The crystalline structures of PFTO and TFTO films are examined by XRD analysis. The SnO₂ tetragonal cassiterite structures (JCPDS 41-1445) are observed from XRD graphs given in Figs.  1 and 2 for PFTO and TFTO films, respectively. The secondary structure peaks, such as SnO, Sn₂O₃, SnF₂, and metallic Pr or Ta compounds have not been identified in the graphs. It could be concluded that doping elements are dispersed homogenously in SnO₂ lattice.^[14]

As seen from XRD patterns, FTO film has (200) preferential orientation and this orientation changes to (211) plane for high Pr and Ta doping level content. Preferred orientation is very important for determining crystal tendencies in XRD graphs. There are many different causes responsible for preferred orientations in a plane. Some of them are precursor solution chemistry, ^{[19, 20]} incorporation of Sn in the SnO₂ structure, ^{[21, 22]} spray precursor solution molarity, ^[23] and substrate temperature of sprayed SnO₂.^[24] In this work, it is shown that the chemical composition of solutions influences the preferential orientations of the films. These kinds of results have been observed by Li et al.^[25] for Pr doping and Lee et al.^[6] for Ta doping.

Fig.  1.

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	Fig.  1. XRD patterns of PFTO.

Fig.  2.

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	Fig.  2. XRD patterns of TFTO.

In PFTO films, peak intensities increase with Pr doping ratio increasing. The 1-at.% and 3-at.% Pr-doped FTO samples have higher intensities than 2-at.% and 4-at.% doped films. In TFTO films, peak intensities also increase with Ta doping level increasing and reach a maximum intensity for 2-at.% Ta doping then decrease with Ta content increasing. The peak intensity of each peak gives information about the growth of polycrystalline thin films. To obtain information about growth mechanism, the texture coefficient (TC) is calculated from the following relation:^[26]

where I_hkl, I_0(hkl), and N are the observed peak intensity, peak intensity in JCPDS card no: 41-1445, and the reflection number, respectively. The changes of TC with Pr and Ta doping ratio are tabulated in Table  1. If a crystal grain is randomly directed, its TC_(hkl) value is equal to one, but the value of TC_(hkl) higher than one indicates that the greater quantity of crystallites directed at the (hkl) orientation.^[26] In this study, for Pr-doped thin films, the values of TC for (210), (200), and (301) directions are all greater than one for all samples.

Table 1.

Table 1.

Table 1. TC values of Pr- and Ta-doped FTO samples for different planes. Samples (110) (101) (200) (210) (211) (220) (301) (112) (310) (202) (321) FTO 0.14 0.14 2.19 4.08 0.63 0.37 1.35 0.19 0.61 0.57 0.73 PFTO-1 0.15 0.10 2.12 4.91 0.45 0.37 0.99 0.20 0.62 0.46 0.64 PFTO-2 0.26 0.15 2.34 3.37 0.71 0.47 1.41 0.24 0.68 0.56 0.81 PFTO-3 0.29 0.19 2.16 3.04 0.90 0.48 1.61 0.22 0.55 0.63 0.94 PFTO-4 0.26 0.14 2.82 3.12 0.55 0.50 1.34 0.22 0.81 0.47 0.77 TFTO-1 0.28 0.27 1.60 3.39 0.90 0.44 1.40 0.30 0.57 0.81 1.05 TFTO-2 0.25 0.15 2.50 3.29 0.65 0.53 1.35 0.21 0.71 0.59 0.81 TFTO-3 0.34 0.19 2.65 2.98 0.64 0.59 1.27 0.21 0.81 0.46 0.86 TFTO-4 0.19 0.19 1.94 3.18 0.91 0.36 1.59 0.23 0.32 1.35 0.75

Table 1. TC values of Pr- and Ta-doped FTO samples for different planes.

FTO film has the highest TC value of (210) and this highest TC value does not change for all Pr-doped FTO films. As FTO is incorporated with 1-at.% Pr, the TC value of (210) peak reaches a maximum value and TC values of (200) and (301) start to decrease. For 2-at.% Pr doping level, TC value of (210) direction decreases and the values for other two peaks increase. After this doping level, TC values of (210) and (301) stay nearly constant but TC value of (200) peak reaches a maximum value for 4-at.% Pr doping level. Moreover, TC value of (321) peak increases up to unity for 3-at.% Pr doping content. All changes for TC values confirm the reorientations in the PFTO films.

TC values of (210), (200), and (301) peaks are higher than unity in all samples for Ta-doped FTO, too. The highest TC values of (210) of FTO film do not change for all Ta-doped FTO films. For FTO, as contribution level is 1-at.% Ta, TC values of (210) and (200) peaks decrease but TC value of (301) stays constant. When contribution level increases until 3-at.% Ta, TC of (210) continues to decrease. However, TC value of (200) continues to increase and TC value of (301) remains constant. For 4-at.% Ta doping, TC quantities of (210) and (301) increase and TC value of (200) decreases. Furthermore, TC value of (321) peak increases up to one with 1-at.% Ta doping content. These changes verify that all TFTO films have reorientations in their structures. This kind of result could be found in Refs.  [23] and [27].

Interplanar distance (d) values of the PFTO and TFTO films are calculated by Bragg’ s law:^[28]

In this equation, n is a constant generally equal to one, λ is the wavelength of x-ray, and θ is the diffraction angle. Interplanar distances given in Table  2 are nearly the same as the standard d values cited from JCPDS card No:41-1445.

The lattice constants of ‘ a = b’ and ‘ c’ of the tetragonal structure are identified by using the following formula:^[29]

where h, k, and l are Miller indices. The determined a and c values in Table  3 agree with those from JCPDS card No.: 41-1445 (a = b = 0.47382  nm, c = 0.31871  nm). Lattice parameters of doped films are higher than the standard lattice values for all doped films, but these parameters do not change with doping contents increasing. The increase in the lattice parameter of the film indicates that F, Pr, and Ta elements penetrate into tin oxide lattice and this case was also observed in earlier studies.^{[25, 26, 30]}

The average crystallite size is evaluated for the most striking peaks with Scherrer relation:^[31]

In Eq.  (4), D is the crystallite size of nanoparticles, β is the full width at half maximum (FWHM), and θ is the Bragg’ s angle. The values of D have also been given in Table  3 and these values generally increase with Pr and Ta doping level. According to Table  3, D values are 24.27  nm, 25.18  nm, 25.62  nm, 24.54  nm, and 48.90  nm for FTO, PFTO-1, PFTO-2, PFTO-3, and PFTO-4, respectively. For TFTO films, these values are 24.31  nm, 25.55  nm, 27.84  nm, and 28.81  nm for TFTO-1, TFTO-2, TFTO-3, and TFTO-4, respectively. The crystallite sizes of Pr- and Ta-doped films are all bigger than that of undoped FTO. The increasing of crystallite size can be attributed to the replacement of Sn⁴⁺ ions with Pr and Ta ions. Such replacements lead to the increases of the crystallite sizes in PFTO and TFTO thin films.^[17] The crystalline size variations demonstrate that the Pr and Ta ions have been doped successfully into SnO₂ lattice and Pr and Ta dopants have an effect on the increment in the crystalline size.^[25]

The geometric mismatchc between film and substrate can cause stress in film^[32] and it has a negative effect on structural feature of material. The dislocation and micro-strain (ɛ ) can result from stresses. The values of dislocation density (δ ) and micro-strain for sample are determined from the following expressions:^{[32, 33]}

Dislocation density value of PFTO film does not so change till 4-at.% Pr doping into FTO, and for 4-at.% Pr doping content, dislocation density value decreases sharply which can be attributed to the reorientation of stabler crystal structure.^[27] Dislocation density value of TFTO film decreases slightly with increasing Ta doping content. The explanation to this case can also be based on the reorientation of crystal structure. Micro-strain value of FTO film exhibits a decreasing tendency from the value of 10.37 × 10^{− 4} to the value of 9.83 × 10^{− 4} in the case of 2-at.% Pr content. Then it increases up to a value of 10.24 × 10 ^{− 4} for 3-at.% Pr, after it decreases sharply to 4.03 × 10 ^{− 4} with 4-at.% Pr dopant. For TFTO samples, micro-strain values continuously decrease from 10.36 × 10^{− 4} to 6.8 × 10^{− 4} with increasing Ta doping level. These reductions also attributed to the fact that Pr and Ta elements penetrate into SnO₂ lattice.^[25]

Table 2.

Table 2.

Table 2. Calculated and standard d values of Pr- and Ta-doped FTO samples for different planes. Samples (110) (101) (200) (210) (211) (220) (310) (112) (301) (202) (321) Standard d/Å 3.347 2.643 2.369 2.118 1.764 1.675 1.498 1.439 1.414 1.322 1.215 FTO 3.477 2.791 2.525 2.273 1.975 1.897 1.758 1.712 1.693 1.634 1.577 PFTO-1 3.475 2.789 2.526 2.272 1.973 1.897 1.757 1.712 1.696 1.636 1.577 PFTO-2 3.470 2.785 2.526 2.271 1.974 1.897 1.757 1.712 1.696 1.633 1.577 PFTO-3 3.470 2.788 2.523 2.294 1.974 1.897 1.757 1.714 1.696 1.636 1.577 PFTO-4 3.468 2.789 2.519 2.283 1.971 1.897 1.754 1.712 1.692 1.633 1.576 TFTO-1 3.485 2.791 2.531 2.294 1.974 1.896 1.756 1.714 1.696 1.636 1.578 TFTO-2 3.483 2.792 2.527 2.274 1.976 1.896 1.757 1.712 1.693 1.634 1.577 TFTO-3 3.473 2.788 2.527 2.286 1.976 1.899 1.757 1.712 1.697 1.633 1.578 TFTO-4 3.478 2.788 2.526 2.274 1.974 1.899 1.757 1.714 1.694 1.633 1.576

Table 2. Calculated and standard d values of Pr- and Ta-doped FTO samples for different planes.

Table 3.

Table 3.

Table 3. Some parameters of Pr- and Ta-doped FTO films for preferred orientations. Samples Preferred orientation θ /(° ) d/Å FWHM/(° ) D/nm δ /(1014 lines/m2) a* /Å c* /Å ɛ /10− 4 FTO (200) 37.74 2.525 0.414 24.27 16.98 5.050 3.349 10.37 PFTO-1 (200) 37.72 2.526 0.399 25.18 15.78 5.052 3.345 10.00 PFTO-2 (200) 37.72 2.526 0.392 25.62 15.23 5.052 3.338 9.83 PFTO-3 (200) 37.84 2.523 0.410 24.54 16.61 5.046 3.345 10.24 PFTO-4 (211) 51.50 1.971 0.261 48.90 4.18 5.038 3.349 4.03 TFTO-1 (200) 37.70 2.531 0.413 24.31 16.92 5.062 3.345 10.36 TFTO-2 (200) 37.70 2.527 0.393 25.55 15.32 5.054 3.350 9.86 TFTO-3 (211) 51.46 1.976 0.458 27, 84 12.90 5.054 3.343 7.08 TFTO-4 (211) 51.50 1.974 0.443 28.81 12.05 5.052 3.343 6.83

Table 3. Some parameters of Pr- and Ta-doped FTO films for preferred orientations.

The effects of Pr and Ta doping on morphological structure of spray deposited FTO thin film are investigated by AFM analysis.

The two-dimensional (2D) AFM images of Pr- and Ta-doped FTO films are given in Fig.  3. When AFM images are analyzed in terms of granule gratitude, it is clearly seen that the smallest granule size is in FTO film, whereas the highest granule size is observed from PFTO-1 film. However, it is the evidence that the surfaces of all the samples in the series are granulized and the morphological structures are homogenous. Ta-doped film has bigger grain sizes than FTO film. In the Ta-doped films, surfaces of all the samples are granulized and the morphological structures are also homogenous. Granule sizes increase nearly with increasing up to 2-at.% Ta doping level then decrease.

The three-dimensional (3D) AFM images of Pr- and Ta-doped films are shown in Fig.  4. In these films, the smallest granule sizes are observed also in the FTO film. In conclusion, it can be said that Pr and Ta elements have been incorporated into FTO structure.^[25]

The topographic characterizations for films are also made by SEM images. The SEM images (in Fig.  5 for Pr- and Ta-doped films) indicate that the films are composed of sub-micron particles. These particles are well-defined dense small needle-shaped and pyramidal-shaped. They nearly homogenously dispersed on glass substrate surface. A similar nanoparticle structure was also observed in Ref.  [34]. The pyramidal and dense small particles probably belong to (211) and (200) peaks.

Fig.  3.

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	Fig.  3. The 2D AFM images of Pr- and Ta-doped FTO thin films.

Fig.  4.

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	Fig.  4. The 3D AFM images of Pr- and Ta-doped FTO thin films.

Fig.  5.

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	Fig.  5. SEM images of Pr- and Ta-doped FTO films.

The electrical features of films are investigated with the four-point probe technique. The sheet resistance (R_s) values of the PFTO and TFTO films are shown in Table  4. The sheet resistance of FTO film is 2.51  Ω /□ and it initially decreases to the value of 1.63  Ω /□ in the case of 1-at.% Pr doping. The higher Pr contribution level causes the increase in sheet resistance. For Ta-doped FTO samples, R_s first decreases to the values of 1.70  Ω /□ and 1.27  Ω /□ at 1-at.% and 2-at.% Ta doping levels, respectively, then they increase to the values of 1.96  Ω /□ and 2.29  Ω /□ at 3-at.% and 4-at.% Ta doping levels respectively. The decreasing of sheet resistance can clarify that (i) Pr and Ta elements can exist in various oxidation states up to 5^{+ [25, 35]} and if their 5⁺ oxidation states are substituted by Sn⁴⁺ ions, there will be an increment in free carrier concentration and sheet resistance will decrease; (ii) Pr and Ta ions can be replaced at interstitial lattice positions and they give free electrons to SnO₂:F lattice. An increase in sheet resistance of FTO can result from substituting low valance states of Pr and Ta atoms with Sn⁴⁺ and structural imperfections.^[30] For Ta-doped SnO₂, Nakao et al. found that low Ta doping ratio induces the decrease in resistivity of tin oxide and then high Ta content leads to an increase in resistivity.^[36] In studies of Sb, ^{[37, 38]} F, ^[22] and doubly doping, ^{[32, 39]} it was also found that sheet resistance sharply decreases to nearly one.

Table 4.

Table 4.

Table 4. Electrical and optical values for samples. Samples Sheet resistance Rs/(Ω /□ ) Eg/eV Eu/meV σ st/10− 2 φ /(Ω /□ )− 1 (IR-R)/% FTO 2.51 3.92 586 4.41 3.67 × 10− 2 97.38 PFTO-1 1.63 3.88 765 3.38 1.60 × 10− 2 98.29 PFTO-2 2.44 3.69 1231 2.10 1.46 × 10− 3 97.45 PFTO-3 3.01 3.64 1286 2.01 1.08 × 10− 3 96.87 PFTO-4 3.66 3.57 1360 1.90 5.48 × 10− 4 96.22 TFTO-1 1.70 3.98 668 3.87 1.75 × 10− 2 98.22 TFTO-2 1.27 3.81 748 3.46 9.08 × 10− 3 98.66 TFTO-3 1.96 3.76 1320 1.96 1.07 × 10− 3 97.95 TFTO-4 2.29 3.35 1836 1.41 1.46 × 10− 5 97.61

Table 4. Electrical and optical values for samples.

The variations of optical transmittance with wavelength for various PFTO and TFTO films are shown in Figs.  6 and 7, respectively. PFTO films have 60%– 90% transmittance, and TFTO films have 40%– 90% transmittance on the visible region. The FTO film has the highest optical transmittance in all films, and all the optical transmittances decrease with Pr and Ta contribution content increasings.

Fig.  6.

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	Fig.  6. Variations of transmittance with wavelength of different PFTO films.

Fig.  7.

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	Fig.  7. Variations of transmittance with wavelength of different TFTO films.

Band gap values of PFTO and TFTO films can be found from transmittance values. The coefficient of absorption (α ) is found from the following equation:^[18]

Then (α hν )² versus hν curves are plotted by using the following equation:^[40]

In Eqs.  (7) and (8), t is the film thickness, A is a constant, and n = 1. The films prepared in the present study have direct allowed transitions. The curves of (α hν )² versus hν are plotted in Figs.  8 and 9 for PFTO and TFTO films, respectively. The band gap values for samples are also shown in Table  4. The band gap value of FTO film continuously decreases from 3.92  eV to 3.57  eV with increasing Pr incorporation ratio, however 1-at.% Ta doping gives rise to an increase in optical band gap value of FTO and then it causes continuous decrease in E_g value. The highest band gap value is observed to be 3.98  eV at 1-at.% Ta doping level, another values gradually decrease. This fluctuation could be based on an increase of carrier concentration in the beginning, but a decrease in carrier concentration is responsible for the dropping of band gap values.^[41]

This band gap increasing could be expressed as follows; SnO₂ is a degenerate semiconductor and its Fermi energy level is inside the conduction band.^[42] The band gap of SnO₂ is based on the excitation of the electrons from the valance band to Fermi level^[7] and an increase in carrier concentration causes Fermi level to increase and optical band gap to broaden. This is known as Moss– Burstein effect.^[43] According to the sheet resistance values, it would be expected that 1-at.% Pr and 2-at.% Ta doping cause E_g values of FTO films to increase. But, the re-orientation effect, which can be clearly seen from XRD analysis, could lead to a shift of E_g to lower energy and a red shift of the transmittance.^[23]

Fig.  8.

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	Fig.  8. Band gap energy graphs of PFTO films.

Fig.  9.

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	Fig.  9. Band gap energy graphs of TFTO films.

The localized states around E_g may cause band tails. The absorption edge is known to be the Urbach tail. The energy of Urbach (E_u) is based on possible defects. E_u is given in the following relation:^[34]

If logarithm of Eq.  (9) is performed, the following equation can be found:

In these equations, E₀ and α ₀ are constants. The values of E_u for samples are determined from the slopes of ln α versus hν graphs (Fig.  10) by using E_u^{− 1} = Δ (lnα )/Δ (hν ). The determined E_u values are given in Table  4. The E_u value of FTO film continuously increases from 586  meV to the values of 761  meV, 1231  meV, 1286  meV, and 1360  meV for PFTO-1, PFTO-2, PFTO-3, and PFTO-4, respectively. Similarly, for Ta-doped samples, they increase up to the values of 668  meV, 748  meV, 1320  meV, and 1836  meV for TFTO-1, TFTO-2 TFTO-3, TFTO-4, respectively. The E_u is based on structural defects and an increase in the number of defects causes an increase in E_u.^[44] But, for this study, there are discrepancies between structural defects and Urbach energy values. These discrepancies can be attributed to the preferred orientations of the films which are effective to determine dislocation densities and structural defects, and other orientations are also effective to dope an element into the structure, sometimes. The discrepancies of this kind could be seen in Refs.  [34] and [45]. The optical band gap values change reversely with E_u values. The broadening of Urbach tail causes the optical band gap of SnO₂ to decrease due to transitions from band to tail and from tail to tail.^[40]

Steepness parameter (σ _st) is temperature-dependent and it characterizes the widening of the absorption edge because of electron– phonon or exciton– phonon interactions.^[46] The steepness (σ _st) parameter of the film at T = 300  K is calculated from the following equation:

Here, K_B is the Boltzmann constant and T is temperature. The steepness value of the sample decreases with adding Pr and Ta dopant into FTO (Table  4). Absorption edge of the film narrows with increasing dopant ratio.^[47]

In order to evaluate the efficiency of SnO₂ for being utilized as a front contact material, the figure of merit parameter is obtained by Turgut et al.^[32] as follows:

The figure of merit values for samples are evaluated to be 550  nm. The determined values are given in Table  4. Figure of merit values of PFTO and TFTO films decrease with increasing doping ratios of Pr and Ta dopants.

Fig.  10.

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	Fig.  10. Plots of ln(α ) versus hν of PFTO and TFTO films.

The infrared (IR) reflectivity value of the film is given by relation:^[48]

where ɛ ₀c₀= 1/376  Ω ^{− 1}. None of the IR values for FTO, Pr-doped, and Ta-doped FTO films are so changed for different doping ratios (Table  4). All IR reflectivity values are so high for IR reflective coating materials which could be used for heating mirrors.