Spray Pyrolysis Deposition and Characterisation of Dielectric SnO 2 Thin Films

Dielectric and optical dispersion properties of thin films of SnO 2 deposited via spray pyrolysis were investigated. These properties are fundamental to new applications of SnO 2 in energy storage and pressure sensing. The composition and thickness of the films were determined using the Rutherford Backscattered Spectroscopic mode of the Pelletron Tandem Accelerator. X-ray diffraction (XRD) technique and scanning electron microscope (SEM) were used to examine the crystal structure and surface morphology of the films. Optical transmission data were analyzed to obtain the optical band gap, dispersion parameters, and dielectric constants. The analyses showed that the films were polycrystalline in nature with the tetragonal rutile crystal structure. It was also observed that annealed films increased in thickness compared to the as-deposited samples. The Urbach tail width of the annealed sample also decresed from 293 to 252 meV indicating an improvement in crystallinity with heat treatment. The refractive index dispersion in the visible region analyzed in terms of long wavelength single-oscillator Sellmier approximation was in the range 1.9 -3.0. The zero and high-frequency dielectric constants were evaluated. The values of these constants could be a justification for further exploration of SnO 2 -based materials for charge storage and capacitive pressure sensing.


Introduction
SnO2 is a wide bandgap semiconducting dielectric material. It belongs to a class of materials that combines high electrical conductivity with optical transparency and thus constitutes an important component for optoelectronic applications such as flat panel displays, solar cells, photodetectors, LEDs and transistors (Abdullah et al., 2012;Bancolo et al., 2012 andBatal et al., 2012). It is the most commonly used metal oxide gas sensing material (Brousseau et al., 1997;Batzill & Diebold, 2005;Borse & Garde, 2008;Caglar et al., 2007;. As a dielectric material it has been explored in fabricating capacitors/supercapacitors for energy storage, supplying burst of power to devices and components and for filtering (Edukondalu et al.,1992;Gordillo, et al., 1994;Galatsis, et al., 2003;Fasasi et al., 2009;Chen et al., 2012;Guan et al., 2014 andHe et al., 2014). There are other applications of SnO2-based dielectrics such as sensors like strain gauge and electrical insulation in wire and cable protection (Hu et al., 2009). In all these applications, it is obvious that the dispersion characteristics and dielectric constants are two important parameters to determine. For instance, the larger the dielectric constant the more energy the capacitor can store with other factors being favourable. In addition, the dielectric change will influence the features and efficiency of the devices, such as thermal loss, current leakage, refractive index, signal response speed, and light transmittance. Despite its wide areas of dielectric applications there are few studies on its dielectric and dispersion properties (Jarzebski & Morton, 1976;Korotcenkov et al., 2001;Lim et al., 2008;Li, et al., 2009;Ikhmayies &Ahmad-Bitar, 2013). Tin oxide films, nano-sized tin oxide particles and nanostructured tin oxides have been produced by a variety of synthesis techniques such as sol-gel processing , e-beam evaporation Wu, et al (2014), thermal co-evaporation, Melsheimer & Ziegler (1985), hydrothermal Natsume et al (1995), screen printing Park et al (2011), ion irradiation Shine et al (2011), gas phase condensation Perednis & Gauckler (2005), mechanochemical processing Pusawale et al (2011), and spray pyrolysis (Serin et al., 2006;Gedanken et al., 2008 andWang et al., 2014). The choice of a synthesis method depends on a number of important factors. These include but not limited to the cost of materials and procedures required, repeatability of properties, scalability for industrial production, continuous versus batch processing, and the number, complexity, and waste associated with the fabrication steps . In order to obtain uniform and quality films of the oxide, we used the chemical spray pyrolysis method. The spray pyrolysis process is a simple and low-cost technique for ceramic thin films deposition and nanometer-sized powder production . The process equipment is rather simple; the method is robust and if properly controlled yields oxide films of high quality at rather low costs. This process has the potential to produce thin layers on different substrates . Therefore, in this study, SnO2 films were deposited on glass substrates by spray pyrolysis method. The optical and dielectric parameters such as the static (zero frequency) and high-frequency dielectric constants and the refractive index dispersion were investigated. These are important parameters for device fabrication and simulation for various energy and optoelectronic applications based on SnO2.

Experimental Procedure
The starting solutions were prepared from tin II chloride dihydrate (purchased from Sigma Aldrich), ethanol, methanol and distilled water. The salt was of analytical grade and was used as purchased without any further purification. All the solutions were 0.1 M in concentration. Soda lime glass substrates were cleaned in dilute hydrochloric acid, alcohol and distilled water and dried in an oven before deposition at a substrate temperature of 320 ± 5 0 C and nozzle -to -substrate distance of 23 cm. Suction-based airbrush and air blast atomization were employed for the deposition. After the deposition, the samples were annealed in a tubular furnace at temperatures ranging from of 500 o C. The RBS experiment was performed using the 1.7 MeV Pelletron Tandem Accelerator at the Centre for Energy Research & Development, Obafemi Awolowo University, Ile-Ife, Nigeria. The resulting spectra was fitted using Windows SIMNRA software for the estimation of composition and thickness of the films. Crystal structures of the film was determined through X-ray diffraction studies carried out with an X-ray Diffractometer model Bruker AXS D8 Advance. The optical properties were studied using UV-Visible -NIR Spectranet Ultraviolet spectrometer model EP2000.

Composition and Thickness Determination by RBS
The RBS spectra of both th as-deposited and annealed samples are shown in Figure 1.The results of the analysis are presented in Table 1. From the analysis, it can be inferred that the annealed sets of samples of the same composition increased in  This corroborated the fact that SnO2 grains grow in size as a result of annealing .

Morphological Observation
The SEM micrographs of the films are shown in Figures 2. The particle structure is spherical in nature for both the annealed and as-deposited samples. It is also obvious that the surface of annealed films showed increased densification after annealing. Of particular importance to device applications is the porosity and the densification of the surfaces of the films. For instance the pressure -resistance relationship of a pressure transducer  Channel   460  450  440  430  420  410  400  390  380  370  360  350  340  330  320  310  300  290  280  270  260  250  240  230  220  210  200  190  180  170  160  150  140  130  120  110

Energy [keV]
could be attributed to decrease in thickness and accordingly the porosity of the sensing materials when pressure is applied. . In addition, the conductivity in metal oxides is considered to be through thermally assisted hopping transitions between spatially separated sites/particles. Hence according to Percolation theory, average conductivity depend on the concentration of sites/particles and the resistance of the path between sites. These two are affected by the porosity because with increase in pressure the concentration of sites increases and the resistance of the path decreases for a porous sample.  (110), (200) and (211) respectively. The dominant peak is the peak at 26.6 degree, which indicates the film is textured. The results were in agreement with other studies carried out by (Summitt, 1968;Brousseau et al., 1997 andSohn et al., 2009). The diffractograph is shown in Figure 3.  (Brousseau et al., 1997;Tiemann, 2007;Batal et al., 2012 andThirumala et al., 2014). Increase in the band gap energy is related to the improved degree of crystallization of the films with annealing (Tsay et al., 2015).

Refractive Index Dispersion
Using the reflectance data derived from transmission data of both the as-deposited and annealed films and employing the relationship between reflectance and refractive index presented in equation 4 (Thirumala et al., 2014). The variation in the refractive index as a function of the photon wavelength was obtained. The result is presented in Figure 6 where it can be seen that annealing has led to a decrease in the refractive indices of the films. The difference between the refractive indices of the annealed and the as-deposited samples is a manifestation of the structural changes that might have occurred in the sample during the process of annealing. The transmittance data of the films were also employed to derive their complex dielectric function. The real and the imaginary parts of the dielectric function ε' and ε" are related to ''n'' and ''k'' by equation 5 ε' = n 2 -k 2 and ε" = 2nk 5 Moreover, if the value of n is much greater than k, then ε' is approximately equal to n 2 and the dependence of ε'̍ on λ can be examined again using the relation in equation 6 (Wu, Li, & Sun, 2010).
where e is the electronic charge, c is the speed of light, Nc is the carrier density, m* is the effective mass of the carrier and ∞ is the high-frequency dielectric constant. From the plot of n 2 as a function of λ 2 , the intersection at λ 2 = 0 for the linear part of the curve at higher wavelength gives the high frequency dielectric constant ∞ as shown in Figure  7.

Figure 7:
Plot showing the determination of the high-frequency dielectric constant (εα) for the asdeposited and annealed SnO2

Oscillator strength determination
The single-term Sellmier long wavelength approximation relation given in equation 7 is used to evaluate the various oscillator parameters (Wu et al., 2010 andTsay &Liang, 2015).
Where λ o is the average oscillator parameter and So is the average oscillator strength. The plot of (n 2 (λ) − 1) -1 as a function of λ -2 given in figure 8 give a straight line where the values of 1/So and 1/S o λ o 2 were evaluated from the slope and the intercept respectively. The dispersion parameter (Eo/So) for the film was also evaluated using equation 8. The values obtained are listed in Table 2. The relationship between the refractive index and the photon energy is given by Wu et al (2010) and Tsay & Liang ( 2015)in equation 9. n 2 (hν) − 1 = where Eso is the single oscillator strength and Ed is the dissipation energy. By linearizing the expression and plotting (n 2 -1) -1 against (hv) 2 as shown in Figure 9 Figure 9: Plot to determine the single oscillator strength, dispersion energy, and zero frequency refractive index and the dielectric constant no and εo for as-deposited and annealed SnO2 using the relation given in equation 10, the zerofrequency refractive index (no) and dielectric constant ε o using the values of E d and E so were determined from the slope (E so /E d ) and the intercept (1/E so E d ). The results obtained from the analysis are presented in Table 2.

Tail width of localized states
The exponential tail appears because disordered and amorphous materials produced localized states extended in the bandgap. In the low photon energy, it is assumed that the spectral dependence of the absorption edge follows the empirical Urbach rule given in equation 11 (Yildirim et al., 2014), where α o is a constant, hν is the photon energy and ΔΕ denotes the width of the tail of localized states in the bandgap (Urbach energy). The plot of ln (α) against the photon energy hν given in Figure  10. The linear fit was established in the linear portions of the curves. The width of the Urbach tail ΔΕ is the inverse of the slope of the linear fit. The results are presented in the Table 3 Figure 10: Ubarch tail plot for the SnO2 Comparing these values with the values reported for SnO2 thin films in the literature by Tiemann (2007) for spray-deposited amorphous (530 -550 meV) and polycrystalline tin oxide (350 -460 meV) and for vacuum-evaporated SnO2 (483 -646 meV) by Zeng et al (2010) showed that our results are a bit lower, this could be attributed to the quality of the thin films. It is also noticeable from the table that the tail width of the annealed films is lower than that of their corresponding as-deposited film. This is most probably due to the smaller density of localized states because of a decrease in disorder with annealing since crystallization becomes better with temperature. .

Conclusions
SnO2 films were deposited on glass substrates by using facile and economical spray pyrolysis method. The properties of as-deposited and annealed films were investigated and compared. The RBS analysis showed the composition and thickness of the films. The SEM also confirmed the spherical nature of the oxide crystallites while the XRD confirmed the cassiterite and polycrystalline structure of the films. The optical band values increased with annealing from 3.86 to 3.88. We also studied the annealing effect on the tail width of the localized states in the band gap of the oxide. We observed that the Urbach tail width of the annealed films is lower than that of the as-deposited films. This is associated with a decrease in disorder as a result of annealing. We evaluated the refractive indices, zero-and high-frequency dielectric constant using the reflection data obtained from the transmission data. The high value of dielectric constant and its frequency dependence are indications of the possibility of using SnO2 in energy storage devices and as a capacitive pressure sensing material. However, as pointed out by Summit, (Korotcenkov, Brinzari, Schwank, DiBattista, & Vasiliev, 2001) this parameter could be well affected by structural modification. An important direction of investigation would be to study the effects of distortion of the centrosymmetric structure of this oxide on its dielectric and dispersion properties.