Comparative Study of Structural, Optical and Electrical Properties of SnO2 Thin Film Growth via CBD, Drop-Cast and Dip-Coating Methods

Abstract

Tin oxide (SnO2) thin films were deposited on glass substrate by Chemical Bath Deposition (CBD), Drop-Cast and Dip-Coating method. The thin films were post-annealed at 500°C for 2 hours. The structural, optical, and electrical properties of the SnO2 thin films were investigated by using XRD, FTIR, SEM, EDX, UV-Vis spectroscopy, and Electrometer experiment. The XRD patterns of SnO2 thin films deposited on glass substrate by CBD method, Drop-Cast method and Dip-Coating method showed cubic, tetragonal and amorphous structures respectively. The FTIR spectrum exhibited the strong presence of SnO2 with the characteristic vibrational mode of Sn-O-Sn. The SEM analysis was observed that the surface morphology of the thin films toughly depends on the deposition methods of the SnO2 thin films. EDX measurement confirmed that the thin films are the composition of Tin (Sn) and Oxygen (O2). The optical band gap of SnO2 thin films deposited by CBD method, Drop-Cast method and Dip-Coating method is found to be 3.12 eV, 3.14 eV and 3.16 eV respectively. Thin films deposited by Dip-Coating method showed the highest band gap. The electrical results confirmed that the SnO2 thin films are good conductors and pursued Ohm’s Law. These properties of the SnO2 thin films brand are appropriate for application in solar cell assembly, gas sensor devices and transparent electrodes of panel displays.

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Rahman, M. , Ahmed, Z. , Islam, M. , Kamaruzzaman,  . , Khatun, M. , Gafur, M. , Bashar, M. and Alam, M. (2021) Comparative Study of Structural, Optical and Electrical Properties of SnO2 Thin Film Growth via CBD, Drop-Cast and Dip-Coating Methods. Materials Sciences and Applications, 12, 578-594. doi: 10.4236/msa.2021.1212038.

1. Introduction

The development of modern society has become dependent on the progress of science and technology, which is not possible without technological improvement in the field of nano thin films. Nowadays microstructural and microelectronics components’ demands rise in several sectors of science and technology which is significantly prolonged the arena of thin film research [1] [2]. Inorganic materials have been in focus because of their multifunctional advantages, for example, their solution-type processing, which allows deposition at room temperature and pressures. Inorganic thin films are used in different modern technological sectors such as coating interference filters, anti-reflection (A.R), solar cells, gas sensors, narrow band filters, diodes, biosensors, photoconductors, humidity sensors, IR detectors, temperature control of satellites, magnetic films, waveguide coatings, anticorrosive films, microelectronics devices, etc. [3] [4] [5] [6] [7]. SnO2 thin film is very effective because of its well structural, superconducting films, optical and electrical properties. SnO2 is one kind of n-type semiconductor that has been familiar to possess several outstanding physical properties such as high transmittance under visible range, high reflectivity for infrared light, high mechanical hardness, low electrical resistivity, wide band gap, excessive conductivity, great chemical stability, thermal stability and good environmental stability [8] [9]. The SnO2 thin films have various effective applications such as in optoelectronic devices, including the gas sensors, solar cells, film resistors, heat-reflective mirror, liquid crystal display (LCD), light detectors, transparent conducting electrodes, electric conversion films, far-infrared detectors, biosensors and high-efficiency solar cells [10] [11] [12] [13] [14]. Generally, researchers have shown more interest in SnO2-based films because SnO2 thin films have a wide application in the modern engineering sectors. Mostly, the structural phases of SnO2 are entirely different at various in constant of optics and subsist in the tetragonal or cubic phases, however, the atomic components are the same in materials. The phase is changed with producing method and annealing temperature. Overall discussion SnO2 thin film is promising useable for optoelectronics and LPG (Liquefied petroleum gas) linkage detection because SnO2 has high oxygen absorption power [1] [2]. The SnO2 thin films can be deposited by several types of techniques such as vacuum evaporation, RF magnetron sputtering, Chemical bath deposition, pulsed laser deposition, chemical vapor deposition, pulsed electron beam deposition, spray pyrolysis, Dip-Coating, spin coating, Drop-Cast method, sol-gel and so on [15] - [20]. Having a vast variety of accessible alternatives, Chemical bath deposition method has been one of the most widely used techniques due to the thin films depositing by chemical bath deposition method has shown outstanding mechanical and physicochemical properties. Therefore, Chemical Bath Deposition method has gained recognition as a significant technique for thin film deposition of absolutely new kinds of oxide films. This technique is beneficial because of its marvelous quality, adherent to the substrate and pinhole free to acquire the thin films [21] [22]. The thickness of the thin film can be easily controlled in a level of λ/4 of visible light at which the thin films can be appointed in optical instruments [23] [24] [25] [26]. Drop-Cast method is a very easy, fast and minimal waste system. Dip-Coating is much simpler, more economical and highly efficient. From the above review, it is considering that exploration of Structural, optical and electrical properties of SnO2 thin films depositing by CBD, Drop-Cast and Dip-Coating methods using different deposition parameter and comparison of the different properties of SnO2 thin films depositing by CBD, Drop-Cast and Dip-Coating methods which give us new aspects to find their suitable applications.

2. Materials and Methods

2.1. Preparation of Tin Oxide (SnO2) Thin Films

The SnO2 thin films were prepared by using SnCl2·2H2O (purity 98%), Sodium Hydroxide (NaOH) and Urea (CO(NH2)2) (purity 99%). The materials were obtained from Merck Germany. All of these raw materials ethanol, double distilled water were analytic reagent grade and were used without any purification. Impurities of the silica glass substrates may influence various properties of thin films. To remove the impurities, the glass substrates were dissolved in a solution where concentrated sulfuric acid (H2SO4) (purity 98%) was added to water. Then the substrates were rinsed with double distilled water. Again ethanol was used to clean the surface of the substrates properly. After that, the substrates were warmed-up in an oven at 80˚C for drying. These cleaned glass substrates were used to deposit thin films. In this research, the SnO2 thin films were deposited via 1) Chemical Bath Deposition method; 2) Drop-Cast method; 3) Dip-Coating method.

1) Chemical Bath Deposition method

The SnO2 thin film was prepared by CBD method using 200 ml 0.5M SnCl2·2H2O solution and 15 ml 1M CO(NH2)2 solutions. These two solutions were mixed together and stirring 30 minutes after that 200 ml of 1M NaOH was added and again stirring 30 minutes at room temperature and subsequently measured the pH value of this solution. The pH value of the solution was maintained at 12. Then the solution was sonicated for 10 minutes by Ultrasonic Cleaner. The cleaned glass substrate immersed in the solution at room temperature for 48 hours. After 48 hours of immersion, the SnO2 thin film was formed on the glass substrate. The film was washed with double distilled water, dehydrated at a vacuum drier at 50˚C for 1 hour and annealed at 500˚C for 2 hours.

2) Drop-Cast method

The SnO2 thin film was prepared by Drop-Cast method using 40 ml 1M SnCl2·2H2O solution and 3 ml 1M CO(NH2)2 solutions. These two solutions were mixed together and stirring 30 minutes and 1M NaOH was added drop wise to the solution until the pH value reached at 12, stirring the solution again for 30 minutes at 75˚C temperature. Then the cleaned glass substrate was kept on hot plate at 200˚C and 3 drops solution was given on the substrate and after some time the substrate was removed from the hot plate and rinsed in double distilled water for removing the unwanted parts. This manner was repeated for 30 times and afterward a homogeneous SnO2 thin film was formed on the substrate’s surface. The film was dried at room temperature for 30 minutes and annealed at 500˚C for 2 hours.

3) Dip Coating method

The SnO2 thin film was prepared by Dip coating method using 40 ml 1M SnCl2·2H2O solution and 3 ml 1M CO(NH2)2 solutions. These two solutions were mixed together and stirring 30 minutes and 1M NaOH was added drop wise to the solution until the pH value was reached at 12, stirring the solution again for 30 minutes at room temperature. Then the washed glass substrate was dipped into the solution by dip-coater, after some time the substrate withdrawal from the solution vertically at a constant speed, then it was dehydrating at room temperature and this manner was repeated for 30 times. After the removal of the substrate from the solution, a homogeneous film of SnO2 was formed on the substrate’s surface. The film was washed with double distilled water, dried at a vacuum drier at 50˚C for 1 hour and annealed at 500˚C for 2 hours.

In CBD method the glass substrate was kept into the solution for long time, when the concentration of SnCl2·2H2O was high, the particle was going down with time. For this reasonm the low concentration of SnCl2·2H2O was used for CBD method than other two methods. The different growth conditions of SnO2 thin films for different methods are summarizes in Table 1.

2.2. Characterization Techniques

The structural properties of SnO2 thin films were characterized by X-ray Diffractometer (D8 advance, Bruker, Germany). X-ray diffraction patterns were investigated from 20˚ to 70˚ with CuKα (λ = 1.5406 Å) and scanning speed was 0.02 degree/sec. Peak intensities were recorded corresponding to their 2θ degree values. The FTIR spectra were recorded using FT-IR/NIR spectrometer (Frontier, PerkinElmer, USA) in the transmission mode and the wavelength range was 400 - 4000 cm1. The SnO2 film was scraped from the glass slide and made pellet for this measurement. Scanning Electron Microscope (Model JSM-6490LA, Jeol, Japan) was used to investigate the surface morphology and chemical composition

Table 1. Different growth conditions for different SnO2 thin films.

of SnO2 thin films in secondary electron emission mode which applied voltage was 10 KV and distance was 8.5 mm. The optical properties of the samples were brought to pass with respect to plain glass substrate by using UV-Vis spectrophotometer (UV-1601, Shimadzu, Japan) in the range of 300 nm to 800 nm. The electrical properties were obtained by (6517B, Keithley, USA) Electrometer analysis system.

3. Results and Discussions

3.1. Structural Properties

The XRD diffraction technique was used to investigate the structural properties such as peak position, reflections plane, interplanar spacing, lattice parameter, crystallite size, dislocation density and strain. Figure 1 shows the XRD patterns of SnO2 thin films were deposited by CBD, Drop-Cast and Dip-Coating method. The film was deposited by CBD method is presented crystalline nature with cubic phase of tin oxide film. The strongest peak is at 2θ = 31.69˚ from diffraction pattern and other peaks are at 40.89˚ and 45.34˚ also observed reflections planes (hkl) corresponding to the peaks are (111), (210) and (211) respectively and the outcomes are almost similar to the reported data of JCPDS card No. 01-071-5329 [27].

The film was deposited by Drop-Cast method is presented crystalline nature with tetragonal phase of tin oxide. The strongest peak is at 2θ = 26.51˚ from diffraction pattern and other peaks are at 33.78˚ and 51.73˚ also observed reflections planes (hkl) corresponding to the peaks are (110), (101) and (211) respectively and the outcomes are almost similar to the reported data of JCPDS card No.041-1445 [28]. The tetragonal phase has shown more thermodynamically stable than cubic phase of SnO2 thin films [29]. The film was deposited by Dip-Coating method is presented amorphous nature that means there are no significant peaks at the XRD result of this film.

From the literature it is observed that the SnO2 crystals appeared at different annealing temperature, some are indicating above 550˚C [30], some are indicating at 500˚C [31] on the other hands some are indicating at 400˚C appeared some weak diffraction peaks and the peak intensity increases with temperature [32]. At this research, all the films are annealed at 500˚C for 2 hours due to observing all properties at the same condition. X-ray diffraction is a worthy method for obtaining the crystallite size of nano crystalline materials. The crystallite size (D) is measured by using the Scherrer formula [33].

D = K λ / β cos θ (1)

where D is the Crystallite size of nano-particles, K is a constant related to crystallite shape and normally taken as 0.9, λ is the wave length of X-ray (1.54056 Å), β is the full width at half maximum (FWHM) intensity of the peak in radian, θ is the Bragg’s diffraction angle. Average Crystallite sizes are given in Table 2. The strain (ε) of SnO2 thin films are calculated by the following formula:

Figure 1. XRD patterns of SnO2 thin films deposited by CBD, Drop-Cast and Dip-Coating method.

Table 2. Peak position, reflections plane, interplanar spacing (d), lattice parameter, average crystallite size, dislocation density and strain of SnO2 thin films.

ε = β cos θ / 4 (2)

where β is the full-width at half-maximum of the preferential peak in radian, the measured values of strain are shown in Table 2. It is detected that the strain reductions with the increase of crystallite size. The dislocation density (δ) is defined as the length of dislocation line per unit volume. The dislocation density (δ) of the SnO2 thin films are estimated from the following equation:

δ = n / D 2 (3)

where n is an element which is equivalent to unity giving minimum dislocation density, usually n is equal to 1 and D is the crystallite size. The lattice constant for SnO2 thin film is determined from the relation:

1 / d 2 = ( h 2 / a 2 + k 2 / b 2 + l 2 / c 2 ) (4)

where d is the spacing between the crystal planes and (hkl) is a miller index. For cubic structure a = b = c and for tetragonal a = b. The values of calculated lattice constants are marginally changed from standard for several orientations of the film. The XRD analyses for SnO2 thin films deposited by different methods are summarized in Table 2.

3.2. Fourier Transform Infrared (FTIR) Spectroscopy

The FTIR spectrum of SnO2 thin films are given in Figure 2. For CBD method, the potential peak at 540 cm–1 is assigned to the fundamental Sn-O-Sn stretching vibration band and growing SnO2 lattice [34]. The peak at 1016 cm–1 represents the stretching vibrations of Sn-OH band. The absorption peak at around 1635 cm–1 occurs due to the bending vibration of hydroxyl groups of molecular water. Furthermore, the broad peaks at 3453 cm–1 observed which are assigned to the O-H stretching vibrations. For Drop-Cast method, the potential peak at 590 cm–1 is assigned to the fundamental stretching vibrations for Sn-O-Sn [35]. The peak at around 1014 cm–1 related to the characteristic of the Sn-OH stretching vibration band. The absorption peak at around 1588 cm–1 represents the bending vibration of hydroxyl groups of molecular water. Besides, the broad peaks at 3441 cm–1 identified which are ensured the stretching vibrations of O-H band. For Dip-Coating method, the potential peak at 567 cm–1 is identified the fundamental stretching vibrations for Sn-O-Sn band and growing SnO2 lattice. The absorption

Figure 2. The FT-IR spectrum of SnO2 thin films prepared by CBD, Drop-Cast and Dip-Coating method.

peak at around 1644 cm–1 occurs due to the bending vibration of hydroxyl groups of molecular water. Furthermore, the broad peaks at 3334 cm–1 identified which are ensured the O-H stretching vibration band. Thus the FTIR spectrum confirmed the presence of Sn-O-Sn bonds in the films.

3.3. Surface Morphology

Figure 3 represents the surface morphology of SnO2 thin films were prepared by (a) CBD method; (b) Drop-Cast method; (c) Dip-Coating method on the glass substrate. These images indicate that the shape and morphology of SnO2 thin films changes with methods. The particle sizes of the different SnO2 thin films are measured using the ImageJ software from the SEM images. The average particle size for (a) CBD method; (b) Drop-Cast method; (c) Dip-Coating method are estimated ~1000 nm, ~500 nm and ~300 nm respectively. The particle sizes are little higher due to the high annealing temperature and layer by layer deposition method.

3.4. Energy Dispersive X-Ray Spectroscopy

Energy-dispersive X-ray is a popular systematic technique used to analyze elemental compositional investigation or chemical characterization of a sample. Figure 4 shows the EDX spectra and distribution and Table 3 shows the elements

Figure 3. SEM images of SnO2 thin films prepared by (a) CBD (25 KX), (b) Drop-Cast 10 KX) and (c) Dip-Coating (10 KX), method.

Figure 4. (a-c) shows the EDX spectra and distribution of elements for SnO2 thin films were prepared by (a) CBD, (b) Drop-Cast and (c) Dip-Coating method deposited on glass substrate.

Table 3. Shows the EDX spectra elements weight and atomic percentages for SnO2 thin films.

weight and atomic percentages of elements of SnO2 thin films. The quantification results in an atomic ratio of O/Sn close to 2.0 on the film surface in the case of CBD and Dip-Coating method and decreasing O/Sn atomic ratio with increasing sputtering time in the case of Drop-Cast method. It is well known that ion bombardment of SnO2 leads to preferential sputtering of oxygen [36]. Therefore, a freshly prepared film in Drop-Cast method, where a sputter cleaning of the surface can be omitted, is best suited for a reliable composition examination [37].

Figures 4(a)-(c) and Table 3 ensures the existence of Sn, Na, Cl, Si, and O in the SnO2 thin films deposited by (a) CBD method, (b) Drop-Cast method and (c) Dip-Coating method and the elements were same in every deposition method. From the energy dispersive x-ray microanalysis measurement the existence of tin (Sn) and oxygen (O) is confirmed, besides tin (Sn) and oxygen (O) the existence of a little amount of other elements are due to the other elements in solution and glass substrate. The energy dispersive x-ray microanalysis measurement confirms that the films are indeed SnO2 thin films.

3.5. Optical Properties

Figure 5 shows the optical absorbance spectra of the SnO2 thin films deposited by CBD, Drop-Cast and Dip-Coating method. It is observed that, the maximum absorbance happen in the ultra-violate region and then absorbance gradually decrease to visible region, reaches a maximum, and then absorbance gradually decreases up to for all the thin films. Above 400 nm, absorbance decreases slowly. All the SnO2 thin films which are prepared by different methods are highly transparent in the visible region. The absorbance increases with increasing thickness of all the method prepared thin films. The wavelengths (ʎmax) at maximum absorbance and maximum absorbance values corresponding to thin films of different thickness prepared by different methods are recorded in Table 4. The ʎmax does not shift much and absorbance increases with increasing the thickness of the films and scattering also increases with increasing the thickness of the films. The transmittance is high at visible region indicates that the films are relatively good homogeneous and formed at small nanometer-sized grains

Table 4. The wavelengths (ʎmax) at maximum absorbance, maximum absorbance values and optical direct band gap corresponding to different method prepared thin film of different thickness

Figure 5. UV-Vis Absorption spectra of SnO2 thin films prepared by CBD, Drop-Cast and Dip-Coating method.

which is opposite of absorbance [31]. The transmittance is high at visible region because of the reflectivity low and very low absorption. This happens due to the transition of electrons from valence band to conduction band with regard to optical interference effects [38] [39]. High transmittance at visible region is an important factors for the semiconducting material in a transparent device such as the SnO2 thin films solar cell or transistor [31].

Figure 5 shows the absorption spectra of the SnO2 thin films. The maximum absorbance of all films is at ultraviolet region and decrease to visible region. This decrease in the absorbance indicates that the existence of optical band gap in the materials. In order to find out the optical band gap (Eg) of thin films, at first the absorption coefficient (α) should be calculated using the following relation [40].

α = ( 1 / t ) ln ( 1 / T ) (5)

where, t is the thickness of the film and T is the transmittance. The absorption coefficient (α) decreases with increasing the value of transmittance. The optical band gaps (Eg) of SnO2 thin films are obtained using the Tauc relation [41].

α h v = A ( h v E g ) n (6)

where “α” is the absorption coefficient, “” is the energy of absorbed photons, “A” is a proportionality constant and “Eg” is the optical band gap, and “n” is an index in regard to the density of state curves for the energy band. This “n” is obtained by the nature of the optical Transition involved in the absorption process. Analyses of the data have been made using n = 1/2 for direct transition and n = 2 for indirect transition [42].

The band gap of a semiconductor is related to the fundamental optical absorption edge. The plot of (αhv)2 vs. Energy (eV) for (direct transition) of SnO2 thin films which were prepared by CBD method, Drop-Cast method and Dip-Coating method are shown in Figure 6 and the direct band gap energy have been obtained from the intercept on the energy axis after extrapolation of the straight

Figure 6. Plot of (αhν)2 vs. Energy (eV) for SnO2 thin films prepared by (a) CBD method, (b) Drop-Cast method and (c) Dip-Coating method for estimation of band gap energy.

line section of (αhv)2 vs. Energy (eV) curve [39]. The optical direct band gap of SnO2 thin films which were prepared by (a) CBD, (b) Drop-Cast and (c) Dip-Coating methods are 3.12 eV, 3.14 eV and 3.16 eV respectively. These values are very close to the reported values [43]. The band gap of amorphous thin film is higher than crystalline thin films [44] and for the crystalline thin films the band gap is increasing with decreasing the crystallite size as the similarity in XRD results and increasing strain of the films [34].

3.6. Electrical Properties

Figure 7 shows the conductivity vs. temperature curve of SnO2 thin films. From Figure 7, it is clear that the conductivity is increased with temperature. When the applied temperature was 10˚C the value of conductivity was around 2.2 × 10−4 S/m, 7.3 × 10−5 S/m and 2.6 × 10−5 S/m respectively for CBD method, Drop-Cast method and Dip-Coating method. After increasing temperature conductivity also increases due to adventitious ions and/or electrons, at 100˚C value of conductivity was around 5.4 × 10−4 S/m, 1.08 × 10−4 S/m and 6.7 × 10−5 S/m respectively for CBD method, Drop-Cast method and Dip-Coating method. Summaries the above it is confirmed that the conductivity is decreased from CBD method to Dip-Coating method because the surface morphology homogenously incorporates and crystallinity size higher in CBD method thin film [45]. For semiconductor, the conductivity increases with increasing the temperature [46].

Figure 7. Conductivity vs. temperature curve of SnO2 thin films prepared by CBD, Drop-Cast and Dip-Coating method.

4. Conclusion

SnO2 thin films were successfully deposited on a glass substrate via different deposition methods and their effects on structural, optical and electrical properties have been investigated. The XRD patterns of SnO2 thin films deposited by CBD, Drop-Cast and Dip-Coating methods are shown crystalline cubic phase, tetragonal phase and amorphous phase of tin oxide (SnO2) respectively. The FT-IR spectrum confirmed the presence of Sn-O-Sn bonds in the films. SEM images show the surface morphology. EDX measurement ensured the existence of Sn and O2 in the thin films. From the UV-visible spectroscopy measurements seen that the maximum absorbance of all films is at ultraviolet region and the absorbance decreases to the visible range. The optical direct band gap of SnO2 thin films prepared by CBD, Drop-Cast and Dip-Coating methods are 3.12 eV, 3.14 eV and 3.16 eV respectively. The band gap of the amorphous thin film is higher than crystallite thin films and for the crystalline thin films, the band gap is increasing with decreasing the crystallite size and increasing strain of the films. The semiconducting behaviour of the SnO2 thin films was confirmed from the electrical analysis. Finally, thin film prepared by CBD method shows better performance than the thin films prepared by Dip-Coating and Drop-Cast methods. The conductivity of thin film via CBD method is 5 times more than the thin films via Dip-Coating and Drop-Cast methods. So, its activation energy is more sensitive to active and it is useable as a gas sensor to identify the gas linkage.

Acknowledgements

Authors would like to thank Islamic University Kushtia for providing the financial support under the UGC allocation, and Bangladesh Council of Scientific and Industrial Research, Dhaka, Bangladesh for providing the instrumental supports to carry out this work.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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