1 Introduction

Transparent conducting oxide (TCO) films have gained a lot of interest owing to their various optoelectronic devices viz., gas sensors, solar cells, varistors, and flat-panel displays [1,2,3]. The TCO films have gained much interest specific to applications in solar cells. Among the TCO materials, Tin oxide makes the best choice because of the chemical inertness, mechanical strength, and heat-resistant nature of this material. The optical properties of any conducting material are governed by the energy bandgap between conduction and valence band, and this bandgap can be tweaked or modified by using various intrinsic or extrinsic dopants. These impurities may improve the free charge carriers, which is found to be important in adjusting optical transparency as well as electrical conductivity in the oxides. TCO material's electrical conductivity can be enhanced by decreasing the bandgap energy which is possible in two ways: either by (i) enhancing the number of free charge carriers via intrinsic imperfections, like oxygen vacancies, or (ii) extrinsic dopants, usually higher-valence metal cations [7]. Antimony is the material of choice for extrinsic doping of SnO2 owing to the non-toxic characteristic properties, it is also gaining versatility due to its cost-effectiveness in comparison to some commonly used dopants, such as indium (In) and fluorine (F) [4,5,6]. Sb:SnO2 thin film can be deposited using various approaches like spray pyrolysis, sol–gel coating, sputtering, and chemical vapor deposition [8,9,10,11,12,13].This study/work focuses on the experimental findings of structural, morphological, optical, and electrical parameters change in pristine SnO2 as well as Sb-doped SnO2 thin films. Simple and economical spray pyrolysis methods were employed to create thin films. In this research, antimony-doped tin dioxide (ATO) films were deposited on glass substrates at T = 550 °C through spray pyrolysis. Different weight percentages from 5% to 25% of Sb were doped in the SnO2.The effects of antimony doping on the structural, optical, and electrical properties of the thin films were investigated. Tin chloride (SnCl4_5H2O) and antimony chloride (SbCl3) were used as a host and a dopant precursor, respectively. X-ray diffraction (XRD) analysis indicated that the undoped SnO2 thin film exhibited a preferred (110) orientation. As the Sb doping concentration increased, a different preferred (200) orientation was observed. As the doping concentration increased, the average grain size initially increased and then decreased; electrical resistance initially decreased and subsequently increased; the carrier concentration and Hall mobility initially increased and then decreased. The optical bandgap of the thin films ranged from 3.92 to 3.98 eV. Hall effect and thermoelectric studies revealed that the films exhibited an n-type conductivity. The innovative part of this work is to bring change in optical and electrical properties of SnO2 by increasing the dopant concentration. We can make new materials with very good optical and electrical properties if we take into consideration materials like SnO2 with dopant concentration.

2 Experimental details

A chemical spray pyrolysis process was used to create Tin oxide (SnO2 and Sb:SnO2) and Sb-doped SnO2 thin films. Stannous chloride (0.2 M SnCl2⋅2H2O) was applied as a precursor which was dissolving in Methanol. Sb (antimony chloride, SbCl3) was dissolved in methanol separately and applied to SnCl2⋅2H2O precursor solution. The concentration of antimony was varied as 0.0, 5.0, 7.0, 15.0, and 25.0 wt% in the starting solution. The solution was kept on a magnetic stirrer to obtain the homogenous solution. Further, the glass substrates were rinsed with organic solvents like acetone, ethanol as well as deionized water in the ultrasonic bath before use. Fine droplets were formed from the spray liquid by passing it through a pyrolysis mechanism. Precursor’s droplets were poured out over the glass substrates. The substrate was kept at a distance of 25 cm from the spray nozzle. A substrate holder heated to a temperature of 450 °C was placed below the glass substrate. FTIR analysis was carried out using a PerkinElmer 2000 infrared spectrometer in the range of 500–2000 cm−1. Powder X-ray diffraction study was carried out using Bruker AXS-8D diffractometer with CuKα X-ray source to determine the crystal structure of SnO2 thin film. To examine the optical properties of deposited thin films, the optical transmission spectrometer of Horiba Jobin Yvon Spectrofluorometer (Fluoromax-4) was used. The morphology of the prepared samples were measured using SEM (JEOL JSM 6390LV). The sheet resistance of the films and the Hall effect data were determined using the van der Pauw configuration.

3 Results and discussions

3.1 Structural properties

3.1.1 XRD analysis

Figure 1 demonstrates the XRD analysis recorded for pristine SnO2 thin films and Sb-doped (Sb: 5, 7, 15, 25 wt%) deposited on glass substrates. It is usual for amorphous glass to have a diffractogram background of 10°–30° [14]. The undoped SnO2 thin film has a preferred to be (110) orientation. The (110) peak's intensity dropped in comparison to the (200) peaks as the quantity of Sb doping raised. Therefore, it can be concluded that the preferred orientation changes with Sb doping. Apart from (110) and (200) orientations, the existence of other peaks like (101), (211). (220), (310), and (301) were also noted with significant intensities for doped and undoped SnO2 films. The patterns of XRD do not show the Sb2O3 and Sb2O5 phases [10,11,12,13,14,15]. It has been found that the concentration of the preferred orientation increases with the rise in doping levels. Increases in the main (200) plane's intensity reveal that crystallinity rises as Sb doping levels rise [16], similar behavior/result was also reported by Elangovan et al. [16]. The tetragonal structure's lattice constants a and c were found using relation (1) [17]:

$$ \frac{1}{{d_{hkl}^{2} }} = \frac{{h^{2} + k^{2} }}{{a^{2} }} + \frac{{l^{2} }}{{c^{2} }} $$
(1)
Fig. 1
figure 1

XRD of Different % of Sb-doped SnO2 thin films

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Using Scherrer's formula (2) [18], the films crystallite size D was determined from the XRD data. Here, β indicates the whole width at half-maximum of the high-intensity peak, λ signifies the X-rays wavelength used, k is the dimensionless shape factor whose value is close to unity, and θ indicates the Braggs’ angle in radians.

$$D_{{({\text{nm}})}} \; = \;\frac{k\lambda }{{\beta \cos \theta }}$$
(2)

Table 1 presents the primary crystalline calculated parameters of the Sb:SnO2 thin films. Figure 2 clearly indicates that the sample undoped presented a much higher crystallite size value of 63 nm in each principal plane (110), (101), and (200) evaluated to the samples Sb-doped SnO2 thin films. The computed D value for undoped film decreases from 58.04 to 26.3 nm with 5 wt% Sb doping and then increases to 49.63 nm with 15 wt% Sb doping. The observed decrease in average crystallite size may be attributed to enhancing (200) orientation [19]. The grain size decreases again to 39.16 nm in doping level 25 wt%. Using Eq. (3) [20], the “texture coefficient” TC(hkl) for the planes is calculated, which provides information on the random or preferential development of polycrystalline thin films.

$$ T_{{{\text{c}}(hkl)}} \; = \;\frac{{{{I_{{\left( {hkl} \right)}} } \mathord{\left/ {\vphantom {{I_{{\left( {hkl} \right)}} } {I_{{0\left( {hkl} \right)}} }}} \right. \kern-\nulldelimiterspace} {I_{{0\left( {hkl} \right)}} }}}}{{\frac{1}{M}\sum {{{I_{{\left( {hkl} \right)}} } \mathord{\left/ {\vphantom {{I_{{\left( {hkl} \right)}} } {I_{{0(hkl)}} }}} \right. \kern-\nulldelimiterspace} {I_{{0(hkl)}} }}} }} $$
(3)
Table 1 Primary crystalline calculated parameters of the Sb:SnO2 thin films
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Fig. 2
figure 2

Change in size of the nanocrystals with different Sb-doped concentrations

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Here, I0(hkl) & I(hkl) represent the measured X-ray reflection intensity and associated standard intensity using JCPDS data card Number 41-1445, respectively. The number of reflections seen in the pattern of the XRD is denoted by M. The computed values of TC (hkl) are represented in Table 1. The films' polycrystalline composition is confirmed by the peaks that are less than unity. Nonetheless, the undoped film represented a possible oriented growth along the (110) plane direction. With an increase in the doping concentration, there occurs further growth-oriented along the (200) plane direction, as shown in (Fig. 3).

Fig. 3
figure 3

Variation of TC (110), TC (200), and TC (211) with different % of Sb-doped SnO2 thin films

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The microstrain (ε) produced in the SnO2 thin films was computed through Eq. (4): [17] Here, β indicates the whole width at half-maximum of the (101) peak and D signifies the crystallite size. The values of ε are shown in Table 1.

$$ \varepsilon = \left( {\frac{1}{\sin \theta }} \right)\left[ {\left( {\frac{\lambda }{D}} \right) - \left( {\beta \cos \theta } \right)} \right]. $$
(4)

3.1.2 FTIR spectroscopic analysis

The “Fourier Transform Infrared” (FTIR) spectrum was recorded to monitor/detect any change in the functional groups from pristine SnO2 to Sb-doped SnO2 thin films. Figure 4 shows the FTIR spectrum of the SnO2 thin films as a dopant function (Sb) content (0.0, 5.0, 7.0, 15.0, and 25.0 wt%) in transmission mode. The physisorbed water molecules’ hydroxyl groups are linked to (OH-) groups on the SnO2 surface by weak hydrogen, as shown by the absorption peak at 3430 cm−1 [21, 22]. The bands found between 1080 and 970 cm−1 are caused by vibration of surface cation–oxygen bonds Sn=O as well as Sn–O, [23]. The weak adsorption at 1635 cm−1 is correlated with the deformation vibration for physisorbed water's H–O–H bonds. The usual M–O–M (in this example, Sn–O–Sn) vibrations are seen in the 530–770 cm−1 range [24].

Fig. 4
figure 4

FTIR spectra of Sb:SnO2 films with varied Sb concentrations

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3.2 Microstructure properties

3.2.1 Scanning electron microscopy (SEM) analysis

SEM descriptions of Sb:SnO2 thin film formed on a glass substrate with different Sb doping concentrations were taken (Fig. 5). Sb doping concentration has a significant impact on the film's surface morphology. As can be seen, the films are crack-free, continuous, and uniform with continuous grain distribution. It indicates that the sample has grains of various sizes [25, 26]. The presence of large grains suggests that the crystallites are designed using coalescence. The grains are randomly distributed, resulting in a dispersion effect that lowers transmittance. The grain size is reduced with rising the doping ratio which is in keeping with the XRD study that indicates the reduction of the crystallite size in the doped film. Figure 6 depicts the cross-sectional view of the SnO2 thin film having 7% by weight Sb concentration.

Fig. 5
figure 5

SEM images of the SnO2 thin film with Sb concentration of a non-doped, b 5 wt%, c 7 wt%, d 15 wt%, e 25 wt%, f Variation of grain size with different Sb concentrations

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Fig. 6
figure 6

Cross-sectional view of the SnO2 thin film with Sb concentration of 7 wt%

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3.3 Optical properties

3.3.1 UV–vis analysis

Figure 7 indicates the change of transmittance concerning the wavelength of SnO2 thin films formed at different substrate temperatures. The visible region's average transmission ranges from 60 to 86%, based on concentration. A rise in the transmission is detected with the rise of Sb content. In the visible portion of the spectra, transmission is often fairly high (high enough to examine interference fringes). Low reflectivity and no (or little) absorption owing to optical interference effects allow for maximum thin-film transmission in a certain wavelength range [21]. A considerably greater transmittance of approximately 87% at 700 nm for both films 5 and 7 wt% Sb:SnO2 has been observed, which is vital for the applications of the Sb:SnO2 films as transparent conducting oxide (TCO). Figure 8 shows the (αhν)1/2 Tauc plot vs. Photon energy (hν) plot for pure SnO2 films and Urbach tail width. For indirect bandgap semiconductors, the Tauc formula (3) is used to compute the film bandgaps [27];

$$ \alpha (h\upsilon )^{1/2} = B(Eg - \, h\upsilon ). $$
(5)

Here, α indicates the absorption coefficient, h represents Planck’s constant, B signifies constant, Eg denotes the energy bandgap, and ν indicates “incident photon frequency.” As shown in Table 2, the film's bandgap is increased from 3.87 to 3.92 eV, with the antimony doping ratio increasing from 5 to 7 wt% and then decreasing from 25 wt% to 3.92 eV value. The undoped SnO2 bandgap value is a good accord with the presented data in the literature; it is recognized that the optical bandgap of SnO2 films ranges from 3.4 to 4.6 eV [28,29,30,31]. The Burstein–Moss effect [32, 33] is responsible for the widening of the bandgap, as the concentration of free carriers increases. The low-energy photon absorption is due to the band tail states induced through film network disorder. The absorption coefficient in this range is determined by the following equation [34]:

$$ \alpha (h\upsilon )\; = \;a_{0} \exp \, \left( {\frac{h\upsilon }{{_{{\mathop E\nolimits_{00} }} }}} \right) $$
(6)

here α0 indicates the pre-exponential factor, E00 denotes the disorder energy or band tail width called Urbach tail and hν signifies the photon energy [35]. E00 may be determined from the “inverse slope” of the linear plot of ln(α) vs. (hν).The fluctuations of the computed bandgap and the band tail width E00 with antimony doping levels are shown in Fig. 9 and Table 2. As could be observed, the fluctuations of bandgap and Urbach energies have opposite trends. That proposed that the disorder also controls the fluctuation of the optical gap in the film network.

Fig. 7
figure 7

Variation of transmittance vs. wavelength for Sb:SnO2 films with varied Sb concentrations

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Fig. 8
figure 8

a Typical variation of (αhv)2 and lnα drawn as a function of photon energy used for optical bandgap and b Urbach tail width determinations

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Table 2 Optical characteristics of undoped and Sb-doped SnO2 thin films deposited with varied Sb doping concentrations
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Fig. 9
figure 9

Variation of optical bandgap and Urbach energy with different Sb concentrations

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3.3.2 Photoluminescence (PL) analysis

The PL spectra of SnO2 and Sb-doped SnO2 films are demonstrated in Fig. 10. The spectrum shows two emission peaks in the Ultraviolet. We observed that with the rise in Sb concentration, the PL peaks to higher wavelengths and reduces their intensity. The existence of the ultraviolet emission peak is owing to the electron's transition from the conduction band (CB) to holes in the valence band (VB) [36,37,38]. As shown in Fig. 10, the PL intensity falls as the Sb level rises and achieves a minimal value of 25 wt% for the Sb doping concentration. This decrease in intensity may be described by reducing the electron–hole combination rate and Sb’s new trapping energy level suppressing the electron–hole pairs, which enhances the photocatalytic efficiency [36]. However, at doping contents higher than 25 wt%, the PL intensity increases where the Sb doping turns into an electron–hole pairs recombination center and reduces the photocatalytic activity of the films [39, 40].

Fig. 10
figure 10

Variation of optical bandgap and Urbach energy with different Sb concentrations

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4 Electrical properties

4.1 Hall Effect

Different electrical properties like resistivity, mobility, as well as free carrier concentration of spray, deposited films of undoped as well as antimony-doped tin oxide, were assessed using Hall Effect tests, which are summarized in Table 3. The n-type film's conductivity was verified by the negative sign of the “Hall coefficient.” Figure 11 depicts the impact of Sb doping on the SnO2 films' mobility (μ) and carrier concentration (n). With a rise in Sb doping concentration, the film's carrier concentration rises, and its mobility decreases, as seen in the figure. While doped films have the lowest electrical resistivity, the undoped film's resistivity is still within acceptable limits when evaluated to the previously noted values. The low resistivity might be explained by the existence of a chlorine impurity as a dopant. In SnO2 lattices, Sb atoms are substituted for the Sn4+ atoms when Antimony is applied. Sb may exist in two distinct oxidation states in antimony-doped SnO2 films: Sb3+ and Sb5+. The Sb5+ replaced on Sn4+ sites operate as donor and generate surplus electrons in the first addition of Sb ([Sb]/[Sn]\(\le\) 5 weight percent) in the film [14, 41,42,43,44]. Therefore, increased carrier concentration and decreased resistivity are seen with Sb incorporation up to 5% by weight. A portion of the Sb5+ ions decrease to the Sb3+ beyond 5 wt% of Sb doping, leading to the creation of acceptor sites and the carrier’s loss [45,46,47]. The Sb3+ to Sb5+ reduction was confirmed by Kojima et al. [42] by estimating the lattice parameter of the doped SnO2 films. The ionic radius of Sn4+ is below that of Sb3+ but greater as compared to Sb5+, a rise in the SnO2 stage’s lattice parameter is noted over 5 wt% percent of Sb doping. The film's figure of merit (ϕ) plays a significant part in device application. The Haacke formula [32] is used to compute it:

$$ \varphi \; = \;\frac{{T^{10} }}{{R_{{\text{s}}} }}. $$
(7)

Here, ‘T’ indicates the transmittance at λ = 550 nm (close to solar maximum) and ‘Rs’ represents the sheet resistance. The sheet resistance is computed with the Van der Pauw method [10]. Table 4 shows the transmittance values at 550 nm, sheet resistance, and computed figures of merits for the investigated films. It is obvious from Eq. (6) that the merit figure is dependent on the sheet resistance. As a consequence, the smaller the value of the figure of merit, the greater the sheet resistance, and vice versa. Furthermore, it is essential to mention that the figure of merit is influenced by the thickness of the film. Thus, the sheet resistance of a thicker film would be reduced, resulting in a greater figure of merit. Thus, the figure of merit is a useful tool for comparing optical and electrical characteristics [34]. It was discovered (Table 4) that the maximum merit is found for the film, which is noted in the case of 5 wt% Sb content (ϕ = 2.5 × 10–3(Ω sq−1)−1. This is possible owing to the creation of good film quality in terms of conductivity and transmittance. The noted (ϕ) values are in good accord with the related fluorine values (F:SnO2) [11], Antimony (Sb:SnO2) [48], and Niobium (Nb:SnO2)-doped SnO2 thin films [49]. The (Sb:SnO2) thin films with high transparency and conducting (namely figure of merit) show promise in a range of optoelectronic usages, particularly as flat-panel displays and solar cells window layers.

Table 3 Electrical parameters of Sb:SnO2 thin films with different doping concentrations
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Table 4 Values of the optical transmittance, sheet resistance, and figure of merit for Sb:SnO2 thin films deposited by spray pyrolysis technique
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Fig. 11
figure 11

Variation of carrier concentration and mobility with Sb dopant concentration

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5 Conclusions

The pure and Sb-doped tin oxide films were prepared using spray pyrolysis technique. Investigations of doping of Sb on structural, morphological, optical, and electrical properties of SnO2 were studied successfully. X-ray diffraction confirmed tetragonal rutile crystalline phase of Sb-doped and undoped tin oxide. It was observed that the preferential orientation changes with the addition of Sb doping from (110) to (200). The electrical study reveals that the films have n-type electrical conductivity and depend upon Sb concentration. We observed a decrease in sheet resistance and resistivity with the increase in Sb dopant concentration.