JAST
2012 March;3(1):85-94.
Published online 2012 January 27.
doi:http://dx.doi.org/10.5355/JAST.2012.85
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| Copyright ⓒ 2010 Journal of Analytical Science & Technology |
| Sn-doped Zinc Oxide thin films for LPG sensors |
| R. K. Nath1,*, S. S. Nath2, Kumar Sunar1 |
1Department of Physics, Guru Charan College, Silchar-788004, Assam, India
2Department of Physics, Assam University, Silchar, Silchar-788018, Assam India |
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Corresponding Author:
R. K. Nath ,Email: nathraj2002@gmail.com |
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ABSTRACT |
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| Sn doped zinc oxide (ZnO:Sn) thin films have been prepared by chemical spray pyrolysis technique using Zn(CH3COO)2 as a precursor solution and SnCl4 as a doping solution respectively. The dopant concentration (Sn/Zn at%) is varied from 0 to 1.5 at%. The structural, morphological, optical and electrical properties of the films are explored and then tested for LPG sensing. The resistivity of the Sn-doped films decreases with the Sn doping up to 0.5at%, while at a higher doping concentration the disorder produced in the lattice causes an increase in resistivity of the films. Exposure of LPG decreases the resistance of undoped and doped films. The response of the film is measured for both ZnO and ZnO:Sn films at different operating temperature (275-400℃) and concentration (vol %) of LPG in air. It is observed that Sn-doped ZnO films are more sensitive to LPG than undoped ZnO film. In this work, maximum response (~88 %) is observed for 0.5at % ZnO:Sn film for 1 vol% of LPG in air at 300℃. Further all the films have shown faster response and recovery times at higher operating temperatures |
| Keywords: ZnO:Sn, thin film, spray pyrolysis, LPG sensor |
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Introduction |
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Studies of semiconductor oxides are growing very rapidly because of the successful production of these thin film materials as detecting devices. In the recent years, there has been great demand for ZnO films because they can be made to possess high electrical conductivity, high infrared reflectance and high visible transmittance. Low resistive zinc oxide films have been achieved by doping with different Group III elements like aluminium, boron, indium, gallium or with Group VI elements like fluorine [1-8]. The electrical resistivity change of semi conducting material upon exposure to reducing gases is used for various gas detection. The sensing capability is due to the surface properties involved in the reaction mechanism with the gas to be detected [9,10]. Zinc oxide (ZnO) thin films are attractive in the semiconductor fields for gas sensor applications because of its chemical sensitivity to volatile and other radical gases, its high chemical stability, suitability to doping, non toxicity, abundance in nature and low cost. Apart from the sensing applications, thin films of ZnO find many promising applications such as transparent electrodes, surface acoustic wave devices, heat mirrors etc.
Many researchers have studied doped and undoped ZnO thin films for the detection of toxic and inflammable gases. Mitra et al [11,12] have studied the LPG sensing properties of zinc oxide thin films sensitized with a Pd layer. They reported a response of 88 % for 1.6 vol% of LPG in air at 250 0C. Shinde et al [13] have examined the effect of molarity of precursor solution on the LPG sensing properties of spray deposited ZnO films and observed the maximum sensitivity of 43 % to 0.4 vol% of LPG in air at an operating temperature of 350 0K. Shamala et al [14] have examined the LPG sensing properties of undoped and Antimony-doped tin oxide films prepared by spray pyrolysis method. LPG sensors based on SnO2/Pd composite films deposited on Si/SiO2 substrates have been studied by Majumder et al. [15] They have reported a response of 72 % an operating temperature of ~250 0C to 3000 ppm of LPG in air. Senguttuvan et al [16] have investigated the LPG gas sensing properties of lead-doped tin oxide thick films and found the maximum response (~48 %) at a low temperature of 150 0C to 1000 ppm of LPG in air. Sahay et al have prepared ZnO:Al films by chemical spray pyrolysysis method and reported a maximum sensitivity of ~89 % to 1 vol% of LPG in air at 325 0C [17] and ~44 % to 500 ppm of methanol vapour in air at 275 0C [18] respectively.
Different deposition techniques have been widely used for the preparation of semiconducting thin films. However, a technique, which is the most reliable and economic, is the main goal. Among the most studied techniques are the chemical vapour deposition, radio frequency magnetron sputtering, sol-gel technique and spray pyrolysis technique. In this work, we have investigated structural, optical, morphological and electrical properties of Sn(Tin)-doped zinc oxide (ZnO:Sn) thin films prepared by spray pyrolysis technique and finally the sensing behavior of these films towards LPG is studied, as there is an increased demand to develope LPG sensor because, it is potentially hazardous due to high possibility of explosion accidents caused by leakage or by human error. |
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Experimental |
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Tin doped zinc oxide (ZnO:Sn) thin films are deposited on to the glass substrates, which are cleaned with freshly prepared chromic acid, detergent solution and distilled water. The schematic representation of the spray system is described elsewhere [18]. The deposition method involves the decomposition of a solution of 0.1 M concentration of high purity zinc acetate dehydrate (Merck, India) prepared in distilled water. The dopant concentration (Sn/Zn at%) is varied from 0 at% to 1.5 at%. The compound source of dopant is stannic chloride penta hydrated (SnCl4. 5H2O; Merck, India). The resulting solution is subsequently sprayed onto heated substrate at a constant temperature of (410 ± 10 0C), which is monitored by a chromel alumel thermocouple fitted close to the substrate with the help of a Motwane Digital Multimeter (Model: 454). The atomization of the solution into a spray of fine droplets is affected by the spray nozzle with the help of compressed air as carrier gas.
The thickness of the films is determined by the weight difference method using an electronic precision balance (Citizen, model: CY 204). Structural analysis of the films is carried out using a PANalytical X’Pert Pro X-ray diffractometer with CuKα radiation (λ = 1.5418 Ǻ) as an X-ray source at 40 kV and 30mA in the scanning angle (2θ) from 30 to 700 with a scan speed 0.020/s. The optical transmission spectra of the films are obtained in the UV/ VIS/ near IR region up to 1100 nm using Perkin Elmer UV-VIS spectrophotometer (Model: Lamda 35). For making ohmic contacts at both the ends of the film, high conducting silver paste is used and is dried at a temperature of 150 0C. The film is mounted on a home made two-probe assembly placed inside a silica tube, which is inserted co-axially inside a resistance-heated furnace. The electrical resistance of the film is measured before and after exposure to LPG using a Keithly System Electrometer (Model: 6514).
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Results and Discussion |
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Film formation
When aerosol droplets arrive close to the heated glass substrates, a pyrolytic process takes place and a highly adherent film is formed on the glass substrates. Possible reaction mechanism in ZnO film formation is as follows [19].
The films thus prepared are found to be almost clear and transparent in physical appearance. All the film thicknesses are found to be within the range 200–300 nm.
Structural analysis
Figure 1 shows the X-ray diffraction patterns of the typical Sn-doped ZnO films.
All the films are found to be polycrystalline in nature, possessing “hexagonal wurzite” structure as per ICDD reference pattern (01-070-8070). No phase corresponding to tin or other tin compound is observed in the XRD patterns, which indicates a low level incorporation of tin in ZnO film [20].
All the Sn-doped ZnO films show the most intense peak corresponding to (101) plane while the other planes corresponding to (100), (002), (102), (103), etc, are present with low relative intensities. The crystallite size of the films is calculated using Debye-Scherrer equation. The equation is written as
It is observed that the crystallite size of 0.5 at% Sn-doped ZnO film is less than that of undoped ZnO of similar thickness. This is due to the lesser ionic radius of Sn+4 (RSn+4 = 0.071 nm) which substitutes Zn+2 (RZn2+ = 0.074 nm), thereby decreasing the crystallite size. However, crystallite size does not vary systematically with Sn dopant concentration, which is attributed to the lattice disorder produced in the films at higher dopant concentrations due to the difference in their ionic radii [17].
Optical studies
Figure 2 shows the optical transmission spectra of zinc oxide films prepared at substrate temperature of 410 ± 10 0C for different Sn doping concentrations (0 to 1.5 at%). The measurements are made in the wavelength-scanning mode under the following
parametric conditions:
Incidence- Normal
Temperature- Room temperature
Reference –glass slide (substrate)
These spectra reveals that in all the doped films, the average transmission over the range 400- 1100 nm is more than 80 % with a sharp fall near the fundamental absorption, whereas fall in transmission is gradual in undoped ZnO film. The ripples observed in these spectra are due to the interference fringes arising from the substrate-film and film-air interfaces [19]. These fringes smoothen out as the doping concentration decreases.
The optical absorption spectra of the ZnO:Sn films with different Sn dopant concentration (0 to 1.5 at%) as a function of wavelength are shown in Figure 3. It is evident from the figure that the films grown under the same process parameter (Table 1) have low absorbance in the visible/near infrared region while the absorbance is high in the ultraviolet region. Further a steep rise in the absorbance near the absorption edge is observed for all the doped films that hint at a direct type transition.
The absorption coefficient (α) is calculated using Lambert law as follows [19].
where Io and I are the intensity of incident and transmitted light respectively, A is the optical absorbance and d the film thickness.
The absorption coefficient (α) was found to follow the relation----
where A is a constant and Eg is the optical band gap.
Plots of (αhv)2 versus photon energy (hv) in the absorption region near the fundamental absorption edge is shown in Figure 4. In this case n = 2 gives the best linear graph which indicates direct allowed transition in the film material.
The optical band gap, estimated by the extrapolation of the linear region of the graph to the photon energy axis is found to be increasing from 3.02 eV to 3.13 eV with Tin (Sn) dopant concentrations from 0 to 1.5 at%. The change in band gap can be attributed due to the Burstein-Moss band gap widening and band gap narrowing due to electron-electron and electron-impurity scattering [19].
Morphological studies
The TEM images of Sn-doped ZnO samples are displayed below.
The crystallite sizes of Sn-doped ZnO samples obtained from TEM are given in table 2.
The resistivity of ZnO thin films for different Tin doping concentrations (0 to 1.5 at%) as a function of temperature is represented in Figure 6. The rate of heating is 50 Kelvin per minute. It is observed in the figure that the resistivity of all the films increases initially at the lower temperature region. This increase in resisitivity is attributed to the chemisorption of oxygen on the film surface, causing a decrease in carrier concentration. This is consistent with the adsorption of oxygen on the surface of polycrystalline ZnO films as reported by other researchers [21]. Further the resistivity of the film is found to be decreased with a small amount (0.5 at%) of Sn dopant. This could be due to the replacement of Zn2+ by Sn4+ ions, which increases electron concentration, thereby decreasing the resistivity. With an increase in Sn dopant above 0.5 at%, the resistivity is found to be increasing significantly. This may be due to the formation of nonconductive tin oxide from the tin atoms and the achievement of equilibrium between the tin atoms contributing conduction electrons and those producing tin oxide. Further, at higher doping concentrations, the disorder produced in the lattice (due to the difference in ionic radii of Zn2+ and Sn4+) increases the efficiency of scattering mechanism such as phonon scattering and ionized impurity scattering which, in turn, causes an increase in resistivity.
As evident from the Figure 6, the resistivity of all the films show a slight increase at lower temperatures afterwards follow the four-region behaviour similar to that reported by other workers [18,19]. In region I, the decrease in resistivity is due to the thermal excitation of electrons into the conduction band. The sharp increase in resistivity in region II is attributed to vigorous oxygen adsorption on the film surface. In region III the resistivity is not much affected by the temperature change. This could be probably due to the equilibrium achieved between the two competing processes of thermal excitation of electrons and oxygen adsorption. Finally, the resistivity in region IV decreases again. This is attributed due to the dominant thermal excitation of electrons and the desorption of oxygen species.
LPG sensing properties
Figure 7 represents the sensing characteristics of the Sn-doped ZnO films as a function of operating temperature for three different concentrations, namely 0.5, 0.75 and 1 vol% of LPG in air. It is observed in the figure that compared to the undoped (0 at% Sn) ZnO film, the Sn dopant enhances the response of the films to LPG.
Among all the films, the 0.5 at% Sn-doped ZnO film shows the highest sensitivity (~88 %) at 300 0C to 1 vol% of LPG in air which corresponds to 50% LEL (lower explosive limit) of LPG in air. This is attributed to the fact that this film is of minimum crystallite (Table 2). As we know, the smaller the crystallite size, the larger the specific surface area, which results in greater oxygen adsorption and hence higher response [18].
The sensitivity of the films is determined using following equation, as LPG (Liquefied Petroleum Gas) possesses the properties of reducing gas.
where Ra is the resistance of the film in air and Rg is that upon exposure to LPG.
It is obvious from the figure that operating temperature plays a vital role in determining the sensitivity of the film. In fact, there exists an optimum operating temperature of a sensor to achieve the maximum response to a gas/vapour of interest, the temperature being dependent upon the kind of gas/vapour, i.e the mechanism of dissociation and
further chemisorption of a gas/vapour on the particular sensor surface [18,19]. Also the adsorption of atmospheric oxygen on the film surface depends upon the operating temperature. In this present work, there exists an optimum operating temperature (300 0C) at which the sensitivity is found to be maximum in all the cases. It may be due to the activation of LPG on the film surface being dominant in this temperature range.
At a low operating temperature of 275 0C, the response of the films to LPG is restricted by the speed of the chemical reaction because the LPG molecules do not have enough thermal energy to react with the surface adsorbed oxygen species. In fact, during adsorption of atmospheric oxygen on the film surface, a potential barrier to charge transport is developed. At higher operating temperatures the thermal energy obtained is high enough to overcome the potential barrier and thus the electron concentration increases significantly due to sensing reaction, which in turn leads to an increase in sensitivity of the films. At temperatures higher than 300 0C, the adsorbed oxygen species available at the sensing sites on the film surface are not enough to react with LPG molecules. This results a small change in resistance of the films at higher operating temperatures, which in turn leads to a decrease in sensitivity of the films.
Figure 8 shows the saturation sensitivity characteristics of 0.5 at% Sn-doped ZnO film as a function of LPG concentration in air at different operating temperatures. It is observed from the figure that the sensitivity increases with increasing LPG concentration in air. At higher concentration, the response is found to be increasing rapidly compared to that at lower concentration. This behaviour is attributed due to the fact that for a low concentration, there is a smaller surface coverage of LPG molecules on the film and hence the surface reaction proceeds slowly. On an increase in concentration, the surface reaction increases due to a larger surface coverage of gas molecules, resulting in a rapid increase in response [18].
The LPG sensing mechanism of the film can be explained as follows. At first, oxygen is chemisorbed on the film surface when it is heated in air. During the chemisorption, atmospheric oxygen forms ionic species such as O2ˉ, O2ˉ and Oˉ which have acquired electrons from the conduction band and which desorbs from the surface at 80 0C, 130 0C and 500 0C respectively [21]. So at the temperatures on which sensors studies have been carried out, only Oˉ species will react with LPG molecules. The reaction kinematics is as follows [21].
The reaction between LPG and ionic oxygen species can be explained as follows [18].
 ˉ
Here Cn H2n+2 represents CH4, C3H8, C4H10 etc. while Cn H2n : O represents partially oxidized intermediates on the film surface. During oxidation LPG liberates electrons in to the conduction band, thereby decreasing the resistance of the film upon exposure to liquefied petroleum gas (LPG).
The response and the recovery times are important parameters for designing sensors forthe desired applications. In this paper the response and recovery times are defined as the time required for a sensor to reach 90 % of its full response/recovery ability with respect to the baseline resistance when the gas is turned on or off. To study the reversibility of the transient response of the film, both the ends of the silica tube are opened and the film resistance is allowed to recover the initial value in air.
Figure 9 represents the transient response characteristics of 0.5 at% Sn-doped ZnO film for various concentrations (0.5, 0.75 & 1 vol%) in air at 300 0C and 400 0C. It is seen from the graph that the response time as well as recovery time increases due to an increase in LPG concentration. This is attributed to an increase in the surface reaction because of a larger surface coverage of LPG molecules on the film surface. Further it is observed that the films show faster recovery to LPG at higher operating temperatures. In this work the average response and recovery times are found to be 5 min and 3 min respectively. |
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Conclusion |
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| The Sn-doped zinc oxide thin films prepared by chemical spray pyrolysis technique have been studied for LPG sensors. The optical studies show an increase in band gap due to Sn doping, which is attributed to the Burstein-Moss (BM) effect. The resistivity of all the films shows typical four-region behaviour, viz (I) electron activation, (II) oxygen adsorption, (III) equilibrium and (IV) oxygen desorption. It is observed that compared to undoped ZnO film, Sn-doped ZnO films are highly sensitive to LPG and the magnitude of response can be varied either by changing the operating temperature or the LPG concentration. Among the all Sn-doped ZnO films studied in this work, the 0.5 at% Sn-doped ZnO film shows the maximum sensitivity (~ 88 %) at 300 0C to 1 vol% of LPG in air, whereas in case of undoped ZnO film, the sensitivity is found to be about 38 % at the same operating temperature and concentration of LPG in air. The average response and recovery times are found to be respectively 5 mins and 3 mins. |
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Acknowledgement |
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| The authors wish to acknowledge Dr. P. P. Sahay, Department of Physics, MNNIT, Allahabad, UP, India and Sanatan Unnyan Sangtha(SUS) for their assistance in this work. |
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FIGURES |
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Fig.1
XRD patterns of the typical Sn-doped ZnO thin films |
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Fig.2
Transmission spectra of undoped and Sn-doped ZnO thin films. |
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Fig.3
Absorbance spectra of undoped and Sn-doped ZnO thin films |
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Fig.4
Plots of (αhv)2 vs hv for different Sn-doped ZnO thin films. |
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Fig.5
TEM images of Sn-doped ZnO samples |
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Fig.6
Plot of resistivity of Sn- doped ZnO thin films as a function of temperature. The heating rate is 5 K min-1 |
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Fig.7
Sensing characteristics of the Sn-doped ZnO thin films as a function of operating temperature to different concentrations of LPG in air. |
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Fig.8
Sensing characteristics of 0.5at% Sn-doped ZnO as a function of LPG concentration in air at various operating temperatures. |
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Fig.9
Transient response characteristics of the 0.5 at% Sn-doped ZnO film to LPG at operating temperatures of 300 0C and 400 0C. |
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TABLES |
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Table.1
Process parameters for the deposition of the films |
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Table.2
Crystallite size of Sn-doped ZnO samples from TEM studies |
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