[13], texture [14], microstructure [15], and doping [16] have been published. E. Elangovan, et al. [11] reported SnO2:F thin films with a resistivity in the order of 2 × 104 Ωcm, starting from stannous chloride (SnCl2) and ammonium fluoride (NH4F). Also, the role of solution preparation in the deposition of tin oxide films by chemical spray technique has been raised [17]. Tin oxide thin films deposited by ultrasonic spray pyrolysis are usually reported starting from SnCl2 and ethanol. In our case due to limitations of equipment, methanol has been adopted as the main solvent, since this can be atomized easily.

In this work, the effect of both water content in the starting solutions and substrate temperature on the physical characteristics of SnO2 films, deposited by ultrasonic spray pyrolysis, starting from tin chloride and hydrofluoric acid, is reported.

2. Experimental

SnO2:F thin films were deposited by ultrasonic spray pyrolysis from a 0.2 M starting solution of tin chloride (SnCl4·5H2O), dissolved in a mix of water and methanol. The water content was varied in five different volumes, namely, 2.5, 5.0, 10.0, 20.0 and 30.0 ml for a total solution volume of 100 ml. Hydrofluoric acid at a fixed [F]/[Sn] ratio of 30 at % was used as F precursor. The selection of this ratio is based in previous experimental work, where we found that the optimum fluorine doping concentration oscillate between 20 and 40 at %.

SnO2:F samples were deposited on 2.5 cm × 5.0 cm clean glass substrates at five different substrate temperatures, namely, 375˚C, 400˚C, 425˚C, and 450˚C, at a fixed deposition time of 12 min. The cleaning process was as follows: 1) a five minutes ultrasonic bath in a trichloroethylene for degreasing the substrates; followed by 2) a five minutes bath in methyl alcohol; 3) a five minutes ultrasonic bath in acetone [CH3COCH3]; and finally, 4) a drying process by a jet of gas nitrogen [N2].

The electrical sheet resistance of all as-deposited samples was measured by the four point probe technique by using a Veeco equipment, with the appropriate geometric correction, p/ln2 = 4.53. The structure of the as-deposited films was characterized by means of X-ray diffraction in a Pan-Analytical XPert Pro system, by using the θ - 2θ technique, based on the Cu-Kα radiation (λ = 1.5405 Å). Scanning electron micrographs were obtained from a Jeol JSM 5400 LV microscope. Chemical composition of all the SnO2 films was determined by energy dispersive spectroscopy (EDS) with a detector MORAN (Quest) having a 136 eV resolution. The optical transmittance at normal incidence was measured with a double-beam UVVis Shimadzu spectrophotometer, in the UV-visible region (300 - 1000 nm) without glass substrate correction. The film thickness was estimated according to the Manifacier’s formula [18], and corroborated by direct measurements with a KLA Tencor P15 profilometer. The values estimated were around 600 nm.

3. Results and Discussion

3.1. Electrical Properties

Electrical resistivities for all as-grown SnO2:F films deposited were calculated from the mathematical product of the respective sheet resistances and thickness values. In Table 1 are listed the results obtained. From these, it can be observed that an increase in the water content leads to a decreasing in the electrical resistivity, reaching a minimum magnitude in samples deposited at 450˚C from a starting solution with. Further increase in the substrate temperature increases the electrical resistance of the films. These results show that both substrate temperature and water content influence the growth kinetic and consequently the physical properties of the films. This behavior is associated with the variation of the F incorporation into the SnO2 lattice as the solution conditions and deposition temperature are changed.

Table 1. Electrical resistivity values of SnO2:F films deposited at different substrate temperatures, from starting solutions with different ratios.

3.2. Structure

Figure 1 shows the X ray diffraction spectra of SnO2:F thin films deposited at 450˚C from starting solutions with different water contents, for 2θ values from 20˚ to 70˚. All samples were polycrystalline, and the peaks fit well with the different reflections of the SnO2 cubic rutile structure, ASTM standard card JCPDS No. 41-1445 [19]. All spectra show the (200) direction as preferred growth orientation, whereas the (110) and (310) directions increase with the water content in the starting solution. These results demonstrate that, as was expected, the preferred growth orientation is sensible to the water content in the starting solutions. Absence of the Sn3O4 phase was also confirmed, as substrate temperatures higher than 400˚C guarantee only SnO2 formation. Additionally, no fluorine phases were detected despite the fact that a high [F/Zn] = 30 at % ratio in starting solution, was added. This result confirms the highly volatile character of fluorine compounds during the growth process, and consequently its low efficiency of incorporation into the SnO2 lattice.

3.3. Morphology

Figures 2 and 3 show the variation in the surface morphology of SnO2:F thin films deposited at 450˚C as a function of the water content in the starting solution at different magnifications.

Figure 1. X-ray diffraction spectra of chemically sprayed SnO2:F thin films deposited at 450˚C, from starting solutions with different water content.

Figure 2. SEM micrographs at low magnification of SnO2:F thin films deposited at 450˚C, from starting solutions with different water content. (a); (b); (c); and (d).

Figure 3. SEM micrographs at high magnification of SnO2:F thin films deposited at 450˚C, from starting solutions with different water content. (a); (b); (c); and (d).

In Figure 2, at low magnification, it can be seen that SnO2 films deposited with the lowest water content (, Figure 2(a)), a few secondary grains in the order of 1.5 μm are formed on the compact and uniform surface of the film; in the case of films deposited from a solution with a higher water content (, Figure 2(b)), the number of secondary particles decreases both in number and size, whereas for the highest water content (, Figure 2(d)) no particles can be observed. From a higher zoom of the images shows that grains of films deposited from solutions with the lowest water content present irregular size and shape, then it can be considered that grain size is ranging from 100 to 200 nm (Figure 3(a)). The surface of films deposited with a higher water content () show an increase in the size, higher than 200 nm, and the geometry of the grains resembles a cylinder-shaped form (Figure 3(b)). The films deposited with a water content of 20/100 show a surface covered by triangular pyramids with irregular size, in the order of 150 nm (Figure 3(c)). Finally, the films deposited with the highest water content, 30/100, shows well-defined triangular pyramids with small dimensions compared with films deposited from a starting solution with a water content of 20/100 (Figure 3(d)).

3.4. Opical Properties

Figure 4 shows the optical transmittance spectrum of a SnO2:F thin film deposited from a starting solution containing the highest water content, 30/100, measured in the wavelength range from 300 to 1000 nm , taking the air as reference. All SnO2:F thin films were highly transparent in the visible range, with an optical transmittance in the order of 80% at the middle of the visible range (550 nm). The optical band-gap values, EG, were determined according to Roth and Webb procedure [20], for heavily doped semiconductors, and from the absorption coefficient spectra α(λ) computed from the BurguerLambert equation (I = I0exp(−αd)), where α(λ) is the absorption coefficient, and d the film thickness [21]. A typical band gap calculation is reported in the inset in Figure 4, corresponding to the SnO2:F sample deposited from the starting solution with the highest water content, namely,.

In general, EG values were high, around 3.9 eV, irespective of both water content in the starting solution and deposition temperature. This band gap increasing can be interpreted according to the Moss Burstein effect, which states that the increase in the free carrier concentrationdue to the high doping level, fills empty states belonging to conduction band, increasing the energy magnitude required for the valence band to conduction band transitions.

3.5. Chemical Composition

Compositional analysis of the elements present in the films was performed on selected samples. Figure 5 shows the EDS measurement of a SnO2:F thin film deposited at 450˚C, from a starting solution. Films are almost stoichiometric, with a Sn:O ratio of 1:2, found in all the films. However, the non-intentional incorporation of undesirable elements was also confirmed by EDS analysis, as Ca, Si, K, Mg were detected in all the samples.

Figure 4. Optical transmittance SnO2:F thin films deposited at 450˚C, from a starting solution.

Figure 5. EDS measurements of SnO2:F thin films deposited at 450˚C, from a starting solution.

These elements come from the glass substrate due to exo-diffusion of impurities during the growth process, and are responsible of the increase in the electrical resistivity of the films. It is worthy of mention that nitrogen element was also detected into the lattice, although in a small quantity as compared with alkaline elements. It is hard to assume that nitrogen can dope the SnO2 films effectively due to the fact that a low electrical resistivity was obtained. In this case a more careful study has to be done in order to figure out the specific sites where nitrogen is hosted. Fluorine was detected into the films and a variation ranging from 4 to 4.5 at % was measured. This value of fluorine atomic content into the SnO2 lattice is consistent with other reported [22], however, there still remain the question about specific sites where F is incorporated, as clustering of fluorine or grain boundary segregation can occur.

4. Conclusion

The characteristics of SnO2:F thin films deposited by ultrasonic spray pyrolysis (USP) as a function of both water content in starting solution and substrate temperature were studied. The minimum electrical resistivity and maximum optical transmittance in the UV-visible achieved for SnO2:F thin films were around 8 × 10−4 Ωcm and 75%, respectively. According to the results obtained, SnO2:F thin films deposited by USP can find application as transparent electrodes in organic thin film solar cells due to both the low resistivity measured and the high chemical stability presented.

5. Acknowledgements

The technical assistance of M. A. Luna-Arias, J. E. Romero-Ibarra, A. Tavira-Fuentes, and A. Palafox is thanked. This work was partially supported by CONACyT under contract Number 166601.

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NOTES

*Corresponding author.

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