Thin film SnO2-based gas sensors: Film thickness influence

Sensors and Actuators B 142 (2009) 321–330

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Thin film SnO2 -based gas sensors: Film thickness influence G. Korotcenkov a,b,∗ , B.K. Cho b,c,∗∗ a

Technical University of Moldova, Chisinau, Republic of Moldova Department of Material Science and Engineering, Gwangju Institute of Science and Technology, 261 Cheomdan-gwagiro (Oryong-dong), Buk-gu, Gwangju, 500-712, Republic of Korea c Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, 261 Cheomdan-gwagiro (Oryong-dong), Buk-gu, Gwangju, 500-712, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 9 June 2009 Received in revised form 1 August 2009 Accepted 4 August 2009 Available online 13 August 2009 Keywords: Gas sensors SnO2 Structure–property relationships Thickness influence Thin films

a b s t r a c t The influence of the thickness of SnO2 films deposited by a spray pyrolysis method on the operating characteristics of gas sensors is analyzed in this paper. It outlines how the thickness of metal oxides is an important parameter for gas sensors in determining the main operating parameters, such as the magnitude and rate of the sensor response and the optimal operating temperature. It is also shown that the optimal film thickness of a gas sensing layer depends on the required sensor parameters. © 2009 Elsevier B.V. All rights reserved.

1. Introduction In dealing with any type of gas sensor, establishing regularities concerning the influence of the thickness of the gas sensing layer on the main operating characteristics of devices is an important task. Not many studies, however, are devoted to these regularities. Instead, the majority of research has preferred to focus on analyzing the properties of sensors that have a gas sensing layer with a fixed thickness. Unfortunately, this does not promote a better understanding of the observed gas sensing effects. For example, in works which analyze the influence of the film thickness on sensor parameters, it is possible to find results showing ambiguous consequences of the film thickness change for gas sensing characteristics. Some reports observed an increase in sensitivity to reducing gases by increasing film thickness [1,2], while others observed a loss in sensitivity for thicker films [3–7]. There are also results indicating that the sensor response would reach its maximum [5,7–12] or minimum at a certain thickness [13].

∗ Corresponding author at: Department of Material Science and Engineering, Gwangju Institute of Science and Technology, 261 Cheomdan-gwagiro (Oryongdong), Buk-gu, Gwangju, 500-712, Republic of Korea. Tel.: +82 62 970 2354; fax: +82 62 970 2304. ∗∗ Corresponding author at: Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, 261 Cheomdan-gwagiro (Oryongdong), Buk-gu, Gwangju, 500-712, Republic of Korea. E-mail addresses: [email protected] (G. Korotcenkov), [email protected] (B.K. Cho). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.08.006

It has also been found that the influence of the film thickness depends on the type of target gas. For example, in Ref. [14] it was established that an increase in film thickness in the range of 70–300 nm promoted both a decrease in sensitivity to H2 and an increase in sensitivity to CO. In both cases, the average size of the SnO2 grains in the gas sensing layer was ∼6 nm. Other research [15] has indicated that the sensitivity to some reducing gases, such as ethanol vapor, decreased as the thickness was increased in the range of 10–200 ␮m. At the same time, the sensitivity to other reducing gases, such as methane, remained unchanged despite the change in thickness. Such disagreement demonstrates once again that the gas sensitivity of metal oxide parameters depends on many factors, which are sometimes difficult to control [16–19]. In light of this, then, it is obvious that a deeper understanding of the main regularities of the film thickness influence on the gas sensing characteristics of metal oxides is necessary. At the same time, however, it must be noted that an indicated analysis based on the results from previously published research is not possible. As is known, the film parameters depend greatly on the technologies employed, which differ considerably from each other. Therefore, it makes sense to base such an analysis only on the results obtained for sensors fabricated using identical technology. We have previously conducted a similar analysis for the In2 O3 sensors, whose fabrication was based on films deposited using the spray pyrolysis technology [20–22]. In the present work, research related to sensors based on SnO2 will be conducted in a similar fashion. It is important to point out that we have previously focused on analyzing the film thickness role in gas sensing properties of the

322

G. Korotcenkov, B.K. Cho / Sensors and Actuators B 142 (2009) 321–330

SnO2 films [6,13,19,23,24]. However, these published results have not been systematized and, as a rule, in each of the articles, the influence of the film thickness was only considered for one fixed sensor parameter. In the present article we will try to analyze the influence of the film thickness on a wider array of sensor parameters. Besides the magnitude of the sensor response, measured at a fixed temperature, such parameters as the dependence of the sensor response on the operating temperature and the kinetics of the sensor response will be considered. As is known, based solely on those parameters, the suitability of the metal oxide gas sensors for various applications can be judged [16]. We would also like to note that, for the present analysis, we use only new experimental results which have not been previously published or discussed. It is necessary to note that analyzing the influence of film thickness on gas sensing characteristics is a more complicated task with sensors fabricated using thin film technology than with sensors fabricated by using ceramics and thick film technologies. As has been established, for “thin” and “thick” film gas sensors, completely different dependencies of the grain size on the film thickness were observed. In both ceramic and thick film gas sensors, the size of the SnO2 grains does not depend on the thickness of the sensing layer. The grain size in such sensors is determined by the conditions of SnO2 synthesis and the parameters of the following thermal treatment [14,18,19]. In contrast, for thin film sensors, which were fabricated using metal oxide deposition at temperatures higher 200–300 ◦ C, the grain size of the metal oxides is usually determined directly by the thickness of the deposited film [25,26]. The strength of the influence on the grain size and the film morphology of SnO2 can be seen by reviewing the results presented in our previous works [26–28]. Such distinctions make it impossible to apply regularities established for sensors fabricated by thick film technology to those fabricated by thin film technology, making the present research necessary.

2. Experimental details SnO2 films of differing thicknesses were prepared using the spray pyrolysis technology. The main regularities of the SnO2 deposition by spray pyrolysis have been previously discussed in several published papers [6,13,28,29]. A 0.2 M SnCl4 water based solution was sprayed onto preheated alumina substrates, the temperature of which was maintained at 450 ◦ C. Previous research [6,26] has shown this deposition temperature to be optimal for forming the gas sensing films. If deposition takes place at a higher (Tpyr > 500 ◦ C) or lower (Tpyr < 370 ◦ C) temperatures, the SnO2 films would have a denser structure. This high density is not optimal for application in gas sensors, as materials need to be highly porous and permeable [16,17]. The volumes of the sprayed tin chloride solution used for SnO2 deposition were 1 ml, 2 ml, and 7 ml and corresponded to the thicknesses (d): 40–50 nm, 80–90 nm, and 270–340 nm, respectively. In our present experiments we did not use films with smaller thickness because this lowest range (<40–50 nm) produces the most ambiguity in the obtained results [3–5,7]. Films with a thickness exceeding 350 nm were also excluded, since their gas sensing characteristics are considerably worse than the characteristics of films with smaller thickness. For structure characterization of the studied films such methods as X-ray diffraction (XRD), transmission electron microscopy (TEM) and the scanning electron microscopy (SEM) were applied. For these purposes we have used a diffractometer Siemens D5000, working with the K˛ of the Cu, a Philips CM30 Super Twin electron microscope operating at 300 keV and a microscope Jeol JSM840, respectively. The sample preparation methods and the measurement modes have been described in detail by Arbiol in Ref. [30].

For fabrication of the gas sensor prototypes, Au electrodes were used and the Au films were deposited by vacuum evaporation. The distance between measured contacts was ∼2 mm, minimizing the influence of the measurement electrodes on the gas sensing characteristics. Before being measured, the samples were annealed at 500 ◦ C in an ambient air atmosphere for 15 min in order to stabilize the sensor parameters. Both an oxidizing gas, ozone (∼1 ppm), and a reducing gas, H2 (1000 ppm), were used as target gases. The gas sensing properties of the deposited films were tested in a flow-type reactor, where the tested sensors were exposed to different atmospheres with a controlled relative humidity (35–45% or 1–2%) and a controlled concentration of the target gas. In those measurements, we used either air → (O3 + air) → air or air → (0.1% H2 + air) → air measurement cycles. The sensor response (S) during ozone detection was determined as the ratio S = Rgas /Rair and during H2 detection as the ratio S = Rair /Rgas , where Rair is the resistance of the SnO2 film in pure air and Rgas is the resistance in the air including the target gas. All measurements were conducted at a steady-state temperature ranging from 25 ◦ C to 450 ◦ C. In order to exclude influences from the sensor prehistory on the research results, all measurements began at the highest temperature of 450 ◦ C and were then decreased to lower temperatures. The response ( res ) and recovery times ( rec ), estimated during “on” and “off” of the target gas correspondingly, were used for characterization of the sensor response kinetics. Time constants of transient processes were determined on the 0.9 level from a steady state value of film conductivity. The methodology of these measurements, including the methodology of the study of sensor response kinetics, has been previously described [31]. During the process of interaction with the surrounding gas, the response and recovery time constants of the film conductivity relaxation have been determined for the operating temperatures at which the time constants exceeded the time of the full change of gas atmosphere in the measuring cell. For our measuring cell, that time did not exceed 2–3 s.

3. Experimental results 3.1. Thickness influence on film morphology A detailed structural analysis of the SnO2 films deposited by spray pyrolysis has previously been conducted and is available in several published papers [6,13,26,28,29]. Therefore, this paper will provide only a short characterization of film thickness influence on their morphology. Typical TEM images of the SnO2 films deposited at different temperatures are shown in Fig. 1. One can see that the SnO2 films deposited at 420–475 ◦ C (Fig. 1c) have a more developed surface and lower crystallite packing density than the SnO2 films deposited at other temperatures. Typical SEM images of the films studied are shown in Fig. 2. It can be seen that the increase of film thickness is accompanied by a strong morphology transformation. We observe the change in both the size and the shape of crystallites forming the gas sensing layer. As the films become thicker, the grain size also increases. For example, according to the SEM measurements, the SnO2 films studied in this paper had grains with sizes (t) equal to ∼25–30 nm, ∼40–60 nm and ∼120–160 nm for films with thicknesses of 40–50 nm, 80–90 nm, and 270–340 nm, respectively. According to the XRD measurements, the same films had grain sizes equal to ∼21 nm, ∼33 nm and ∼51 nm, respectively. Large discrepancies observed between the results of the SEM and XRD measurements are explained in Refs. [26,28,29] by both the presence of mechanical strains in the deposited films and the distinctions in methods of structural parameters’ measurement. As is

G. Korotcenkov, B.K. Cho / Sensors and Actuators B 142 (2009) 321–330

323

Fig. 1. Cross-section TEM micrographs of the SnO2 films deposited at different temperatures: (a) Tpyr = 530 ◦ C; (b) Tpyr = 330 ◦ C; (c) Tpyr = 435 ◦ C.

known, using SEM images provides the in-plane size of grains in the top layer of analyzed films, while the grain sizes calculated from the XRD data are thickness-averaged magnitudes. From the SEM images, it is also clear that “thick films” (at least their top layers) are more porous than thin films. Due to their smaller grain size, thin films seem to be more densely packaged. Thus, presented results of the SEM and TEM measurements confirm that the SnO2 films deposited at 420–475 ◦ C have a more porous structure compared with films deposited at both lower and higher pyrolysis temperatures. 3.2. Thickness influence on sensor response In analyzing the experimental data presented in the literature for all types of SnO2 -based conductometric gas sensors, it can be concluded that, in spite of the identity of the gas sensing material, sensors fabricated using the “thin film” and “thick film” technologies have their own specific character of behavior during interaction with a target gas and their own peculiarities. It has been shown that “ceramic” and “thick film” sensors have a better sensitivity to reducing gases [10,16,18,32,33], while “thin film” gas sensors show a higher sensitivity to oxidizing gases [18,20,21,34].

Fig. 2. SEM imagers of the SnO2 films with different thickness (Tpyr = 450–475 ◦ C): (a) d ∼ 40–50 nm; (b) d ∼ 80–90 nm; (c) d ∼ 310–380 nm.

Results obtained during the present research are also in the frame of this indicated regularity. As the results in Fig. 3 show, an increase of the film thickness is accompanied by both a drop in sensitivity to ozone and an increase in sensitivity to the reducing gases. This regularity was established for a fixed operating temperature of 300 ◦ C. An interesting fact is that air humidity also has an opposite influence on sensor response to H2 (CO) and ozone, as it increases sensitivity to ozone and decreases sensitivity to reducing gases. At present there is no generally accepted explanation for this effect [25,34–36]. The simplest explanation is based on the humidity’ influence humidity on the initial resistance of the gas sensing layer. As is known, the appearance of water vapor in the surrounding atmosphere decreases the resistance of the n-SnO2 [37–40]. Due to differences in the direction of the SnO2 resistance change during interaction with reducing and oxidizing gases, the state with a smaller initial resistance is better for ozone detection and worse for CO and H2 detection. However, in reality, the mechanism of water influence is much more complex because a change in air humidity

324

G. Korotcenkov, B.K. Cho / Sensors and Actuators B 142 (2009) 321–330

Fig. 3. Influence of the film thickness on sensor response to ozone and hydrogen (Toper = 300 ◦ C).

is also accompanied by a change in film resistance in the final state after interaction with a target gas [35,36,41]. Due to the complexity of the processes taking place on a hydroxylated metal oxide surface [37,38,42–45], this paper will not discuss the peculiarities of SnO2 interaction with O3 , CO and H2 over OH-groups. Every target gas has its own peculiarities when interacting with SnO2 and water. Moreover, it should be noted that we do not have a definite understanding of these processes at this point. A description of several possible mechanisms of the above mentioned interactions can be found in Refs. [25,35,36,40,41,46–48]. In the frame of modern models suggested for explanation of the gas sensing effects, grain size is one of the most important parameters for gas sensing materials [49–51]. According to the conclusions made in previous research, it is necessary to use a material with minimal grain size in order to maximize sensitivity. Considering that the grain size in the SnO2 films increases with the growth of the film thickness (see Fig. 2), one can conclude that, as in the above mentioned case, the correlation between sensitivity and grain size occurs only for ozone detection. When gasses are reducing ones, we obtain the opposite affect for sensor response dependence on film thickness. Such behavior of the sensor characteristics leads to the conclusion that the grain size in the “thin film” gas sensors plays an important role, but at the same time, this parameter is not always determinative. A similar conclusion has previously been made in several published papers [1,10,17], where the influence of the films porosity on the gas sensing effects was analyzed. For example, Dolbec et al. [1] and Yamazaki et al. [10] have reported that the gas sensitivity of SnO2 films is due to the porosity rather than the size of the crystal grains. A more detailed description of grain size influence on gas sensing characteristics of SnO2 and In2 O3 -based sensors can be found in a review paper devoted to the analysis of this topic [52].

Fig. 4. The SnO2 film thickness influence on normalized S(Toper )/Smax dependencies of sensor response to a reducing gas: (1) d ∼ 40–50 nm; (2) d ∼ 80–90 nm; (3) d ∼ 280–320 nm.

al. [13] for the SnO2 -based CO sensors. In their study, they analyzed the influence of changes in film thickness in the range of 22–170 nm on the gas sensing characteristics of SnO2 films deposited by spray pyrolysis. The same effect was also observed in Ref. [53] for “thick film” gas sensors during interaction with NO2 , O3 , CO and CH4 . Comparable sensors had films with thicknesses equal to 300 nm and 30 ␮m. These results indicate that the established regularity of the thickness influence on the maximum temperature of a sensor response is really a general one for SnO2 gas sensors independent of the type of target gas. It is important to note that while analyzing the gas sensing characteristics of ozone sensors tested over a wide range of operating temperatures, we established that the film thickness has a specific influence on the temperature dependence of the sensor response. We found that an increasing the film thickness in the range of 40–350 nm, thereby shifting the sensitivity maximum towards a range of lower temperatures, had little influence on the maximum magnitude of sensitivity for this type of sensor. Even relatively “thick” films were observed to have a high sensitivity to ozone.

3.3. Thickness influence on the temperature of the sensor response maximum During our experiments, we also established that an increase in film thickness is accompanied by a shift in the maximum temperature of the sensor response to lower operating temperatures. Results showing these changes are given in Figs. 4 and 5. The shift of the sensitivity maximum is observed for both the sensors for the reducing gases (see Fig. 4), and the sensors for ozone (see Fig. 5). This means that in both cases the film thickness influences on the temperature of the sensitivity maximum have the same nature. A similar effect was recorded earlier by Korotcenkov et

Fig. 5. The influence of film thickness on the temperature dependencies of sensor response to ozone: (1) d ∼ 40–50 nm; (2) d ∼ 80–90 nm; (3) d ∼ 280–320 nm.

G. Korotcenkov, B.K. Cho / Sensors and Actuators B 142 (2009) 321–330

This once again confirms the previous statement [52] that in “thin film” sensors the role of grain size, even for ozone detection, is not always a determinative factor. As shown in Fig. 5, sensors fabricated on the basis of films with a thickness of 350 nm also have a high sensitivity to ozone. Therefore, the previous conclusion regarding the film thickness influence on the sensor response cannot be applied to the entire range of operating temperatures. While analyzing the S = f(Toper ) dependences shown in Fig. 5, one can see that the S = f(t) dependence presented in Fig. 3 is correct only for the temperature range of 250–400 ◦ C. Certainly, when a wider range of operating temperatures is reviewed, one can see that the influence of the film thickness on sensor response in other working temperature ranges, for example <220 ◦ C, could be directly opposite. However, it is clear that the range of sensor operating temperatures of 250–400 ◦ C is exactly the range in which it is possible to attain an acceptable combination of high sensitivity and short times for both the response and recovery processes. It should be pointed out that the results obtained for our “thick” film sensors (d ∼ 350 nm) cannot be extended to really thick films with thicknesses exceeding tens and hundred of microns. As we indicated earlier, films with such a thickness have very low sensitivity to ozone [53]. Due to the high activity of ozone, the top layers of the SnO2 film could act as a filter [54], which at a sufficient film thickness would prevent a deep penetration of ozone into the gas sensing material. The obtained results also cannot be applied to the SnO2 films deposited at temperatures higher than 500 ◦ C. As has already been established [13], the decrease in sensor response for these films already takes place at d > 100 nm. Such behavior is in accordance with the distinction in the structures of the SnO2 films deposited at various temperatures described in Sections 2 and 3.1. 3.4. Thickness influence on kinetics of sensor response Prima facie, which is the observed change of the sensor working characteristics with an increase in the SnO2 film thickness, ensures that sensors based on “thick” films have a considerable advantage over sensors fabricated on thinner films. Higher sensitivity at low operating temperatures could provide an opportunity for “thick” film sensors to work at lower temperatures, decreasing the power dissipated by the sensor for the attainment of the required working temperature and, as a result, could be accompanied by an increase in their service life. However, as a study of the kinetics of sensor response has shown, such a change is accompanied by an increase in response time. Fig. 6 clearly indicates that when ozone sensors have a thicker film, they also have a greater response time,  res . This increase is not always acceptable for real devices, especially those assigned for in situ control. Interestingly, as the film thickness increases, the difference between  rec and  res becomes considerably smaller. This means that along with the film thickness increase, changes in the processes controlling the kinetics of sensor response also take place. As is known, during ozone detection, a fulfillment of the correlation  rec   res , peculiar to adsorption mechanism, is typical [21,34]. Recovery processes have a weaker dependence on film thickness. We observed a similar situation for sensors of reducing gases (see Fig. 7), in that the time constants also depend on the film thickness. This dependence manifests as a considerable growth in the response time of very thick sensors. The recovery time in this case, as well as in the case of ozone sensors, shows almost no change with an increase in film thickness. Due to this, the difference between  rec and  res increases as the film thickness grows. If  rec and  res are almost equal in very thin films, then  res starts to exceed  rec in thicker films. However, we have only observed the regularity indicated above for sensors tested in a dry atmosphere (RH ∼1–2%). In a wet atmo-

325

Fig. 6. Film thickness influence on the time constants of the response and recovery processes during ozone detection at Toper = 220 ◦ C in a wet atmosphere.

sphere (RH ∼35–45%), we have observed no noticeable influence of the film thickness on the kinetics of sensor response while using films in the thickness range of 40–350 nm. We observed very similar behavior earlier while studying the kinetics of sensor response for devices fabricated based on porous In2 O3 films. The growth in time response with the increase of thickness during detection in a wet atmosphere was observed only for films deposited under conditions which formed denser (i.e. less gas penetrable) films. It is necessary to note that the established regularity was also observed for other reducing gases. In particular, we found such dependence during the detection of CO and CH4 [24]. However, the growth of the response time for detection of CO and CH4 with an increase in film thickness was greater than in the case of H2 detection. Thus, the results of the conducted research once again confirm our previous conclusion [20]. If high-speed sensors for detection of either reducing or oxidizing gases are required, it is necessary to use thin films, since thicker films produce a slower rate of response. The same conclusion can be found in Ref. [55], where the influence of film thickness in the range of 125–400 nm on the parameters of CO2 gas sensors was analyzed. These films were fabricated based on BaTiO3 –CuO films deposited by a sputtering method. In that case, the sensor response had the same magnitude for all thicknesses used. 4. Discussion At present two approaches are being used to explain the operating characteristics of conductometric metal oxide gas sensors. One is an electron-chemisorption approach [23,56–61] and the other is a diffusion-reacting approach suggested at the beginning of 1990s [61–68]. According to the second approach, the gas sensing properties of metal oxide devices are determined by the competitive influence of gas diffusion and surface reactions taking place in pores. Unfortunately, further work in this direction had been suspended until a diffusion-reacting approach was explored once again in recent years in order to interpret the obtained results [32,69–72]. Moreover, Yamazoe and co-workers in his research tried to design a phenomenological model of metal oxide gas sensors based on the indicated approach [14,73,74]. However, those works were focused mainly on analyzing diffusion processes and not surface reactions. This model can therefore be

326

G. Korotcenkov, B.K. Cho / Sensors and Actuators B 142 (2009) 321–330

ifying the structural properties of the gas sensing layer, such as film porosity, instead of optimizing the energy parameters of the chemical reactions that control the kinetics of the sensor response. However, in the frame of a diffusion model, the temperature shift of the sensor response maximum to lower temperatures should be accompanied by a drop in sensitivity as the film thickness increases [73]. According to this model [73], the sensor response (S = Rgas /Rair ) decreases sigmoidally to 1.0 with an increase in



d k/Dk , where d is the film thickness, k is the rate constant, and Dk is the Knudsen diffusion coefficient, which can be presented as [76],

 Dk = 9700 · r

Fig. 7. The film thickness influence on response and recovery times during H2 detection at Toper = 220 ◦ C in (a) dry and (b) wet atmospheres.

characterized as a model of gas sensitivity controlled by gas diffusion. In general, both electron-chemisorption and diffusion-reacting approaches provide explanations for the regularities obtained in our research. The first approach does so based on the change in the energy parameters of the adsorption–desorption processes caused by an increase in film thickness. According to electronchemisorption model [17,75], these changes take place due to the change in both the crystallographic planes faceting crystallites and the electro-physical properties of the material conditioned by the change in crystallite size [26–28]. The second approach provides an explanation based on the change in distribution profiles of either oxygen or target gas concentrations throughout the thickness of the gas sensing layer. In thicker films, a gas with a lower diffusion coefficient has a smaller concentration in the depth of the gas sensing layer than does a gas having high diffusion coefficient [53,73]. In this case, with an increase of film thickness, the indicated difference rises as well. According to the second approach, an improvement in the performance of the gas sensors may be achieved by mod-

T , M

(1)

where T is the temperature (K), r is the pore radius (cm), and M is the molecular weight of the gas. However, as it is seen from our experimental results, this correlation is not observed for our thin film sensors. Our research has shown that sensors with either a large or small thickness of the gas sensing layer can have equal sensor responses. Further, for films with a thickness of 40–70 nm, it is difficult to imagine the existence of a structure in which sensitivity is being limited by gas diffusion along the pores. Typical structures of the thin and ultra-thin SnO2 films deposited by spray pyrolysis on the surface of an oxidized Si substrate are shown in Fig. 8. The presented images show that in films with a thickness of 40–70 nm it is impossible to find pores with a path at which the time of diffusion will exceed even 10−1 s. It is necessary to note that in real experiments the response times and recovery processes exceed tens of seconds, even at Toper > 200 ◦ C [24,31,34]. According to a calculation based on the expressions designed by Yamazoe and co-workers [73,74], the time necessary for gas diffusion through the total thickness of the gas sensing layer only at a thickness exceeding 200 ␮m can attain values, which is comparable with the time constants of the conductivity change observed during the process of metal oxide interaction with the target gas. Using the diffusion model framework, it is also impossible to explain the presence of a maximum in the temperature dependence of the sensitivity for ultra-thin films. In the frame of model, which supposes a dominant role of Knudsen diffusion, we also cannot explain the exponential character of the dependence of response and recovery times on operating temperatures observed in many experiments [24,58,61]. Therefore, from our point of view, the electronic chemisorption model, suggested in our papers [23,59,61], is a more realistic one, especially for the thin film sensors fabricated based on gas sensing layers with a thickness of less than 100 nm. The chemisorption approach also more efficiently explains the effects discussed earlier in this paper. For example, using only the chemisorption approach, one can understand the reasons leading to a violation in the range of temperatures <200 ◦ C of regularities, which were established for operating temperatures of 200–400 ◦ C. As noted in Refs. [31,50,77], at T ∼ 200 ◦ C a change takes place in the nature of the adsorbed species dominating the SnO2 surface and controlling the kinetics of sensor response. As was established earlier, when T > 200 ◦ C at the SnO2 surface, oxygen is present in the atomic chemisorbed state and when T < 200 ◦ C, a molecular form becomes the dominant oxygen state. However we have to admit that even in thin film gas sensors diffusion processes participate in gas sensing effects and under certain conditions they can limit both the kinetics and magnitude of the sensor response. Thus, the integration of the above mentioned approaches for consideration of gas sensing effects would be the most optimal approach and could contribute to the design of a more general phenomenological model of solid state gas sensors. It is necessary to point out, though, that under the diffusion processes

G. Korotcenkov, B.K. Cho / Sensors and Actuators B 142 (2009) 321–330

327

Fig. 8. (a) TEM and (b) SEM images of the SnO2 films deposited by spray pyrolysis: (a) Tpyr = 435 ◦ C; d ∼ 40–50 nm; (b) Tpyr = 375 ◦ C; d ∼ 25–30 nm.

we refer not only to gas diffusion along pores, but also to surface (intercrystallite) diffusion (see Fig. 9). For sensors fabricated according to thin film technology, the above mentioned definition of the nature of the diffusion processes is particularly pertinent because, in spite of a considerably smaller thickness, they do not have the porosity found in sensors fabricated by thick or ceramic technologies [50]. A TEM and SEM study of deposited films also shows a clear interface between crystallites. This fact points to the absence of necks between grains, whose overlap could be used to explain the high sensitivity of films with a thickness of 300–350 nm during interaction with ozone [50,51]. As we have shown before, SnO2 films deposited at temperatures higher than 350 ◦ C have columnar structure in which grains can grow through the entire film thickness (see Fig. 1). This means that films deposited by thin film technology have a larger contact area between crystallites. Moreover, the comparison of the images of films having a different thickness (see Figs. 1 and 8) shows that the area of indicated contacts increases considerably with the film growth. Thus, the time required for the diffusion of oxygen or oxygen vacancies into intercrystallite space should increase along with the film thickness growth. The specific character of such film structure is shown in Fig. 9. It is necessary to note that the above mentioned view on the nature of diffusion processes taking place in thin film gas sensors was developed based on results presented by Korotcenkov et al. [25]. This work showed that surface (intercrystallite) diffusion at the interface of two crystallites, which is being controlled in most cases by surface reactions, has the ability to determine the

kinetics of the sensor response of the In2 O3 -based thin film gas sensors. Under a diffusion controlled by surface reactions, Korotcenkov et al. [25] understood a process in which the concentration of oxygen, capable of diffusion and thus able to affect the metal oxide stoichiometry at the intercrystallite interface, is controlled by the rate of surface reactions. This last case could be used to explain the absence of the SnO2 film thickness influence on recovery time, in spite of the fact that the grain size growth takes place with a corresponding increase of the area of intercrystallite contacts. From our point of view, intercrystallite (surface) diffusion through the change of intercrystallite metal oxide stoichiometry can alone affect the value of the intercrystalline barrier (ϕb ), even in cases where the relation (t/2) < L is not being executed. Here, t is the grain size and L is the width of surface space charge region, formed by the chemisorbed species. As established on many occasions [50,51,77–79], as a rule, only a change of ϕb is responsible for changing the conductivity of polycrystalline gas sensing films ((Rg /Ra )∼exp(ϕb )) for interaction with a target gas. Fig. 10 shows the d influence on the film resistance normalized to its thickness (R·d). It is seen that R·d grows with the d increasing. Such behavior can be considered as a confirmation of the of intercrystallite barriers’ responsibility for the layer resistance of polycrystalline films. As the graphs in Fig. 10 show, the clearest indicated growth for the value of R·d is observed in the SnO2 film resistance measured in an ozone atmosphere. For the resistance measurement in air, the growth of R·d is significantly smaller. While measuring in an atmosphere containing a reducing gas (H2 ), the influence of the film thickness on the value of R·d is sig-

Fig. 9. Models, illustrating diffusion processes dominating in films formed using (a) thick film and (b and c) thin film technologies.

328

G. Korotcenkov, B.K. Cho / Sensors and Actuators B 142 (2009) 321–330

higher than 200 ◦ C, a smaller film thickness also supports a higher sensitivity to ozone. If the rate of sensor response is not a critical parameter, one should use thicker films for the sensor design, which will provide better sensitivity at lower operating temperatures. Acknowledgements

Fig. 10. Influence of film thickness on the normalized resistance of films (R·d) measured at T = 300 ◦ C in air (1), in an atmosphere containing 0.1% H2 (2), and in an atmosphere containing ozone (3).

This work was supported by the World Class University (WCU) program at GIST (No. R31-2008-000-10026-0) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) (No. 2009-0078928). It has been supported by a grant provided by the Ministry of Education, Science, and Technology (MEST) of Korea and by the project titled “Development of Maritime Environmental Sensor using Nano and Photonic Technology,” funded by the Ministry of Land, Transport and Maritime Affairs, Korea. The authors are also thankful to Professors J.R. Morante and A. Cornet from the University of Barcelona (Spain) and Prof. J. Schwank from the University of Michigan (USA) for their help in structural characterization of the studied SnO2 films, as well as to I. Blinov from the Technical University of Moldova for his help in gas sensor characterization and Dr. A. England from the National Fusion Research Institute (Korea) for useful discussions. References

nificantly smaller. It is necessary to note that the above mentioned dependences are in agreement with the results of gas influence on the height of surface potential barriers. As established earlier, the interaction with ozone increases the height of the potential barrier at the inter-grain boundary, while the interaction with hydrogen decreases it [34,77,78,80]. Therefore, such a strong influence of the detected gas nature on the features of R·d dependences confirms that the conductivity of the analyzed SnO2 films in the temperature range 200–400 ◦ C is actually controlled by the resistance of the intercrystallite barriers [79,81]. Based on the conducted analysis, we can therefore conclude that, due to the more compact structure of the films prepared by thin film technology, a diffusion limitation in the kinetics of sensor response in thin film sensors can appear at considerably smaller film thicknesses than in sensors fabricated by “thick film” technology. For “thick film” sensors, this advantage could be realized only after resolving the problem of small grains agglomerating in metal oxide ceramics prepared by the thick film technology. We need to note that the resolution of this task is quite difficult. A large contact area between crystallites in more compact “thin film” gas sensors could also prevent an attainment of high sensitivity. However, the indicated shortcoming for “thin film” sensors, as a rule, starts to appear at film thicknesses exceeding 100 nm. 5. Conclusions The conducted research has shown that for “thin film” sensors, the film thickness is a very important parameter and determines their main operating characteristics, such as sensor response, rate of response, and working temperature. Given this, choosing the optimal thickness of the gas sensing layer (in the range of 40–350 nm) used for sensor fabrication should be determined by three factors: (1) the required rate of sensor response, (2) the nature of the target gas which the sensor is designed to detect, and (3) the required operating temperature. If a high-speed sensor is needed, it is necessary to use thin films because the thinner is the film, the shorter the response time of metal oxide sensors is. In this case the sensitivity is determined mainly by the efficiency of surface reactions, while the process of gas diffusion inside the gas sensing matrix, which could limit the kinetics of sensor response, does not influence the gas sensing effects. For operating temperatures

[1] R. Dolbec, M.A. El Khakani, A.M. Serventi, R.G. Saint-Jacques, Influence of the nanostructural characteristics on the gas sensing properties of pulsed laser deposited tin oxide thin films, Sens. Actuators B 93 (2003) 566–571. [2] T.-R. Ling, C.-M. Tsai, Influence of nano-scale dopants of Pt, CaO and SiO2 , on the alcohol sensing of SnO2 thin films, Sens. Actuators B 119 (2006) 497–503. [3] L. Bruno, C. Pijolat, R. Lalauze, Tin dioxide thin-film gas sensor prepared by chemical vapour deposition. Influence of grain size and thickness on the electrical properties, Sens. Actuators B: Chem. 18–19 (1994) 195–199. [4] T. Mochida, K. Kikuchi, T. Kondo, H. Ueno, Y. Matsuura, Highly sensitive and selective H2 S gas sensor from r.f. sputtered SnO2 , thin film, Sens. Actuators B 24–25 (1995) 433–437. [5] P. Montmeat, R. Lalauze, J.-P. Viricelle, G. Tournier, C. Pijolat, Model of the thickness effect of SnO2 thick film on the detection properties, Sens. Actuators B 103 (2004) 84–90. [6] G. Korotcenkov, V. Brinzari, M. DiBattista, J. Schwank, A. Vasiliev, Peculiarities of SnO2 thin film deposition by spray pyrolysis for gas sensor application, Sens. Actuators B 77 (2001) 244–252. [7] J. KIiiber, M. Ludwig, H.A. Schneider, Effects of thickness and additives on thinfilm SnO2 gas sensors, Sens. Actuators B 3 (1991) 69–74. [8] G. Sberveglieri, Classical and novel techniques for the preparation of SnO2 thinfilm gas sensors, Sens. Actuators B 6 (1992) 239–247. [9] G. Williams, G.S.V. Coles, Gas sensing properties of nanocrystalline metal oxide films, in: Sensors Applications VII, IOP, London, 1995, pp. 69–73. [10] T. Yamazaki, H. Okumura, C.-J. Jin, A. Nakayama, T. Kikuta, N. Nakatani, Effect of density and thickness on H2 -gas sensing property of sputtered SnO2 films, Vacuum 77 (2005) 237–243. [11] T. Suzuki, T. Yamazaki, M. Azumaya, Hydrogen gas sensing properties in polycrystalline tin oxide films of submicron thickness, J. Ceram. Soc. Jpn. 97 (1989) 1268–1273. [12] S.-S. Park, J.D. Mackenzie, Thickness and microstructure effects on alcohol sensing of tin oxide thin films, Thin Solid Films 274 (1996) 154–159. [13] G. Korotcenkov, V. Brinzari, J. Schwank, A. Cerneavschi, Possibilities of aerosol technology for deposition of SnO2 -based films with improved gas sensing characteristics, Mater. Sci. Eng. C 19 (1–2) (2001) 73–77. [14] G. Sakai, N.S. Baik, N. Miura, N. Yamazoe, Gas sensing properties of tin oxide thin films fabricated from hydrothermally treated nanoparticles. Dependence of CO and H2 response on film thickness, Sens. Actuators B 77 (2001) 116–121. [15] L. de Angelis, R. Riva, Selectivity and stability of a tin dioxide sensor for methane, Sens. Actuators B 28 (1995) 25–29. [16] G. Korotcenkov, Metal oxides for solid state gas sensors. What determines our choice? Mater. Sci. Eng. B 139 (2007) 1–23. [17] G. Korotcenkov, The role of morphology and crystallographic structure of metal oxides in response of conductometric-type gas sensors, Mater. Sci. Eng. R 61 (2008) 1–39. [18] G. Korotcenkov, Gas response control through structural and chemical modification of metal oxides: state of the art and approaches, Sens. Actuators B 107 (2005) 209–232. [19] V. Brinzari, G. Korotcenkov, V. Golovanov, Factors influencing the gas sensing characteristics of tin dioxide films deposited by spray pyrolysis: understanding and possibilities for control, Thin Solid Films 391 (2) (2001) 167–175. [20] G. Korotcenkov, V. Brinzari, A. Cerneavschi, M. Ivanov, V. Golovanov, A. Cornet, J. Morante, A. Cabot, J. Arbiol, The influence of film structure on In2 O3 gas response, Thin Solid Films 460 (2004) 308–316.

G. Korotcenkov, B.K. Cho / Sensors and Actuators B 142 (2009) 321–330 [21] G. Korotcenkov, A. Cerneavschi, V. Brinzari, A. Vasiliev, A. Cornet, J. Morante, A. Cabot, J. Arbiol, In2 O3 films deposited by spray pyrolysis as a material for ozone gas sensors, Sens. Actuators B 99 (2004) 304–310. [22] G. Korotcenkov, V. Brinzari, A. Cerneavschi, M. Ivanov, A. Cornet, J. Morante, A. Cabot, J. Arbiol, In2 O3 films deposited by spray pyrolysis: gas response to reducing (CO, H2 ) gases, Sens. Actuators B 98 (2004) 236–243. [23] V. Brinzari, G. Korotcenkov, J. Schwank, Optimization of thin film gas sensors for environmental monitoring through theoretical modeling, In: S. Buettgenbach (Ed.), Chemical Microsensors and Applications II, Proc. of SPIE, vol. 3857, 1999, pp. 186–197. [24] G. Korotchenkov, V. Brynzari, S. Dmitriev, Kinetics characteristics of SnO2 thin film gas sensors for environmental monitoring. In: S. Buettgenbach (Ed.), Chemical Microsensors and Applications, Proc. of SPIE, vol. 3539, 1998, pp. 196–204. [25] G. Korotcenkov, V. Brinzari, J.R. Stetter, I. Blinov, V. Blaja, The nature of processes controlling the kinetics of indium oxide-based thin film gas sensor response, Sens. Actuators B 128 (2007) 51–63. [26] G. Korotcenkov, A. Cornet, E. Rossinyol, J. Arbiol, V. Brinzari, Y. Blinov, Faceting characterization of SnO2 nanocrystals deposited by spray pyrolysis from SnCl4 –5H2 O water solution, Thin Solid Films 471 (2005) 310–319. [27] E. Elangovan, M.P. Singh, K. Ramamurthi, Studies on structural and electrical properties of spray deposited SnO2 :F thin films as a function of film thickness, Mater. Sci. Eng. B 113 (2004) 143–148. [28] V. Brinzari, G. Korotcenkov, J. Schwank, V. Lantto, S. Saukko, V. Golovanov, Morphological rank of nano-scale tin dioxide films deposited by spray pyrolysis from SnCl4 ·5H2 O water solution, Thin Solid Films 408 (2002) 51–58. [29] G. Korotcenkov, M. DiBattista, J. Schwank, V. Brinzari, Structural characterization of SnO2 gas sensing films deposited by spray pyrolysis, Mater. Sci. Eng. B 77 (2000) 33–39. [30] J. Arbiol, Metal additive distribution in TiO2 and SnO2 semiconductor gas sensor nanostructured materials, PhD Thesis, University of Barcelona, Barcelona, Spain, 2001. [31] G. Korotcenkov, M. Ivanov, I. Blinov, J.R. Stetter, Kinetics of In2 O3 -based thin film gas sensor response: the role of “redox” and adsorption/desorption processes in gas sensing effects, Thin Solid Films 515 (2007) 3987–3996. [32] Y. Shimuzu, T. Maekawa, T. Nakamura, M. Egashira, Effects of gas diffusivity and reactivity on gas sensing properties of thick film SnO2 -based sensors, Sens. Actuators B 46 (1998) 163–168. [33] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sens. Actuators B 5 (1991) 7–19. [34] G. Korotcenkov, I. Blinov, M. Ivanov, J.R. Stetter, Ozone sensors on the base of SnO2 thin films deposited by spray pyrolysis, Sens. Actuators B 120 (2007) 679–686. [35] G. Korotcenkov, I. Blinov, V. Brinzari, J.R. Stetter, Effect of air humidity on gas response of SnO2 thin film ozone sensors, Sens. Actuators B 122 (2007) 519–526. [36] G. Korotcenkov, V. Brinzari, Y. Boris, M. Ivanov, J. Schwank, J. Morante, Surface Pd doping influence on gas sensing characteristics of SnO2 thin films deposited by spray pyrolysis, Thin Solid Films 436 (2003) 119–126. [37] M. Batzill, Surface science studies of gas sensing materials: SnO2 , Sensors 6 (2006) 1345–1366. [38] D.F. Cox, S. Semancik, P.D. Szuromi, Structural and electronic-properties of clean and water dosed SnO2 (1 1 0), J. Vac. Sci. Technol. A 4 (1986) 627–628. [39] J.F. Boyle, K.A. Jones, The effects of CO, water vapor and surface temperature on the conductivity of a SnO2 gas sensor, J. Electronic Mater. 6 (1977) 717–720. [40] G. Korotchenkov, V. Brynzari, S. Dmitriev, Electrical behavior of SnO2 thin films in humid atmosphere, Sens. Actuators B 54 (1999) 197–201. [41] S.H. Hahn, N. Bârsan, U. Weimar, S.G. Ejakov, J.H. Visser, R.E. Soltis, CO sensing with SnO2 thick film sensors: role of oxygen and water vapour, Thin Solid Films 436 (2003) 17–24. [42] A. Verdaguer, G.M. Sacha, H. Bluhm, M. Salmeron, Molecular structure of water at interfaces: wetting at the nanometer scale, Chem. Rev. 106 (2006) 1478–1510. [43] J. Gonialowski, M.J. Gilan, The adsorption of H2 O on TiO2 and SnO2 (1 1 0) studied by first-principles calculations, Surf. Sci. 350 (1996) 145–158. [44] N. Yamazoe, J. Fuchigami, M. Kishikawa, T. Seiyama, Interactions of tin oxide with O2 , H2 O and H2 , Surf. Sci. 86 (1979) 335–344. [45] M. Egashira, M. Nakashima, S. Kawasumi, T. Seiyama, Temperature programmed desorption study of water adsorbed on metal oxides. 2. Tin oxide surfaces, J. Phys. Chem. 85 (1981) 4125–4130. [46] D. Koziej, N. Barsan, U. Weimar, J. Szuber, K. Shimanoe, N. Yamazoe, Water–oxygen interplay on tin dioxide surface: implication on gas sensing, Chem. Phys. Lett. 410 (2005) 321–323. [47] R. Ionescu, A. Vancu, C. Moise, A. Tomescu, Role of water vapour in the interaction of SnO2 gas sensors with CO and CH4 , Sens. Actuators B 61 (1999) 39–42. [48] V. Golovanov, T. Pekna, A. Kiv, V. Litovchenko, G. Korotcenkov, V. Brinzari, A. Cornet, J. Morante, The influence of structural factors on sensitivity of SnO2 based gas sensors to CO in humid atmosphere, Ukr. Phys. J. 50 (4) (2005) 374–380. [49] N. Matsunaga, G. Sakai, K. Shimanoe, N. Yamazoe, Diffusion equation-based study of thin film semiconductor gas sensor-response transient, Sens. Actuators B 83 (2002) 216–221. [50] N. Barsan, M. Schweizer-Berberich, W. Göpel, Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: a status report, Fresenius J. Anal. Chem. 365 (1999) 287–304.

329

[51] X. Wang, S.S. Yee, W.P. Carey, Transition between neck-controlled and grainboundary-controlled sensitivity of metal-oxide gas sensors, Sens. Actuators B 24–25 (1995) 454–457. [52] G. Korotcenkov, S.D. Han, B.K. Cho, V. Brinzari, Grain size effects in sensor response of nanostructured SnO2 - and In2 O3 -based conductometric gas sensor, Crit. Rev. Solid State Mater. Sci. 34 (1–2) (2009) 1–17. [53] Th. Becker, S. Ahlers, C. Bosch-v.Braunmühl, G. Müller, O. Kiesewetter, Gas sensing properties of thin- and thick-film tin-oxide materials, Sens. Actuators B 77 (2001) 55–61. [54] C. Pijolat, J.P. Viricelle, G. Tournier, P. Montmeat, Application of membranes and filtering films for gas sensors improvements, Thin Solid Films 490 (2005) 7–16. [55] J. Herran, G.G. Mandayo, E. Castano, Solid state gas sensor for fast carbon dioxide detection, Sens. Actuators B 129 (2008) 705–709. [56] J. Ding, T.J. McAvoy, R.E. Cavicchi, S. Semancik, Surface state trapping model for SnO2 -based microplate sensor, Sens. Actuators B 77 (2001) 597–613. [57] H. Geistlinger, Electron theory of thin-film gas sensors, Sens. Actuators B 17 (1993) 47–60. [58] H. Geistlinger, Accumulation layer model for Ga2 O3 thin-film gas sensors based on the Volkenstein theory of catalysis, Sens. Actuators B 18–19 (1994) 125–131. [59] V. Brynzari, G. Korotchenkov, S. Dmitriev, Theoretical study of semiconductor thin film gas sensitivity: attempt to consistent approach, J. Electron Technol. 33 (2000) 225–235. [60] V. Brinzari, G. Korotcenkov, J. Schwank, Y. Boris, Chemisorptional approach to kinetic analysis of SnO2 :Pd-based thin film gas sensors (TFGS), J. Optoelectron. Adv. Mater. 4 (1) (2002) 147–150. [61] V. Brynzari, G. Korotchenkov, S. Dmitriev, Simulation of thin film gas sensor kinetics, Sens. Actuators B 61 (1–3) (1999) 143–153. [62] D.E. Williams, K.F.E. Pratt, Theory of self-diagnostic sensor array devices using gas-sensitive resistors, J. Chem. Soc., Faraday Trans. 91 (1995) 1961–1966. [63] J.W. Gardner, A diffusion-reaction model of electrical conduction in tin oxide gas sensors, Semicond. Sci. Technol. 4 (1989) 345–350. [64] J.W. Gardner, Electrical conduction in solid-state gas sensors, Sens. Actuators 18 (1989) 373–387. [65] J.W. Gardner, A non-linear diffusion-reaction model of electrical conduction in semiconductor gas sensors, Sens. Actuators B 1 (1999) 166–170. [66] J.W. Gardner, M. Iskandarani, Effect of electrode geometry on gas sensitivity of lead phthalocyanine thin films, Sens. Actuators B 9 (1992) 133–142. [67] U. Jain, A.H. Harker, A.M. Stoneham, D.E. Williams, Effect of electrode geometry on sensor response, Sens. Actuators B 2 (1990) 111–114. [68] D.E. Williams, G.S. Hensham, K.F.E. Pratt, Reaction-diffusion effects and systematic design of gas-sensitive resistors based on semiconducting oxides, J. Chem. Soc., Faraday Trans. 91 (23) (1995) 4299–4307. [69] H. Lu, W. Ma, J. Gao, J. Li, Diffusion-reaction theory for conductance response in metal oxide gas sensing thin films, Sens. Actuators B 66 (2000) 228–231. [70] F. Rettig, R. Moos, C. Plog, Sulfur adsorber for thick-film exhaust gas sensors, Sens. Actuators B 93 (2003) 36–42. [71] K. Darcovich, F.F. Garcia, C.A. Jeffrey, J.J. Tunney, M.L. Post, Coupled microstructural and transport effects in n-type sensor response modeling for thin layers, Sens. Actuators A 147 (2008) 378–386. [72] P. Boeker, O. Wallenfang, G. Horner, Mechanistic model of diffusion and reaction in thin sensor layers—the DIRMAS model, Sens. Actuators B 83 (2002) 202–208. [73] G. Sakai, N. Matsunaga, K. Shimanoe, N. Yamazoe, Theory of gas-diffusion controlled sensitivity for thin film semiconductor gas sensor, Sens. Actuators B 80 (2001) 125–131. [74] N. Matsunaga, G. Sakai, K. Shimanoe, N. Yamazoe, Formulation of gas diffusion dynamics for thin film semiconductor gas sensor based on simple reaction–diffusion equation, Sens. Actuators B 96 (2003) 226–233. [75] F.F. Volkenstein, The Electronic Theory of Catalysis on Semiconductors, Pergamon, Oxford, 1963. [76] C.N. Satterfield, Mass Transfer in Heterogeneous Catalysis, MIT Press, Cambridge, MA, 1970. [77] N. Barsan, D. Koziej, U. Weimar, Metal oxide-based gas sensor research: how to? Sens. Actuators B 121 (2007) 18–35. [78] A. Karthigeyan, R.P. Gupta, K. Scharnagl, M. Burgmair, M. Zimmer, S.K. Sharma, I. Eisele, Low temperature NO2 sensitivity of nano-particulate SnO2 film work function sensors, Sens. Actuators B 78 (2001) 69–72. [79] K.-D. Schierbaum, Engineering of oxide surfaces and metal/oxide interfaces for chemical sensors: recent trends, Sens. Actuators B 24–25 (1995) 239–247. [80] B. Yea, R. Konishi, T. Osaki, S. Abe, H. Tanioka, K. Sugahara, Analysis of the sensing mechanism of tin dioxide thin film gas sensors using the change of work function in flammable gas atmosphere, Appl. Surf. Sci. 100/101 (1996) 365–369. [81] A. Chandra Bose, P. Thangadurai, S. Ramasamy, Grain size dependent electrical studies on nanocrystalline SnO2 , Mater. Chem. Phys. 95 (2006) 72–78.

Biographies G. Korotcenkov received his PhD in physics and technology of semiconductor materials and devices from Technical University of Moldova in 1976 and his Habilitat degree in physics and mathematics from Academy of Science of Moldova in 1990.

330

G. Korotcenkov, B.K. Cho / Sensors and Actuators B 142 (2009) 321–330

Currently he is a research professor at Gwangju Institute of Science and Technology (GIST) in Korea. Long time his research activity was focused on the study of Schottky barriers, MOS structures, and various photoreceivers on the base of III-Vs compounds. His present scientific interests include material sciences, focusing on metal oxide film deposition and characterization, surface science, and thin film gas sensor design.

B.K. Cho is a professor in Department of Nanobio Materials and Electronics, and Department of Materials Science and Engineering at Gwangju Institute of Science and Technology (GIST) in Korea. He received his PhD in physics and astronomy from Iowa State University, USA in 1995. His current research project is focused on material science and the applications of nano-spin devices, especially development of biosensors using magnetic thin film technology.