Comparative Study of Structural, Optical and Electrical Properties of SnO 2 Thin Film Growth via CBD, Drop-Cast and Dip-Coating Methods

Tin oxide (SnO 2 ) 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 SnO 2 thin films were investigated by using XRD, FTIR, SEM, EDX, UV-Vis spectroscopy, and Electrometer experiment. The XRD patterns of SnO 2 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 SnO 2 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 SnO 2 thin films. EDX measurement confirmed that the thin films are the composition of Tin (Sn) and Oxygen (O 2 ). The optical band gap of SnO 2 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 SnO 2 thin films are good conductors and pursued Ohm’s Law. These properties of the SnO 2 thin films brand are appropriate for application in solar cell assembly, gas sensor devices and transparent electrodes of panel displays.


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]. SnO 2 thin film is very effective because of its well structural, superconducting films, optical and electrical properties. SnO 2 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 SnO 2 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 SnO 2 -based films because SnO 2 thin films have a wide application in the modern engineering sectors. Mostly, the structural phases of SnO 2 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 SnO 2 thin film is promising useable for optoelectronics and LPG (Liquefied petroleum gas) linkage detection because SnO 2 has high oxygen absorption power [1] [2]. The SnO 2 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

Characterization Techniques
The structural properties of SnO 2 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 cm −1 . The SnO 2 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

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 SnO 2 thin films were deposited by CBD, Drop-Cast and Dip-Coating method. No.041-1445 [28]. The tetragonal phase has shown more thermodynamically stable than cubic phase of SnO 2 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 SnO 2 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].
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 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 SnO 2 thin films are estimated from the following equation: where n is an element which is equivalent to unity giving minimum dislocation

Fourier Transform Infrared (FTIR) Spectroscopy
The FTIR spectrum of SnO 2 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 SnO 2 lattice [34].   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.

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    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]. Table 3 Table 4.   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 SnO 2 thin films solar cell or transistor [31]. Figure 5 shows the absorption spectra of the SnO 2 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 (E g ) of thin films, at first the absorption coefficient (α) should be calculated using the following relation [40].

Optical Properties
where, t is the thickness of the film and T is the transmittance. ( ) g hv A hv E n α = − (6) where "α" is the absorption coefficient, "hν" is the energy of absorbed photons, "A" is a proportionality constant and "E g " 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 SnO 2 thin films which were prepared by CBD method, Drop-Cast method and Dipcoating 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 [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]. Figure 7 shows the conductivity vs. temperature curve of SnO 2 thin films. From 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].

Electrical Properties
For semiconductor, the conductivity increases with increasing the temperature [46]. The semiconducting behaviour of the SnO 2 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.