Optoelectronic Properties of Antimony Doped Tin Oxide Thin Films Obtained by Spray Pyrolysis

2021 Antimony doped tin oxide (ATO) thin films are deposited on corning glass substrate using the spray pyrolysis technique. The experimental parameters such as distance between the substrate and source (10-30 cm), substrate temperature (350-450°C) and atmospheres (Nitrogen and Forming gas) are varied to study their effect on the properties of ATO thin films. The ATO thin film annealed at 425°C exhibits the lowest electrical resistivity of 2.23×10 -2 Ω-cm. Besides, the film annealed in the nitrogen atmosphere showed a less resistivity value of 9.06×10 -3 (Ω-cm) than the forming gas atmosphere. The film doped with 3 at% of Sb revealed the highest figure of merit value of 11.45x10 -2 Ω -1 . The preferential orientation is observed at the (200) diffraction plane in all the cases from the structural studies. Furthermore, the intensity of the diffraction planes decreases as the temperature increases. The average transmittance of 75% is obtained for ATO thin


Introduction
Transparent conductive oxide (TCO) materials have conductive properties close to metals and also allow the electromagnetic spectrum of visible light 1 . The TCO materials have been used in different fields such as screens 2 , sensors 3 , transparent electronic devices, electrochromic 4 and solar cells 5 . TCO material possesses band gap ≥ 3 eV and resistivity ˂10 -3 Ω-cm 6 .TCO could transmit a limited light close to electrical conductivity, which is relatively low compared to metals such as silver or gold 7 . The properties of these materials are contradictory in the band structure view: a transparent material is an insulator that possesses filled valence and empty conduction bands 8 , whereas metallic conductivity appears when the Fermi level lies within a band with a large density of states to provide high carrier concentration 9 . Therefore, researchers have focused on developing TCO materials as window layers and electrical contacts in photovoltaic devices 10 . Nowadays, TCO materials with high mobility and increased conductivity in conjunction with good transparency are extensively studied 11 . In the last two decades, tin-doped indium oxide (ITO) and fluorine-doped tin oxide (FTO) thin films have been essentially employed in the semiconductor industries in highly sophisticated fields of science and technology. Despite excellent optoelectronic properties, ITO and FTO suffer from instability under reducing plasma at elevated temperatures 9,12,13 . Furthermore, the deficiency in the supply of rare, expensive, toxic indium and fluorine for the production of ITO and FTO thin films is an environmental concern 14 . Therefore, many efforts have been made for the substitution of these materials. In TCO, the sheet resistance is considered the essential property: ITO changes from 7 to 50 ohms per frame depending on the deposition technique and the work surface. Besides, FTO shows the sheet resistance values in the range between 15 and 150 ohms per square. However, Sb doped tin oxide (ATO) thin films demonstrate competitive sheet resistance values 15,16 . The ATO thin films are considered an alternative to ITO and FTO due to their excellent calorific properties 5,17,18 .
In this work, ATO thin films are deposited by spray pyrolysis to use as transparent electrodes 19 . The experimental parameters such as temperature, dopant concentration, deposition distance, and post-treatment at different atmospheres are varied to study their effect on the ATO thin films' structural, electrical, and optical properties 20,21 . This is the first report for the systematic investigation of ATO thin films by varying all these parameters together. Moreover, the ATO thin films demonstrated excellent electrical properties (Resistivity ≤ 10 -3 Ω-cm, carrier concentration> 10 20 cm -3 ) and high visible transparency (> 75%) which revealed their suitability in optoelectronic devices 22-25 .

Methodology
The corning glass substrates were cleaned with acetic acid and acetone in an ultrasonic bath. The antimony doped tin oxide film was deposited onto the corning glass substrate by spray pyrolysis technique with a thickness of ~ 400 nm. For the preparation of the films, a 0.7 M solution of stannous *e-mail: jsantos@uaq.edu.mx II chloride and antimony chloride were dissolved in methanol with a few drops of hydrochloric acid under constant stirring for about 2 hours. The ratio between Sb/Sn was varied from 1 to 5%, and the substrate temperature was altered from 350 to 450°C. The distance between the source and substrate was adjusted from 10 to 40 cm. Subsequently, the films were annealed in an inert atmosphere (N 2 ) and a reducing atmosphere (N 2 :H 2 , 95:5).

Characterization
The structural properties were analyzed with X-ray diffraction, using the Rigaku Miniflex X-ray diffractometer. The electrical properties of the thin films were carried out using the four-probe Mitsubishi Loresta-GP equipment and the Hall effect ECOPIA 3000. SEM and EDS measurements were performed using the Hitachi TM-1000 equipment. The optical properties were evaluated using the UV-Vis, ThermoScientific S10. The surface elemental composition of the thin films was analyzed by X-ray photoelectron spectroscopy using a Thermo Fisher Scientific K-Alpha spectrometer. The resolution of the spectrometer is around ± 0.2 eV. Besides, an Al Kα X-ray source with an excitation wavelength of 1487 eV was used. For the measurements, the energy of 50 eV, a step size of 0.10 eV, and a spot size of 400 μm were employed. The experiments were repeated three times under the same conditions to check their reproducibility and the standard deviation values.  intensity for the film annealed under nitrogen atmosphere than the samples annealed in Forming gas 29,30 .

Structural analysis
The crystallite size was calculated using the Williamson-Hall equation. As shown in Table 1, a smaller crystallite size of 23 nm was observed for the film annealed at 425°C in air, which was then increased to 25.3 nm for 450°C. The observed peak shift in the (200) plane indicates stress in the lattice 31 . This stress is related to the substitution of a larger ionic radius of tin (162pm) with a smaller ionic radius of antimony (159pm) 32,33 . The obtained stress values increased from 1 to 1.54 x10 -3 as the annealing temperature increased between 350 and 425°C, whereas the stress decreased to 1.54 x10 -3 for the film annealed at 450°C. Furthermore, the dislocation density values increased from 0.945 to 2.09 x10 11 lin cm -2 as a function of annealing temperature. The grain size, stress and dislocation density were calculated from the Williamson-Hall equation 1.
Where B is FWHM of the diffraction planes, (hkl) is miller indices, K is the Shape factor (in this case 0.9), Δ is the wavelength of CuK radiation (0.154 nm), D is the crystallite size, ε is the stress and θ is the diffraction Bragg angle.
The film annealed at 450°C showed a greater (200) plane intensity than the other temperatures, but a smaller crystallite size was found at 425°C. A smaller crystal size favors electrical properties, which could be verified with resistivity measurements 34 . The crystallite size, stress, and dislocation density were calculated for the film annealed at 425°C at different atmospheres (Figure 1 c)). The calculated crystallite size was 23.1 nm and 23.4 nm for nitrogen and forming gas atmospheres, respectively ( Table 2). Crystallite size is associated with electrical properties. Lopez-Maldonado et al. reported that increasing crystallite size decreases the grain boundaries, decreasing the energy barrier width, which influences the electrical conduction properties 35 . There is no significant difference in FWHM for other diffraction planes. Figure 2 depicts surface scanning electron microscopy images of ATO thin films. Figure 2 a-c) show as-prepared ATO films, film annealed in a nitrogen and Forming gas (95% N2: 5% H2) atmospheres at 425°C, respectively. The average thickness of 400 nm was observed from the profilometer. The as-prepared film exhibited inhomogeneity grains while film annealed at different atmospheres showed a more homogeneous surface with a grain size of 40 nm. This could be due to a greater atomic diffusion that occurred during annealing and a concentration gradient in the flow of the atmosphere [36][37][38] . The bonding between the atoms in the crystalline structure is energetically favored, resulting in a larger grain size with a lower density 31,39 . Figure 3 displays the effect of the different atmospheres on the electrical properties of ATO films. The lowest resistivity was observed at 425°C. Lower resistivities and a greater figure of merit values were obtained for the film annealed at 400°C in a nitrogen atmosphere compared to forming gas atmosphere. As the temperature increases, crystallinity increases, improving the electrical properties of the film. This may be due to a higher generation of Sb 5+ in the SnO 2 structure 13,17,40 . Another possible explanation is the generation of a concentration gradient of the oxygen present in the film with the environment. The mobility to the film's surface is favored, which generates an oxygen deficiency. Oxygen vacancies are associated with higher conductivity. Vacancies and doping of the film do not affect the transmission of the film, resulting in a significant figure of merit 41 . Figure 4 represents the UV-Vis transmittance of ATO thin films annealed at different atmospheres. Film annealed at 375°C (Figure 4a) with Forming gas atmosphere exhibited an average transmittance of 70%, while film annealed in nitrogen showed 57% of transmittance. As shown in Figure 4b, film   annealed at 425°C demonstrated the average transmittance of 73% in the range between 550 and 100 nm, whereas 65% of transmittance was obtained in the forming gas. Therefore, the highest transmittance was attained for the film annealed at 425°C in a nitrogen atmosphere. Figure 5 depicts the high-resolution XPS (HR-XPS) spectra of ATO thin films. The measurements were calibrated to the C 1s peak at 284.6 eV, ascribed to the adventitious hydrocarbon deposited on the thin film surface. Also, the Shirley-type method was used to obtain the background.    Figure 5a displays the HR-XPS spectrum of the Sn 3d level that corresponds to the spin-orbit splitting, which is a characteristic of the tin. The spectrum was deconvoluted into two doublets, associated with the Sn 3d3/2 and Sn 3d5/2 peaks, which were located at 495.13 and 486.57 eV, respectively. For the deconvolution process, the peak-topeak separation (ΔSn 3d) was obtained by the following expression: ΔSn 3d = Sn 3d3/2 − Sn 3d5/2 42 . The obtained value of 8.56 eV matches well with Choi et al. 43 . Figure 5b shows the HR-XPS of the Sb 4s level, which confirms the presence of antimony in the tin oxide film. The obtained peak at 153.57 eV agrees with other reports 44,45 . Figure 5c represents the HR-XPS signal of the O 1s; the observed peak at 530.50 eV could be associated with the oxygen of tin in the bulk as observed in other materials 46 .

Conclusions
ATO thin films were obtained by spray pyrolysis technique by varying the experimental parameters such as distance, temperature and atmosphere. XPS measurements confirm the presence of antimony in the film. The highest figure of merit value of 9.83x10 -2 Ω -1 was obtained for the film annealed at 425°C in a nitrogen atmosphere with a distance of 30 cm. The antimony doped tin oxide films exhibited the lowest resistivity value of 1.23x10 -2 Ωcm and the highest transmittance of 73%.

Acknowledgments
We acknowledge the National Council of Science and Technology (CONACYT) for its financial support to the student BRFH.