 at different working temperatures. Note that 100 ppm NO2 was introduced into the detection chamber and stabilized during measurement. The working temperature (Tw ) of gas sensors was varied from 160 to 200 C. Here, the sensitivity of a gas sensor is S¼
Rgas R ¼1þ ; Rair Rair
ð1Þ
where Rair and Rgas (¼ Rair þ R) are the measured resistances of the gas sensor in open air and a gas environment, respectively. The resistance of the prepared sensor increased after NO2 was introduced, which is attributed to the surface of TONWs being covered by negatively charged oxygen adsorbates in NO2 ambient and the adjacent space charge region being electron-depleted.10) It is seen that the sensitivity of the prepared TONW gas sensors increases strongly with increasing working temperature. However, at 200 C (the upper limit of our measurement system), the measured sensitivity of the TONW gas sensor is only 1.06. An insufficiently high working temperature might be responsible for the low sensitivity. The search for another gas sensing measurement system applicable at higher heating temperature is now under way. Nevertheless, such a low sensitivity should be due in part to the window area of the present structure being insufficiently large and to the fact
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6
11
6.5x10
Tw : 200°C
9 8
Gas in
S8 = 9.3
SEqui.
100%
A
Gas cut
6
6
sensor 2
10%
S6 = 5.9
0
SAir
sensor n
SAir
t1
5 4
6 ppm NO2
5.5x10
t2 Time
S4 = 3.7
6
5.0x10
4 ppm NO2
6
4.5x10
2 ppm NO2
6
4.0x10
6
3.5x10
6
3
3.0x10
2
2.5x10
S2 = 2.8
AIR
AIR
AIR
6
0
300
AIR
2.0x10
0 0
AIR
AIR
6
1
10 ppm NO2
8 ppm NO2
90%
sensor 1
7
8series 4series
6
6.0x10
Resistance (ohm)
Sensitivity ( RNO / RAIR ) 2
10
Sensitivity
NO2 : 100 ppm
600
900
300
600
1200
900 1200 1500 1800 2100 2400 2700 3000 3300
Time (s)
Time (s)
Fig. 7. (Color online) Transient response of sensors connected in series. The concentration of NO2 and working temperature were kept at 100 ppm and 200 C, respectively.
Table I. Measured sensitivities and response/recovery times for seriesconnected sensors. Number of sensors
Sensitivity (Rgas =Rair )
8
9.3
9
154
6
5.9
18
150
4
3.7
60
145
2
2.8
110
130
1
1.06
150
50
Response time t1 (s)
Recovery time t2 (s)
that only a small number of TONWs with lengths > 0.3 mm in the window area link the top and bottom electrodes. Furthermore, since these TONWs were in a parallel configuration, only the TONWs which have much lower resistance (i.e., much larger diameter) predominate the measured I–V characteristics. As a result, the sensitivity of the parallel-connection TONW sensor is limited because only a very small amount of gas adsorption would occur in a limited number of TONWs. To improve the performance of TONW sensors, connections of 2, 4, 6, and 8 sensors in series were also investigated, and the results are shown in Fig. 7 and listed in Table I. Note that the concentration of NO2 and the working temperature were kept at 100 ppm and 200 C, respectively. It is very interesting that the sensitivity of the seriesconnected sensors increases with increasing number of sensors used in the connection. Our results reveal that each cell in the series connection might have different NO2 sensing performances. Essentially, the sensor with the highest Rair and R should determine the sensitivity of the series-connected sensors. The improvement in the sensitivity of the series-connected device might be due to only one cell that has the highest sensitivity among all of the seriesconnected cells. Using the definition of the response time (t1 ) and recovery time (t2 ) illustrated in the inset of the figure, it is noted that the response times of the 2-, 4-, 6-, and 8-series-connected sensors were about 110, 60, 18, and 9 s, while the recovery times were about 50, 130, 145, 150, and 154 s, respectively. It is thought that the series connection of
Fig. 8. (Color online) Transient response of serial connections of four and eight sensors. The concentration of NO2 was varied in the range of 0 –10 ppm.
sensors increases the number of TONWs responding to the detected gas, and thus causes response time to decrease and recovery time to increase with increasing number of sensors in the series connection. The transient responses of the 4- and 8-series-connected sensors to NO2 at 200 C in the range of 0 –10 ppm are shown in Fig. 8. Note that the concentration of NO2 was increased in steps of 2 ppm, and a 5 min purge of the measurement chamber by dried air during the change of NO2 concentration was carried out. For the 4-series-connected sensor, the measured resistances in air and NO2 were about 2 and 2.6 M, respectively. However, the variations in the measured resistance at different NO2 concentrations were not significant. For the 8-series-connected sensors, the measured resistances after NO2 was introduced increased with increasing NO2 concentration. The present results suggest that the number of TONWs in the series-connected sensors plays a crucial role in determining both the sensitivity and detectability of sensors. The sensitivity is only about 1.65 in 10 ppm NO2 , indicating that the number of TONWs active in gas sensing must be further increased. Nevertheless, the results showed that the proposed device has a good NO2 detectability as low as 2 ppm. It is expected that by further increasing the number of sensors in the series connection proposed in the present work or by using a horizontal multiple-electrode geometry with two neighboring electrodes separated by a distance 0.3 mm, fabricated by submicron photolithography, further improvement in both the sensitivity and transient response of the TONWbased gas sensor can be obtained. 4.
Conclusions
In this study, a three-layer structure TONW-based gas sensor was fabricated and its sensing performance for NO2 of different concentrations at different working temperatures was reported. Experimental results showed that self-growth TONWs have main phase of W18 O49 (010) and typical lengths and diameters of 0.15 – 0.4 mm and 17 – 25 nm, respectively. The present sandwich structure allows TONWs to firmly link the top and bottom electrodes without the use of submicron photolithography. Because of the presence of oxide vacancies and large SVR of TONWs for gas
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absorption, good sensor performance with a sensitivity as high as 9.3, a short response time (about 9 s), and a detectability of 2 ppm of NO2 was achieved using eight sensors in a series connection. It is expected that the sensitivity of gas sensors can be further increased by increasing the number of TONWs in each cell in series-connected sensors. Acknowledgments This work was supported by the National Science Council (NSC) of Taiwan, Republic of China, under Contract Nos. NSC 94-2215-E-006-056 and NSC 95-2215-E-006014. The technical assistance by staff of the Centre for Micro/Nano Technology Research, National Cheng Kung University, Tainan, Taiwan, and Nano Materials Laboratory, Materials Engineering, Tatung University is appreciated.
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