1 Introduction

Transparent conducting oxides (TCOs) have been researched because they exhibit combined optical and electrical features that grant them useful access in batteries, sensing devices, optoelectronics, solar cells, and photocatalyst devices (Batzill and Diebold 2005). Tin (IV) oxide is an important TCO that undergoes phase transition during the synthesis process, has a transparent conducting surface, great sensitivity, and a useful catalyst during oxidation processes (Batzill and Diebold 2005). The different forms of fabricating SnO2 sensors could be as whiskers, pallets, thick or thin films (Mitra and Mondal 2008). Tin (IV) oxide is an n-type semiconducting material with good stability, oxidation state of + 4, has a wide and direct bandgap (Eifert et al. 2017), high conductivity (Kılıç and Zunger 2002), great optical features (Kang 2010), and can exist in the tetragonal or orthorhombic phase. It finds useful application in optical devices, solar cells, gas sensors, perovskite cells, and dye-sensitized solar cells (Kumara et al. 2001).

Several methods of synthesizing tin (IV) oxide films include chemical bath deposition (Amma et al. 2005), spray deposition (Baranauskas et al. 2005; Kasar et al. 2008; Chacko 2006; Thangaraju 2002), successive ionic layer adsorption and reaction (Mitra and Mondal 2008; Deshpande et al. 2008; Pusawale et al. 2011), evaporation technique (Geurts et al. 1984; Jaiswal et al. 2013), sequential infiltration synthesis (Barick et al. 2019), ion beam irradiation (Kang 2010), chemical vapor deposition (Nagirnyak et al. 2016; Naeem et al. 2015), magnetron sputtering (Adamchuk et al. 2019; Chub et al. 2020), atomic layer deposition (Mai 2019; Maximov 2017). The diverse synthesis methods influence the quality and properties exhibited by the films. Sputter deposition allows film materials to be ejected from a target to a substrate with minimal heating effects. Sputtered films adhere more on substrate surfaces, sputter materials of high melting point, and produce films with similar compositions as the source material. Annealing films improve the crystal structure, enhances surface features, reduces strain, and improves optical features (Nkele 2019). Tin (IV) oxide films are usefully applied in optoelectronic, catalytic devices (Barick et al. 2019), and sensors (Chub et al. 2020).

Several works have been carried out on the synthesis of SnO2 by the sputtering technique. Sangaletti et al. (1997) thermally treated tin films in the air via RF sputtering to obtain a mixed orthorhombic and tetragonal phase. Camacho-Lopéz (2013) characterized reactive DC-sputtered SnO2 film and obtained tetragonal-phased and high transmittance films. The purpose of this research is to synthesize and characterize the obtained morphologies, structure, optical, and photoluminescence characteristics of the sputtered tin (IV) oxide films.

2 Experimental details

2.1 Materials and methods

The SnO2 material was synthesized on the glass substrate via an RF magnetron sputtering device. SnO2 ceramic target (Kurt. J. Lesker, 99.99% pure) and 2 mm diameter with 6 mm thickness was deposited by RF power supply. First, the substrates were cleaned intensively before coating. The layering stage was performed in the argon (99.99% pure) atmosphere. The chamber was first vacuumed by a turbo molecular pump to an initial pressure of 5 × 10–5 torr while the working pressure was kept constant at 4.5 × 10–3 torr. Before starting the coating process, the chamber was flashed thrice with argon (Ar) gas to eliminate oxygen and other contaminants from the chamber. A thin film of SnO2 with a power of 80 watts was layered to a thickness of 100 nm. After coating, the samples were heated at 300 °C (S3), 400 °C (S2) and 500 °C (S1) for 2 h in the air atmosphere. One sample (S4) was left without heating treatment.

The Au metal interdigital electrodes (IDEs) were deposited on the prepared SnO2 samples by the sputtering technique as schematically illustrated in Fig. 1. The thickness and active area of the patterned IDEs were kept constant at 100 nm and 3.5 mm2, respectively.

Fig. 1
figure 1

Schematic of the interdigitated electrode and SnO2 thin film device

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2.2 Characterizations

The synthesized tin (IV) oxide films were respectively analyzed to understand the morphology, structure, optical, photoluminescence, and photoresponse features using field electric scanning electron microscope (FESEM) (Sigma 300-HV Zeiss), X-ray diffraction (XRD) (ADVANCE-D8 Bruker) equipped with Cukα radiation source with λ = 1.5406 Å, UV–Vis spectrometer (1800 UV/Vis SHIMADZU), Spectro fluorophotometer (RF-6000 SHIMADZU). I–V features of the films were obtained at room temperature and atmospheric pressure. The LED was blue with 0.11 W/m2 intensity with 2 min OFF/ON switching cycles under 5 V bias voltage.

3 Results and discussion

3.1 Morphological studies

FESEM images revealed tiny clusters of nanoparticles distributed over the substrate surface, as seen in Fig. 2. Similar tiny morphology has also been reported by Barick et al. (2019). Synthesizing the SnO2 film produced clustered nano grains evenly distributed with surface cracks. The surface cracks are usually attributed to SnO2 films because developing oxide films leads to stress development in the films. The cracks could also be due to deposition conditions, low surface porosity, and diffusion of oxygen into the pores of the oxide layer (Camacho-López et al. 2013).

Fig. 2
figure 2

FESEM images of the films synthesized at a 500 b 400 c 300 d 0 °C

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A cross-section of the FESEM images for the synthesized films has been shown in Fig. 3a–d. Uniform film distribution can be observed throughout the substrate surface with efficient material build-up. The most annealed film recorded the highest optical transparency due to the applied post-heating effect. This property makes the films useful in optical devices and light-emitting diodes (Kang 2010).

Fig. 3
figure 3

Cross-sectional FESEM images of the films synthesized at a 500 b 400 c 300 d 0 °C

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Atomic force microscope images of the films obtained at several temperatures in tapping modes are shown in Fig. 4a–d. Round homogenous bulges were seen as agglomerations on the substrate surface. Spikes of different densities emanating from the substrate surface can be observed at varying temperatures. Varying the annealing temperature significantly affected the density of the tin (IV) oxide nanostructures. Thickness values of 240 nm, 219 nm, 210 nm, and 200 nm were obtained for the SnO2 films annealed respectively at 0 °C, 300 °C, 400 °C, and 500 °C with the help of Digimizer software program. The film’s thickness decreased with increasing annealing temperature because the thermal energy overcomes the adhesion and bonding energies of loosely bound atoms, thereby reducing the number of atoms that are adsorbed on the surface.

Fig. 4
figure 4

AFM images of the samples synthesized at a 500 b 400 c 300 d 0 °C

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The roughness values for the samples were obtained using Eqs. (1) and (2) and are displayed in Table 1.

$${\text{R}}_{{\text{a}}} = \frac{1}{{\text{n}}}\sum {\text{y}}_{{\text{i}}}$$
(1)
$${\text{R}}_{{\text{q}}} = \sqrt {\frac{1}{{\text{n}}}\sum {\text{y}}_{{\text{i}}}^{2} }$$
(2)
Table 1 Roughness values for the deposited films
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It can be observed that the unannealed film recorded the maximum roughness value while the film annealed at 500 °C had the least roughness value. Increasing the annealing temperature led to more growth of the nanoclusters, and accounts for the thickness variations.

3.2 Structural analysis

Figure 5 displays a tetragonal crystalline structure of the SnO2 films at 2theta degrees ranging from 10° to 80°. The lattice planes of the X-ray diffractograms and their corresponding 2theta angles are 26.57° (1 1 0), 33.86° (1 0 1), 37.94° (2 0 0), 51.76° (2 1 1), 54.74° (2 2 0), 57.81° (0 0 2), 61.86° (3 1 0), 64.72° (1 1 2), 71.26° (2 0 2), 78.68° (3 2 1). Tamilalagan et al. (2020) obtained similar lattice planes. The non-distinct peaks were indicative of sputtered tin (IV) oxide films (Popovich et al. 2016). Applying low and high temperatures to tin oxide produced poorly crystalline films (Singh 2019). Annealing the films led to a transformation to the polycrystalline tin (IV) oxide phase, reduction of water content, and stabilization the TiO2 structure (Adamchuk et al. 2019). The structural parameters obtained from the most prominent peaks have been outlined in Table 2.

Fig. 5
figure 5

Structural patterns of the SnO2 films synthesized at varying temperatures

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Table 2 Some structural parameters for the SnO2 films
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Table 2 showed that increasing the annealing temperature reduced the crystallite size, D and interplanar distance, d. The decreasing crystallite size could be attributed to the high density of the localized state. The lattice constants, a and c have also been outlined in Table 2, where the lattice constant a equals that of b.

3.3 Optical studies

Figure 6 gives the optical transmittance, absorbance, and reflectance plots for the deposited films. The deposited films recorded high transmittance and absorbance. The films were transparent to light in the visible electromagnetic spectrum. Reduced and fluctuating reflectance values were observed at increasing wavelength regions. Annealing the films reduced the reflectance of the films in the visible spectrum. Similar transmittance and absorbance features have been reported in the literature (Singh 2019; Ivanova 2020). These optical features make the SnO2 films potential materials for solar cells.

Fig. 6
figure 6

Optical a transmittance, b absorbance, and c reflectance plots of the synthesized samples

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The absorption coefficient versus wavelength plot of the SnO2 films are exhibited in Fig. 7a. A similar absorption coefficient trend was obtained with the most annealed film recording the highest absorption. Figure 7b shows the bandgap energy plots of the synthesized films. Tauc plot was employed in determining the bandgap energies of the films. The bandgap energy values of the films increased from 3.18 to 3.21 eV as the annealing temperature increased, due to confinement of the electrons and holes. Subjecting the films to annealing conditions did not significantly alter the band structure of the films (Bazargan et al. 2012). Similar results have also been obtained (Sangaletti 1997; Kong et al. 2010).

Fig. 7
figure 7

Plots showing the a absorption coefficient and b bandgap energies of the samples

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3.4 Photoluminescence (PL) studies

Photoluminescence analysis is a non-destructive technique that gives information on the optical, point defects, and photochemical features of the films under study. Figure 8 provides the photoluminescence plot of the unannealed and annealed SnO2 films measured at a PL wavelength of 200 nm. The unannealed sample exhibited higher photoluminescence quenching than the annealed samples. Maximum PL emission occurred at 369 nm within the visible emission region. Strong emission bands were obtained at 369 nm and 766 nm, while weak emission bands were visible at 470 nm, 626 nm, and 739 nm. The unannealed sample exhibited lower luminous intensity than the annealed samples; thereby making the heat-treated SnO2 films suitable for optical materials. The role of oxygen vacancies in PL analysis is evident from the decrease in PL intensity (Nehru et al. 2012).

Fig. 8
figure 8

Photoluminescence spectra of the deposited films

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3.5 Photoresponse analysis

The photoresponse (current versus time) plots of the films are shown in Fig. 9. Figure 9 depicts the time-dependent photoresponse behavior when the devices are under darkness and irradiation by blue lights with 2 min OFF/ON switching cycles under 5 V bias voltage. The more the annealing temperature, the better the photoresponse of the films (Breddels and Blasse 1984). The film annealed at 500 °C recorded the best photoresponse as outlined in Table 3.

Fig. 9
figure 9

Plot showing the photoresponse spectra of the annealed SnO2 films

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Table 3 Photoresponse parameters of the samples
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The photocurrent Iph is defined in Eq. (3) (Tian and Fan 2018) as:

$${\text{I}}_{{{\text{ph}}}} {\text{ = I}}_{{{\text{illuminated}}}} {-}{\text{I}}_{{{\text{dark}}}}$$
(3)

Two parameters of photodetector are calculate (Zhong 2017):

$${\text{Photo responsivity }}\left( {\text{R}} \right) \, = {\text{ I}}_{{{\text{ph}}}} /{\text{PS}}$$
(4)
$${\text{Detectivity}}({\text{D}}*) = [({\text{RS}})^{1} /2]/[(2{\text{eIdark}})^{1} /2]$$
(5)

where Iph is the photocurrent, P is the light power intensity, and S is the effective exposure area of the photodetector, and Idark is the dark current. The results obtained are displayed in Table 1.

Table 3 shows increased photoresponse of the tin (IV) oxide films at the higher annealing temperatures. The sample annealed at 400 °C recorded the highest photodetection ability because more photogenerated carriers were created at that temperature. This increased sensitivity makes SnO2 films useful as sensors (Bazargan et al. 2012; Savaniu 1999).

4 Conclusion

This work successfully synthesized SnO2 films unannealed and annealed at 300 °C, 400 °C, and 500 °C via the RF magnetron sputtering technique. The films were characterized for their morphological, structural, optical, photoluminescence, and photoresponse features. The unannealed film had compact nanog rained morphology with surface cracks, while AFM images showed spikes emanating from the substrate surface. The films exhibited a tetragonal crystal structure with the high absorbance feature. The bandgap energy value of the SnO2 films increased with annealing temperature. The highest photoluminescence intensity was recorded at 369 nm within the visible electromagnetic region. The films also recorded good photoresponse to current. The synthesized SnO2 films could be potentially applied in optical and solar cell devices.