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Structure and NH3 sensing properties of SnO thin film deposited by RF magnetron sputtering
Accepted Manuscript Title: Structure and NH3 sensing properties of SnO thin film deposited by RF magnetron sputtering Author: Vu Xuan Hien Joon-Hyung Lee Jeong-Joo Kim Young-Woo Heo PII: DOI: Reference:
S0925-4005(13)01567-0 http://dx.doi.org/doi:10.1016/j.snb.2013.12.086 SNB 16384
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
3-10-2013 18-12-2013 21-12-2013
Please cite this article as: V.X. Hien, J.-H. Lee, J.-J. Kim, Y.-W. Heo, Structure and NH3 sensing properties of SnO thin film deposited by RF magnetron sputtering, Sensors and Actuators B: Chemical (2013), http://dx.doi.org/10.1016/j.snb.2013.12.086 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structure and NH3 sensing properties of SnO thin film deposited by RF magnetron sputtering Vu Xuan Hien, Joon-Hyung Lee, Jeong-Joo Kim, Young-Woo Heo* School of Materials Science and Engineering, Kyungpook National University, Daegu 702-701
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Republic of Korea
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Highlights: ! ! SnO gas sensors were fabricated by RF magnetron sputtering
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! ! A thin layer of SnO2 was found on the surface of the SnO thin film
! ! The electrical properties of the sensor was recorded during heat treatment
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! ! NH3 sensing properties of the SnO sensor was carried out
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! ! The sensing mechanism of the device was proposed and explained
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Abstract
SnO thin films, 100 nm in thickness, were deposited on glass substrates by RF magnetron
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sputtering. A stack structure of SnO2/SnO, where few nanometers of SnO2 were determined on the SnO thin film by X-ray photoelectron spectroscopy. In addition, XPS depth profile analysis of the pristine and heat treated thin films were introduced. The electrical behavior of the as-sputtered films during heat treatment in air and nitrogen was recorded to investigate the working conditions for the SnO sensor. Subsequently, The NH3 sensing properties of the SnO sensor at operating temperature of 50-200oC were examined, in which the p-type semiconducting sensing properties of the thin film were noted. The sensor shows good sensitivity and repeatability to NH3 vapor. Finally, a sensing mechanism was proposed and discussed.
Keywords: Gas sensor, SnO thin film, Sputtering, p-type semiconductor.
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* Corresponding author: Young-Woo Heo (Tel.:+82 53 950 7587; Fax: +82 53 950 5465, E-mail: [email protected])
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1. Introduction
Stannous oxide is a p-type metal oxide semiconductor with a band gap ranging from 2.5 eV
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to 3 eV. SnO is a well-known coating material [1] and catalyst [2], and has recently been applied to thin film transistors [3-7]. However, sensing properties of stannous oxide is still lack of concern.
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Main reason for this problem may relate to the disproportion of SnO to Sn and SnO2 (SnO Sn3O4 + Sn SnO2 + Sn) at elevated temperatures [8]. This phenomenon has also been studied by Raman
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scattering, infra-red (IR) reflectivity and X-ray diffraction (XRD) [9]. Moreover, a match between
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SnO (001) and SnO2 (101) with similar Sn-Sn coordination (0.38 nm for SnO and 0.37 nm for SnO2) may lead to conversion from the (001)-textured layers of SnO to the (101)-texture layers of SnO2 [9-
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11]. Consequently, the layers of SnO2 may be naturally formed on the surface of SnO. Thus, minimizing the thickness as well as the sensing property of SnO2 layers on the surface of SnO is a key point to measure p-type semiconducting sensing properties of SnO. The formation and expansion of the depletion and accumulation zones is resulted in electron
trapping and hole generation of the oxygen-adsorbed layers for n-type and p-type materials, respectively [12]. Therefore, the fluctuation of the material conductivity during heat treatment may provide us information about the formation priority of a depletion/accumulation zone at discrete points of treatment temperature. As the depletion zone plays an important role in the sensing properties of n-type semiconductors, selecting a right temperature where the formation/expansion of
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the depletion zone is weaker than those of the accumulation zone is a technique to measure the gas sensing properties of SnO. In this study, SnO sensors were fabricated by depositing SnO thin films on glass-supported
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Ni/Au interdigitated electrodes using RF magnetron sputtering. The resistance-temperature (R-T) characteristic was carried out to indicate the operating temperature of the sensors. Subsequently, NH3
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sensing properties of the sensor were examined using a dynamic gas testing system. Finally, a possible hypothesis for the behavior of the R-T curves during heat treatment and the sensing
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mechanism of the sensors are also introduced.
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2. Experimental procedure
SnO thin films, 100 nm in thickness, were deposited by RF magnetron sputtering on Corning glass substrates and a Ni(40 nm)/Au(60 nm) interdigitated electrode (IDEs with electrode width: 100
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μm and electrode spacing: 50 μm) at 200oC. The sputter power, sputter rate and the ratio of Ar:O2
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were 50 W, 1.16 nm/sec and 97:3, respectively. The total gas flow rate was modulated by mass flow
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controllers with the working pressure maintained at 10 mtorr. The R-T behavior and NH3 sensing properties of the as-prepared samples were examined using a dynamic gas testing system. For the
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NH3 sensing measurements, high-purity dry air was used as the base gas at a flow rate of 100 sccm. Before the measurements, the chamber was pumped to just below 8 mtorr to check for gas leakage and remove contamination, such as moisture and residual gases. Dry air was used as the base gas during the measurements.
The surface morphologies of all samples were characterized by field emission scanning
electron microscopy (FE-SEM: JSM-6701F). X-ray Diffraction (XRD) measurements were performed on XPERT-PRO X-Ray Diffraction System using the CuKα1 radiation (
)
with a potential of 40 kV and a current of 30 mA. For the chemical composition, the depth profile and X-ray photoelectron spectroscopy (XPS) high resolution spectra were characterized by XPS analysis (Quantera SXM) in which the binding energy data was calibrated due to C1s signal of
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ambient hydrocarbons (C-H and C-C) at 284.6 eV. Transmission electron microscopy (TEM) was performed on Telnai G2F20 S-TWIN, Philips. 3. Results and discussion Structures of the thin film before/after heat treatment
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3.1.
XRD spectra of the pristine thin film and the film treated from RT to 300oC are shown in fig.
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1. Those spectra show the amorphous phase of the pristine thin film. For the film treated from RT to 300oC, 3 peaks at 29.8o, 37.1o and 50.7o indexing to tetragonal structure of SnO(101), SnO(002) and
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SnO(112) (Reference: Joint Committee for Powder Diffraction Standard – JCPDS: 01-072-1012) were found. Besides, no impurity peaks were observed in those patterns.
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Figure 2 introduces FESEM images of the pristine and the treated film (from RT to 300oC)
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which were deposited on IDEs. It can be easily seen on the figure that the thin film surfaces were constructed of grains. In addition, the grain sizes of both samples were similar (below 100 nm).
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XPS depth-profiling of the as-deposited thin film was probed as a function of layer thickness
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(fig. 3a). As follow this data, about 19 sputtering cycles (nearly 19 min in total) were required to remove a 100 nm thickness of the pristine thin film and reach the glass substrate. Therefore, an
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approximate sputtering rate was calculated of 5.26 nm/min. The drop of C1s concentration after 1s sputtering indicated an elimination of the ambient hydrocarbons on the sample surface. The elemental distributions of Sn and O in the thin film can also be found in fig. 3a where the Sn3d5 concentration increased from 50% to 62% and inversely, the O1s concentration decrease from 50% down to 35%.
The XPS high-resolution O1s (fig. 3b) and Sn3d5 (fig. 3c,d) spectra introduce in more detail
the chemical composition of the pristine thin film followed the distance from the thin film surface. The O1s spectrum could be Gaussian fitted to 3 peaks at 530.1 eV, 531.5 eV and 532.4 eV (fig. 3b). On the thin film surface, the high band energy component at 530.3 eV indicated the formation of the SnO2 phase [13]. Besides, the O1s peaks at 531.7 eV and 532.6 eV was assigned to the presence of a
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hydroxyl groups and water molecules adsorbed on the thin film surface [14], respectively. For the underneath layers, a single peak at 529.9 eV was assigned to SnO phase [13]. The Sn3d5/2 spectra of layers near the thin film surface composed of 3 peaks at 483.3 eV, 485.6 eV and 486.3 eV (fig. 3c).
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Sn3d5/2 peaks at 486.3 eV and 485.8 eV confirmed the appearance of SnO2 on the thin film surface and SnO underneath [13]. Matched with the peak from Sn metal in the pristine thin film at 17 nm
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below the surface was a single Sn3d5/2 peak at 484.3 eV [15]. The peak shifts of O1s, Sn3d3/2 and Sn3d5/2 depending on layers of the pristine film in fig. 3d suggests that the thin film structure was not
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fully stoichiometric.
Figure 4 shows the depth-profiling curve and the XPS high-resolution spectra of SnO thin
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film which was treated from RT to 300oC. As for the depth-profiling curve, the O1s concentration was higher than Sn3d5 concentration at the layers near surface indicating that the oxygen adsorption was
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carried out after heat treatment. Three peaks of SnO2, H2O and OH- were also recorded at the surface
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of this sample. The Sn metal peak in this sample was indicated at layer, which was 61 nm distance
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from the thin film surface. Therefore, Sn metal which was found at 17 nm below the surface was oxidized after treatment. Interestingly, there were linear shifts from SnO2 peak to SnO peak as a
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function of layer position in the treated film. Especially, no shifted peaks were found at layers, which were deeper than 50 nm from the surface. Thus, the oxygen adsorption process was completed and the thin film structure was stoichiometric. 3.2.
Electrical behaviors of the sensors during heat treatment
Figure 5 shows the electrical behavior of the SnO sensor during heat treatment (RT to 300oC
in air). In this data, the black solid, red dash and blue dot curves correspond to the first, second and third heat treatments, respectively. According to the first curve, at least two special regions were observed: from 38oC to 70oC and from 160oC to 260oC. In the first region, a slightly upward trend was noted indicating that the sample resistance had stabilized and decreased slowly with the increasing temperature. Similarly, the same conduct was also detected in the early stages of region 2,
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where the resistance remained until 180oC. Surprisingly, due to the increasing temperature from just above 200oC, the film resistance magnified rapidly from 1.5 kΩ to hit a peak of ~2.5 kΩ (235oC), and then decreased to 200 Ω at the end of region 2. These above electrical behaviors may be related
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to phenomena: the adsorption and desorption of water and oxygen on SnO2 surface. Most metal oxide semiconductors tend to adsorb oxygen from the outer environment because
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of their non-stoichiometric structure because of oxygen vacancies. Consequently, the formations of a depletion zone and accumulation zone corresponding to n-type and p-type semiconductors were
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carried out during adsorption [12,16]. In addition, these processes are related to electron-trapping in the depletion zone and hole-generation in the accumulation zone, which directly affects the variation
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of the material conductivity. Moreover, adsorption also plays an important role in converting the 2+
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oxidation state of Sn to the 4+ oxidation state [10]. Therefore, a knowledge of the oxygen adsorption process is essential for understanding the complicated behavior in the conductivity of the sensor
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during heat treatment.
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The oxygen adsorption process has been studied by Mizokawa [17], Bielanski [18], Chang [19], Kohl [20] and Batzill [21], where a transition of oxygen species occurs from physisorption to
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chemisorption with the aid of temperature according to the following process: (3)
In this process, oxygen is initially diffused into the material structure, particularly in a region near the surface. For tin oxide, chemical bonding between adsorbed oxygen and Sn results in (for SnO2) or
160oC, a transition from ;
to
:
(for SnO). At a working temperature of approximately or
occurs (
;
or
), which expands the depletion zone in SnO2 or the
accumulation zone in SnO. With these expansions, a large number of carriers is taken and generated, causing a decrease and increase in the conductivity of SnO2 and SnO, respectively. Therefore, the conductivity of SnO thin film tends to increase with increasing working temperature because of the
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higher diffusion rate and transition among oxygen species, as mentioned above. Nevertheless, if species at approximately 160oC, a downward curve of
or
there had been a transition to form
the resistance would have been received around this point. In this case, however, upward instead of
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downward trends in the resistance of the sample were observed at closely 160oC (fig. 5). Therefore, another process instead of adsorption should be considered.
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The water molecule is one of the most abundant molecules on the earth, and all living things as well as many nonliving things are affected directly and indirectly by it. Although SnO2 is sensitive
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to inflammable/ flammable gases, it has been studied as a material for humidity sensors [21-23].
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Those studies indicated the intensification of SnO2 conductance while being exposed to water vapor might result in a dissociation and reduction process [20]. In addition to water molecules, hydroxyl
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groups were observed to cause changes in the conductivity of SnO2 during water adsorption. This is related to a study by Yamazoe et al. [24], who reported a decrease of SnO2 conductance on a
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desorption peak at high temperatures in TPD. In addition, according to Kohl [20], the formation of
(4)
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or
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the adsorbed hydroxyl group can be expressed as the follow equations:
(5)
The generations of e- or oxygen vacancies on the surface in equations 3 and 4 directly and
indirectly increase the conductance of SnO2. The decomposition of H2O to OH- at elevated temperatures can lead to electron-trap or hole-generation. Inversely, the generation of e- or oxygen vacancy is equal to the disappearance of h+ in p-type materials, such as SnO, which leads to a decrease in conductivity. As follow the XPS data, the H2O molecular as well as OH- were adsorbed by SnO2 layer on the thin film surface. Besides, the initial water adsorption on SnO2 (101) was observed at 110 K [25], whereas the adsorbed oxygen species appeared only on the SnO2 surface at higher temperatures [19]. Therefore, the adsorption of water molecules is the dominant process at
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room temperature. This process may also be responsible for the drift phenomenon of the resistance (to a lower value) to time or the aging effect of metal oxide semiconductors. Thornton and Harrison indicated that for SnO2 under atmospheric pressure, water molecules
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were removed at 150oC, but hydroxyl groups desorbed at higher temperatures (~250oC) [26]. Similarly, Yamazoe et al. [24] and Egashira et al. [27] observed desorption peaks of water molecules
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in the SnO2 structure below 200oC by thermal desorption spectroscopy. These results explain the change in resistance of the sensor near 160oC. In addition, the upward trend below 80oC for the 1st
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treatment might be related to water molecule desorption at the thin film surface, whereas the change
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in the resistance at 160oC may be related to the desorption of water molecules in the deeper layers of the thin film.
Similar behavior for the 2nd and 3rd measurements (red dash line and blue dot curve in figure
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2) was observed for the sensor. A linear decrease in resistance below 160oC for the 2nd and 3rd
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measurements indicated a small amount of water molecules on the thin film surface. It should be
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noted that the sample was kept in the chamber without loading out. The resistances of the thin films in those measurements showed a slight increase just above 200oC. The cause of the increase in film
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resistance at ~200oC is unclear because oxidation and water desorption occurred simultaneously after 150oC. Therefore, the sensor was measured under the same conditions except N2, which flew into the chamber (total flow rate: 200 sccm) during the measurements to restrict the oxidation. Figure 6 shows the effect of temperature on the electrical properties of the SnO sensor in N2.
The initial resistance of this sample was 20 kΩ, which was lower than that of the previous sensor (approximately 100 kΩ). This behavior suggests a large amount of water molecules on the film surface, which caused the decrease in film resistance, which was comparable to the previous sample treated in dry air. The oxidation was restricted in N2 medium, a significant increase in film resistance at ~250oC was fitted to the point where the desorption of hydroxyl groups was observed in SnO2, as mentioned above. This desorption point was observed easily again at 270oC in the second
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measurement (red dash curve). In addition, the upward shift of the thin film resistance with each measurement shows that the desorption of water molecules and hydroxyl groups dominated oxidation in this experiment. According to the 1st measurement, there was no desorption point at
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approximately 205oC. Therefore, water desorption might not be the reason for the increase in the resistance of the sample treated in air at 205oC. The adsorption of oxygen on the surface of SnO2
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which expand the deletion zone or increase the sensor resistance may respond to this effect. Besides, the oxidations of Sn to SnO2 or SnO to SnO2 may also be processes causing the same phenomenon.
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The sharp decrease in thin film resistance above 236oC may relate to the diffusion and adsorption of
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oxygen from the external medium into the deeper layer of the film, where SnO dominated. This resulted in an expansion of the accumulation zone, which enhanced the carrier quantity or caused a decrease in film resistance. According to these arguments, the operating temperature for the SnO
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sensor should be below 200oC to avoid the oxidation of Sn, SnO to SnO2 as well as the oxygen
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semiconducting sensing properties.
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adsorption of SnO2 (expand depletion zone) which leads the gas sensor behave as n-type
Influence of heat treatment time (1st measurement) on the thin film resistance is introduced in
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fig. 7. There are 3 number15ed regions in which region 1 (from 0 to 40 min), region 2 (from 40 to 340 min) and region 3 (from 340 to 430) shows the thin film resistances when the treatment temperature is increased from room temperature to 200oC, is stabilized at 200oC and is cooled naturally, respectively. As can be seen in region 2, the thin film resistance was slightly increased in the first 5 minutes (oxygen adsorption on SnO2 surface), then the resistance was decreased (oxygen adsorption on SnO surface) and was stabilized at around 950 Ω when the treatment time was 140 min. This saturation state implies that there were no phase transitions or other processes (adsorption, desorption, grain size growth, etc.) happened after 140 min heat treatment. 3.3.
NH3 sensing properties of the SnO sensor
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Figure 8 introduces the response-recovery curves of the SnO sensor to 200 ppm NH3 at operating temperatures of 50oC, 100oC, 150oC and 200oC. When NH3 was injected into the measuring chamber at 200oC, a slight decrease of the sensor resistance was observed which is
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behavior of n-type semiconductor to a reduced gas. Subsequently, the resistance was rapidly increased and stabilized at around 972 Ω and for the recovery, a gradual decline of the sensor
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resistance was recorded. This gas sensing behavior represented p-type semiconducting properties of the sensor. However, the sensor resistance after recovery was slightly decreased comparing with the
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initial value. This phenomenon was clearly seen in the sensing curves which were measured at 150oC, 100oC and 50oC. Besides, the n-type semiconducting sensing properties were not shown in case the
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SnO sensor was measured at 100oC and 50oC.
Figure 9 presents the influences of NH3 concentration to the sensing properties of the SnO
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sensor at 200oC. After injecting 50 ppm of NH3, the sensor resistance was slightly decreased from its
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initial value before increasing. This was not happened in the next concentrations of 100 ppm and 200
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ppm NH3. An explanation for this phenomenon is based on the sensor structure which is illustrated in fig. 10. Fig. 10c shows the p-n junction between SnO2 on the thin film surface and SnO in the deeper
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layers. The NH3 sensing behavior of the sensor can be described by 3 steps. First, NH3 vapor reacts with adsorbed oxygen adatoms of SnO2 on the sensor surface. This reaction removes the oxygen adatoms and also releases free electrons to SnO2 structure. The SnO2 layers in this case, become conductive layers, which make the sensor conductivity surge. Second, the NH3 vapor diffused into deeper layers of the thin film, when adsorbed oxygen of SnO2 was completely reacted. Third, the reduced gas reacts with oxygen adatoms of SnO and takes out holes from the SnO structure which then leads the sensor resistance increase. The reactions between NH3 and surface adsorbed oxygen species for the n-type and p-type semiconductor occur according to the following equations: (6) (7)
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The sensor structure can also be illustrated as an equivalent circuit in fig. 10d. In this figure, are resistance and capacitance of 2 electrodes.
layers on the surface whether For this circuit,
is resistance of SnO2
are resistances of SnO layers under the thin film surface.
was decreased when the sensor exposed to NH3 which make the total
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and
. In this case, the SnO2
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resistance of the sensor decrease because
layers were quite thin (less than 6 nm), therefore, the main sensing area was the SnO layers
subsequently increase
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underneath. The reactions between NH3 and oxygen adatoms of SnO make
increase which
. Finally, the p-type semiconducting sensing properties of the sensor
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was shown. If there is a conversion from SnO to SnO2 during the test, the influence of
on
will be more significant which may reduce the sensor response (S = Rg/Ra).
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The repeatability of the SnO sensor at 150oC and 200oC for 5 pulses of 200 ppm NH3 was
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shown in fig. 11. Both measurements proved the good repeatability of the sensor. Nevertheless, the
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sensor resistance was gently dropped during the tests. This effect was believed as a consequence of the uncompleted recovery process of SnO2. The recovery of SnO2 can be understood as the re-
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adsorption of oxygen adatoms the the SnO2 surface after they were removed by reactions with NH3 (equation 6). Since the adsorption of external oxygen to SnO2 structure was found to be happened at 205oC,
(low value) after reacting with NH3 could not completely recover to the initial stage
(high value) at lower operating temperatures (less than 200oC, in this situation). Ultimately, was dropped after several pulses. During the measurement, the gas response was a constant value, therefore the transformation of SnO to SnO2 which affect the sensor response was probably not occurred. Conclusion SnO thin films, 100 nm thickness, were deposited by RF magnetron sputtering. The extremely thin layers of SnO2 (less than 6 nm) was found on the surface of the thin film. The
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structure of thin films after treating from RT to 300oC was stoichiometric. The effects of heat treatment on the electrical behavior of the as-prepared samples were investigated. Based on the resistance-temperature curves, the adsorption of external oxygen into SnO2 structure was found to be
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occurred at approximately 205oC in air. For the NH3 sensing test, the sensor exhibited a p-type semiconducting sensing characteristics at working temperature below 200oC. In addition, the sensor
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was sensitive to 50 ppm NH3 at 200oC with a good repeatability. The sensing behavior of the SnO sensor was explained in detail using equivalent circuit. Finally, the sensing properties of the sensor to
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several reduced gases at 200oC were introduced. Acknowledgement:
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This work was supported by the Mid-career Researcher Program, and the Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the
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Korean government (MSIP) (No. 2011-0017245, 2008-0062617). References:
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Biographies:
Vu Xuan Hien is presently a PhD student at School of Materials Science and Engineering -
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Kyungpook National University, Daegu, Korea. He earned a MS from School of Engineering Physics,
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Hanoi University of Science and Technology, Vietnam. His research interests cover syntheses of multi-morphology metal oxide semiconductors using physical/chemical methods and their
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applications in gas sensor.
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Jeong-Joo Kim is a professor in School of Materials Science and Engineering, Kyungpook National University, Korea. He received a BS, MS and PhD in Inorganic Materials Engeering, Seoul National University, Korea. His research interests includes Sintering, Transparent Conducting Oxides, and
d
M
Solid Oxide Fuel Cells.
Jopon Hyung Lee is a professor in School of Materials Science and Engineering, Kyungpook
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National University., Korea. He received a BS in Department of Electronics Engineering, Youngnam University and MS and PhD in Inorganic Materials Engineering, Kyungpook National University,
Ac ce p
Korea. His research interests includes sensors, Transparent Conducting Oxides, Solid Oxide Fuel Cells and Sensors.
Young-Woo Heo is an associate professor in School of Materials Science and Engineering, Kyungpook National University., Korea. He received a BS and MS in Inorganic Materials Engeering, Kyungpook National University and PhD in Department of Materials Science and Engineering, University of Florida. He was selected a Top 100 Materials Scientists of The Past Decade, 2000~2010 released by Thomson Reuters (2011). His research interests includes Sensors and Transparent Electronic Materials/Devices.
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Figure 1. XRD spectra of thin film before and after heat treatment (from RT to 300oC). Figure 2. FESEM images of the pristine thin film (a) and thin film treated from RT to 300oC (b).
ip t
Figure 3. Depth profiles of the as-deposited thin film. Sn3d5 and O1s atomic concentration as a function of sputtering time and film thickness (a); XPS high-resolution O 1s (b) and Sn3d5 (c,
cr
d) signal as a function of film thickness.
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Figure 4. Depth profiles of the thin film treated from RT to 300oC. Sn3d5 and O 1s atomic concentration as a function of sputtering time and film thickness (a); XPS high-resolution O 1s
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(b) and Sn3d5 (c, d) signal as a function of film thickness.
Figure 5. Effect of temperature on the thin film resistance of the sensor in dry air.
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Figure 6. Effect of temperature on the thin film resistance of the sensor in pure N 2.
d
Figure 7. Effect of heat treatment time on the thin film resistance at 200oC in dry air.
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Figure 8. Response-recovery curves of the SnO sensor to 200 ppm NH3 at operating temperatures of 50oC, 100oC, 150oC and 200oC.
Ac ce p
Figure 9. Response-recovery curves of the SnO sensor to 50 - 200 ppm NH3 at 200oC. Figure 10. Structure and equivalent circuit of the SnO sensor. Figure 11. Repeatability of the SnO sensor at 150oC and 200oC for 5 pulses of 200 ppm NH3.
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SnO (101)
10
20
30
40
M
cr
an
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SnO (112)
SnO (002)
Intensity (a.u.)
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Pristine thin film Thin film treated from RT to 300oC
50
60
70
80
90
2*theta (degree)
Ac ce p
te
d
Figure 1.
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Page 17 of 29
ip t cr us an M Ac ce p
te
d
Figure 2.
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Page 18 of 29
0
Approx. thickness (nm) 40
60
80
100
(a)
100
80
ip t
C1s
60
O1s Sn3d5
40
cr
Atomic Concentration (%)
20
20
5
10
15
20
25
Sputtering time (minute)
30
35
Ac ce p
te
d
M
an
0
us
0
19
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ip t cr us an M d te Ac ce p
Figure 3.
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Approx. thickness (nm) 0
40
60
80
100
(a)
100
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80
C1s
60
O1s Sn3d5
40
cr
Atomic Concentration (%)
20
20
5
10
15
20
25
Sputtering time (minute)
30
35
Ac ce p
te
d
M
an
0
us
0
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ip t cr us an M d te Ac ce p
Figure 4.
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1st measurement 2nd measurement 3rd measurement
ip t cr
104
us
103
an
Resistance (Ohm)
105
102 0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320
M
Temperature (oC)
Ac ce p
te
d
Figure 5.
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1st measurement 2nd measurement 3rd measurement
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108
cr
106
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Resistance (Ohm)
107
105
0
20
40
60
M
103
an
104
80 100 120 140 160 180 200 220 240 260 280 300 320
Ac ce p
te
d
Temperature (oC) Figure 6.
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ip t cr us an M Ac ce p
te
d
Figure 7.
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Page 25 of 29
ip t cr us an M d te Ac ce p
Figure 8.
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954 952
Air
cr
946
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200ppm NH3
948
940
50ppm NH3
938 936
10
20
30
Time (minute) Figure 9.
40
50
Ac ce p
te
d
0
an
100ppm NH3
942
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944
M
Resistance (ohm)
950
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ip t cr us an M
Ac ce p
te
d
Figure 10.
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1.02
150oC 200oC
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1.00
cr
0.99
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0.98
0.97
NH3 (200ppm) + Air
Air
0.96 10
20
30
40
50
M
0
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Relative Resistance
1.01
60
70
80
90
Ac ce p
te
d
Time (minute) Figure 11.
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S0925-4005(13)01567-0 http://dx.doi.org/doi:10.1016/j.snb.2013.12.086 SNB 16384
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
3-10-2013 18-12-2013 21-12-2013
Please cite this article as: V.X. Hien, J.-H. Lee, J.-J. Kim, Y.-W. Heo, Structure and NH3 sensing properties of SnO thin film deposited by RF magnetron sputtering, Sensors and Actuators B: Chemical (2013), http://dx.doi.org/10.1016/j.snb.2013.12.086 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structure and NH3 sensing properties of SnO thin film deposited by RF magnetron sputtering Vu Xuan Hien, Joon-Hyung Lee, Jeong-Joo Kim, Young-Woo Heo* School of Materials Science and Engineering, Kyungpook National University, Daegu 702-701
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Republic of Korea
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Highlights: ! ! SnO gas sensors were fabricated by RF magnetron sputtering
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! ! A thin layer of SnO2 was found on the surface of the SnO thin film
! ! The electrical properties of the sensor was recorded during heat treatment
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! ! NH3 sensing properties of the SnO sensor was carried out
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! ! The sensing mechanism of the device was proposed and explained
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Abstract
SnO thin films, 100 nm in thickness, were deposited on glass substrates by RF magnetron
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sputtering. A stack structure of SnO2/SnO, where few nanometers of SnO2 were determined on the SnO thin film by X-ray photoelectron spectroscopy. In addition, XPS depth profile analysis of the pristine and heat treated thin films were introduced. The electrical behavior of the as-sputtered films during heat treatment in air and nitrogen was recorded to investigate the working conditions for the SnO sensor. Subsequently, The NH3 sensing properties of the SnO sensor at operating temperature of 50-200oC were examined, in which the p-type semiconducting sensing properties of the thin film were noted. The sensor shows good sensitivity and repeatability to NH3 vapor. Finally, a sensing mechanism was proposed and discussed.
Keywords: Gas sensor, SnO thin film, Sputtering, p-type semiconductor.
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* Corresponding author: Young-Woo Heo (Tel.:+82 53 950 7587; Fax: +82 53 950 5465, E-mail: [email protected])
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1. Introduction
Stannous oxide is a p-type metal oxide semiconductor with a band gap ranging from 2.5 eV
an
to 3 eV. SnO is a well-known coating material [1] and catalyst [2], and has recently been applied to thin film transistors [3-7]. However, sensing properties of stannous oxide is still lack of concern.
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Main reason for this problem may relate to the disproportion of SnO to Sn and SnO2 (SnO Sn3O4 + Sn SnO2 + Sn) at elevated temperatures [8]. This phenomenon has also been studied by Raman
d
scattering, infra-red (IR) reflectivity and X-ray diffraction (XRD) [9]. Moreover, a match between
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SnO (001) and SnO2 (101) with similar Sn-Sn coordination (0.38 nm for SnO and 0.37 nm for SnO2) may lead to conversion from the (001)-textured layers of SnO to the (101)-texture layers of SnO2 [9-
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11]. Consequently, the layers of SnO2 may be naturally formed on the surface of SnO. Thus, minimizing the thickness as well as the sensing property of SnO2 layers on the surface of SnO is a key point to measure p-type semiconducting sensing properties of SnO. The formation and expansion of the depletion and accumulation zones is resulted in electron
trapping and hole generation of the oxygen-adsorbed layers for n-type and p-type materials, respectively [12]. Therefore, the fluctuation of the material conductivity during heat treatment may provide us information about the formation priority of a depletion/accumulation zone at discrete points of treatment temperature. As the depletion zone plays an important role in the sensing properties of n-type semiconductors, selecting a right temperature where the formation/expansion of
2
Page 2 of 29
the depletion zone is weaker than those of the accumulation zone is a technique to measure the gas sensing properties of SnO. In this study, SnO sensors were fabricated by depositing SnO thin films on glass-supported
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Ni/Au interdigitated electrodes using RF magnetron sputtering. The resistance-temperature (R-T) characteristic was carried out to indicate the operating temperature of the sensors. Subsequently, NH3
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sensing properties of the sensor were examined using a dynamic gas testing system. Finally, a possible hypothesis for the behavior of the R-T curves during heat treatment and the sensing
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mechanism of the sensors are also introduced.
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2. Experimental procedure
SnO thin films, 100 nm in thickness, were deposited by RF magnetron sputtering on Corning glass substrates and a Ni(40 nm)/Au(60 nm) interdigitated electrode (IDEs with electrode width: 100
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μm and electrode spacing: 50 μm) at 200oC. The sputter power, sputter rate and the ratio of Ar:O2
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were 50 W, 1.16 nm/sec and 97:3, respectively. The total gas flow rate was modulated by mass flow
te
controllers with the working pressure maintained at 10 mtorr. The R-T behavior and NH3 sensing properties of the as-prepared samples were examined using a dynamic gas testing system. For the
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NH3 sensing measurements, high-purity dry air was used as the base gas at a flow rate of 100 sccm. Before the measurements, the chamber was pumped to just below 8 mtorr to check for gas leakage and remove contamination, such as moisture and residual gases. Dry air was used as the base gas during the measurements.
The surface morphologies of all samples were characterized by field emission scanning
electron microscopy (FE-SEM: JSM-6701F). X-ray Diffraction (XRD) measurements were performed on XPERT-PRO X-Ray Diffraction System using the CuKα1 radiation (
)
with a potential of 40 kV and a current of 30 mA. For the chemical composition, the depth profile and X-ray photoelectron spectroscopy (XPS) high resolution spectra were characterized by XPS analysis (Quantera SXM) in which the binding energy data was calibrated due to C1s signal of
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Page 3 of 29
ambient hydrocarbons (C-H and C-C) at 284.6 eV. Transmission electron microscopy (TEM) was performed on Telnai G2F20 S-TWIN, Philips. 3. Results and discussion Structures of the thin film before/after heat treatment
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3.1.
XRD spectra of the pristine thin film and the film treated from RT to 300oC are shown in fig.
cr
1. Those spectra show the amorphous phase of the pristine thin film. For the film treated from RT to 300oC, 3 peaks at 29.8o, 37.1o and 50.7o indexing to tetragonal structure of SnO(101), SnO(002) and
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SnO(112) (Reference: Joint Committee for Powder Diffraction Standard – JCPDS: 01-072-1012) were found. Besides, no impurity peaks were observed in those patterns.
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Figure 2 introduces FESEM images of the pristine and the treated film (from RT to 300oC)
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which were deposited on IDEs. It can be easily seen on the figure that the thin film surfaces were constructed of grains. In addition, the grain sizes of both samples were similar (below 100 nm).
d
XPS depth-profiling of the as-deposited thin film was probed as a function of layer thickness
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(fig. 3a). As follow this data, about 19 sputtering cycles (nearly 19 min in total) were required to remove a 100 nm thickness of the pristine thin film and reach the glass substrate. Therefore, an
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approximate sputtering rate was calculated of 5.26 nm/min. The drop of C1s concentration after 1s sputtering indicated an elimination of the ambient hydrocarbons on the sample surface. The elemental distributions of Sn and O in the thin film can also be found in fig. 3a where the Sn3d5 concentration increased from 50% to 62% and inversely, the O1s concentration decrease from 50% down to 35%.
The XPS high-resolution O1s (fig. 3b) and Sn3d5 (fig. 3c,d) spectra introduce in more detail
the chemical composition of the pristine thin film followed the distance from the thin film surface. The O1s spectrum could be Gaussian fitted to 3 peaks at 530.1 eV, 531.5 eV and 532.4 eV (fig. 3b). On the thin film surface, the high band energy component at 530.3 eV indicated the formation of the SnO2 phase [13]. Besides, the O1s peaks at 531.7 eV and 532.6 eV was assigned to the presence of a
4
Page 4 of 29
hydroxyl groups and water molecules adsorbed on the thin film surface [14], respectively. For the underneath layers, a single peak at 529.9 eV was assigned to SnO phase [13]. The Sn3d5/2 spectra of layers near the thin film surface composed of 3 peaks at 483.3 eV, 485.6 eV and 486.3 eV (fig. 3c).
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Sn3d5/2 peaks at 486.3 eV and 485.8 eV confirmed the appearance of SnO2 on the thin film surface and SnO underneath [13]. Matched with the peak from Sn metal in the pristine thin film at 17 nm
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below the surface was a single Sn3d5/2 peak at 484.3 eV [15]. The peak shifts of O1s, Sn3d3/2 and Sn3d5/2 depending on layers of the pristine film in fig. 3d suggests that the thin film structure was not
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fully stoichiometric.
Figure 4 shows the depth-profiling curve and the XPS high-resolution spectra of SnO thin
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film which was treated from RT to 300oC. As for the depth-profiling curve, the O1s concentration was higher than Sn3d5 concentration at the layers near surface indicating that the oxygen adsorption was
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carried out after heat treatment. Three peaks of SnO2, H2O and OH- were also recorded at the surface
d
of this sample. The Sn metal peak in this sample was indicated at layer, which was 61 nm distance
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from the thin film surface. Therefore, Sn metal which was found at 17 nm below the surface was oxidized after treatment. Interestingly, there were linear shifts from SnO2 peak to SnO peak as a
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function of layer position in the treated film. Especially, no shifted peaks were found at layers, which were deeper than 50 nm from the surface. Thus, the oxygen adsorption process was completed and the thin film structure was stoichiometric. 3.2.
Electrical behaviors of the sensors during heat treatment
Figure 5 shows the electrical behavior of the SnO sensor during heat treatment (RT to 300oC
in air). In this data, the black solid, red dash and blue dot curves correspond to the first, second and third heat treatments, respectively. According to the first curve, at least two special regions were observed: from 38oC to 70oC and from 160oC to 260oC. In the first region, a slightly upward trend was noted indicating that the sample resistance had stabilized and decreased slowly with the increasing temperature. Similarly, the same conduct was also detected in the early stages of region 2,
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where the resistance remained until 180oC. Surprisingly, due to the increasing temperature from just above 200oC, the film resistance magnified rapidly from 1.5 kΩ to hit a peak of ~2.5 kΩ (235oC), and then decreased to 200 Ω at the end of region 2. These above electrical behaviors may be related
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to phenomena: the adsorption and desorption of water and oxygen on SnO2 surface. Most metal oxide semiconductors tend to adsorb oxygen from the outer environment because
cr
of their non-stoichiometric structure because of oxygen vacancies. Consequently, the formations of a depletion zone and accumulation zone corresponding to n-type and p-type semiconductors were
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carried out during adsorption [12,16]. In addition, these processes are related to electron-trapping in the depletion zone and hole-generation in the accumulation zone, which directly affects the variation
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of the material conductivity. Moreover, adsorption also plays an important role in converting the 2+
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oxidation state of Sn to the 4+ oxidation state [10]. Therefore, a knowledge of the oxygen adsorption process is essential for understanding the complicated behavior in the conductivity of the sensor
d
during heat treatment.
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The oxygen adsorption process has been studied by Mizokawa [17], Bielanski [18], Chang [19], Kohl [20] and Batzill [21], where a transition of oxygen species occurs from physisorption to
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chemisorption with the aid of temperature according to the following process: (3)
In this process, oxygen is initially diffused into the material structure, particularly in a region near the surface. For tin oxide, chemical bonding between adsorbed oxygen and Sn results in (for SnO2) or
160oC, a transition from ;
to
:
(for SnO). At a working temperature of approximately or
occurs (
;
or
), which expands the depletion zone in SnO2 or the
accumulation zone in SnO. With these expansions, a large number of carriers is taken and generated, causing a decrease and increase in the conductivity of SnO2 and SnO, respectively. Therefore, the conductivity of SnO thin film tends to increase with increasing working temperature because of the
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Page 6 of 29
higher diffusion rate and transition among oxygen species, as mentioned above. Nevertheless, if species at approximately 160oC, a downward curve of
or
there had been a transition to form
the resistance would have been received around this point. In this case, however, upward instead of
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downward trends in the resistance of the sample were observed at closely 160oC (fig. 5). Therefore, another process instead of adsorption should be considered.
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The water molecule is one of the most abundant molecules on the earth, and all living things as well as many nonliving things are affected directly and indirectly by it. Although SnO2 is sensitive
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to inflammable/ flammable gases, it has been studied as a material for humidity sensors [21-23].
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Those studies indicated the intensification of SnO2 conductance while being exposed to water vapor might result in a dissociation and reduction process [20]. In addition to water molecules, hydroxyl
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groups were observed to cause changes in the conductivity of SnO2 during water adsorption. This is related to a study by Yamazoe et al. [24], who reported a decrease of SnO2 conductance on a
d
desorption peak at high temperatures in TPD. In addition, according to Kohl [20], the formation of
(4)
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or
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the adsorbed hydroxyl group can be expressed as the follow equations:
(5)
The generations of e- or oxygen vacancies on the surface in equations 3 and 4 directly and
indirectly increase the conductance of SnO2. The decomposition of H2O to OH- at elevated temperatures can lead to electron-trap or hole-generation. Inversely, the generation of e- or oxygen vacancy is equal to the disappearance of h+ in p-type materials, such as SnO, which leads to a decrease in conductivity. As follow the XPS data, the H2O molecular as well as OH- were adsorbed by SnO2 layer on the thin film surface. Besides, the initial water adsorption on SnO2 (101) was observed at 110 K [25], whereas the adsorbed oxygen species appeared only on the SnO2 surface at higher temperatures [19]. Therefore, the adsorption of water molecules is the dominant process at
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room temperature. This process may also be responsible for the drift phenomenon of the resistance (to a lower value) to time or the aging effect of metal oxide semiconductors. Thornton and Harrison indicated that for SnO2 under atmospheric pressure, water molecules
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were removed at 150oC, but hydroxyl groups desorbed at higher temperatures (~250oC) [26]. Similarly, Yamazoe et al. [24] and Egashira et al. [27] observed desorption peaks of water molecules
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in the SnO2 structure below 200oC by thermal desorption spectroscopy. These results explain the change in resistance of the sensor near 160oC. In addition, the upward trend below 80oC for the 1st
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treatment might be related to water molecule desorption at the thin film surface, whereas the change
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in the resistance at 160oC may be related to the desorption of water molecules in the deeper layers of the thin film.
Similar behavior for the 2nd and 3rd measurements (red dash line and blue dot curve in figure
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2) was observed for the sensor. A linear decrease in resistance below 160oC for the 2nd and 3rd
d
measurements indicated a small amount of water molecules on the thin film surface. It should be
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noted that the sample was kept in the chamber without loading out. The resistances of the thin films in those measurements showed a slight increase just above 200oC. The cause of the increase in film
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resistance at ~200oC is unclear because oxidation and water desorption occurred simultaneously after 150oC. Therefore, the sensor was measured under the same conditions except N2, which flew into the chamber (total flow rate: 200 sccm) during the measurements to restrict the oxidation. Figure 6 shows the effect of temperature on the electrical properties of the SnO sensor in N2.
The initial resistance of this sample was 20 kΩ, which was lower than that of the previous sensor (approximately 100 kΩ). This behavior suggests a large amount of water molecules on the film surface, which caused the decrease in film resistance, which was comparable to the previous sample treated in dry air. The oxidation was restricted in N2 medium, a significant increase in film resistance at ~250oC was fitted to the point where the desorption of hydroxyl groups was observed in SnO2, as mentioned above. This desorption point was observed easily again at 270oC in the second
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measurement (red dash curve). In addition, the upward shift of the thin film resistance with each measurement shows that the desorption of water molecules and hydroxyl groups dominated oxidation in this experiment. According to the 1st measurement, there was no desorption point at
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approximately 205oC. Therefore, water desorption might not be the reason for the increase in the resistance of the sample treated in air at 205oC. The adsorption of oxygen on the surface of SnO2
cr
which expand the deletion zone or increase the sensor resistance may respond to this effect. Besides, the oxidations of Sn to SnO2 or SnO to SnO2 may also be processes causing the same phenomenon.
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The sharp decrease in thin film resistance above 236oC may relate to the diffusion and adsorption of
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oxygen from the external medium into the deeper layer of the film, where SnO dominated. This resulted in an expansion of the accumulation zone, which enhanced the carrier quantity or caused a decrease in film resistance. According to these arguments, the operating temperature for the SnO
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sensor should be below 200oC to avoid the oxidation of Sn, SnO to SnO2 as well as the oxygen
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semiconducting sensing properties.
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adsorption of SnO2 (expand depletion zone) which leads the gas sensor behave as n-type
Influence of heat treatment time (1st measurement) on the thin film resistance is introduced in
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fig. 7. There are 3 number15ed regions in which region 1 (from 0 to 40 min), region 2 (from 40 to 340 min) and region 3 (from 340 to 430) shows the thin film resistances when the treatment temperature is increased from room temperature to 200oC, is stabilized at 200oC and is cooled naturally, respectively. As can be seen in region 2, the thin film resistance was slightly increased in the first 5 minutes (oxygen adsorption on SnO2 surface), then the resistance was decreased (oxygen adsorption on SnO surface) and was stabilized at around 950 Ω when the treatment time was 140 min. This saturation state implies that there were no phase transitions or other processes (adsorption, desorption, grain size growth, etc.) happened after 140 min heat treatment. 3.3.
NH3 sensing properties of the SnO sensor
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Figure 8 introduces the response-recovery curves of the SnO sensor to 200 ppm NH3 at operating temperatures of 50oC, 100oC, 150oC and 200oC. When NH3 was injected into the measuring chamber at 200oC, a slight decrease of the sensor resistance was observed which is
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behavior of n-type semiconductor to a reduced gas. Subsequently, the resistance was rapidly increased and stabilized at around 972 Ω and for the recovery, a gradual decline of the sensor
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resistance was recorded. This gas sensing behavior represented p-type semiconducting properties of the sensor. However, the sensor resistance after recovery was slightly decreased comparing with the
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initial value. This phenomenon was clearly seen in the sensing curves which were measured at 150oC, 100oC and 50oC. Besides, the n-type semiconducting sensing properties were not shown in case the
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SnO sensor was measured at 100oC and 50oC.
Figure 9 presents the influences of NH3 concentration to the sensing properties of the SnO
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sensor at 200oC. After injecting 50 ppm of NH3, the sensor resistance was slightly decreased from its
d
initial value before increasing. This was not happened in the next concentrations of 100 ppm and 200
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ppm NH3. An explanation for this phenomenon is based on the sensor structure which is illustrated in fig. 10. Fig. 10c shows the p-n junction between SnO2 on the thin film surface and SnO in the deeper
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layers. The NH3 sensing behavior of the sensor can be described by 3 steps. First, NH3 vapor reacts with adsorbed oxygen adatoms of SnO2 on the sensor surface. This reaction removes the oxygen adatoms and also releases free electrons to SnO2 structure. The SnO2 layers in this case, become conductive layers, which make the sensor conductivity surge. Second, the NH3 vapor diffused into deeper layers of the thin film, when adsorbed oxygen of SnO2 was completely reacted. Third, the reduced gas reacts with oxygen adatoms of SnO and takes out holes from the SnO structure which then leads the sensor resistance increase. The reactions between NH3 and surface adsorbed oxygen species for the n-type and p-type semiconductor occur according to the following equations: (6) (7)
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The sensor structure can also be illustrated as an equivalent circuit in fig. 10d. In this figure, are resistance and capacitance of 2 electrodes.
layers on the surface whether For this circuit,
is resistance of SnO2
are resistances of SnO layers under the thin film surface.
was decreased when the sensor exposed to NH3 which make the total
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and
. In this case, the SnO2
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resistance of the sensor decrease because
layers were quite thin (less than 6 nm), therefore, the main sensing area was the SnO layers
subsequently increase
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underneath. The reactions between NH3 and oxygen adatoms of SnO make
increase which
. Finally, the p-type semiconducting sensing properties of the sensor
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was shown. If there is a conversion from SnO to SnO2 during the test, the influence of
on
will be more significant which may reduce the sensor response (S = Rg/Ra).
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The repeatability of the SnO sensor at 150oC and 200oC for 5 pulses of 200 ppm NH3 was
d
shown in fig. 11. Both measurements proved the good repeatability of the sensor. Nevertheless, the
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sensor resistance was gently dropped during the tests. This effect was believed as a consequence of the uncompleted recovery process of SnO2. The recovery of SnO2 can be understood as the re-
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adsorption of oxygen adatoms the the SnO2 surface after they were removed by reactions with NH3 (equation 6). Since the adsorption of external oxygen to SnO2 structure was found to be happened at 205oC,
(low value) after reacting with NH3 could not completely recover to the initial stage
(high value) at lower operating temperatures (less than 200oC, in this situation). Ultimately, was dropped after several pulses. During the measurement, the gas response was a constant value, therefore the transformation of SnO to SnO2 which affect the sensor response was probably not occurred. Conclusion SnO thin films, 100 nm thickness, were deposited by RF magnetron sputtering. The extremely thin layers of SnO2 (less than 6 nm) was found on the surface of the thin film. The
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Page 11 of 29
structure of thin films after treating from RT to 300oC was stoichiometric. The effects of heat treatment on the electrical behavior of the as-prepared samples were investigated. Based on the resistance-temperature curves, the adsorption of external oxygen into SnO2 structure was found to be
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occurred at approximately 205oC in air. For the NH3 sensing test, the sensor exhibited a p-type semiconducting sensing characteristics at working temperature below 200oC. In addition, the sensor
cr
was sensitive to 50 ppm NH3 at 200oC with a good repeatability. The sensing behavior of the SnO sensor was explained in detail using equivalent circuit. Finally, the sensing properties of the sensor to
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several reduced gases at 200oC were introduced. Acknowledgement:
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This work was supported by the Mid-career Researcher Program, and the Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the
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Korean government (MSIP) (No. 2011-0017245, 2008-0062617). References:
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[1] Z. Han, N. Guo, F. Li, W. Zhang, H. Zhao, Y. Qian, Solvothermal preparation and morphological
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evolution of stannous oxide powders, Mater. Lett. 48 (2001) 99-103.
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[2] X. Song, S. Zhang, D. Zhang, Catalysis investigation of PET depolymerization under metal oxides by microwave irradiation, J. Appl. Polym. Sci. 117 (2010) 3155-3159. [3] Y. Ogo, H. Hiramatsu, K. Nomura, H. Yanagi, T. Kamiya, M. Hirano, H. Hosono, P-channel thinfilm transistor using p-type oxide semiconductor, SnO, Appl. Phys. Lett. 93 (2008) 032113. [4] K. Okamura, B. Nasr, R.A. Brand, H. Hahn, Solution-processed oxide semiconductor SnO in pchannel thin-film transistors, J. Mater. Chem. 22 (2012) 4607-4610. [5] L.Y. Liang, Z.M. Liu, H.T. Cao, Z. Yu, Y.Y. Shi, A.H. Chen, H.Z. Zhang, Y.Q. Fang, X.L. Sun, Phase and optical characterizations of annealed SnO thin films and their p-type TFT application semiconductor devices, materials, and processing, J. Electrochem. Soc. 157 (2010) H598-H602.
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[6] L.Y. Liang, H.T. Cao, X.B. Chen, Z.M. Liu, F.Zhuge, H.Luo, J.Li, Y.C. Lu, W. Lu, Ambipolar inverters using SnO thin-flim transistors with balanced electron and hole mobilities, Appl. Phys. Lett. 100 (2012) 263502.
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[7] H. Yabuta, N. Kaji, R. Hayashi, H. Kumomi, K. Nomura, T. Kamiya, M. Hirano, H. Hosono, Sputtering formation of p-type SnO thin-film transistors on glass toward oxide complimentary
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Biographies:
Vu Xuan Hien is presently a PhD student at School of Materials Science and Engineering -
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Kyungpook National University, Daegu, Korea. He earned a MS from School of Engineering Physics,
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Hanoi University of Science and Technology, Vietnam. His research interests cover syntheses of multi-morphology metal oxide semiconductors using physical/chemical methods and their
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applications in gas sensor.
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Jeong-Joo Kim is a professor in School of Materials Science and Engineering, Kyungpook National University, Korea. He received a BS, MS and PhD in Inorganic Materials Engeering, Seoul National University, Korea. His research interests includes Sintering, Transparent Conducting Oxides, and
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Solid Oxide Fuel Cells.
Jopon Hyung Lee is a professor in School of Materials Science and Engineering, Kyungpook
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National University., Korea. He received a BS in Department of Electronics Engineering, Youngnam University and MS and PhD in Inorganic Materials Engineering, Kyungpook National University,
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Korea. His research interests includes sensors, Transparent Conducting Oxides, Solid Oxide Fuel Cells and Sensors.
Young-Woo Heo is an associate professor in School of Materials Science and Engineering, Kyungpook National University., Korea. He received a BS and MS in Inorganic Materials Engeering, Kyungpook National University and PhD in Department of Materials Science and Engineering, University of Florida. He was selected a Top 100 Materials Scientists of The Past Decade, 2000~2010 released by Thomson Reuters (2011). His research interests includes Sensors and Transparent Electronic Materials/Devices.
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Figure 1. XRD spectra of thin film before and after heat treatment (from RT to 300oC). Figure 2. FESEM images of the pristine thin film (a) and thin film treated from RT to 300oC (b).
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Figure 3. Depth profiles of the as-deposited thin film. Sn3d5 and O1s atomic concentration as a function of sputtering time and film thickness (a); XPS high-resolution O 1s (b) and Sn3d5 (c,
cr
d) signal as a function of film thickness.
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Figure 4. Depth profiles of the thin film treated from RT to 300oC. Sn3d5 and O 1s atomic concentration as a function of sputtering time and film thickness (a); XPS high-resolution O 1s
an
(b) and Sn3d5 (c, d) signal as a function of film thickness.
Figure 5. Effect of temperature on the thin film resistance of the sensor in dry air.
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Figure 6. Effect of temperature on the thin film resistance of the sensor in pure N 2.
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Figure 7. Effect of heat treatment time on the thin film resistance at 200oC in dry air.
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Figure 8. Response-recovery curves of the SnO sensor to 200 ppm NH3 at operating temperatures of 50oC, 100oC, 150oC and 200oC.
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Figure 9. Response-recovery curves of the SnO sensor to 50 - 200 ppm NH3 at 200oC. Figure 10. Structure and equivalent circuit of the SnO sensor. Figure 11. Repeatability of the SnO sensor at 150oC and 200oC for 5 pulses of 200 ppm NH3.
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SnO (101)
10
20
30
40
M
cr
an
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SnO (112)
SnO (002)
Intensity (a.u.)
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Pristine thin film Thin film treated from RT to 300oC
50
60
70
80
90
2*theta (degree)
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te
d
Figure 1.
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te
d
Figure 2.
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0
Approx. thickness (nm) 40
60
80
100
(a)
100
80
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C1s
60
O1s Sn3d5
40
cr
Atomic Concentration (%)
20
20
5
10
15
20
25
Sputtering time (minute)
30
35
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te
d
M
an
0
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0
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Figure 3.
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Approx. thickness (nm) 0
40
60
80
100
(a)
100
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80
C1s
60
O1s Sn3d5
40
cr
Atomic Concentration (%)
20
20
5
10
15
20
25
Sputtering time (minute)
30
35
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te
d
M
an
0
us
0
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Figure 4.
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1st measurement 2nd measurement 3rd measurement
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104
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103
an
Resistance (Ohm)
105
102 0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320
M
Temperature (oC)
Ac ce p
te
d
Figure 5.
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1st measurement 2nd measurement 3rd measurement
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108
cr
106
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Resistance (Ohm)
107
105
0
20
40
60
M
103
an
104
80 100 120 140 160 180 200 220 240 260 280 300 320
Ac ce p
te
d
Temperature (oC) Figure 6.
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te
d
Figure 7.
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Figure 8.
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954 952
Air
cr
946
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200ppm NH3
948
940
50ppm NH3
938 936
10
20
30
Time (minute) Figure 9.
40
50
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te
d
0
an
100ppm NH3
942
us
944
M
Resistance (ohm)
950
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Ac ce p
te
d
Figure 10.
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1.02
150oC 200oC
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1.00
cr
0.99
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0.98
0.97
NH3 (200ppm) + Air
Air
0.96 10
20
30
40
50
M
0
an
Relative Resistance
1.01
60
70
80
90
Ac ce p
te
d
Time (minute) Figure 11.
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