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New gas sensing mechanism for SnO2 thin-film gas sensors fabricated by using dual ion beam sputtering
Sensors and Actuators B 77 (2001) 200±208
New gas sensing mechanism for SnO2 thin-®lm gas sensors fabricated by using dual ion beam sputtering Yong-Sahm Choe* Materials Research Laboratory, Tong Yang R&D Institute, 378-1 Maeng-ri, Wonsam-myun, Yongin-si, Kyungki-do 449-870, South Korea
Abstract A new gas sensing mechanism for SnO2 thin-®lms fabricated by using dual ion beam sputtering, was proposed. The features of the SnO2 thin-®lm considered in this study had been characterized as having extremely smooth surface, dense microstructure and near stoichiometric SnO2 phase. In this study, thus, the problems of reported gas sensor models were noticed brie¯y and next, a novel sensing model for the SnO2 thin-®lms prepared in this study was approached by geometrical considerations. The effects of resistivity in the bulk regions and in the depletion regions on the gas sensitivities were studied by fabrication of thin-®lm gas sensors having various ®lm thickness. The proposed gas sensing mechanism was veri®ed by the fabrication of a H2S sensor with p±n junctions. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Semiconductor gas sensor; Gas sensing mechanism; Debye length; SnO2 thin-®lm; Dual ion beam sputtering
1. Introduction Semiconductor SnO2 gas sensors have been developed with various types such as powder-sintered, thick-®lm and thin-®lm sensors. To understand gas-sensing mechanisms of SnO2 gas sensors fundamentally, two groups of studies have been conducted. One group is related to atomistic models of molecular recognition with SnO2 sensors. The other group concerns geometric effects, in which in¯uence of contact geometry and crystallinity on sensor response signal is mainly discussed [1]. Among latter studies, the ultra®ne particle ®lm and the grain size effects models on gas-sensing response are worth noticing [2,3], which are applicable for porous SnO2 elements [4]. The gas-sensing response of sensor, S is de®ned as the ratio of sensor resistance in air to that in the relevant gas mix, Ra/Rg. In the model for grain-size effects, S is determined by two factors. One factor is a diameter D of SnO2 crystallite and the other is a thickness L of electron depletion layer (space charge layer) in air, where L is determined by the Debye length and the strength of the oxygen chemisorption. The L of the SnO2 ultra®ne particle ®lms sputtered in [2] was estimated to be as thin as 3 nm, and L of the sintered SnO2 in [3] also was evaluated as the same L value. It was also proposed that when D decreases to be comparable to 2L,
* Tel.: 82-31-334-8219; fax: 82-31-334-8224. E-mail addresses: [email protected], [email protected] (Y.-S. Choe).
S could increase drastically. Consequently, in order to make highly sensitive gas sensors, the gas sensors should have very small grain sizes and high speci®c surface areas of SnO2 powders. However, such small crystallites of pure SnO2 are only stable at temperatures below 4008C and hardly maintained in practical sensor fabrications due to grain growth or coalescence during calcinations or sintering processes at temperatures around 7008C [5]. In addition, a thin-®lm gas sensor having high speci®c surface area tends to be fragile. As shown in example of the ultra®ne particle ®lm in [2], the ®lm structure was spongy and the ®lm packing density was as low as 1/300 compared to density of SnO2 single crystal. As reviewed in aforementioned models, traditional approaches have been conducted to decrease D near or below 2L and to maintain such small L by lowering process temperatures or doping foreign oxides for inhibiting the grain growth. In all these cases, however, L was ®xed as about 3 nm. Although L increased up to 20 nm or above in [3] by doping Al for valency control of SnO2, the electric resistance of the sensor was too high as 109 O, which was about three-orders of magnitude larger than that of the pure SnO2 in [3] and was not proper to practical use. On the other hand, a semiconductor gas sensor of thin ®lm type becomes more important recently, because of the advantages in miniaturization, processing, and potential as an electronic nose by integrated circuits. The author, therefore, intended to deposit a robust and highly sensitive SnO2 thin-®lms. For suf®cient mechanical
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 7 3 1 - 6
Y.-S. Choe / Sensors and Actuators B 77 (2001) 200±208
strength, dense structure was preferred to porous one. Instead of obtaining small crystallite, L was designed to extend through whole ®lm thickness for highly sensitive sensors by synthesizing near-stoichiometric SnO2 ®lms. A ®lm thickness of the sensors also could be expected to be thick enough for practical use. For this purpose, SnO2 thin®lms were newly fabricated by using dual ion beam sputtering, whose crystallinity, chemical compositions and physical properties were controllable [6±8]. Based on the above studies, the novel SnO2 thin-®lms were obtained, which were characterized as dense and highly packed SnO2 textures with extremely smooth surface [9]. The purpose of this paper is to elucidate a gas sensing mechanism for the novel SnO2 thin-®lms and to verify the gas sensing mechanism by fabricating thin-®lm gas sensors. Firstly, sensor responses were measured as a function of ®lm thickness, and relationships between thickness of electron depletion layer and gas-sensing response were examined. Secondly, a new gas-sensing mechanism for the novel SnO2 thin-®lms was proposed, in which effects of electrical resistivity in bulk region on gas-sensing response were studied by geometrical consideration. After resistivity in air was compared that in vacuum with each various ®lm thickness, meaning of the new gas-sensing mechanism was discussed. Finally, proof of the new gas-sensing mechanism was proposed, based on H2S sensors employing the novel SnO2 ®lms with new concepts in electrode arrays and catalytic layer. 2. Experimental To investigate a ®lm thickness effect on gas-sensing response, various SnO2 thin-®lms having a thickness of 10, 20, 50, 100, 200, 500 nm were deposited on alumina substrates (5 mm 10 mm 0:4 mm) by using a dual ion beam sputtering system (DIBS). As a primary ion source for sputtering pure tin target, Ar ion gun was employed, where acceleration voltage and current density were 1.0 keV and 1.5 mA cm 2, respectively. As a secondary ion source for bombarding a growing thin ®lm on the substrate, oxygen ion gun was used, where acceleration voltage and current density were 200 eV and 0.2 mA cm 2, respectively. Deposited SnO2 thin-®lms were annealed in an electrical furnace in air at 4008C for 5 h. As a consequence, all samples were deposited and annealed under an identical condition, except for ®lm thickness. A gas sensor for testing was composed of the novel SnO2 thin-®lm by DIBS and an alumina substrate having a pair of inter-digitated electrodes on the front and a Pt heater beneath the backside. For electrical contacts, Pt wires were bonded to a pair of inter-digitated electrodes with Pt paste. The electrical resistance of the sensor in vacuum and air at various temperatures was recorded continuously to evaluate a gassensing response, S of each gas sensors, by a computercontrolled digital multimeter.
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A gas chamber with inlet and outlet was used to measure the gas-sensing response of the sensors. As testing gases, N2-balanced 1% H2 and 100 ppm H2S gases were used, respectively. A gas concentration in the gas chamber was determined ®nally by adjusting gas ¯ow rate to air¯ow rate. For examples, in order to test sensors at 1000 ppm of H2, the ¯ow rate of the H2 gas and air were controlled as 100 standard cubic centimeter per minute (SCCM) and 900 SCCM, respectively. As for 2 ppm of H2S, the ¯ow rate of the H2S gas and air were adjusted to 20 and 1000 SCCM, respectively. The gas ¯ow rate was controlled precisely, using mass ¯ow controller. For air¯ow, ball ¯ow meter was used. The ®nally diluted gas by mixing with air was injected into the inlet of the gas chamber continuously during measurement, and was ejected through the outlet. The Hall effect measurement for the SnO2 ®lms was conducted using the van der Pauw con®guration, a magnetic ®eld of 6300 G and constant current source, whose current was raised from 1.0 to 4.0 mA in step of 1.0 mA. 3. Results and discussion 3.1. Relationship between thickness of electron depletion layer and gas-sensing response in a semiconductor gas sensor Grain boundary effects on electrical properties in the novel SnO2 ®lms were assumed to be negligible, since the novel SnO2 ®lms had been analyzed as the very compact and dense texture structures [6±8], which had no grain boundaries to block current ¯ow along a thickness direction of the novel SnO2 ®lms. Therefore, electrical properties of the novel SnO2 ®lm was expected to be changed mainly by adsorbed oxygen on a surface of the ®lm, which extracts electrons from the ®lm, and by the concentration of charge carriers introduced the ®lm during producing the ®lm. Calculation of donor concentration by Hall effect measurement might be a good way to evaluate a depth of electron depletion layer in a SnO2 thin-®lm. In this study, however, resistance of the SnO2 ®lms with a thickness of below 100 nm was too high for Hall effect measurement. For thicker SnO2 ®lms of 200 and 500 nm, charge carrier concentration was measured in air at room temperature as 1:95 1015 and 1:36 1015 cm 3, respectively. For more precise and systematic analysis, changes in gassensing response of the gas sensors using the novel SnO2 thin-®lms, as a function of ®lm thickness, were measured with 1000 and 400 ppm of H2 gas at 250, 300 and 3508C, respectively, as shown in Fig. 1. In Fig. 1, following four ®ndings are noteworthy. First, the novel SnO2 ®lms have an optimum thickness for the highest gas-sensing response. This is explained as follows: if a ®lm thickness is greater than the optimum thickness, the other part of the ®lm which exceed the optimum thickness would not participate in gas sensing reaction and could not contribute a resistance
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Finally, it is very noteworthy that although the other thin ®lms deviate from the optimum ®lm thickness of the best gas-sensing response, the thin ®lms show still good sensitivity with some degree. According to some former theories, inner SnO2 layer beneath the electron depletion layer was considered to have very low electrical resistivity [13]. Therefore, in the case that a SnO2 ®lm is thicker than a depth of electron depletion layer, gas-sensing response of the ®lm will be reduced drastically [14]. In Fig. 1, however, there are no such abrupt changes in gas-sensing response as a function of ®lm thickness, like Fig. 5 in [14]. A hypothetic depth of electron depletion layer was calculated as a function of charge carrier concentration as shown in Fig. 2. Difference between both formulas proposed is only in coef®cient of kT term. Charge carrier concentration is a function of temperature, but ranges of concentration calculated with temperature compensation are within 5%. In Fig. 2, temperature was assumed as 3008C. When a thickness of the electron depletion layer was assumed as 100 nm, the charge carrier concentration, based on the formula by Ogawa et al. [2] and McAleer et al. [15] in Fig. 2, was estimated as about 3:8 1015 and 6:3 1015 cm 3, respectively, which are in the order of the results of the Hall measurement. Fig. 1. Measured gas-sensing response as a function of film thickness, for various H2 concentration: (a) 1000 ppm of H2, (b) 400 ppm of H2.
change of the ®lm. This explanation is veri®ed in detail by the following results of the Section 3.3. In a case that the ®lm thickness is less than the optimum thickness, the ®lms show lower gas-sensing response. The case is thought to originate from the followings: no the Schottky-barrier mechanism is valid, because the band bending at the surface can be neglected, based on the overall shift of the Fermi level in the grains during gas exposure [10]. Furthermore, it is considered that all surface charge becomes converted to the nonreactive O2 Ð there are too few electrons to reduce the O2 to O [11]. As an example, Egashira et al. obtained single-crystal sensors thin enough for sensitivity by using SnO2 whiskers [12]. In this reference, the maximum sensitivity was recorded in a sample with a thickness of 5 mm, but the samples lose their sensitivity if the sensor thickness is less than a few micrometers. Second, the optimum thickness is reduced by increasing the operating temperature of the ®lm. This result agrees with the fact that generally charge carrier concentration increases in proportion to temperature, because a thickness of electron depletion layer in n-type semiconductors decreases with increasing charge carrier concentration. Third, the optimum thickness is about 100 nm, indicating that the novel SnO2 ®lm fabricated in this study can be thick enough to be used practically as a gas sensor. It is also notable that the thickness of 100 nm is approximately equal to a depth of electron depletion layer estimated by Hall effect measurement.
3.2. Modeling of a new gas sensing mechanism in the novel SnO2 thin-film For modeling of a novel gas-sensing mechanism, the schematic drawing of the SnO2 thin-®lm gas sensor was proposed in Fig. 3a. Strictly speaking, resistance in the electron depletion layer would be a function of distance from the ®lm surface according to the band bending in the Schottkey barrier model, but the resistance in the layer was assumed as to be constant for simplicity. The other
Fig. 2. Calculation of depletion layer length (Debye length), based on the results of Ogawa et al. [2] and McAleer et al. [15], as a function of carrier concentration in SnO2 films; Ogawa: D ekT=e2 N d 1=2 ; McAleer: D 2ekT=e2 N d 1=2 ; D: Debye length; e: static dielectric constant, 13:5 8:85 10 12 F/m in SnO2; k: Boltzmann's constant, 1:38 10 23 J/K; T: absolute temperature; Nd: carrier concentration per unit volume (cm 3).
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Fig. 3. Schematic drawings demonstrating (a) the formation of a depletion layer on the SnO2 thin-film by oxygen chemisorption and (b) an electrical equivalent circuit of (a) used a gas sensor.
assumptions were as followings. First, a resistance of bulk layer beneath the depletion layer would not be changed after or before gas sensing reaction. Second, a resistivity of the electron depletion layer in air is constant, regardless of a resistivity of bulk layer. Third, grain boundary effect on ®lm resistance is ignored, since the SnO2 ®lm in this study has a dense texture structure grown by granular epitaxy and there is no grain boundary barrier for electron current along a ®lm thickness direction [9]. Finally, electrodes have ohmic contacts with both layers simultaneously. As a result, an electrical equivalent circuit was deduced as two parallel resistors shown in Fig. 3b. Next, equations for the calculation of the gas-sensing response were examined. Ra,d and Ra,b represent the resistance in air on the depletion layer and the bulk layer, respectively. The Rg,d and Rg,b mean the resistance in gas on the depletion layer and the bulk layer, respectively. The gas-sensing response, S is de®ned as Ra /Rg. The resistance, Ra, of the sensor in air can be expressed as Ra;d Ra;b Ra Ra;b Ra;d
(1)
After sensor is exposed in gas, the resistance, Rg, of the sensor, can be described as Rg
Rg;d Rg;b Rg;b Rg;d
(2)
Assume that the gas-sensing reaction occurs mainly in the depletion layer and the resistance of the depletion layer is not changed before and after exposure to gas Ra;b Rg;b Rb
(3)
Therefore, the gas-sensing response is given by S
Ra;d Rb Rg;d Rg;d Rb Ra;d
(4)
Since resistance, R, can be expressed with resistivity, r, and thickness, t, under the condition of unit length and unit width, resistance is r R (5) t For the depletion layer, t is substituted with the thickness of the depletion layer, L. As for the bulk layer, t becomes (T ). The gas-sensing response, therefore, is given by S
ra;d L rb rg;d L rb
rg;d Trg;d ra;d Tra;d
(6)
Consequently, five parameters were needed to evaluate the new model: resistivity of the depletion layer in air, and gas; resistivity of bulk; thickness of depletion layer, and bulk layer. For this simulation, following considerations were introduced. Firstly, let whole film thickness, T be 500 nm and let depletion layer thickness, L be 0 L T, for considering that the depletion layer thickness is thinner than
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Fig. 4. Simulated gas-sensing response as functions of the bulk layer resistivity, r and the depletion layer thickness, L. The vertical dotted arrow-line indicates that the response change drastically with r of bulk layer in gas at the thickness of depletion layer assumed as 100 nm. In case that the thickness of depletion layer equals the whole film thickness, the response always show maximum regardless with r of bulk region. Assumptions: (1) thin film thickness, T: 500 nm; (2) ra,d of depletion layer in air: 103 O cm; (3) rg,d of depletion layer in gas: 0.1 O cm; (4) rb of bulk layer: 0.1±102 O cm.
the whole film thickness. Secondly, when the film was exposed to air, let the resistivity of the depletion layer, ra,d be 103 O cm. Thirdly, when the sensor was exposed to the reducing gas, the decreased resistivity of the depletion layer, rg,d was presumed as 0.1 O cm. Finally, the resistivity of bulk layer, rb was considered as 0.1, 1, 10 and 100 O cm, respectively. The simulated result was presented as Fig. 4, suggesting very decisive clues to propose the new gas sensing mechanism. First, in a case that the depletion layer extends through the whole ®lm, the best gas-sensing response can be obtained. This result agrees with former established models. Second, in a case that the depletion layer would be thinner than the whole ®lm, if the bulk layer (the inner part of the ®lm) has high resistivity, the ®lm shows still somewhat good gas-sensing response. For examples, in case of a low resistivity of 0.1 O cm that was so far measured in former SnO2 gas sensors, if a thickness of the electron depletion layer is 480 nm, gassensing response decreases abruptly. Considering a thickness difference between the electron depletion layer and whole ®lm was so small as to be only 4% of the ®lm thickness, such drastic decrease of gas-sensing response is very signi®cant. Inversely, in case of a high resistivity of 102 O cm, if a thickness of the electron depletion layer is so small as to be 50 nm, gas-sensing response shows a good gas-sensing response of 100 times. It was considered, next, that a thickness of the electron depletion layer was assumed to be a 100 nm, which was approximately corresponded with that of the SnO2 ®lm in this study. A vertical dotted arrow-line in Fig. 4. indicated gas-sensing response changes varying with resistivity changes in bulk layer. In that case, although the electron
depletion layer was so small as 100 nm, if a bulk resistivity is as high as 102 O cm, gas-sensing response is also so high as 200.8 times. However, if a bulk resistivity is as low as 0.1 O cm, gas-sensing response is only 1.25 times. Based on the above results, the gas-sensing response changes in Fig. 1 could be understood only by approaching with the novel gas sensing mechanism. In former proposed models, a resistivity of bulk layer was assumed to be much lower than that of the electron depletion layer whose surface was adsorbed with oxygen. As a result, abrupt changes of gas-sensing response which occurred in case of D > 2L and D < 2L, could be explained. The former models, however, could not explain such gentle changes of gas-sensing response with ®lm thickness as shown in Fig. 1. The author proposes that such gentle changes of gas-sensing response originate from the fact that the ®lm used in this study has a high resistivity of bulk layer as about 102 O cm. As a way to verify the newly proposed model, resistivity of the SnO2 ®lms with various thickness was measured in air and vacuum. First, each resistance of the ®lms was measured (Fig. 5), and then resistivity was approximately calculated from resistance by considering ®lm thickness and dimension
Fig. 5. Resistance changes in the SnO2 thin-films under (a) air, and (b) vacuum, for various temperatures, as a function of the film thickness.
Y.-S. Choe / Sensors and Actuators B 77 (2001) 200±208
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Kohl presented another reaction [17]. H2 O SnL OL HO SnL OL H e H2 O 2SnL OL 2 HO SnL VO
Fig. 6. Estimated resistivity changes in the SnO2 thin-films under (a) air, and (b) vacuum, for various temperatures, as a function of the film thickness.
(Fig. 6). In a case that measuring temperature increased over 2508C, ®lm resistance was very unstable and needed a long time for measuring. For this reason, resistance measurement was conducted in air and vacuum at room temperature, 100, 150 and 2008C. As shown in Figs. 5a and 6a, the resistivity of the ®lm in air was constant within order of magnitude, regardless of its thickness, and decreases with increasing the measured temperature. Resistance values measured in air at room temperature show relatively large deviation compared to other samples. It might be inferred as an effect of water adsorbed on the ®lm surface. Samples at 150 and 2008C have a tendency that the resistance slightly increases with the ®lm thickness. In Figs. 5b and 6b, on the other hand, the resistivity of the ®lms in vacuum increases in proportion to the ®lm thickness, except for the samples measured at room temperature. With elevated measurement temperature, the resistivity of the ®lms in vacuum decreases with a greater extent than in air. As the ®lm thickness decreases, the resistivity is reduced signi®cantly. Yamazoe et al. proposed the following reaction to explain an effect of water on resistivity reduction [16]: H2 O O2 VOn 2 OH ne
According to above reactions, the adsorbed water on the surface decreases the resistivity of the film. When a SnO2 film is located in vacuum, adsorbed water tends to be volatilized for equilibrium vapor pressure, even though at room temperature. For this reason, the resistivity of the film becomes to be higher in vacuum than in air. As reasons for the resistivity reduction in the heated ®lms, following two factors were considered. First factor is desorption of adsorbed oxygen from the surface of the ®lm, and next factor is escape of oxygen from the surface lattice of the ®lm. When the SnO2 ®lms are heated in vacuum, a part of oxygen adsorbed on the surface tends to depart from the surface for the equilibrium state of vapor pressure. Additionally, based on the fact that the resistivity in vacuum was much lower than in air, it is also inferred that a part of oxygen atoms in the surface lattice escapes from the ®lm. Cox et al. reported that heating the ideal surface of SnO2 in vacuum removes a large fraction of surface bridging oxygens at temperatures as low as 500 K [18]. As a result, the resistivity becomes to decrease, because surface oxygen vacancies are known to act as n-type donors. The escape and adsorption of oxygen can be presumed to occur mainly near the ®lm surface, where the electron depletion layer is formed. Therefore, it is noteworthy that the ®lm with a thickness of 500 nm did not show appreciable difference in resistivity in air and in vacuum. It means that the bulk layer, which does not participate in oxygen surface reaction, has so high resistivity equal to a resistivity of electron depletion layer covered with adsorbed oxygen. Consequently, this analysis is also in agreement with the results of Figs. 1 and 4, and such analysis distinguishes the novel gas sensing model presented in this study from former models, which proposed that bulk layer has much less resistivity compared to electron depletion layer. 3.3. Proof of the novel gas-sensing model by using the SnO2 thin-film gas sensor Recently, it was reported by several groups that CuOadded SnO2 shows very high sensitivity and selectivity to hydrogen sul®de [19±21]. This sensor was reported to have very stable and excellent sensing characteristics to hydrogen sul®de in the range of 10±100 ppm at 2008C. Such excellent sensing characteristics against hydrogen sul®de have been explained as p±n junction between CuO and SnO2. While oxygen-de®cient SnO2 shows n-type conductivity by electrons, oxygen-excess CuO shows p-type conductivity by holes. When both oxides contact with each other, a thick charge depletion layer is formed at the interface as a p±n junction and the resistance is highly increased. However, if
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the above CuO layer is exposed to H2S or other sulfur compound gases, it is converted to CuS with metallic conductivity by following equations. CuS H2 S ! CuS H2 O CuO CH3 SH 32 O2 ! CuS CuO 2H2 O Thus, as p-type CuO is converted to metallic CuS, the p±n junction as well as charge depletion layer disappears and the resistance drops drastically. In this study, fabricating thin ®lm gas sensor with p±n junction was tried to prove a novel gas sensing model under following three premises. First, sensing reaction of hydrogen sul®de occurs at only a surface of the ®lm and diffusion of chemical species into inner part of the ®lm is negligible, since the ®lm was proved to have a compact and dense texture. Second, p±n junction is formed only interface between CuO and SnO2. Third, electrodes for sensor are formed beneath SnO2 ®lm. For examples, Fig. 7a shows a typical gas sensor with p±n junction where each electrodes are connected to each layer, CuO and SnO2 [22], and Fig. 7b present a newly proposed gas sensor with p±n junction where both electrodes are connected only to backside of SnO2 ®lm. Sintered gas sensors are composed of a mixture of CuO and SnO2, and thus, CuO is dispersed here and there in sensor, where position of electrodes might be unimportant in a viewpoint of gas sensor performance. In the thin ®lm of this study, however, catalytic layer, CuO, was formed only atop the SnO2 ®lm. Since, in this case, electron depletion layer was proposed to be as thick as ®lm thickness, CuO as a
Fig. 8. Resistance changes in variously CuO-doped SnO2 sensors, for H2S, 2 ppm at 1808C, showing p±n junctions formed between CuO layers and SnO2 films.
p-type semiconductor could extract electrons from the SnO2 ®lm as an n-type semiconductor. As a result, even if electrodes is beneath the SnO2 ®lm, resistance change could be monitored. On the other hand, if a sensor with p±n junction has electrodes like Fig. 7a, p±n junction effect might be monitored but it would not be applicable to estimate a thickness of electron depletion layer. Fig. 8 shows resistance changes of CuO-doped SnO2 sensors, in 2 ppm of H2S at 1808C. In Fig. 8, the thickness of CuO was evaluated from multiplication of the deposition rate by the deposition time, because clusters or islands form thin ®lms during the earliest stages of ®lm formation. The sensor resistance increased in proportion to CuO thickness, as shown in Fig. 8. Considering that CuO resistance is lower than that of SnO2, the increase of resistance is attributed to formation of p±n junction between CuO layer and SnO2 ®lm, by which electrons of n-type semiconductor SnO2 is depleted and the sensor resistance is raised. For a sensor Ê , however, the sensor resiswith a CuO thickness of 12.8 A tance decreased. This fact is understood from followings: numbers and sizes of CuO islands atop SnO2 increase, and then a distance between each island is closer, and ®nally electrical conduction by tunneling decreases the sensor resistance [23]. To examine the resistance changes of the sensors in Fig. 8, Table 1. was presented, where each resistance of CuO, SnO2 and CuO/SnO2 ®lm were measured. It shows that resistance Table 1 Summaries of resistance in CuO, SnO2, CuO/SnO2 films Thin film materials
Fig. 7. Schematic diagrams for two kinds of CuO±SnO2 sensors: (a) electrodes are connected to SnO2 and CuO layers, respectively, [22]; (b) all electrodes are connected with SnO2 layers (used in this study).
CuO SnO2 CuO/SnO2
Thickness (nm) 100 100 1/100
Resistance of thin films (O) 1508C
2008C 4
3.19 10 6.30 106 >1.20 108
2508C 4
1.04 10 4.23 106 >1.20 108
1.09 104 3.98 105 1.58 107
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of CuO ®lm at 2008C is as low as 1/400 of that of the SnO2 ®lm, but resistance of CuO/SnO2 ®lm at 2008C is as higher as 25 times than that of the SnO2 ®lm. Based on the results of Fig. 8 and Table 1, therefore, it is also supported that p±n junction is formed in CuO/SnO2 ®lm and the ®lm in this study has a thick electron depletion layer enough to extend through the ®lm. As another proof, a sensor with a CuO thickness of Ê is notable. After exposed to 2 ppm of H2S, the 12.8 A resistance of this sensor was lower than a resistance of pure SnO2 ®lm in air. It indicates that after exposed to H2S, CuO is substituted by CuS having a very low resistance and the p±n junction is relieved, and CuS provides its electrons to the SnO2 ®lm, and raises electrical conduction in the sensor. From the viewpoint of H2S sensor performance, response behavior of the sensor with a CuO thickness of Ê is remarkable. This sensor shows as high sensitivity 12.8 A as about 600 times and very low operating temperature as a 1808C, and response time was within 2 min. For reference, a former study reported 79 times as sensitivity for 2 ppm of H2S at 3008C and about 10 min as a response time. In case Ê , it is thought that since of a thickness of 4.3 and 7.1 A quantity of doped CuO is so small, p±n junction depth is so small that it could not extend to the bottom of the SnO2 ®lm. 4. Summary and conclusions In this study, the novel gas-sensing mechanism was proposed for the new SnO2 thin-®lms, which had been fabricated by using a dual ion beam sputtering. The deposited ®lms also had been characterized as to have nearstoichiometry, smooth surface, and compact and dense texture structures. While traditional studies have been focusing on fabrication of porous SnO2 sensors with a small grain diameter of a few nanometers, this study tried to prove that the robust SnO2 ®lms have enlarged electron depletion layers enough to extend through the ®lms. The SnO2 thin-®lms with various ®lm thickness were employed to examine effects of electron depletion layer on the gas-sensing response. The optimum thickness for the gas-sensing response was evaluated as about 100 nm thick enough for practical uses. While the previous models assumed a very low bulk resistivity in bulk layer, the new model proved that the resistivity of the bulk layer is very high, and nearly equal to that in the oxidized depletion layer, and advantages from such high resistivity of the bulk layer were also discussed. The reasonability of the novel model was veri®ed using custom-made H2S sensors, composed of SnO2 thin-®lms, lower electrodes and CuO-doping surfaces. The H2S sensors were operable at temperatures as low as 1808C, much lower than the conventional 3008C, and addition to the most excellent sensitivity and response characteristics among those reported.
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References [1] W. GoÈpel, K.D. Schierbaum, SnO2 sensors: current status and future prospects, Sens. Actuators B 26/27 (1995) 1±12. [2] H. Ogawa, M. Nishikawa, A. Abe, Hall measurement studies and an electrical conduction model of tin oxide ultrafine particle films, J. Appl. Phys. 53 (1982) 4448. [3] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Grain size effects on gas sensitivity of porous SnO2-based elements, Sens. Actuators B 3 (1991) 147±155. [4] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Correlation between gas sensitivity and crystallite size in porous SnO2-based sensors, Chem. Lett. (1990) 441±444. [5] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sens. Actuator B 5 (1991) 7. [6] Y.-S. Choe, J.-H. Chung, D.-S. Kim, H.K. Baik, Synthesis of tin oxide films by dual ion beam sputtering using Sn target and oxygen ion beam, Surf. Coatings Technol. 112 (1990) 267±270. [7] Y.-S. Choe, J.-H. Chung, D.-S. Kim, H.K. Baik, Ion beam sputtering of SnO2 with low energy oxygen ion beams, Thin Solid Films 341 (1999) 230±233. [8] J.-H. Chung, Y.-S. Choe, D.-S. Kim, Effect of low energy oxygen ion beam on optical and electrical characteristics of dual ion beam sputtered SnO2 thin films, Thin Solid Films 349 (1999) 126±129. [9] Y.-S. Choe, Phase stability and microstructural changes of SnO2 thin films deposited by using dual ion beam sputtering, in: Proceedings of the 8th International Meetings on Chemical Sensors, Basel, Switzerland, 2000, p. 521. [10] K.D. Schierbaum, U. Weimar, W. GoÈpel, R. Kowalkowski, Conductance, work function and catalytic activity of SnO2-based gas sensors, Sens. Actuators B 3 (1991) 205±214. [11] M.J. Madou, S.R. Morrison, Chemical Sensing with Solid State Devices, Academic Press, New York, 1989, p. 425. [12] M. Egashira, Y. Yoshida, S. Kawasumi, Gas sensing characteristics of tin oxide whiskers, Sens. Actuators 9 (1986) 147±155. [13] M. Ippommatsu, H. Ohnishi, H. Sasaki, T. Matsumoto, Study on the sensing mechanism of tin oxide flammable gas sensors using the Hall effect, J. Appl. Phys. 69 (1991) 8368±8374. [14] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Relationship between gas sensitivity and microstructure of porous SnO2, Denki Kagaku 58 (1990) 1143±1148. [15] J.F. McAleer, P.T. Moseley, J.O.W. Norris, D.E. Williams, Tin dioxide gas sensors part 1. Aspects of the surface chemistry revealed by electrical conductance variations, J. Chem. Soc., Faraday Trans. 1 (83) (1987) 1323±1346. [16] N. Yamazoe, J. Fuchigami, M. Kishikawa, Interations of tin oxide surface with O2, H2O and H2, Surf. Sci. 86 (1979) 334±335. [17] D. Kohl, Surface processes in the detection of reducing gases with SnO2-based devices, Sens. Actuators 18 (1989) 71±113. [18] D.F. Cox, T.B. Fryberger, S. Semancik, Oxygen vacancies and defect electronic states on the SnO2 (110) 1 1 surface, Physical Rev. B 38 (1988) 2072±2083. [19] T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, Sensing behavior of CuO-loaded SnO2 element for H2S detection, Chem. Lett. 1991 (1991) 575±578. [20] J. Tamaki, T. Maekawa, N. Miura, N. Yamazoe, CuO±SnO2 element for highly sensitive and selective detection of H2S, Sens. Actuators B 9 (1992) 197±203. [21] M. Rumyantseva, M. Labeau, G. Delabouglise, L. Ryabova, I. Kutsenok, A. Gaskov, Copper and nickel doping effect on interaction of SnO2 films with H2S, J. Mater. Chem. 7 (1997) 1785±1790.
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[22] R.B. Vasiliev, M.N. Rumyantseva, N.V. Yakovlev, A.M. Gaskov, CuO/SnO2 thin film heterostructures as chemical sensors to H2S, Sens. Actuators B 50 (1998) 186±193. [23] A. Horaeau, J.X. Hu, P. Jensen, P. Melinon, M. Treileux, B. Cabaud, Study of the crystallization of antimony thin films by transmission electron microscopy observations and electrical measurements, Thin Solid Films 209 (1992) 161±164.
Biography Yong-Sahm Choe is the leader in materials research lab at TYM R&D Institute of Tong Yang Moolsan Co. Ltd. since 1999. His research interests include synthesis and applications of functional thin films, and are currently focusing on the development and application of thin film gas sensors. He obtained his PhD degree in metallurgical engineering at Yonsei University in 1999.
New gas sensing mechanism for SnO2 thin-®lm gas sensors fabricated by using dual ion beam sputtering Yong-Sahm Choe* Materials Research Laboratory, Tong Yang R&D Institute, 378-1 Maeng-ri, Wonsam-myun, Yongin-si, Kyungki-do 449-870, South Korea
Abstract A new gas sensing mechanism for SnO2 thin-®lms fabricated by using dual ion beam sputtering, was proposed. The features of the SnO2 thin-®lm considered in this study had been characterized as having extremely smooth surface, dense microstructure and near stoichiometric SnO2 phase. In this study, thus, the problems of reported gas sensor models were noticed brie¯y and next, a novel sensing model for the SnO2 thin-®lms prepared in this study was approached by geometrical considerations. The effects of resistivity in the bulk regions and in the depletion regions on the gas sensitivities were studied by fabrication of thin-®lm gas sensors having various ®lm thickness. The proposed gas sensing mechanism was veri®ed by the fabrication of a H2S sensor with p±n junctions. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Semiconductor gas sensor; Gas sensing mechanism; Debye length; SnO2 thin-®lm; Dual ion beam sputtering
1. Introduction Semiconductor SnO2 gas sensors have been developed with various types such as powder-sintered, thick-®lm and thin-®lm sensors. To understand gas-sensing mechanisms of SnO2 gas sensors fundamentally, two groups of studies have been conducted. One group is related to atomistic models of molecular recognition with SnO2 sensors. The other group concerns geometric effects, in which in¯uence of contact geometry and crystallinity on sensor response signal is mainly discussed [1]. Among latter studies, the ultra®ne particle ®lm and the grain size effects models on gas-sensing response are worth noticing [2,3], which are applicable for porous SnO2 elements [4]. The gas-sensing response of sensor, S is de®ned as the ratio of sensor resistance in air to that in the relevant gas mix, Ra/Rg. In the model for grain-size effects, S is determined by two factors. One factor is a diameter D of SnO2 crystallite and the other is a thickness L of electron depletion layer (space charge layer) in air, where L is determined by the Debye length and the strength of the oxygen chemisorption. The L of the SnO2 ultra®ne particle ®lms sputtered in [2] was estimated to be as thin as 3 nm, and L of the sintered SnO2 in [3] also was evaluated as the same L value. It was also proposed that when D decreases to be comparable to 2L,
* Tel.: 82-31-334-8219; fax: 82-31-334-8224. E-mail addresses: [email protected], [email protected] (Y.-S. Choe).
S could increase drastically. Consequently, in order to make highly sensitive gas sensors, the gas sensors should have very small grain sizes and high speci®c surface areas of SnO2 powders. However, such small crystallites of pure SnO2 are only stable at temperatures below 4008C and hardly maintained in practical sensor fabrications due to grain growth or coalescence during calcinations or sintering processes at temperatures around 7008C [5]. In addition, a thin-®lm gas sensor having high speci®c surface area tends to be fragile. As shown in example of the ultra®ne particle ®lm in [2], the ®lm structure was spongy and the ®lm packing density was as low as 1/300 compared to density of SnO2 single crystal. As reviewed in aforementioned models, traditional approaches have been conducted to decrease D near or below 2L and to maintain such small L by lowering process temperatures or doping foreign oxides for inhibiting the grain growth. In all these cases, however, L was ®xed as about 3 nm. Although L increased up to 20 nm or above in [3] by doping Al for valency control of SnO2, the electric resistance of the sensor was too high as 109 O, which was about three-orders of magnitude larger than that of the pure SnO2 in [3] and was not proper to practical use. On the other hand, a semiconductor gas sensor of thin ®lm type becomes more important recently, because of the advantages in miniaturization, processing, and potential as an electronic nose by integrated circuits. The author, therefore, intended to deposit a robust and highly sensitive SnO2 thin-®lms. For suf®cient mechanical
0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 7 3 1 - 6
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strength, dense structure was preferred to porous one. Instead of obtaining small crystallite, L was designed to extend through whole ®lm thickness for highly sensitive sensors by synthesizing near-stoichiometric SnO2 ®lms. A ®lm thickness of the sensors also could be expected to be thick enough for practical use. For this purpose, SnO2 thin®lms were newly fabricated by using dual ion beam sputtering, whose crystallinity, chemical compositions and physical properties were controllable [6±8]. Based on the above studies, the novel SnO2 thin-®lms were obtained, which were characterized as dense and highly packed SnO2 textures with extremely smooth surface [9]. The purpose of this paper is to elucidate a gas sensing mechanism for the novel SnO2 thin-®lms and to verify the gas sensing mechanism by fabricating thin-®lm gas sensors. Firstly, sensor responses were measured as a function of ®lm thickness, and relationships between thickness of electron depletion layer and gas-sensing response were examined. Secondly, a new gas-sensing mechanism for the novel SnO2 thin-®lms was proposed, in which effects of electrical resistivity in bulk region on gas-sensing response were studied by geometrical consideration. After resistivity in air was compared that in vacuum with each various ®lm thickness, meaning of the new gas-sensing mechanism was discussed. Finally, proof of the new gas-sensing mechanism was proposed, based on H2S sensors employing the novel SnO2 ®lms with new concepts in electrode arrays and catalytic layer. 2. Experimental To investigate a ®lm thickness effect on gas-sensing response, various SnO2 thin-®lms having a thickness of 10, 20, 50, 100, 200, 500 nm were deposited on alumina substrates (5 mm 10 mm 0:4 mm) by using a dual ion beam sputtering system (DIBS). As a primary ion source for sputtering pure tin target, Ar ion gun was employed, where acceleration voltage and current density were 1.0 keV and 1.5 mA cm 2, respectively. As a secondary ion source for bombarding a growing thin ®lm on the substrate, oxygen ion gun was used, where acceleration voltage and current density were 200 eV and 0.2 mA cm 2, respectively. Deposited SnO2 thin-®lms were annealed in an electrical furnace in air at 4008C for 5 h. As a consequence, all samples were deposited and annealed under an identical condition, except for ®lm thickness. A gas sensor for testing was composed of the novel SnO2 thin-®lm by DIBS and an alumina substrate having a pair of inter-digitated electrodes on the front and a Pt heater beneath the backside. For electrical contacts, Pt wires were bonded to a pair of inter-digitated electrodes with Pt paste. The electrical resistance of the sensor in vacuum and air at various temperatures was recorded continuously to evaluate a gassensing response, S of each gas sensors, by a computercontrolled digital multimeter.
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A gas chamber with inlet and outlet was used to measure the gas-sensing response of the sensors. As testing gases, N2-balanced 1% H2 and 100 ppm H2S gases were used, respectively. A gas concentration in the gas chamber was determined ®nally by adjusting gas ¯ow rate to air¯ow rate. For examples, in order to test sensors at 1000 ppm of H2, the ¯ow rate of the H2 gas and air were controlled as 100 standard cubic centimeter per minute (SCCM) and 900 SCCM, respectively. As for 2 ppm of H2S, the ¯ow rate of the H2S gas and air were adjusted to 20 and 1000 SCCM, respectively. The gas ¯ow rate was controlled precisely, using mass ¯ow controller. For air¯ow, ball ¯ow meter was used. The ®nally diluted gas by mixing with air was injected into the inlet of the gas chamber continuously during measurement, and was ejected through the outlet. The Hall effect measurement for the SnO2 ®lms was conducted using the van der Pauw con®guration, a magnetic ®eld of 6300 G and constant current source, whose current was raised from 1.0 to 4.0 mA in step of 1.0 mA. 3. Results and discussion 3.1. Relationship between thickness of electron depletion layer and gas-sensing response in a semiconductor gas sensor Grain boundary effects on electrical properties in the novel SnO2 ®lms were assumed to be negligible, since the novel SnO2 ®lms had been analyzed as the very compact and dense texture structures [6±8], which had no grain boundaries to block current ¯ow along a thickness direction of the novel SnO2 ®lms. Therefore, electrical properties of the novel SnO2 ®lm was expected to be changed mainly by adsorbed oxygen on a surface of the ®lm, which extracts electrons from the ®lm, and by the concentration of charge carriers introduced the ®lm during producing the ®lm. Calculation of donor concentration by Hall effect measurement might be a good way to evaluate a depth of electron depletion layer in a SnO2 thin-®lm. In this study, however, resistance of the SnO2 ®lms with a thickness of below 100 nm was too high for Hall effect measurement. For thicker SnO2 ®lms of 200 and 500 nm, charge carrier concentration was measured in air at room temperature as 1:95 1015 and 1:36 1015 cm 3, respectively. For more precise and systematic analysis, changes in gassensing response of the gas sensors using the novel SnO2 thin-®lms, as a function of ®lm thickness, were measured with 1000 and 400 ppm of H2 gas at 250, 300 and 3508C, respectively, as shown in Fig. 1. In Fig. 1, following four ®ndings are noteworthy. First, the novel SnO2 ®lms have an optimum thickness for the highest gas-sensing response. This is explained as follows: if a ®lm thickness is greater than the optimum thickness, the other part of the ®lm which exceed the optimum thickness would not participate in gas sensing reaction and could not contribute a resistance
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Finally, it is very noteworthy that although the other thin ®lms deviate from the optimum ®lm thickness of the best gas-sensing response, the thin ®lms show still good sensitivity with some degree. According to some former theories, inner SnO2 layer beneath the electron depletion layer was considered to have very low electrical resistivity [13]. Therefore, in the case that a SnO2 ®lm is thicker than a depth of electron depletion layer, gas-sensing response of the ®lm will be reduced drastically [14]. In Fig. 1, however, there are no such abrupt changes in gas-sensing response as a function of ®lm thickness, like Fig. 5 in [14]. A hypothetic depth of electron depletion layer was calculated as a function of charge carrier concentration as shown in Fig. 2. Difference between both formulas proposed is only in coef®cient of kT term. Charge carrier concentration is a function of temperature, but ranges of concentration calculated with temperature compensation are within 5%. In Fig. 2, temperature was assumed as 3008C. When a thickness of the electron depletion layer was assumed as 100 nm, the charge carrier concentration, based on the formula by Ogawa et al. [2] and McAleer et al. [15] in Fig. 2, was estimated as about 3:8 1015 and 6:3 1015 cm 3, respectively, which are in the order of the results of the Hall measurement. Fig. 1. Measured gas-sensing response as a function of film thickness, for various H2 concentration: (a) 1000 ppm of H2, (b) 400 ppm of H2.
change of the ®lm. This explanation is veri®ed in detail by the following results of the Section 3.3. In a case that the ®lm thickness is less than the optimum thickness, the ®lms show lower gas-sensing response. The case is thought to originate from the followings: no the Schottky-barrier mechanism is valid, because the band bending at the surface can be neglected, based on the overall shift of the Fermi level in the grains during gas exposure [10]. Furthermore, it is considered that all surface charge becomes converted to the nonreactive O2 Ð there are too few electrons to reduce the O2 to O [11]. As an example, Egashira et al. obtained single-crystal sensors thin enough for sensitivity by using SnO2 whiskers [12]. In this reference, the maximum sensitivity was recorded in a sample with a thickness of 5 mm, but the samples lose their sensitivity if the sensor thickness is less than a few micrometers. Second, the optimum thickness is reduced by increasing the operating temperature of the ®lm. This result agrees with the fact that generally charge carrier concentration increases in proportion to temperature, because a thickness of electron depletion layer in n-type semiconductors decreases with increasing charge carrier concentration. Third, the optimum thickness is about 100 nm, indicating that the novel SnO2 ®lm fabricated in this study can be thick enough to be used practically as a gas sensor. It is also notable that the thickness of 100 nm is approximately equal to a depth of electron depletion layer estimated by Hall effect measurement.
3.2. Modeling of a new gas sensing mechanism in the novel SnO2 thin-film For modeling of a novel gas-sensing mechanism, the schematic drawing of the SnO2 thin-®lm gas sensor was proposed in Fig. 3a. Strictly speaking, resistance in the electron depletion layer would be a function of distance from the ®lm surface according to the band bending in the Schottkey barrier model, but the resistance in the layer was assumed as to be constant for simplicity. The other
Fig. 2. Calculation of depletion layer length (Debye length), based on the results of Ogawa et al. [2] and McAleer et al. [15], as a function of carrier concentration in SnO2 films; Ogawa: D ekT=e2 N d 1=2 ; McAleer: D 2ekT=e2 N d 1=2 ; D: Debye length; e: static dielectric constant, 13:5 8:85 10 12 F/m in SnO2; k: Boltzmann's constant, 1:38 10 23 J/K; T: absolute temperature; Nd: carrier concentration per unit volume (cm 3).
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Fig. 3. Schematic drawings demonstrating (a) the formation of a depletion layer on the SnO2 thin-film by oxygen chemisorption and (b) an electrical equivalent circuit of (a) used a gas sensor.
assumptions were as followings. First, a resistance of bulk layer beneath the depletion layer would not be changed after or before gas sensing reaction. Second, a resistivity of the electron depletion layer in air is constant, regardless of a resistivity of bulk layer. Third, grain boundary effect on ®lm resistance is ignored, since the SnO2 ®lm in this study has a dense texture structure grown by granular epitaxy and there is no grain boundary barrier for electron current along a ®lm thickness direction [9]. Finally, electrodes have ohmic contacts with both layers simultaneously. As a result, an electrical equivalent circuit was deduced as two parallel resistors shown in Fig. 3b. Next, equations for the calculation of the gas-sensing response were examined. Ra,d and Ra,b represent the resistance in air on the depletion layer and the bulk layer, respectively. The Rg,d and Rg,b mean the resistance in gas on the depletion layer and the bulk layer, respectively. The gas-sensing response, S is de®ned as Ra /Rg. The resistance, Ra, of the sensor in air can be expressed as Ra;d Ra;b Ra Ra;b Ra;d
(1)
After sensor is exposed in gas, the resistance, Rg, of the sensor, can be described as Rg
Rg;d Rg;b Rg;b Rg;d
(2)
Assume that the gas-sensing reaction occurs mainly in the depletion layer and the resistance of the depletion layer is not changed before and after exposure to gas Ra;b Rg;b Rb
(3)
Therefore, the gas-sensing response is given by S
Ra;d Rb Rg;d Rg;d Rb Ra;d
(4)
Since resistance, R, can be expressed with resistivity, r, and thickness, t, under the condition of unit length and unit width, resistance is r R (5) t For the depletion layer, t is substituted with the thickness of the depletion layer, L. As for the bulk layer, t becomes (T ). The gas-sensing response, therefore, is given by S
ra;d L rb rg;d L rb
rg;d Trg;d ra;d Tra;d
(6)
Consequently, five parameters were needed to evaluate the new model: resistivity of the depletion layer in air, and gas; resistivity of bulk; thickness of depletion layer, and bulk layer. For this simulation, following considerations were introduced. Firstly, let whole film thickness, T be 500 nm and let depletion layer thickness, L be 0 L T, for considering that the depletion layer thickness is thinner than
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Fig. 4. Simulated gas-sensing response as functions of the bulk layer resistivity, r and the depletion layer thickness, L. The vertical dotted arrow-line indicates that the response change drastically with r of bulk layer in gas at the thickness of depletion layer assumed as 100 nm. In case that the thickness of depletion layer equals the whole film thickness, the response always show maximum regardless with r of bulk region. Assumptions: (1) thin film thickness, T: 500 nm; (2) ra,d of depletion layer in air: 103 O cm; (3) rg,d of depletion layer in gas: 0.1 O cm; (4) rb of bulk layer: 0.1±102 O cm.
the whole film thickness. Secondly, when the film was exposed to air, let the resistivity of the depletion layer, ra,d be 103 O cm. Thirdly, when the sensor was exposed to the reducing gas, the decreased resistivity of the depletion layer, rg,d was presumed as 0.1 O cm. Finally, the resistivity of bulk layer, rb was considered as 0.1, 1, 10 and 100 O cm, respectively. The simulated result was presented as Fig. 4, suggesting very decisive clues to propose the new gas sensing mechanism. First, in a case that the depletion layer extends through the whole ®lm, the best gas-sensing response can be obtained. This result agrees with former established models. Second, in a case that the depletion layer would be thinner than the whole ®lm, if the bulk layer (the inner part of the ®lm) has high resistivity, the ®lm shows still somewhat good gas-sensing response. For examples, in case of a low resistivity of 0.1 O cm that was so far measured in former SnO2 gas sensors, if a thickness of the electron depletion layer is 480 nm, gassensing response decreases abruptly. Considering a thickness difference between the electron depletion layer and whole ®lm was so small as to be only 4% of the ®lm thickness, such drastic decrease of gas-sensing response is very signi®cant. Inversely, in case of a high resistivity of 102 O cm, if a thickness of the electron depletion layer is so small as to be 50 nm, gas-sensing response shows a good gas-sensing response of 100 times. It was considered, next, that a thickness of the electron depletion layer was assumed to be a 100 nm, which was approximately corresponded with that of the SnO2 ®lm in this study. A vertical dotted arrow-line in Fig. 4. indicated gas-sensing response changes varying with resistivity changes in bulk layer. In that case, although the electron
depletion layer was so small as 100 nm, if a bulk resistivity is as high as 102 O cm, gas-sensing response is also so high as 200.8 times. However, if a bulk resistivity is as low as 0.1 O cm, gas-sensing response is only 1.25 times. Based on the above results, the gas-sensing response changes in Fig. 1 could be understood only by approaching with the novel gas sensing mechanism. In former proposed models, a resistivity of bulk layer was assumed to be much lower than that of the electron depletion layer whose surface was adsorbed with oxygen. As a result, abrupt changes of gas-sensing response which occurred in case of D > 2L and D < 2L, could be explained. The former models, however, could not explain such gentle changes of gas-sensing response with ®lm thickness as shown in Fig. 1. The author proposes that such gentle changes of gas-sensing response originate from the fact that the ®lm used in this study has a high resistivity of bulk layer as about 102 O cm. As a way to verify the newly proposed model, resistivity of the SnO2 ®lms with various thickness was measured in air and vacuum. First, each resistance of the ®lms was measured (Fig. 5), and then resistivity was approximately calculated from resistance by considering ®lm thickness and dimension
Fig. 5. Resistance changes in the SnO2 thin-films under (a) air, and (b) vacuum, for various temperatures, as a function of the film thickness.
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Kohl presented another reaction [17]. H2 O SnL OL HO SnL OL H e H2 O 2SnL OL 2 HO SnL VO
Fig. 6. Estimated resistivity changes in the SnO2 thin-films under (a) air, and (b) vacuum, for various temperatures, as a function of the film thickness.
(Fig. 6). In a case that measuring temperature increased over 2508C, ®lm resistance was very unstable and needed a long time for measuring. For this reason, resistance measurement was conducted in air and vacuum at room temperature, 100, 150 and 2008C. As shown in Figs. 5a and 6a, the resistivity of the ®lm in air was constant within order of magnitude, regardless of its thickness, and decreases with increasing the measured temperature. Resistance values measured in air at room temperature show relatively large deviation compared to other samples. It might be inferred as an effect of water adsorbed on the ®lm surface. Samples at 150 and 2008C have a tendency that the resistance slightly increases with the ®lm thickness. In Figs. 5b and 6b, on the other hand, the resistivity of the ®lms in vacuum increases in proportion to the ®lm thickness, except for the samples measured at room temperature. With elevated measurement temperature, the resistivity of the ®lms in vacuum decreases with a greater extent than in air. As the ®lm thickness decreases, the resistivity is reduced signi®cantly. Yamazoe et al. proposed the following reaction to explain an effect of water on resistivity reduction [16]: H2 O O2 VOn 2 OH ne
According to above reactions, the adsorbed water on the surface decreases the resistivity of the film. When a SnO2 film is located in vacuum, adsorbed water tends to be volatilized for equilibrium vapor pressure, even though at room temperature. For this reason, the resistivity of the film becomes to be higher in vacuum than in air. As reasons for the resistivity reduction in the heated ®lms, following two factors were considered. First factor is desorption of adsorbed oxygen from the surface of the ®lm, and next factor is escape of oxygen from the surface lattice of the ®lm. When the SnO2 ®lms are heated in vacuum, a part of oxygen adsorbed on the surface tends to depart from the surface for the equilibrium state of vapor pressure. Additionally, based on the fact that the resistivity in vacuum was much lower than in air, it is also inferred that a part of oxygen atoms in the surface lattice escapes from the ®lm. Cox et al. reported that heating the ideal surface of SnO2 in vacuum removes a large fraction of surface bridging oxygens at temperatures as low as 500 K [18]. As a result, the resistivity becomes to decrease, because surface oxygen vacancies are known to act as n-type donors. The escape and adsorption of oxygen can be presumed to occur mainly near the ®lm surface, where the electron depletion layer is formed. Therefore, it is noteworthy that the ®lm with a thickness of 500 nm did not show appreciable difference in resistivity in air and in vacuum. It means that the bulk layer, which does not participate in oxygen surface reaction, has so high resistivity equal to a resistivity of electron depletion layer covered with adsorbed oxygen. Consequently, this analysis is also in agreement with the results of Figs. 1 and 4, and such analysis distinguishes the novel gas sensing model presented in this study from former models, which proposed that bulk layer has much less resistivity compared to electron depletion layer. 3.3. Proof of the novel gas-sensing model by using the SnO2 thin-film gas sensor Recently, it was reported by several groups that CuOadded SnO2 shows very high sensitivity and selectivity to hydrogen sul®de [19±21]. This sensor was reported to have very stable and excellent sensing characteristics to hydrogen sul®de in the range of 10±100 ppm at 2008C. Such excellent sensing characteristics against hydrogen sul®de have been explained as p±n junction between CuO and SnO2. While oxygen-de®cient SnO2 shows n-type conductivity by electrons, oxygen-excess CuO shows p-type conductivity by holes. When both oxides contact with each other, a thick charge depletion layer is formed at the interface as a p±n junction and the resistance is highly increased. However, if
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the above CuO layer is exposed to H2S or other sulfur compound gases, it is converted to CuS with metallic conductivity by following equations. CuS H2 S ! CuS H2 O CuO CH3 SH 32 O2 ! CuS CuO 2H2 O Thus, as p-type CuO is converted to metallic CuS, the p±n junction as well as charge depletion layer disappears and the resistance drops drastically. In this study, fabricating thin ®lm gas sensor with p±n junction was tried to prove a novel gas sensing model under following three premises. First, sensing reaction of hydrogen sul®de occurs at only a surface of the ®lm and diffusion of chemical species into inner part of the ®lm is negligible, since the ®lm was proved to have a compact and dense texture. Second, p±n junction is formed only interface between CuO and SnO2. Third, electrodes for sensor are formed beneath SnO2 ®lm. For examples, Fig. 7a shows a typical gas sensor with p±n junction where each electrodes are connected to each layer, CuO and SnO2 [22], and Fig. 7b present a newly proposed gas sensor with p±n junction where both electrodes are connected only to backside of SnO2 ®lm. Sintered gas sensors are composed of a mixture of CuO and SnO2, and thus, CuO is dispersed here and there in sensor, where position of electrodes might be unimportant in a viewpoint of gas sensor performance. In the thin ®lm of this study, however, catalytic layer, CuO, was formed only atop the SnO2 ®lm. Since, in this case, electron depletion layer was proposed to be as thick as ®lm thickness, CuO as a
Fig. 8. Resistance changes in variously CuO-doped SnO2 sensors, for H2S, 2 ppm at 1808C, showing p±n junctions formed between CuO layers and SnO2 films.
p-type semiconductor could extract electrons from the SnO2 ®lm as an n-type semiconductor. As a result, even if electrodes is beneath the SnO2 ®lm, resistance change could be monitored. On the other hand, if a sensor with p±n junction has electrodes like Fig. 7a, p±n junction effect might be monitored but it would not be applicable to estimate a thickness of electron depletion layer. Fig. 8 shows resistance changes of CuO-doped SnO2 sensors, in 2 ppm of H2S at 1808C. In Fig. 8, the thickness of CuO was evaluated from multiplication of the deposition rate by the deposition time, because clusters or islands form thin ®lms during the earliest stages of ®lm formation. The sensor resistance increased in proportion to CuO thickness, as shown in Fig. 8. Considering that CuO resistance is lower than that of SnO2, the increase of resistance is attributed to formation of p±n junction between CuO layer and SnO2 ®lm, by which electrons of n-type semiconductor SnO2 is depleted and the sensor resistance is raised. For a sensor Ê , however, the sensor resiswith a CuO thickness of 12.8 A tance decreased. This fact is understood from followings: numbers and sizes of CuO islands atop SnO2 increase, and then a distance between each island is closer, and ®nally electrical conduction by tunneling decreases the sensor resistance [23]. To examine the resistance changes of the sensors in Fig. 8, Table 1. was presented, where each resistance of CuO, SnO2 and CuO/SnO2 ®lm were measured. It shows that resistance Table 1 Summaries of resistance in CuO, SnO2, CuO/SnO2 films Thin film materials
Fig. 7. Schematic diagrams for two kinds of CuO±SnO2 sensors: (a) electrodes are connected to SnO2 and CuO layers, respectively, [22]; (b) all electrodes are connected with SnO2 layers (used in this study).
CuO SnO2 CuO/SnO2
Thickness (nm) 100 100 1/100
Resistance of thin films (O) 1508C
2008C 4
3.19 10 6.30 106 >1.20 108
2508C 4
1.04 10 4.23 106 >1.20 108
1.09 104 3.98 105 1.58 107
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of CuO ®lm at 2008C is as low as 1/400 of that of the SnO2 ®lm, but resistance of CuO/SnO2 ®lm at 2008C is as higher as 25 times than that of the SnO2 ®lm. Based on the results of Fig. 8 and Table 1, therefore, it is also supported that p±n junction is formed in CuO/SnO2 ®lm and the ®lm in this study has a thick electron depletion layer enough to extend through the ®lm. As another proof, a sensor with a CuO thickness of Ê is notable. After exposed to 2 ppm of H2S, the 12.8 A resistance of this sensor was lower than a resistance of pure SnO2 ®lm in air. It indicates that after exposed to H2S, CuO is substituted by CuS having a very low resistance and the p±n junction is relieved, and CuS provides its electrons to the SnO2 ®lm, and raises electrical conduction in the sensor. From the viewpoint of H2S sensor performance, response behavior of the sensor with a CuO thickness of Ê is remarkable. This sensor shows as high sensitivity 12.8 A as about 600 times and very low operating temperature as a 1808C, and response time was within 2 min. For reference, a former study reported 79 times as sensitivity for 2 ppm of H2S at 3008C and about 10 min as a response time. In case Ê , it is thought that since of a thickness of 4.3 and 7.1 A quantity of doped CuO is so small, p±n junction depth is so small that it could not extend to the bottom of the SnO2 ®lm. 4. Summary and conclusions In this study, the novel gas-sensing mechanism was proposed for the new SnO2 thin-®lms, which had been fabricated by using a dual ion beam sputtering. The deposited ®lms also had been characterized as to have nearstoichiometry, smooth surface, and compact and dense texture structures. While traditional studies have been focusing on fabrication of porous SnO2 sensors with a small grain diameter of a few nanometers, this study tried to prove that the robust SnO2 ®lms have enlarged electron depletion layers enough to extend through the ®lms. The SnO2 thin-®lms with various ®lm thickness were employed to examine effects of electron depletion layer on the gas-sensing response. The optimum thickness for the gas-sensing response was evaluated as about 100 nm thick enough for practical uses. While the previous models assumed a very low bulk resistivity in bulk layer, the new model proved that the resistivity of the bulk layer is very high, and nearly equal to that in the oxidized depletion layer, and advantages from such high resistivity of the bulk layer were also discussed. The reasonability of the novel model was veri®ed using custom-made H2S sensors, composed of SnO2 thin-®lms, lower electrodes and CuO-doping surfaces. The H2S sensors were operable at temperatures as low as 1808C, much lower than the conventional 3008C, and addition to the most excellent sensitivity and response characteristics among those reported.
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Biography Yong-Sahm Choe is the leader in materials research lab at TYM R&D Institute of Tong Yang Moolsan Co. Ltd. since 1999. His research interests include synthesis and applications of functional thin films, and are currently focusing on the development and application of thin film gas sensors. He obtained his PhD degree in metallurgical engineering at Yonsei University in 1999.