1. Introduction

SnO$_{2}$ is an n-type semiconductor material. Because of its good adsorptive properties and chemical stability, it can be deposited on glass, ceramics, oxides, and substrate materials of other types^{[1, 2]}. It has a high melting point and good transmission, and does not easily react with oxygen and water vapor in the air, so it has a high specific volume and good cycling performance. Gas sensors based on SnO$_{2}$ thin films are used to detect a variety of hazardous gases, combustible gases, industrial emissions, and pollution gases^{[1, 3]}. In addition, SnO$_{2}$ thin films are also used for film resistors, electric conversion films, heat reflective mirrors, semiconductor–insulator–semiconductor (SIS) heterojunction structures, and surface protection layers of glass. At present, the most common application of SnO$_{2}$ is as the anode material of solar cells^[1-5].

Undoped SnO$_{2}$ is a highly transparent, widely applicable material with n-type conductivity and a wide band gap energy ($E_{\rm g}$ $>$ 3.7 eV)^[6]. Thin films of SnO$_{2}$ have been fabricated using a variety of methods, including spray pyrolysis^[4], ultrasonic spray^[5], chemical vapour deposition^[7], activated reactive evaporation^[8], ion-beam assisted deposition, sputtering^[9], and sol–gel methods^[10]. Among these, we will focus in particular in this paper on the ultrasonic spray technique, which is suitable for large-scale production. It has several advantages in producing nanocrystalline thin films, such as a relatively homogeneous composition, simple deposition on glass substrates because of the low substrate temperatures involved, easy control of film thickness, and a fine and porous microstructure. It is possible to alter the mechanical, electrical, optical and magnetic properties of SnO$_{2}$ nanostructures.

In the experiment, transparent SnO$_{2}$ thin films were prepared using the ultrasonic spray technique on glass substrates. The films were grown at a substrate temperature of 350 ℃ and a pending time of 60 and 90 s. We studied the crystalline structure, conductivity and optical properties of the transparent SnO$_{2}$, :, F thin film.

2. Experimental procedure

2.1 Preparation of the spray solution

The spray solution was prepared by dissolving 0.2 M of Tin (Ⅱ) chloride dihydrate (SnCl$_{4}$, 2H$_{2}$O) in a solvent containing an equal volume of absolute ethanol solution (99.995%) purity. Then, a drop of NaOH solution was added as a stabilizer, after which a 5 wt% ammonium fluoride dehydrate (NH$_{4}$F, 2H$_{2}$O) molar ratio was added to the solution. The mixture was stirred at 60 ℃ for 180 min to yield a clear and transparent solution.

The substrate was R217102 glass of size 1 $\times$ 1 $\times$ 0.1 cm$^3$, and prior to pumping the substrate was cleaned with alcohol in an ultrasonic bath and blow-dried with dry nitrogen gas.

2.2 Deposition of thin films

The resulting solutions were sprayed onto the heated glass substrates using the ultrasonic nebulizer system (Sonics), which transforms the liquid to a steam formed of uniform and fine droplets of, 35 $\mu$m average diameter (according to the manufacturer). The deposition was performed at various times between 1 and 4 min, and the films were realized at a substrate temperature of 350 ℃ ^[11].

2.3 Characterization

The crystallographic and phase structures of the thin films were determined by X-ray diffraction (XRD, Bruker AXS-8D) with CuK$\alpha$ radiation ($\lambda$ $=$ 0.15406 nm) in the scanning range between 2$\theta$ $=$ 20$^\circ$ and 80$^\circ$. The optical properties of the deposited films were measured over the range 300–2500 nm using an ultraviolet–visible spectrophotometer (UV, Lambda 35), and the electrical conductivity of the films was measured in a coplanar structure obtained by the evaporation of four golden strips on the surface of the film. All the spectra were measured at room temperature (RT).

3. Results and discussion

The XRD patterns of the SnO$_{2}$, :, F thin films grown at 60 and 90 s are shown in Fig. 1. One can note the progressive emergence of the diffraction peak located at 31$^\circ$, underlining a strong preferential orientation growth perpendicular to the crystallographic plan (101) with an interplanar distance ($d$ $=$ 0.2833 nm), which corresponds to an SnO$_{2}$ wurtzite structure^[10]. The intensity of the peak is enhanced at 90 s. The result indicates that the growth time improves the SnO$_{2}$, :, F crystallinity, with more atoms moved to the favorable energy position in the SnO$_{2}$, :, F wurtzite structure^[12].

The average grain size ($G$ $=$ 31.82 nm) can be measured from the full width at half maximum (FWHM $=$ 0.259$^\circ$) value of the SnO$_{2}$, :, F (101) diffraction peak ($\theta$ $=$ 31.78$^\circ$) using Scherrer's formula^[13]:

$G=\frac{0.9\lambda }{\beta \cos \theta },$

(1)

where $G$, $\lambda$, $\beta$ and $\theta$ denote the grain size, X-ray wavelength, FWHM and Bragg angle of the (101) peak, respectively; these parameters are attained for SnO$_{2}$, :, F film grown at 90 s. Our results correspond with those of Benrabah et al.^[14].

The Bragg equation was used to calculate the interplanar distance ($d$ $=$ 0.2833 nm).

The transmittance, absorbance and reflectance of the SnO$_{2}$, :, F thin films were measured over the range 300–2500 nm for the film grown at 90 s, as shown in Fig. 2. As a general trend, the transmittance of the SnO$_{2}$, :, F thin films becomes high in the visible light region. For the longer wavelengths ($\lambda$ $>$ 400 nm), the thin film becomes transparent and no light is scattered or absorbed at the non-absorbing region (i.e. $R$ $+$ $T$ $=$ l). The inequality ($R$ $+$ $T$ < 1) at shorter wavelengths ($\lambda$ $>$ 400 nm), known as the absorbing region, is due to the existence of absorption. The transmittance is greater than 80% in the visible region. It is seen that the transmittance is limited only by the surface reflectance of about 18% in the visible region. As can be seen, $\lambda$ < 400 nm is the region of the absorption edge in the layers due to the transition between the valence band and the conduction band. At this region, the transmittance is decreased because of the onset of fundamental absorption.

SnO$_{2}$ is a semiconductor with a large direct band gap; the optical gap energy $E_{\rm g}$ of the film grown at 90 s (Table 1) was obtained from the transmission spectra using the following relations^[15]:

$A=\alpha d=-\ln T,$

(2)

$(Ah\nu )^2=C(h\nu -E_{\rm g})/A=\alpha d,$

(3)

where $A$ is the absorbance; $d$ is the film thickness equal to 0.513 $\mu$m; $T$ the transmittance spectra of the thin films; $\alpha$ the absorption coefficient values; $C$ a constant; $h\nu$ the photon energy; and $E_{\rm g}$ the energy band gap of the semiconductor. Figure 3 shows the typical variation of $(\alpha h\nu )^2$ as a function of photon energy ($h\nu )$, and is 4.05 eV as determined by extrapolation of the linear region to $(\alpha h\nu )^2$ $=$ 0^[12]. Moreover, we used the Urbach energy of the film grown at 90 s, which is related to the disorder in the film network, and expressed as^[13]:

$A=A_0 \exp \frac{h\nu }{E_{\rm u}},$

(4)

where $A_0$ is a constant, and $E_{\rm u}$ is the Urbach energy, as shown in Table 1.

Figure 4 shows the variation in the electrical conductivity $\sigma$ measured as a function of the sample temperature of SnO$_{2}$, :, F grown at 90 s. We found that the electrical conductivity increases with increasing temperature from 25 to 400 ℃ . The electrical conductivity curve of the SnO$_{2}$, :, F film shows a linear and a transition region^[16]. In the linear region, the electrical conductivity increases with the increase in temperature. Thus, the electrical conductivity of the film can be analyzed by the following relation^[11]:

$\sigma =\sigma _0 \exp \frac{E_{\rm a}}{KT},$

(5)

where $\sigma$ is the electrical resistivity, $\sigma _0$ the pre-exponential factor, $K$ the Boltzmann constant, $T$ the temperature of the sample, and $E_{\rm a}$ the activation energy for the conductivity calculated from the linear portion of Fig. 4.

Figure 5 shows the plot of ln$\sigma$ versus 1000/$T$ of the SnO$_{2}$, :, F thin film, which is used to extrapolate the activation energy. The obtained value is $E_{\rm a}$ $=$ 22.85 meV, and this activation energy value suggests that the activation of electrons is from the donor level to the conduction band. In n-type semiconducting nature, there are electron energy levels near the top of the band gap, so they can be easily excited into the conduction band. This shifts the effective Fermi level to a point about halfway between the donor levels and the conduction band. The measurement of the activation energy of the SnO$_{2}$, :, F thin films in these preparation conditions shows that the transparent conducting SnO$_{2}$, :, F exhibits an n-type semiconducting nature; where we apply the relation: $E_{\rm a}$ < $\frac{E_{\rm g}}{2}$^[11].

4. Conclusion

In summary, high-quality transparent SnO$_{2}$, :, F film was grown on a glass substrate at a temperature of 350 ℃ using the ultrasonic spray technique. The as-grown films exhibited a hexagonal wurtzite structure and (101) orientation. The $G$ $=$ 31.82 nm value of the grain size was attained from SnO$_{2}$, :, F film grown at 90 s. The films demonstrated an optical transparency of greater than 80% in the visible region and show conductivity with a height electrical conductivity of 166.61 $(\Omega \cdot$cm)$^{-1}$ at 25 ℃ . The optical band gap of the F-doped SnO$_{2}$ film was 4.05 eV, and the maximum activation energy value of the films at 350 ℃ was 22.85 meV, indicating that the films exhibit an n-type semiconducting nature.