Study of structural and morphological properties of RF-sputtered SnO thin films and their effect on gas-sensing phenomenon

Ajay Kumar Arora; Sandeep Mahajan; Maya Verma; Divya Haridas

Détails du journal et du numéro

International Journal on Smart Sensing and Intelligent Systems

Détails du magazine

Format: Magazine
eISSN: 1178-5608
Première parution: 01 Jan 2008
Périodicité: 1 fois par an
Langues: Anglais

Gas sensors are nowadays playing a pivotal role at many industrial and domestic sites. Extensive research is going on in designing efficient gas sensors that can be deployed at important sites to avoid any fatal accidents due to the leakage of inflammable or toxic gas. The three most important parameters that characterize sensor performance are sensitivity, selectivity, and stability (SSS). Thus, an efficient gas sensor is considered to be one that has high sensitivity, good selectivity, and good stability. Apart from the SSS parameters, room temperature operation, low power requirement, fast response and recovery time, and low detection limit are other important parameters that are considered while designing an efficient gas sensor. Semiconductor gas sensors have emerged as one of the most popular contenders for developing commercial gas sensors. A semiconductor gas sensor, also known as chemoresistive gas sensor, changes its resistance/conductance when the sensor comes in contact with the target gas. Semiconductor gas sensors can be fabricated as thin films, thick films, and ceramic gas sensors. Among the three, thin film gas sensors are considered to be the best as the surface-to-volume ratio in a thin film is the highest. Apart from the high surface-to-volume ratio of thin film gas sensors, their low cost, small size, and low power consumption are also attractive features, making them potential candidates for commercial gas sensors. Thin films of metal oxides are widely used for gas-sensing applications.

Semiconducting metal oxides are generally the most preferred gas-sensing materials because they readily adsorb oxygen on the surface in a reversible way. In view of this phenomenon, a comprehensive investigation was carried out in Europe as part of the Basic Research in Industrial Technologies for Europe/European Research In Advanced Materials (BRITE/EURAM) project [1], and >50 different oxide materials were examined for their gas-sensing properties. Mostly, sintered ceramic samples were prepared, and a few attempts on thick- and thin-film structures were also made. SnO₂ exhibited sensitivity to most of the gases at a low operating temperature of 300°C, followed by TiO₂. SnO₂ is an n-type extrinsic semiconductor due to the presence of large amounts of native defects related to oxygen vacancies, which creates donor energy levels (0.03 eV and 0.14 eV) that are below the conduction band (related to single and double ionization of oxygen vacancies). SnO₂ possesses excellent capability for exchange of oxygen from the atmosphere due to natural nonstoichiometry, which makes it the most suitable material for gas-sensing application.

The performance of metal-oxide gas sensors can be improved by integrating a suitable catalyst with the sensing layer in desired quantities. The effect of various catalytic additives (noble metals and transition metal oxides) on the gas-sensing properties of metal oxides has been widely studied [2]. Additives not only enhance the gas-sensing response but also influence the response speed and operating temperature [3], besides improving the selectivity of the sensor for a specific target gas. Moreover, gas sensing is known to be primarily a surface phenomenon; so, thin films are expected to exhibit enhanced sensitivity because of the pronounced surface-to-volume ratio. Adsorption of the sensing gas is a surface phenomenon, and the sensitivity of the sensor is therefore expected to depend on the surface-to-volume ratio of the SnO₂ particle size. The amount of oxygen adsorbates per volume of the SnO₂ grain increases with decrease in crystallite grain. Literature indicates an increase in sensitivity with decrease in grain size in the range of 4–30 nm [4]. The reported data show that, initially, the increase in sensitivity with decreasing grain size [5] is slow and then proceeds rapidly. Debye length for a thin sputtered SnO₂ film is reported to be around 3 nm at 250°C [6]. Thus, at a crystallite size of around 6 nm (comparable to twice the Debye length), the observed high sensitivity can be attributed to the formation of a space-charge region in the entire crystallite. Hence, nanocrystalline SnO₂ films are expected to exhibit enhanced sensitivity.

The shape and size of crystallites, as well as their orientation, also play crucial roles in the entire adsorption/desorption process [7,8,9] taking place in the gas-sensing phenomenon. Lucas et al. [10] reported the impact of shape and orientation of crystallites of the gas-sensing matrix on sensing phenomenon.

It is also reported in the literature that the gas-sensing characteristics of semiconducting metal oxides depend on the thickness of the sensing layer [11]. An enhanced sensing response has been reported by various workers by increasing the thickness of the sensing layer [12,13,14,15,16], while few have reported plummeting sensing response on increasing the thickness of the film [17, 18]. The grain size in ceramic or thick film sensors is governed by the conditions of SnO₂ synthesis and thermal treatment [19, 20]. However, for thin-film-based sensors, the grain size is usually influenced by the thickness of the sensing film, apart from the growth kinetics [21]. A semiquantitative model is proposed by Xu et al. [22]. Deshwal and Arora [23] studied the sensing response for acetone gas by optimizing the thickness of ZnO thin films and reported a sensing response of 63.3 measured at 320°C for an optimized thickness of 410 nm for a ZnO-sensing layer. Hassan et al. [24] deposited 50-nm-, 100-nm-, and 150-nm-thin NiO- and NiO–Cu-doped films by radiofrequency (RF) magnetron sputtering technique. It was reported that 50-nm-thick NiO- and NiO–Cu-doped thin films gave the highest sensitivity toward NO₂ gas.

Gas-sensing characteristics are strongly influenced by the structural properties of the sensing layer. In the present work, the thickness of SnO₂ thin films is varied and their structural properties and surface morphologies are studied and are correlated for understanding the gas-sensing phenomenon in a better perspective. In the present work, the thickness of SnO₂ film (the base layer) is optimized, which may be further modified in the future using various catalysts for the efficient detection of LPG gas.

Experimental

In the present study, the RF-sputtering technique was used to deposit the SnO₂ films (having thickness varying from 30 nm to 180 nm) in a reactive atmosphere (50% Ar and 50% O₂) at 14 mTorr [25]. The typical processing conditions used for SnO₂ thin films are presented in Table 1.

Table 1

Sputtering parameters used for deposition of SnO₂ thin film.

Technique	RF diode sputtering
Target	Tin (99.99%)
Gas	50% Ar + 50% O₂
Sputtering pressure	14 mTorr
Power	150 W
Substrate-to-target distance	7.5 cm
Substrate temperature	25°C (no heating)
Rate of deposition:	60 nm/h

The surface morphology of SnO₂ thin films of varying thicknesses was studied over an area of 2 × 2 μm² in the noncontact mode using an atomic force microscope (MultiMode^TM SPM; Manufacturer: Digital Instrument, Veeco Metrology Group, NY, USA).

The sensing characteristics were recorded by depositing the SnO₂ thin films onto the entire Pt-interdigitated electrodes (IDEs), protecting the contact pads with a shadow mask as shown in Figure 1. Before the deposition of SnO₂ thin film, IDEs of Pt/Ti are patterned on Corning glass substrates using photolithography. The typical dimensions are shown in Figure 1, where the distance between the IDEs was 0.8 mm.

The SnO₂ film was deposited on a 2.5 × 2.1 cm² area on the substrate, as shown in Figure 1. The sensing area of all the sensor structures was kept the same in order to maintain uniformity, but the thickness of the sensing layer was varied in the present work. It is known that the gas-sensing characteristics of a semiconducting metal oxide depend strongly on the thickness of the sensing layer [26].

The sensing response of the sensors toward 200 ppm of LPG was measured as a function of temperature (50°C–300°C). The sensing measurements were carried out in a test chamber having a relative humidity (RH) of around 30%. A Keithley digital multimeter (Model-2700) and an automated data acquisition system were used to record the sensing response. Before the introduction of LPG in the chamber, the sensor was stabilized in air for each temperature measurement. The sensing response can be calculated as R_a/R_g, where R_g is the resistance in the presence of LPG. A Philips X-ray diffractometer was used to study the crystallographic structure of the thin film. The surface morphology of SnO₂ thin films of varying thicknesses was studied over an area of 2 × 2 μm² in the noncontact mode using an atomic force microscope (MultiMode^TM SPM; Manufacturer: Digital Instrument, Veeco Metrology Group, NY, USA).

Results and Discussion

3.1

Structural and morphological studies

The X-ray diffraction (XRD) pattern obtained for the as-grown SnO₂ thin film deposited by RF sputtering is shown in Figure 2 (graph a). No reflection in XRD corresponding to any plane of SnO₂ thin film was observed for the as-grown thin film, confirming its amorphous nature. Annealing treatment of thin films at 300°C in air was done for 2 h after deposition, and its XRD pattern is shown in Figure 2 (graph b). Broad and well-defined reflections were observed at 2θ = 25.88°, 33.89°, and 51.79°, corresponding to the (110), (101), and (211) planes, respectively, in the XRD pattern of the annealed SnO₂ thin film; the results are in good agreement with the reported values for the rutile structure [27, 28]. Scherrer's formula was used to find the crystallite size of the annealed SnO₂ thin film, and it was found to be about 4.8 nm. The effect of film thickness (30–180 nm) on the XRD pattern of the SnO₂ thin films deposited by RF sputtering is shown in Figure 3.

XRD of the as-grown SnO₂ thin film (graph a) and the SnO₂ thin film annealed in air at 300°C for 2 h (graph b). XRD, X-ray diffraction.

XRD pattern of the annealed SnO₂ thin film of varying thicknesses. XRD, X-ray diffraction.

Three characteristic XRD peaks corresponding to (110), (101), and (211) planes are observed for SnO₂ thin films of all thicknesses (30–180 nm). It may be observed from Figure 3 that the intensity of all XRD peaks increases as the thickness of the film is increased from 30 nm to 180 nm. With increase in the thickness of the SnO₂ film, the length of the X-ray path increases in the film. This results in an increase in the number of atoms that contribute to the XRD processes, and thus, the peak height of a diffraction peak increases [29].

The peak corresponding to the (110) plane is dominant as compared to other reflections in all the prepared SnO₂ samples. A small peak at around 2θ =38.23°, corresponding to the (200) plane of SnO₂, was seen to appear at higher thicknesses (>150 nm) of the SnO₂ film.

Normalized intensities of all XRD peaks were calculated as a ratio, namely, I(hkl)/ΣI(hkl), to find the preferred orientation of the SnO₂ crystallites in the annealed samples, where I(hkl) is the intensity of the XRD peak under analysis, and ΣI(hkl) is the sum of the intensities of all peaks appearing in the XRD pattern. Normalized intensities of all the observed peaks [(110), (101), (200), and (211)] for varying thicknesses of the SnO₂ thin film are shown in Figure 4.

Influence of the thickness of the SnO₂ thin film on the normalized intensities of various XRD peaks. XRD, X-ray diffraction.

It may be noted from Figure 4 that all SnO₂ thin films exhibit the maximum normalized intensity (in the range of 32%–38%) corresponding to the (110) plane among all the four reflections, indicating that the (110) plane is the preferred orientation plane for the growth of SnO₂ crystallites. The normalized intensity of the (110) plane was in the range of 32%–38% and was the maximum (38%) for the 90 nm SnO₂ thin film. The normalized intensity corresponding to the (101) plane is also reasonably good (~27%) and is almost constant with film thickness. The observed preferred orientation of crystallites along the (110) plane is in agreement with the reports available in the literature [9] on the growth of SnO₂ film.

Crystallite size of all postdeposition annealed SnO₂ films is calculated using Scherrer's formula. Figure 5 shows the variation of crystallite size as a function of film thickness. The (110) peak is chosen to calculate the crystallite size as it is the preferred orientation plane.

Variation of crystallite size of annealed SnO₂ thin film as a function of film thickness.

As the thickness of film increases from 30 nm to 90 nm, the crystallite size was observed to decrease from 8.2 nm to 4.8 nm and, afterward, a rise in its value was observed with increase in the thickness of the film (Figure 5). Similar variation of crystallite size with thickness (0.43–2.5 μm) of the SnO₂ film was reported by Salunkhe and Lokhande [30]; a minimum value of crystallite size (~8 nm) was reported at 1.5 μm thickness of the SnO₂ film. The effect of crystallite size of SnO₂ thin film on the gas-sensing characteristics was reported by Xu et al. [22]. It was reported that a smaller crystallite size is better for gas-sensing applications. Therefore, in the present case, 90-nm SnO₂ thin film having minimum crystallite size (4.8 nm) is expected to give enhanced sensing response characteristics.

Lattice constants (“a” and “c”) of the annealed SnO₂ thin films are calculated and are shown in Figure 6 as a function of SnO₂ film thickness. The lattice constants “a” and “c” are found to be almost constant with respect to the film thickness. The values are observed to be slightly higher than the corresponding bulk values [a = 4.738Å; and c = 3.187Å (Joint Committee on Powder Diffraction Standards {JCPDS} 41-1445)], which indicates that the SnO₂ lattice is in a state of compressive stress with an expanded unit cell. Higher values of lattice constants have been reported by other researchers also [31].

Variation of lattice constants “c” and “a” with the thickness of SnO₂ thin films.

In summary, the as-grown SnO₂ thin film is found to be amorphous in nature; however, the X-ray analysis confirms that it becomes polycrystalline after a postdeposition annealing treatment in atmospheric air at 300°C for 2 h. Orientation of the crystallites along the (110) plane is found to be the most preferred one, especially in the 90 nm SnO₂ thin film. Furthermore, the smallest crystallite size of around 4–5 nm was observed for the 90-nm-thick SnO₂ thin film and it is important for obtaining enhanced gas-sensing characteristics.

The surface morphology of the films was examined over an area of 2 × 2 μm² in the noncontact mode, and the atomic force microscopy (AFM) images of the as-grown and postdeposition annealed SnO₂ thin films are shown in Figure 7 [25]. A rough surface morphology with randomly distributed channels can be seen in the as-grown SnO₂ thin film, as confirmed from the AFM image (Figure 7A).

AFM images of 90 nm SnO₂ thin films: (A) as-deposited film; and (B) postdeposition annealing in air at 300°C. AFM, atomic force microscopy.

A rough surface having elongated channel-like microstructures can be observed from the AFM image (Figure 7B) of SnO₂ thin film that was annealed at 300°C for 2 h. The presence of elongated channel-like microstructures may enhance the gas adsorption on the surface of the sensing layer [25]. AFM images of the surfaces of postdeposition-annealed SnO₂ thin films of varying thicknesses (30–180 nm) are shown in Figure 8. The images were acquired over an area of 1 × 1 μm².

AFM images of SnO₂ films of varying thicknesses: (A) 30 nm; (B) 60 nm; (C) 90 nm; (D) 120 nm; (E) 150 nm; and (F) 180 nm. AFM, atomic force microscopy.

All SnO₂ thin films show a rough surface with nanosized grains. The size and shape of the grains are found to be highly dependent on thickness. For a 30 nm SnO₂ thin film, the grains are elongated and of larger size, but as the film thickness increases, the grain size is found to reduce, leading to the formation of spherical grains, for a 90-nm-thick film (Figure 8C). The surface roughness of 90-nm-thick SnO₂ film is increased, which is a desirable property. The smooth elongated structures start disappearing after increasing the thickness (>90 nm) of the film further from 90 nm and almost disappear for 180-nm-thick SnO₂ film. One can see a maximum roughness and surface-to-volume ratio for 90 nm SnO₂ thin film. The initial decrease in the crystallite size may be attributed to the presence of microstrain in the sample, which is introduced during the stretching and compression of the lattice, leading to the displacement of atoms with respect to their reference lattice place [32]. Shano et al. [32] also reported a similar trend. With further increase in the thickness of the film, the crystallization approaches greater perfection, which leads to a decrease in the internal microstrain in the films and an increase in the crystallite size [33].

3.2

Response of SnO₂ film for LPG sensor: effect of thickness

3.2.1

Sensing response

Figure 9 shows the sensing response for 200 ppm LPG as a function of temperature in the range of 60°C–260°C for SnO₂ thin films of different thicknesses (30–180 nm).

Response of SnO₂ thin film sensors of varying thicknesses (30–180 nm) to 200 ppm of LPG as a function of temperature. LPG, liquefied petroleum gas.

3.2.2

Sensing mechanism

For all SnO₂ thin film sensors, with increasing temperature, the sensing response is observed to increase initially until a maximum value is reached at a particular temperature, which is known as the operating temperature (T_opt) and then it starts decreasing. It may be noted from Figure 9 that with further increase in temperature beyond T_opt, the sensor response starts decreasing. The operating temperature of the SnO₂ sensor is noticeably different at different thickness values (Figure 9). It was observed that the sensing response increases with increasing thickness of the sensing layer from 30 nm to 90 nm. However, as the thickness of the SnO₂ film increases from 90 nm, the sensing response was observed to decrease. A maximum sensor response of S = 2.90 is noted at around 240°C operating temperature for the 90 nm SnO₂ thin sensing film. To investigate the dependence of the LPG response on the thickness of the SnO₂ film, the behavior of sensor resistance in the absence (R_a) and presence (R_g) of LPG has been analyzed.

Variation in the measured values of sensor resistances R_a and R_g of SnO₂ films of different thicknesses are shown, respectively, in Figures 10 and 11 as a function of temperature. The initial starting resistance (R_a) of the sensor measured at 60°C was found to decrease continuously from 9.0 × 10⁴ Ω to 4.2 × 10⁴ Ω as the thickness of SnO₂ film increases from 30 nm to 180 nm (Figure 10). Since the design and dimension of the Pt-IDEs are the same for all the prepared SnO₂ thin film sensors, the value of R_a is expected to fall with increase in the thickness of the sensing layer due to increase in the cross-sectional area between the finger electrodes of the IDEs. A continuous decrease in the value of R_a was observed with increase in the temperature from 60°C to 260°C and is expected due to the normal semiconducting nature of SnO₂ thin film (Figure 10).

Variation of sensor resistance in air (R_a) as a function of temperature for different thickness values of SnO₂ film.

Temperature dependence of sensor resistance (R_g) measured in the presence of 200 ppm LPG for different thickness values of the SnO₂ film. LPG, liquefied petroleum gas.

It is interesting to note from Figure 10 that temperature dependent behavior of R_a for SnO₂ thin film having large thickness (>60 nm) is slightly different especially at higher temperatures. The rate of decrease in R_a is reduced significantly and even R_a shows a slight increase with temperature at around 240°C (Figure 10). The observed change in conductivity is due to the trapping of electrons by the oxygen adsorbed on the SnO₂ surface. Electrons get trapped when O₂ molecules get adsorbed on the surface or grain boundaries of metal oxides, and they form a depletion or space–charge region, whose thickness is the length of band bending (ω). Electrons in the conduction band have to overcome a barrier (eV_s), where V_s is the potential at the surface of the film, associated with this electronic field, in order to move to the neighboring grain. The conductance is proportional to the density of electrons (n_s) having sufficient energy to cross the potential barrier. Further, n_s is given by the Boltzmann equation and is equal to N_d exp(−qV_s/kT), where N_d is the density of donors, q is the charge, k is the Boltzmann constant, and T is the temperature. The conductance will be proportional to n_s. The potential barrier rises as the amount of oxygen (O₂⁻) present on the surface increases, so fewer electrons participate in the conduction process, resulting in higher sensor resistance. Thus, the adsorption/desorption of oxygen on the surface of SnO₂ is the key parameter for change in conductance and thus accounts for the nonlinear response of the sensor. It is important to point out that chemisorption of oxygen is crucial for the gas-sensing mechanism. Thus, the following conversion scheme of adsorbed oxygen to charged O⁻ species has been proposed by various researchers [34]: (1) $O_{2} (gas) \Leftrightarrow O_{2} (ad) \Leftrightarrow O_{2}^{-} (ad) \Leftrightarrow O^{-} (ad)$ {O_2}\left( {gas} \right) \Leftrightarrow {O_2}\left( {ad} \right) \Leftrightarrow O_2^ - \left( {ad} \right) \Leftrightarrow {O^ - }\left( {ad} \right)

The adsorbed oxygen on the surface of SnO₂ at room temperature becomes molecular oxygen (O₂⁻) after capturing electrons. The molecular oxygen gets converted to atomic oxygen (O⁻ or O₂⁻) with increase in temperature. The oxygen ion (O⁻) is reported to be dominant at higher temperatures (>200°C) [35], which decreases the conductivity at higher temperatures. The competing effect of this decrease in conductivity due to the presence of chemisorbed oxygen and the increase in conductivity with temperature due to its semiconducting behavior lead to a nonlinear response. It is because of this nonlinear response that a maximum sensing response is observed at a certain temperature (T_opt), known as the operating temperature.

Variation in sensor resistance (R_g) as a function of temperature in the presence of 200 ppm LPG (Figure 11) shows a similar trend as that observed for R_a. The value of R_g shows a decrease with increasing temperature and increases with decrease in the thickness of the SnO₂ film (Figure 11). When an n-type semiconducting metal oxide is exposed to a reducing target gas, it is oxidized by O⁻ and releases trapped electrons to the base material (SnO₂). The thickness of the space–charge layer decreases due to either a decrease in the number of surface O⁻ or release of trapped charge carriers. The barrier between two grains at the surface is lowered, and electrons are able to move easily in the sensing layer through different grains. Therefore, modulation of the space–charge region in the presence and absence of reducing gas is the key for the sensing response characteristics of metal oxides [36, 37]. At the SnO₂ sensor surface or grain boundaries, the reducing gas or dissociated reducing ion is able to interact with adsorbed oxygen in one of the following ways: [38]. (2) $R + O_{2}^{-} \to {RO}_{2} + e^{-}$ {\rm{R}} + {\rm{O}}_2^ - \to {\rm{R}}{{\rm{O}}_2} + {{\rm{e}}^ - } (3) $R + O^{-} \to RO + e^{-}$ {\rm{R}} + {{\rm{O}}^ - } \to {\rm{RO}} + {{\rm{e}}^ - }

The trapped electrons are released when the reducing gas interacts with adsorbed oxygen ion, which increases the electron carrier concentration.

In summary, semiconducting metal oxides adsorb on the surface a large amount of oxygen from the atmosphere, which becomes O₂⁻ or O⁻ after capturing electrons from the bulk, and this results in high resistance (R_a). The concentration of O⁻ decreases on interaction with reducing gases, and the resistance (R_g) of oxide support starts decreasing. It may also be noted from Figure 11 that the value of R_g starts increasing with increase in temperature for thicker SnO₂ films (>60 nm) beyond a critical temperature, and the value of this critical temperature decreases from 240°C to 200°C with increase in the thickness of the thin film from 90 nm to 180 nm.

Specific values of R_a and R_g of all the sensor structures at room temperature and their respective operating temperatures are listed in Table 2. From Table 2, it is noted that the 30 nm SnO₂ thin film showed a higher value of starting resistance (R_a = 1.2 × 10⁵ Ω) in comparison to the values of SnO₂ thin films of other thickness values. The corresponding change (fall) in sensor resistance (R_g) in the presence of LPG was significant for all SnO₂ thin films at their respective operating temperatures.

Table 2

Resistance and sensor response variations of SnO₂ films as a function of thickness

SnO₂ thickness (nm)	R_a (Ω) (at room temperature)	R_g (Ω) (at operating Temperature)	Operating temperature (°C)	Sensor response
30	1.2 × 10⁵	3.6 × 10⁴	240	1.05
60	1.0 × 10⁵	2.7 × 10⁴	220	1.19
90	8.6 × 10⁴	1.3 × 10⁴	240	2.90
120	6.0 × 10⁴	1.4 × 10⁴	220	1.97
150	5.8 × 10⁴	1.2 × 10⁴	200	1.40
180	5.4 × 10⁴	1.1 × 10⁴	180	1.15

An increase in the thickness of SnO₂ film is found to be accompanied by a shift in the operating temperature (corresponding to the maximum response), which is observed to move toward lower values (Table 2), with an exception for the 90 nm SnO₂ thin film. The observed behavior of operating temperature with the thickness of the sensing layer is in agreement with the result reported by Korotcenkov et al. [21], where they analyzed the influence of thickness (22–170 nm) of SnO₂ thin films deposited by spray pyrolysis on the CO gas-sensing characteristics. Figure 12 shows the variation of sensor response obtained at the operating temperatures as a function of thickness of the SnO₂ thin film.

Sensor response of SnO₂ thin film sensor as a function of sensing layer thickness for detection of 200 ppm of LPG. LPG, liquefied petroleum gas.

It was observed that sensor response gradually increases with increasing thickness of SnO₂ sensing layer until it reaches a maximum response (S = 2.9) at 90 nm thickness. However, response starts decreasing from 2.90 to 1.15 as the thickness of film increases from 90 nm to 180 nm. A similar trend of response with larger thickness was reported by Salunkhe et al. [39] for CdO film-based gas sensor, where a maximum response (~34%) was observed at a thickness of 0.97 μm. The observed higher sensor response for the 90 nm SnO₂ thin film can be attributed to the small crystallite size, preferred crystallographic orientation of crystallites, and large porosity. It is clear that the thickness of sensing layer has a great impact on the sensing response. Five samples for each thickness were prepared under similar processing conditions in different batches and tested for LPG detection in the present study. The sensing results (response at respective operating temperature) of these sensor structures were found to be reproducible within an accuracy of ±2%.

Adsorption of the sensing gas is a surface phenomenon, and sensitivity of the sensor is expected to depend on the surface-to-volume ratio of the SnO₂ particle size. The amount of oxygen adsorbates per volume of the SnO₂ grain increases with decrease in crystallite grain. Thus, the sensing response in this case is strongly dependent on the size of the grain. Rothschild and Komem [40] have conducted simulation studies on SnO₂ grains having diameters ranging from 5 nm to 80 nm and reported a significant influence of grain size on the sensor performance. The model predicted a rise in the sensing response with decreasing grain size.

The depth of the space–charge layer (L) for a thin sputtered SnO₂ film has been reported to be about 3 nm [6]. The space–charge layer (L) is reported to be the same for sintered specimens [41] also. It is interesting to recognize that in the present study, the crystallite size for the 90 nm SnO₂ thin film was calculated to be 4 nm, which is <2L(~6 nm). The crystallite size of all other thin film samples is >2L. When the diameter (D) of the crystallite is <2L (~6 nm), then the whole crystallite is supposed to be depleted of electrons and the grains control the sensing response as they influence the conductance of the chain. Therefore, the 90 nm SnO₂ thin film having D<2L exhibits the maximum sensing response to 200 ppm LPG in comparison to other samples in the present study. Similar results were obtained in a previous study for sensing H₂ and CO gases using SnO₂ films [42], where the highest sensing response was observed when the diameter is comparable or <2L.

One of the main features of any gas sensor is how well it is able to reproduce and sustain its sensing response characteristics over a period of time. Looking at the promising results obtained with the 90 nm SnO₂ thin film, this behavioral aspect was investigated by subjecting the sensor to repeated exposures to 200 ppm LPG (fixed concentration) The transient response of the 90 nm SnO₂ thin film is given in Figure 13.

Transient response of the 90 nm SnO₂ thin film under repeated exposures to 200 ppm LPG. LPG, liquefied petroleum gas.

In summary, the orientation (110) of the crystallites constituting the gas-sensitive surface in the polycrystalline SnO₂ thin film, the presence of large porosity, and the high surface roughness were reported to be significant in influencing the sensing response [9]. The intensity of the (110) XRD peak was found to be the maximum in comparison to other XRD peaks for all the prepared SnO₂ thin films (Figure 4). It was observed that the relative intensity of XRD peak of (110) plane with respect to (101) peak, I(110)/I(101) is observed to be maximum for 90 nm thin SnO₂ among all other SnO₂ films of different thickness. Furthermore, a large surface-to-volume ratio is desirable because gas sensing is a surface phenomenon. Presence of porous morphology and high surface roughness were noted in the SnO₂ thin film having 90 nm thickness. The (110) plane is reported to be sensitive to the adsorption of oxygen from the atmosphere [34]. Since most of the crystallites in the 90 nm SnO₂ thin films are oriented along the preferred (110) direction, a large amount of oxygen gets adsorbed on the surface or grain boundaries. The free electrons from the bulk of the SnO₂ film are trapped by the adsorbed oxygen on the sensing surface, and the small size of the crystallite becomes fully depleted, thereby giving a relatively large value of R_a in comparison to the expected value based on the film dimensions only (Table 2). Due to the small grain size, large porosity, and the preferred orientation of the surface crystallites, the SnO₂ thin film with 90 nm thickness exhibits the maximum sensing response to 200 ppm LPG.

Conclusions

In the present study, the thickness of SnO₂ thin films was varied from 30 nm to 180 nm and the films were tested for application in LPG detection. The films were polycrystalline and possessed porous structure and rough channel-like microstructure after post-deposition annealing treatment in air. The structural and morphological properties of the SnO₂ thin films were highly dependent on the film thickness. A 90 nm SnO₂ thin film possessed the smallest crystallite size, with the majority of the crystallites oriented along the preferred (110) plane. AFM confirmed the rough microstructure and the maximum surface-to-volume ratio for the 90 nm SnO₂ thin film, which is desirable for an enhanced gas-sensing response. Hence, the 90 nm thin film exhibits the maximum sensing response to 200 ppm LPG.

Figures et tableaux

XRD of the as-grown SnO2 thin film (graph a) and the SnO2 thin film annealed in air at 300°C for 2 h (graph b). XRD, X-ray diffraction.

XRD pattern of the annealed SnO2 thin film of varying thicknesses. XRD, X-ray diffraction.

Influence of the thickness of the SnO2 thin film on the normalized intensities of various XRD peaks. XRD, X-ray diffraction.

Variation of crystallite size of annealed SnO2 thin film as a function of film thickness.

Variation of lattice constants “c” and “a” with the thickness of SnO2 thin films.

AFM images of 90 nm SnO2 thin films: (A) as-deposited film; and (B) postdeposition annealing in air at 300°C. AFM, atomic force microscopy.

AFM images of SnO2 films of varying thicknesses: (A) 30 nm; (B) 60 nm; (C) 90 nm; (D) 120 nm; (E) 150 nm; and (F) 180 nm. AFM, atomic force microscopy.

Response of SnO2 thin film sensors of varying thicknesses (30–180 nm) to 200 ppm of LPG as a function of temperature. LPG, liquefied petroleum gas.

Variation of sensor resistance in air (Ra) as a function of temperature for different thickness values of SnO2 film.

Temperature dependence of sensor resistance (Rg) measured in the presence of 200 ppm LPG for different thickness values of the SnO2 film. LPG, liquefied petroleum gas.

Sensor response of SnO2 thin film sensor as a function of sensing layer thickness for detection of 200 ppm of LPG. LPG, liquefied petroleum gas.

Transient response of the 90 nm SnO2 thin film under repeated exposures to 200 ppm LPG. LPG, liquefied petroleum gas.

Sputtering parameters used for deposition of SnO2 thin film.

Technique	RF diode sputtering
Target	Tin (99.99%)
Gas	50% Ar + 50% O₂
Sputtering pressure	14 mTorr
Power	150 W
Substrate-to-target distance	7.5 cm
Substrate temperature	25°C (no heating)
Rate of deposition:	60 nm/h

Resistance and sensor response variations of SnO2 films as a function of thickness

SnO₂ thickness (nm)	R_a (Ω) (at room temperature)	R_g (Ω) (at operating Temperature)	Operating temperature (°C)	Sensor response
30	1.2 × 10⁵	3.6 × 10⁴	240	1.05
60	1.0 × 10⁵	2.7 × 10⁴	220	1.19
90	8.6 × 10⁴	1.3 × 10⁴	240	2.90
120	6.0 × 10⁴	1.4 × 10⁴	220	1.97
150	5.8 × 10⁴	1.2 × 10⁴	200	1.40
180	5.4 × 10⁴	1.1 × 10⁴	180	1.15

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International Journal on Smart Sensing and Intelligent Systems

Study of structural and morphological properties of RF-sputtered SnO2 thin films and their effect on gas-sensing phenomenon

Ajay Kumar Arora,Ajay Kumar Arora,Sandeep Mahajan,Sandeep Mahajan,Maya Verma etMaya Verma etDivya HaridasDivya Haridas

Publié en ligne: 16 Feb 2023

Volume & Edition: Volume 16 (2023) - Edition 1 (January 2023)

Pages: -

Reçu: 14 Jun 2022

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International Journal on Smart Sensing and Intelligent Systems

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Sputtering parameters used for deposition of SnO2 thin film.

Resistance and sensor response variations of SnO2 films as a function of thickness

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