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H2 sensing properties in highly oriented SnO2 thin films
Sensors and Actuators B 125 (2007) 504–509
H2 sensing properties in highly oriented SnO2 thin films Yun-Hyuk Choi, Seong-Hyeon Hong ∗ Department of Materials Science and Engineering and Nano Systems Institute – National Core Research Center, Seoul National University Seoul 151-742, Republic of Korea Received 30 November 2006; received in revised form 26 February 2007; accepted 26 February 2007 Available online 2 March 2007
Abstract Highly oriented polycrystalline SnO2 films were deposited on the sapphire substrates with various orientations using rf magnetron sputtering, and the effects of the crystallographic orientation on the H2 gas sensing performance were investigated. The orientation of the SnO2 films was varied with the substrate orientation such that (1 0 1), (0 0 2), and (1 0 1) oriented films were grown on (1 1 2¯ 0) (a-cut), (1 0 1¯ 0) (m-cut), and (1 1¯ 0 2) (r-cut) Al2 O3 substrates, respectively. More than one preferred orientation was observed in the films deposited on (0 0 0 1) (c-cut) Al2 O3 , quartz, and ˚ SiO2 (20,000 A)/Si substrates. All the films had a similar thickness (∼115 nm), root-mean-square (rms) roughness (∼1 nm), and surface area, and therefore the sensing performance of each film was little affected by the microstructure. The (1 0 1) SnO2 films grown on (1 1¯ 0 2) Al2 O3 exhibited the highest H2 gas response (Ro /Rg ) of ∼300 to 1.0% H2 /air, and the other films showed an order of magnitude lower gas response. The chemical composition and surface state of the films were further examined by AES and XPS to find out the reasons for the different H2 gas response of the (1 0 1) films grown on (1 1 2¯ 0) and (1 1¯ 0 2) Al2 O3 . © 2007 Elsevier B.V. All rights reserved. Keywords: SnO2 film; Gas sensor; Orientation dependence; Sapphire substrate
1. Introduction Tin dioxide (SnO2 ) is widely used as a gas sensor detecting reducing gases, such as H2 , CO, and H2 S, based on the resistivity changes with gas adsorption and desorption [1,2]. The sensor performance such as sensitivity, selectivity, and long-term stability is strongly dependent on the particle (grain) size, pore size, and grain boundary characteristics [3], and nanoscale crystallites [4–6], additives or surface functionalization [7–9], and solid solution [10,11] methods have been employed to improve the sensor performance. Recently, various SnO2 nanostructures such as nanotubes [12,13], nanoribbons [14], nanodiskettes [15], and nanocubes [16] are fabricated and applied to gas sensors. It is expected that the high surface-to-volume ratio associated with nanostructured materials makes their electrical responses extremely sensitive to the species adsorbed on the surface. Most works have focused on their high surface area and little attention was paid to the effect of crystallographic orientation on gas sens-
ing performance though nanostructures commonly exhibit the dominant or preferred orientations. The study of the relationship between gas sensing characteristics and surface crystallographic orientation was limited to the whiskers [3] or epitaxial films, which were deposited by MOCVD [17], ALD [18–20], MBE [21], reactive rf magnetron sputtering [22–24] and femtosecond pulsed laser deposition [25,26]. In the previous report [27], we have shown that highly oriented polycrystalline TiO2 films were grown on the sapphire substrates with various orientations, and H2 gas sensing properties are strongly dependent on the substrate orientations and the resulting TiO2 film orientations as well as the surface morphology. Such a detailed study has not been performed in SnO2 films. In this study, differently oriented polycrystalline SnO2 films were fabricated by rf magnetron sputtering, and their structural characteristics and H2 gas sensing performance were investigated. The film orientation was controlled by the orientation of the sapphire substrates (a-, m-. r-, and c-cut). 2. Experimental
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Corresponding author. Tel.: +82 2 880 6273; fax: +82 2 884 1413. E-mail address: [email protected] (S.-H. Hong).
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SnO2 thin films were deposited by rf magnetron sputtering using a tin dioxide target (99.99% purity) on sapphire
Y.-H. Choi, S.-H. Hong / Sensors and Actuators B 125 (2007) 504–509
substrates (Al2 O3 ). The substrates used were (1 1 2¯ 0) (a-cut), (1 0 1¯ 0) (m-cut), (1 1¯ 0 2) (r-cut), and (0 0 0 1) (c-cut) Al2 O3 . For comparison, the films were also deposited on quartz and ˚ SiO2 (20,000 A)/Si substrates. Film deposition was carried out in an Ar environment with an rf power of 100 W at 25 mTorr for 30 min. During the deposition, the substrate temperature was maintained at 350 ◦ C, which resulted in the crystalline films. Further heat treatment at 600 ◦ C slightly enhanced the crystallinity of the films. The phases and orientations of the films were determined by the X-ray diffraction θ–2θ method and the X-ray pole figure. The surface morphology and cross-section of the films were characterized by a field emission scanning electron microscope (FE-SEM) and an atomic force microscope (AFM). The depth profile of elements in the films was obtained by Auger electron spectroscopy (AES, Model 660, Perkin-Elmer) and the chemical bonding information was examined by X-ray photoelectron spectroscopy (XPS, Model AXIS, KRATOS) with a Mg K␣ radiation (1253.6 eV). The core level XPS spectra for O1s and Sn3d were measured and energy calibration was achieved by setting the hydrocarbon C1s line at 284.6 eV. For the electrical measurements, a pair of comb-like Pt electrodes were formed by sputtering on the SnO2 films through a mask and Pt lead wires were attached to them using a Pt paste [27]. Thereafter, all the sensors were fired at 600 ◦ C for 1 h in order to make the electrical contact between the Pt paste and the Pt lead wires. The H2 and CO sensing properties were determined by measuring the changes in electric resistance between 500 and 10,000 ppm H2 balanced with air and pure air at 550 ◦ C. The electrical resistance was measured using a multimeter (2000 multimeter, Keithley). The magnitude of gas response (S) was defined as the ratio (Ro /Rg ) of the resistance in air (Ro ) to that in a sample gas (Rg ). The response time (t90% ) was defined as the time required for the sensor to reach 90% of the final signal. 3. Results and discussion 3.1. Structural characteristics XRD patterns of the SnO2 films deposited on the various substrates are shown in Fig. 1. Both as-deposited and annealed (at 600 ◦ C) films were crystalline and oriented. All the diffraction peaks were indexed based on the tetragonal rutile structure. The highly oriented (1 0 1), (0 0 2), and (1 0 1) films were grown on (1 1 2¯ 0) (a-cut), (1 0 1¯ 0) (m-cut), and (1 1¯ 0 2) (r-cut) Al2 O3 , respectively. The (1 0 1) SnO2 film on (1 1¯ 0 2) Al2 O3 is consistent with the reported epitaxial relationship [18–22,24,25]. The XRD pattern of the SnO2 film deposited on (0 0 0 1) (c-cut) Al2 O3 showed (1 0 1), (2 0 0), and (2 1 1) reflections (Fig. 1(D)). It is known that the (1 0 0) film was generally obtained in the (0 0 0 1) sapphire substrate [17,22,23,26]. However, it was reported that the (1 0 1) oriented grains could be formed on the top of (1 0 0) oriented grains when the film thickness was above the critical value (∼60 nm) [26], which appears to be true of the present case. The (1 0 1) and (2 1 1) ori˚ ented films were deposited in the quartz and SiO2 (20,000 A)/Si substrates.
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Fig. 1. XRD patterns of SnO2 films deposited on (A) (1 1 2¯ 0) (a-cut), (B) (1 0 1¯ 0) (m-cut), (C) (1 1¯ 0 2) (r-cut), (D) (0 0 0 1) (c-cut) Al2 O3 , and (E) quartz.
Typical surface morphology and cross-section image of the SnO2 films are shown in Fig. 2. The deposited films were polycrystalline and the average grain size was ∼20 nm. The film surface was smooth with a root-mean-square (rms) roughness of ∼1 nm (scan area 1 m × 1 m) and the surface area determined by AFM was similar for all the films. The cross-section image indicated that the films grew in a columnar fashion and the estimated film thickness was ∼115 nm. All the films had similar microstructural characteristics and it is expected that the sensing performance of each film is little affected by the microstructure. Highly oriented (1 0 1) SnO2 films were further investigated using X-ray pole figure. Fig. 3 shows the {1 0 1} pole figures of the SnO2 films on (1 1 2¯ 0) (a-cut) and (1 1¯ 0 2) (r-cut) Al2 O3 . Both pole figures are similar and only reflections from the {1 0 1} family of planes ((0 1 1), (1¯ 0 1), (0 1¯ 1)) appeared with additional contribution from 180◦ rotation of the (1 0 1) plane marked as T1 in the figures. These pole figures exactly match with that for the (1 0 1) epitaxial SnO2 film on (1 1¯ 0 2) (r-cut) Al2 O3 , which has an in-plane rela¯ SnO [1¯ 2¯ 1]Al O tionship of [0 1 0]SnO2 [1 0 0]Al2 O3 and [1 0 1] 2 2 3 [18,28,29]. This means that there are in-plane orientation relationships between the films and the substrates even though the deposited films are polycrystalline. Based on these observations, it is believed that the (1 0 1) SnO2 polycrystalline films obtained in this study are highly textured close to the epitaxial films. Further analyses are required to find out the in-plane orientation relationships for the films deposited on (1 1 2¯ 0) (a-cut) and (1 0 1¯ 0) (m-cut) Al2 O3 , which were not reported yet.
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Fig. 2. Surface morphology and cross-section view of the SnO2 films grown on (1 1 2¯ 0) (a-cut) Al2 O3 .
3.2. Gas sensing properties The response transient and sensing properties of the SnO2 films to 1.0% H2 balanced with air are shown in Fig. 4. The sensor measurements were carried out at 550 ◦ C due to the equipment limitation (up to ∼108 ) and the high resistance of the films. Upon injecting a sample gas (1.0% H2 /air), the resistance decreased rapidly by more than two orders of magnitude (Fig. 4(A)). The recovery was slow in some cases but the sensing signal was quite stable and reversible. The determined gas response (S) and response time (t90% ) varied with the substrate orientations and resulting film orientations (Fig. 4(B)). The highest H2 gas response of ∼300 was obtained in the (1 0 1) SnO2 films deposited on (1 1¯ 0 2) (r-cut) Al2 O3 , which is comparable to the high gas response reported previously [30–32]. However, the other films exhibited an order of magnitude lower gas response with a prolong response time, and the (0 0 2) film on (1 0 1¯ 0) (m-cut) Al2 O3 exhibited the lowest gas response. It was noticeable that highly textured (1 0 1) SnO2 films on (1 1 2¯ 0) and (1 1¯ 0 2) Al2 O3 had an order of magnitude different gas response. Based on these observations, it was speculated that the dependence of H2 gas response on the orientation of SnO2 film appears
Fig. 3. {1 0 1} pole figures for the SnO2 films deposited on (A) (1 1 2¯ 0) (a-cut) and (B) (1 1¯ 0 2) (r-cut) Al2 O3 .
to be small, and other aspects such as chemical composition and bonding state might play an important role for determining the sensing performance. The concentration dependence of each film was investigated in the range of 500–10,000 ppm H2 /air and the log–log plots of the gas response against the gas concentration are shown in Fig. 5. The gas response increased almost linearly with increasing H2 concentration, irrespective of the substrate orientation. Among them, the film on (0 0 0 1) Al2 O3 showed the strong concentration dependence. However, the gas response of the films on (1 1¯ 0 2) Al2 O3 was superior in all the concentrations investigated. In addition, the SnO2 films showed a response to CO gas balanced with air. The CO gas response was an order of magnitude smaller than that to H2 and the difference between films are quite small. Thus, its H2 selectivity (ratio of the H2 gas response to the CO gas response) was highest (∼150) in the film
Y.-H. Choi, S.-H. Hong / Sensors and Actuators B 125 (2007) 504–509
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Fig. 6. H2 selectivity against CO in the SnO2 films deposited on a-, r-, m-, c-cut Al2 O3 , and quartz substrates.
deposited on (1 1¯ 0 2) Al2 O3 at 10,000 ppm and the other films had similar low values (Fig. 6). 3.3. Comparison of (1 0 1) SnO2 films As mentioned earlier, highly oriented (1 0 1) SnO2 films were obtained on the (1 1 2¯ 0) and (1 1¯ 0 2) Al2 O3 substrates (Fig. 1), but their gas sensing properties were quite different (Fig. 4) though they had similar surface area, surface roughness and even in-plane texture (Figs. 2 and 3). Besides the structural factors, the chemical composition and surface states of the films can influence the gas sensing properties [33]. The overall Fig. 4. (A) Response transient of the SnO2 film grown on (1 1¯ 0 2) (r-cut) Al2 O3 and (B) sensing properties to 1% H2 /air of the films grown on a-, r-, m-, c-cut Al2 O3 , and quartz substrates, measured at 550 ◦ C.
Fig. 5. Concentration dependence of the H2 gas response in the SnO2 films deposited on a-, r-, m-, c-cut Al2 O3 , and quartz substrates.
Fig. 7. Depth profiles determined by AES in the SnO2 films deposited on (A) (1 1 2¯ 0) (a-cut) and (B) (1 1¯ 0 2) (r-cut) Al2 O3 .
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Fig. 8. The O1s and Sn3d core level XPS spectra of the SnO2 films deposited on (A and B) (1 1 2¯ 0) (a-cut) and (C and D) (1 1¯ 0 2) (r-cut) Al2 O3 , respectively.
chemical composition measured by Rutherford backscattering spectroscopy (RBS) was same for both films ([O]/[Sn] = 2.3), and the deviation from the stoichiometry of SnO2 could be attributed to the high alumina background and consequently poor oxygen determination [34]. The depth profiles of these films determined by AES indicated that the chemical composition was uniform throughout the films except the near surface region of ∼10 nm (Fig. 7). The film deposited on the (1 1¯ 0 2) Al2 O3 exhibited the relatively higher oxygen content at the near surface region, which appears to be closely related to the higher gas sensing performance. The films were further investigated by XPS and the core level O1s and Sn3d spectra are shown as a function of the sputtering time in Fig. 8. The peak for O1s at 530.5 eV and the peak for Sn3d5/2 at 486.6 eV in the films deposited on (1 1¯ 0 2) Al2 O3 are in good agreement with the values for the lattice oxygen and tin of SnO2 , respectively [35]. One clear distinction is that the peaks were rather broad and peak positions shifted slightly to the lower energy with sputtering time in the film deposited on (1 1 2¯ 0) Al2 O3 (Fig. 8(A and B)), while it remained unchanged in the case of the films on (1 1¯ 0 2) Al2 O3 (Fig. 8(C and D)). The broad peaks with a shift toward lower energy can be attributed to the presence of the Sn2+ component along with the Sn4+ component according to the previous works [36,37]. Therefore, although the overall chemical compositions are quite similar, the film deposited on the (1 1 2¯ 0) Al2 O3 contains a considerable amount of SnO at the near surface region, whereas the film deposited on the (1 1¯ 0 2) Al2 O3 is close to stioichiometic SnO2 . It has been reported that the SnO2−x thin films with a low value of x have a high sensitivity to reducing gases compared with those with a high x value because there is more Sn4+ that can be reduced to Sn2+ by reducing gases [38,39]. Based on the arguments above, it is speculated that the surface region of the (1 0 1) film on (1 1¯ 0 2) Al2 O3 is more stoichiometric which provides a high gas response to reducing gases, particularly H2 gas.
4. Conclusion The orientation of the polycrystalline SnO2 films was strongly dependent on the sapphire (Al2 O3 ) orientation. The (1 0 1) film on (1 1¯ 0 2) (r-cut) Al2 O3 is consistent with the reported epitaxial relationship. The (0 0 2) and (1 0 1) oriented films were formed on (1 1 2¯ 0) (a-cut) and (1 0 1¯ 0) (m-cut) Al2 O3 , respectively, which are new findings and require further analysis of in-plane orientation relationships. The SnO2 film deposited on (0 0 0 1) (c-cut) Al2 O3 showed (1 0 1), (2 0 0), and (2 1 1) diffraction peaks instead of the reported (1 0 0) orientation. The (1 0 1) films grown on (1 1¯ 0 2) Al2 O3 exhibited the highest H2 gas response of ∼300 and the other films showed an order of magnitude lower gas response. Two (1 0 1) oriented films grown on (1 1 2¯ 0) and (1 1¯ 0 2) Al2 O3 had an order of magnitude different gas response, which is believed to be due to the presence of the Sn2+ component (SnO) at the near surface region along with the Sn4+ component (SnO2 ). Based on these observations, it can be summarized that the effect of crystallographic orientation on the H2 gas response appears to be small and the chemical composition and surface state play an important role. Acknowledgment This work was supported by the Ministry of Information and Communication, Republic of Korea, under project no. A11000602-0101. References [1] K. Ihokura, J. Watson, The Stannic Oxide Gas Sensor—Principles and Applications, CRC Press, Boca Raton, Florida, 1994, pp. 49–89. [2] W. G¨opel, K.D. Schierbaum, SnO2 sensors: current status and future prospects, Sens. Actuators B 26 (1995) 1–12. [3] M. Egashira, Y. Shimizu, in: N. Yamazoe (Ed.), Chemical Sensor Technology, vol. 3, Kodansha, Tokyo, 1991, pp. 1–17.
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Biographies Yun-Hyuk Choi studied materials science and engineering and received his BS in 2005 at Hankuk Aviation University in Korea. He is currently studying for a MS degree at Seoul National University. His research interests are semiconducting gas sensor and thin film deposition. Seong-Hyeon Hong has been an associate professor at Seoul National University since 1998. He received his MS degree in 1990 from Seoul National University and PhD degree in 1996 from Pennsylvania State University. His current research interests are to develop the nanostructured materials for the sensor applications.
H2 sensing properties in highly oriented SnO2 thin films Yun-Hyuk Choi, Seong-Hyeon Hong ∗ Department of Materials Science and Engineering and Nano Systems Institute – National Core Research Center, Seoul National University Seoul 151-742, Republic of Korea Received 30 November 2006; received in revised form 26 February 2007; accepted 26 February 2007 Available online 2 March 2007
Abstract Highly oriented polycrystalline SnO2 films were deposited on the sapphire substrates with various orientations using rf magnetron sputtering, and the effects of the crystallographic orientation on the H2 gas sensing performance were investigated. The orientation of the SnO2 films was varied with the substrate orientation such that (1 0 1), (0 0 2), and (1 0 1) oriented films were grown on (1 1 2¯ 0) (a-cut), (1 0 1¯ 0) (m-cut), and (1 1¯ 0 2) (r-cut) Al2 O3 substrates, respectively. More than one preferred orientation was observed in the films deposited on (0 0 0 1) (c-cut) Al2 O3 , quartz, and ˚ SiO2 (20,000 A)/Si substrates. All the films had a similar thickness (∼115 nm), root-mean-square (rms) roughness (∼1 nm), and surface area, and therefore the sensing performance of each film was little affected by the microstructure. The (1 0 1) SnO2 films grown on (1 1¯ 0 2) Al2 O3 exhibited the highest H2 gas response (Ro /Rg ) of ∼300 to 1.0% H2 /air, and the other films showed an order of magnitude lower gas response. The chemical composition and surface state of the films were further examined by AES and XPS to find out the reasons for the different H2 gas response of the (1 0 1) films grown on (1 1 2¯ 0) and (1 1¯ 0 2) Al2 O3 . © 2007 Elsevier B.V. All rights reserved. Keywords: SnO2 film; Gas sensor; Orientation dependence; Sapphire substrate
1. Introduction Tin dioxide (SnO2 ) is widely used as a gas sensor detecting reducing gases, such as H2 , CO, and H2 S, based on the resistivity changes with gas adsorption and desorption [1,2]. The sensor performance such as sensitivity, selectivity, and long-term stability is strongly dependent on the particle (grain) size, pore size, and grain boundary characteristics [3], and nanoscale crystallites [4–6], additives or surface functionalization [7–9], and solid solution [10,11] methods have been employed to improve the sensor performance. Recently, various SnO2 nanostructures such as nanotubes [12,13], nanoribbons [14], nanodiskettes [15], and nanocubes [16] are fabricated and applied to gas sensors. It is expected that the high surface-to-volume ratio associated with nanostructured materials makes their electrical responses extremely sensitive to the species adsorbed on the surface. Most works have focused on their high surface area and little attention was paid to the effect of crystallographic orientation on gas sens-
ing performance though nanostructures commonly exhibit the dominant or preferred orientations. The study of the relationship between gas sensing characteristics and surface crystallographic orientation was limited to the whiskers [3] or epitaxial films, which were deposited by MOCVD [17], ALD [18–20], MBE [21], reactive rf magnetron sputtering [22–24] and femtosecond pulsed laser deposition [25,26]. In the previous report [27], we have shown that highly oriented polycrystalline TiO2 films were grown on the sapphire substrates with various orientations, and H2 gas sensing properties are strongly dependent on the substrate orientations and the resulting TiO2 film orientations as well as the surface morphology. Such a detailed study has not been performed in SnO2 films. In this study, differently oriented polycrystalline SnO2 films were fabricated by rf magnetron sputtering, and their structural characteristics and H2 gas sensing performance were investigated. The film orientation was controlled by the orientation of the sapphire substrates (a-, m-. r-, and c-cut). 2. Experimental
∗
Corresponding author. Tel.: +82 2 880 6273; fax: +82 2 884 1413. E-mail address: [email protected] (S.-H. Hong).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.02.043
SnO2 thin films were deposited by rf magnetron sputtering using a tin dioxide target (99.99% purity) on sapphire
Y.-H. Choi, S.-H. Hong / Sensors and Actuators B 125 (2007) 504–509
substrates (Al2 O3 ). The substrates used were (1 1 2¯ 0) (a-cut), (1 0 1¯ 0) (m-cut), (1 1¯ 0 2) (r-cut), and (0 0 0 1) (c-cut) Al2 O3 . For comparison, the films were also deposited on quartz and ˚ SiO2 (20,000 A)/Si substrates. Film deposition was carried out in an Ar environment with an rf power of 100 W at 25 mTorr for 30 min. During the deposition, the substrate temperature was maintained at 350 ◦ C, which resulted in the crystalline films. Further heat treatment at 600 ◦ C slightly enhanced the crystallinity of the films. The phases and orientations of the films were determined by the X-ray diffraction θ–2θ method and the X-ray pole figure. The surface morphology and cross-section of the films were characterized by a field emission scanning electron microscope (FE-SEM) and an atomic force microscope (AFM). The depth profile of elements in the films was obtained by Auger electron spectroscopy (AES, Model 660, Perkin-Elmer) and the chemical bonding information was examined by X-ray photoelectron spectroscopy (XPS, Model AXIS, KRATOS) with a Mg K␣ radiation (1253.6 eV). The core level XPS spectra for O1s and Sn3d were measured and energy calibration was achieved by setting the hydrocarbon C1s line at 284.6 eV. For the electrical measurements, a pair of comb-like Pt electrodes were formed by sputtering on the SnO2 films through a mask and Pt lead wires were attached to them using a Pt paste [27]. Thereafter, all the sensors were fired at 600 ◦ C for 1 h in order to make the electrical contact between the Pt paste and the Pt lead wires. The H2 and CO sensing properties were determined by measuring the changes in electric resistance between 500 and 10,000 ppm H2 balanced with air and pure air at 550 ◦ C. The electrical resistance was measured using a multimeter (2000 multimeter, Keithley). The magnitude of gas response (S) was defined as the ratio (Ro /Rg ) of the resistance in air (Ro ) to that in a sample gas (Rg ). The response time (t90% ) was defined as the time required for the sensor to reach 90% of the final signal. 3. Results and discussion 3.1. Structural characteristics XRD patterns of the SnO2 films deposited on the various substrates are shown in Fig. 1. Both as-deposited and annealed (at 600 ◦ C) films were crystalline and oriented. All the diffraction peaks were indexed based on the tetragonal rutile structure. The highly oriented (1 0 1), (0 0 2), and (1 0 1) films were grown on (1 1 2¯ 0) (a-cut), (1 0 1¯ 0) (m-cut), and (1 1¯ 0 2) (r-cut) Al2 O3 , respectively. The (1 0 1) SnO2 film on (1 1¯ 0 2) Al2 O3 is consistent with the reported epitaxial relationship [18–22,24,25]. The XRD pattern of the SnO2 film deposited on (0 0 0 1) (c-cut) Al2 O3 showed (1 0 1), (2 0 0), and (2 1 1) reflections (Fig. 1(D)). It is known that the (1 0 0) film was generally obtained in the (0 0 0 1) sapphire substrate [17,22,23,26]. However, it was reported that the (1 0 1) oriented grains could be formed on the top of (1 0 0) oriented grains when the film thickness was above the critical value (∼60 nm) [26], which appears to be true of the present case. The (1 0 1) and (2 1 1) ori˚ ented films were deposited in the quartz and SiO2 (20,000 A)/Si substrates.
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Fig. 1. XRD patterns of SnO2 films deposited on (A) (1 1 2¯ 0) (a-cut), (B) (1 0 1¯ 0) (m-cut), (C) (1 1¯ 0 2) (r-cut), (D) (0 0 0 1) (c-cut) Al2 O3 , and (E) quartz.
Typical surface morphology and cross-section image of the SnO2 films are shown in Fig. 2. The deposited films were polycrystalline and the average grain size was ∼20 nm. The film surface was smooth with a root-mean-square (rms) roughness of ∼1 nm (scan area 1 m × 1 m) and the surface area determined by AFM was similar for all the films. The cross-section image indicated that the films grew in a columnar fashion and the estimated film thickness was ∼115 nm. All the films had similar microstructural characteristics and it is expected that the sensing performance of each film is little affected by the microstructure. Highly oriented (1 0 1) SnO2 films were further investigated using X-ray pole figure. Fig. 3 shows the {1 0 1} pole figures of the SnO2 films on (1 1 2¯ 0) (a-cut) and (1 1¯ 0 2) (r-cut) Al2 O3 . Both pole figures are similar and only reflections from the {1 0 1} family of planes ((0 1 1), (1¯ 0 1), (0 1¯ 1)) appeared with additional contribution from 180◦ rotation of the (1 0 1) plane marked as T1 in the figures. These pole figures exactly match with that for the (1 0 1) epitaxial SnO2 film on (1 1¯ 0 2) (r-cut) Al2 O3 , which has an in-plane rela¯ SnO [1¯ 2¯ 1]Al O tionship of [0 1 0]SnO2 [1 0 0]Al2 O3 and [1 0 1] 2 2 3 [18,28,29]. This means that there are in-plane orientation relationships between the films and the substrates even though the deposited films are polycrystalline. Based on these observations, it is believed that the (1 0 1) SnO2 polycrystalline films obtained in this study are highly textured close to the epitaxial films. Further analyses are required to find out the in-plane orientation relationships for the films deposited on (1 1 2¯ 0) (a-cut) and (1 0 1¯ 0) (m-cut) Al2 O3 , which were not reported yet.
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Fig. 2. Surface morphology and cross-section view of the SnO2 films grown on (1 1 2¯ 0) (a-cut) Al2 O3 .
3.2. Gas sensing properties The response transient and sensing properties of the SnO2 films to 1.0% H2 balanced with air are shown in Fig. 4. The sensor measurements were carried out at 550 ◦ C due to the equipment limitation (up to ∼108 ) and the high resistance of the films. Upon injecting a sample gas (1.0% H2 /air), the resistance decreased rapidly by more than two orders of magnitude (Fig. 4(A)). The recovery was slow in some cases but the sensing signal was quite stable and reversible. The determined gas response (S) and response time (t90% ) varied with the substrate orientations and resulting film orientations (Fig. 4(B)). The highest H2 gas response of ∼300 was obtained in the (1 0 1) SnO2 films deposited on (1 1¯ 0 2) (r-cut) Al2 O3 , which is comparable to the high gas response reported previously [30–32]. However, the other films exhibited an order of magnitude lower gas response with a prolong response time, and the (0 0 2) film on (1 0 1¯ 0) (m-cut) Al2 O3 exhibited the lowest gas response. It was noticeable that highly textured (1 0 1) SnO2 films on (1 1 2¯ 0) and (1 1¯ 0 2) Al2 O3 had an order of magnitude different gas response. Based on these observations, it was speculated that the dependence of H2 gas response on the orientation of SnO2 film appears
Fig. 3. {1 0 1} pole figures for the SnO2 films deposited on (A) (1 1 2¯ 0) (a-cut) and (B) (1 1¯ 0 2) (r-cut) Al2 O3 .
to be small, and other aspects such as chemical composition and bonding state might play an important role for determining the sensing performance. The concentration dependence of each film was investigated in the range of 500–10,000 ppm H2 /air and the log–log plots of the gas response against the gas concentration are shown in Fig. 5. The gas response increased almost linearly with increasing H2 concentration, irrespective of the substrate orientation. Among them, the film on (0 0 0 1) Al2 O3 showed the strong concentration dependence. However, the gas response of the films on (1 1¯ 0 2) Al2 O3 was superior in all the concentrations investigated. In addition, the SnO2 films showed a response to CO gas balanced with air. The CO gas response was an order of magnitude smaller than that to H2 and the difference between films are quite small. Thus, its H2 selectivity (ratio of the H2 gas response to the CO gas response) was highest (∼150) in the film
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Fig. 6. H2 selectivity against CO in the SnO2 films deposited on a-, r-, m-, c-cut Al2 O3 , and quartz substrates.
deposited on (1 1¯ 0 2) Al2 O3 at 10,000 ppm and the other films had similar low values (Fig. 6). 3.3. Comparison of (1 0 1) SnO2 films As mentioned earlier, highly oriented (1 0 1) SnO2 films were obtained on the (1 1 2¯ 0) and (1 1¯ 0 2) Al2 O3 substrates (Fig. 1), but their gas sensing properties were quite different (Fig. 4) though they had similar surface area, surface roughness and even in-plane texture (Figs. 2 and 3). Besides the structural factors, the chemical composition and surface states of the films can influence the gas sensing properties [33]. The overall Fig. 4. (A) Response transient of the SnO2 film grown on (1 1¯ 0 2) (r-cut) Al2 O3 and (B) sensing properties to 1% H2 /air of the films grown on a-, r-, m-, c-cut Al2 O3 , and quartz substrates, measured at 550 ◦ C.
Fig. 5. Concentration dependence of the H2 gas response in the SnO2 films deposited on a-, r-, m-, c-cut Al2 O3 , and quartz substrates.
Fig. 7. Depth profiles determined by AES in the SnO2 films deposited on (A) (1 1 2¯ 0) (a-cut) and (B) (1 1¯ 0 2) (r-cut) Al2 O3 .
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Fig. 8. The O1s and Sn3d core level XPS spectra of the SnO2 films deposited on (A and B) (1 1 2¯ 0) (a-cut) and (C and D) (1 1¯ 0 2) (r-cut) Al2 O3 , respectively.
chemical composition measured by Rutherford backscattering spectroscopy (RBS) was same for both films ([O]/[Sn] = 2.3), and the deviation from the stoichiometry of SnO2 could be attributed to the high alumina background and consequently poor oxygen determination [34]. The depth profiles of these films determined by AES indicated that the chemical composition was uniform throughout the films except the near surface region of ∼10 nm (Fig. 7). The film deposited on the (1 1¯ 0 2) Al2 O3 exhibited the relatively higher oxygen content at the near surface region, which appears to be closely related to the higher gas sensing performance. The films were further investigated by XPS and the core level O1s and Sn3d spectra are shown as a function of the sputtering time in Fig. 8. The peak for O1s at 530.5 eV and the peak for Sn3d5/2 at 486.6 eV in the films deposited on (1 1¯ 0 2) Al2 O3 are in good agreement with the values for the lattice oxygen and tin of SnO2 , respectively [35]. One clear distinction is that the peaks were rather broad and peak positions shifted slightly to the lower energy with sputtering time in the film deposited on (1 1 2¯ 0) Al2 O3 (Fig. 8(A and B)), while it remained unchanged in the case of the films on (1 1¯ 0 2) Al2 O3 (Fig. 8(C and D)). The broad peaks with a shift toward lower energy can be attributed to the presence of the Sn2+ component along with the Sn4+ component according to the previous works [36,37]. Therefore, although the overall chemical compositions are quite similar, the film deposited on the (1 1 2¯ 0) Al2 O3 contains a considerable amount of SnO at the near surface region, whereas the film deposited on the (1 1¯ 0 2) Al2 O3 is close to stioichiometic SnO2 . It has been reported that the SnO2−x thin films with a low value of x have a high sensitivity to reducing gases compared with those with a high x value because there is more Sn4+ that can be reduced to Sn2+ by reducing gases [38,39]. Based on the arguments above, it is speculated that the surface region of the (1 0 1) film on (1 1¯ 0 2) Al2 O3 is more stoichiometric which provides a high gas response to reducing gases, particularly H2 gas.
4. Conclusion The orientation of the polycrystalline SnO2 films was strongly dependent on the sapphire (Al2 O3 ) orientation. The (1 0 1) film on (1 1¯ 0 2) (r-cut) Al2 O3 is consistent with the reported epitaxial relationship. The (0 0 2) and (1 0 1) oriented films were formed on (1 1 2¯ 0) (a-cut) and (1 0 1¯ 0) (m-cut) Al2 O3 , respectively, which are new findings and require further analysis of in-plane orientation relationships. The SnO2 film deposited on (0 0 0 1) (c-cut) Al2 O3 showed (1 0 1), (2 0 0), and (2 1 1) diffraction peaks instead of the reported (1 0 0) orientation. The (1 0 1) films grown on (1 1¯ 0 2) Al2 O3 exhibited the highest H2 gas response of ∼300 and the other films showed an order of magnitude lower gas response. Two (1 0 1) oriented films grown on (1 1 2¯ 0) and (1 1¯ 0 2) Al2 O3 had an order of magnitude different gas response, which is believed to be due to the presence of the Sn2+ component (SnO) at the near surface region along with the Sn4+ component (SnO2 ). Based on these observations, it can be summarized that the effect of crystallographic orientation on the H2 gas response appears to be small and the chemical composition and surface state play an important role. Acknowledgment This work was supported by the Ministry of Information and Communication, Republic of Korea, under project no. A11000602-0101. References [1] K. Ihokura, J. Watson, The Stannic Oxide Gas Sensor—Principles and Applications, CRC Press, Boca Raton, Florida, 1994, pp. 49–89. [2] W. G¨opel, K.D. Schierbaum, SnO2 sensors: current status and future prospects, Sens. Actuators B 26 (1995) 1–12. [3] M. Egashira, Y. Shimizu, in: N. Yamazoe (Ed.), Chemical Sensor Technology, vol. 3, Kodansha, Tokyo, 1991, pp. 1–17.
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Biographies Yun-Hyuk Choi studied materials science and engineering and received his BS in 2005 at Hankuk Aviation University in Korea. He is currently studying for a MS degree at Seoul National University. His research interests are semiconducting gas sensor and thin film deposition. Seong-Hyeon Hong has been an associate professor at Seoul National University since 1998. He received his MS degree in 1990 from Seoul National University and PhD degree in 1996 from Pennsylvania State University. His current research interests are to develop the nanostructured materials for the sensor applications.