An investigation of the Nb doping effect on structural, morphological, electrical and optical properties of spray deposited F doped SnO₂ films

The structural properties of F and F + Nb doped samples have been investigated by XRD spectra. Typical XRD spectra of the films are shown in figure 1. All diffractograms show only characteristic SnO₂ peaks with cassiterite tetragonal structure and these peaks collaborate with those from JCPDS 41-1445. Other peaks belonging to SnO, Sn₂O₃, SnF₂ and metallic Nb are not observed in the deposited films. The observed d values are presented in table 1 and these values are compared with the standard ones from JCPDS card no. 41-1445. The (200) peak is the strongest peak observed for all samples. The (200) preferential orientations have also been observed for Nb doped films [21] and F doped films [22]. The films oriented along the (200) direction have better electronic transport properties for TCO applications [17]. Therefore, the growth of the films in this direction is very important. The other peaks observed are (110), (101), (211), (220), (310), (301) and (321).

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The addition of Nb does not affect the preferred (200) orientational growth of the FTO film. One significant difference is the intensity of this peak. Once FTO films are doped with Nb, the relative intensities of (211) and (301) show an increasing tendency and this indicates a slight reorientation effect. This effect has also been observed by Babar et al [23, 24] at spray deposited Sb doped SnO₂ films. The reflection intensities from each peak contain information related to the preferential or random growth of polycrystalline thin films, which is studied by calculating the texture coefficient TC_(hkl) for the planes using the equation [25]

$$\begin{equation} {\rm{TC}}_{(hkl)} = \frac{{I_{(hkl)} /I_0 }}{{\left( {\frac{1}{N}} \right)\mathop \sum /I_0 }}{\rm{\quad ,}} \end{equation} \tag{ 1 }$$

where I_(hkl) is the measured intensity of x-ray reflection, I_0(hkl) is the corresponding standard intensity from JCPDS data card no. 41-1445 and N is the number of reflections observed in the XRD pattern. The calculated TC values are presented in table 1. Figure 2 depicts the variation of the texture coefficient of 5 wt% FTO films with Nb doping for each peak. A sample which has randomly oriented crystallites presents TC_(hkl) = 1, while the larger this value, the larger the abundance of crystallites oriented at the (hkl) direction [26]. In the present study, the TC values of (200), (310) and (301) peaks for all the samples are larger than unity. The F doped film (FTO) has the highest TC value of (200), but when FTO films are first doped with 1 at.% Nb, the TC value of (301) peak is highest. When the Nb doping content is 2 at.%, the TC value of (200) increases and the value of (301) decreases. After this doping content, the TC value of (301) peak increases again and reaches the highest value. These results confirm reorientation with increasing Nb doping concentration for FTO films.

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The lattice constants a and c of the tetragonal structure were determined by the following relation [27]

$$\begin{equation} \frac{1}{{d^2 }} = \left( {\frac{{h^2 + k^2 }}{{a^2 }}} \right) + \left( {\frac{{l^2 }}{{c^2 }}} \right){\rm{,\quad \quad \quad \quad \quad \quad \quad \quad }} \end{equation} \tag{ 2 }$$

where d is the interplanar distance and h, k and l are Miller indices. The calculated lattice constants are given in table 1. The calculated a and c values are slightly more than JCPDS card no. 41-1445 (a = b = 4.7382 Å, c = 3.1871 Å). It is evident from table 1 that the lattice parameters are not affected much by the incorporation of the Nb dopants into the films.

The grain size is usually calculated for the most striking peak. However, in the present study, the grain sizes of the films are calculated from (200) and (301) peaks using the Scherrer formula [5]

$$\begin{equation} D = {\rm{\quad }}\frac{{0.9\lambda }}{{(\beta \,{\rm{cos}}\,{{\theta}})}}, \end{equation} \tag{ 3 }$$

where D is the grain size of nano-particles, β is the full-width at half of the peak maximum (FWHM) in radians and θ is Bragg's angle. The grain size values for (200) and (301) peaks are given in table 1. For (200) peak, when Nb doping content increases up to 3 at.%, the calculated D value of the FTO sample continuously increases from 28.81 to 32.19 nm. Then it decreases to 25.11 nm with further doping content. The grain size values of (301) peaks are found between 100.6 and 183.7 nm. These values increase from 100.6 to 183.7 nm with 2 at.% Nb doping and then its value decreases to 111.4 nm for 3 at.% Nb doping content. And finally, it again increases to the value of 157.8 nm.

The stresses, which are among the most prominent factors adversely affecting the structural properties, can result from geometric unconformity at interphase boundaries between crystalline lattices of films and the substrate [25], and these stresses may cause microstrains in the films. The microstrain (ε ) values of F and F + Nb doped SnO₂ films for (200) of the most striking peak are calculated by the relation [28]

$$\begin{equation} \varepsilon = \left( {\frac{1}{{{\rm{sin\,\theta }}}}} \right)\left[ {\left( {\frac{{\lambda _x }}{D}} \right) - (\beta \,{\rm{cos\,\theta }})} \right], \end{equation} \tag{ 4 }$$

where β is the FWHM of the preferential peak and D is the average grain size; the calculated values are given in table 1. It is observed that the microstrain first exhibits a decreasing tendency from the value of 12.00 × 10⁻⁴ to the value of 11.00 × 10⁻⁴ with 1 at.% Nb content and then remains constant for 2 and 3 at.% Nb doping content. However, finally, it increases to the value of 15.00 × 10⁻⁴ for 4 at.% Nb doping content. The microstrains cause crystal disorders and dislocations at the films. The dislocation density (δ) is defined as the length of dislocation lines per unit volume (lines m⁻²). The dislocation density (δ) of the films is estimated using the equation [29]

$$\begin{equation} \delta = 1/D^2 . \end{equation} \tag{ 5 }$$

The smallest values are calculated as 9.651 × 10¹⁴ lines m⁻² for the 3 at.% Nb doped FTO sample. The variation of dislocation density with Nb doping is the same as the variation of microstrain and we may conclude that microstrain is a major factor in the formation of dislocation densities.

The film morphology of FTO and NFTOs films was investigated by scanning electron microscope (SEM) micrographs in figures 3(a)–(e). It is observed that the surface morphology of the FTO film depends on Nb doping concentration. As seen from figure 3(a), FTO films are composed of dense small particles with a pyramidal shape, and have a smooth surface. This particle structure is similar to those obtained in other studies [17, 19, 20, 30]. Also, FTO films have well formed and densely packed crystallites and the distribution of these particles is homogeneous. When Nb is first inserted into the FTO structure, the bigger and polyhedron-like grains appear on the surface of films and the smaller and pyramidal grains appear in the spaces between the bigger polyhedron-like grains. For NFTO films, the distribution of the smaller pyramidal and the bigger polyhedron-like grains on the film surface is non-homogeneous. The size of bigger polyhedron-like grains first increases with 2 at.% Nb doping and then their sizes decrease for 3 at.% Nb doping. For 4 at.% Nb doping, the sizes of polyhedron-like grains again increase. The polyhedron structure has also been observed by Babar et al [23, 24] and Yao [31]. The particle sizes of both the smaller pyramidal and the bigger polyhedron grains observed from SEM images have the same variation as those (shown in figure 3(f)) calculated with Scherrer's formulation by XRD analysis. The SEM observations are well correlated with the different orientations of the grains as observed from XRD studies. For FTO films, the smaller and pyramidal grains indicate (200) orientation. The bigger and polyhedron-like grains give rise to high intensity (301) reflection. These results also suggest a slight reorientation effect with Nb doping.

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It is found, by the hot probe technique, that the all the samples have n-type conduction. The electrical properties of FTO and NFTOs films were investigated by the four-point probe method. The sheet resistance and resistivity values of the films are given in table 2 and figure 4. The sheet resistance and resistivity values of the films vary between 2.50 and 0.97 Ω and between 2.25 × 10⁻⁴ and 0.87 × 10⁻⁴ Ω cm, respectively. As seen in figure 4, there is a decrease in the sheet resistance and resistivity of FTO films with 1 at.% Nb doping. The sheet resistance and resistivity values of FTO films decrease from 2.04 to 0.97 Ω and from 1.84 × 10⁻⁴ to 0.87 × 10⁻⁴ Ω cm for 1 at.% Nb content and then these values continuously increase with increasing doping content. Namely, in the present study, it is found that the 1 at.% Nb doping ratio was a special doping level for the lowest sheet resistance and resistivity for FTO films. However, in the earlier studies, this special Nb doping level has been found to be 1.5 wt% Nb [32], 1.5 at.% [21], 1 at.% [33, 34], 4 wt% [8] and 2 at.% [35] for undoped SnO₂ films. The resistivity values of FTO films have the same order as the values obtained in some studies [1, 17–20] and the resistivity values of co-doped films are one order of magnitude smaller than the values of Nb doped SnO₂ deposited by spray pyrolysis [21]. The variation of the sheet resistance and resistivity of tin oxide with F + Nb doping can be explained on the basis of the presence of different valance states of Nb element (+3, +4, +5 oxidation states). At the low doping ratios, some of the Sn⁴⁺ ions in the lattice can be replaced by Nb⁵⁺, resulting in a decrease at the sheet resistance [8, 21, 31, 33, 36]. Such niobium acts as a donor. At high doping ratios, a part of the Nb⁵⁺ ions is reduced to Nb³⁺, resulting in the formation of acceptor states and a concomitant loss of carriers [35, 37, 38] and thus it results in increasing sheet resistance. Another reason for the increasing sheet resistance may that Nb + F atoms may not be placed in a proper lattice position with increasing doping ratios.

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The effect of Nb doping level on optical properties of F doped SnO₂ thin films have been investigated by a UV–VIS spectrometer at room temperature. Figure 5(a) shows optical transmittance curves as a function of the wavelength of FTO and NFTOs thin films. From the transmittance curves, it is seen that the transmittance of FTO films is strongly affected by the Nb doping concentration. It is observed that the transmittance of FTO films has a decreasing tendency with Nb doping concentration. Further, the absorption edge shifts toward the longer wavelength region (red shift) with increasing the doping concentration. The FTO and NFTO films have interference fringes, and with increasing the concentration of Nb dopant, the interference fringes start diminishing, which indicates an increase in the roughness of film surfaces (see figure 5(a)). The appearance of interference fringes indicates both the smooth reflecting surface of the films and minimum scattering loss at the surface and this is indirect proof of the homogeneous film deposition [16]. The transmittance value of FTO films at 550 nm is 89.52% and this value continuously decreases with Nb content inserted into FTO structure and reaches a minimum value of 69.73% for the NFTO-4 sample. Similar to this, the transmittance values of FTO films at 650, 750 and 850 nm are 90.28, 94.84 and 92.94%, respectively. These values generally have a decreasing tendency with increasing Nb doping content.

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The absorption coefficient (α) is determined by the equation [14]

$$\begin{equation} \alpha = {\rm ln}\left( {1/{{T}}} \right)/{{d}}, \end{equation} \tag{ 6 }$$

where T is the transmittance and d is the film thickness. The optical band gap of FTO and NFTO thin films is obtained by the following relation [16]:

$$\begin{equation} {\rm{\alpha }}h{\rm{\nu }} = A(h{\rm{\nu }} - E_{\rm{g}} )^{1/2} . \end{equation} \tag{ 7 }$$

According to the relationship equation (7), where hν and A are the photon energy and a constant, respectively. E_g values are determined by plotting (α hν )² versus hν and extrapolating the linear region of the plot to zero absorption (α hν )² = 0) . It is clearly seen in figure 5(b) that with increasing the Nb content in the SnO₂ structures, the band gap values decrease. The band gap values of FTO, NFTO-1, NFTO-2, NFTO-3 and NFTO-4 thin films are found to be 4.21, 4.15, 4.10, 4.02 and 3.93 eV, respectively. SnO₂ is one of the degenerate semiconductors [39] whose Fermi level lies within the conduction band [40]. Thus, the optical band gaps are related to the excitation of the electrons from the valance band to Fermi level [23, 24]. In such semiconductors, there is lifting of the Fermi level into the conduction band due to an increase in carrier density, which leads to energy band broadening (shifting) with some of the Sn⁴⁺ and O²⁻ ions in the lattice replaced by ions such as X⁵⁺ and Y⁻; this is called the Moss–Burstein effect [41]. Thus, according to electrical studies, it is expected from electrical properties that the energy band broadens with some of the Sn⁴⁺ ions in the lattice replaced by Nb⁵⁺ for 1 at.% Nb doping content. It is probably because the grains of layers are randomly grown, giving rise to a scattering effect, thereby reducing the transmittance because of crystal defects, impurities and lattice strain created by niobium and F doping [42–44].

The figure-of-merit is an important parameter for interpreting transparent conducting oxide thin films for their use in solar cells [45]. To compare the performance of various transparent conductors, the figure-of-merit that can be widely used is as defined by the Haacke formulation [23]

$$\begin{equation} \Phi = T^{10} /R_{{\rm s}} , \end{equation} \tag{ 8 }$$

where T is the transmittance at 550 nm and R_s is the sheet resistance. This formula gives more weight to the transparency and thus is better adapted to solar cell technology. It is clear that the figure-of-merit is dependent on the sheet resistance. The calculated values of figure-of-merit are given in table 2. The variation of the figure-of-merit of FTO films with Nb doping can be seen in figure 4. As soon as the Nb atoms are inserted into the FTO lattice, the figure-of-merit decreases from the value of 16.2 × 10⁻² Ω⁻¹ to the value of 11.6 × 10⁻² Ω⁻¹. The values of figure-of-merit continuously decrease to the minimum value of 1.09 × 10⁻² Ω⁻¹. It is found that FTO films have the highest of the obtained values in the present study.

The high infrared (IR) reflectivity is one of the main requirements for high-quality solar window materials. Further, the efficiency of flat plate collectors reduces considerably [46] because of the escape of thermal energy in the form of IR radiation. This problem is overcome by the use of high-IR-reflectivity transparent conducting oxides. The reflectivity (R) of the films can be calculated using the relation [47]

$$\begin{equation} {{R}} = (1 + 2{\rm{ }}\varepsilon _0 {{c}}_0 {{ R}}_{{\rm s}} )^{ - 2} , \end{equation} \tag{ 9 }$$

where ε₀c₀ = 1/376 Ω⁻¹. This relation is valid over a wide range in the IR region. The estimated values of R are in the range 97.39–98.98% and these values are very good for IR reflective coating.