- Email: [email protected]
Investigation of F doped SnO2 thin films properties deposited via ultrasonic spray technique for several applications
Surfaces and Interfaces 15 (2019) 244–249
Contents lists available at ScienceDirect
Surfaces and Interfaces journal homepage: www.elsevier.com/locate/surfin
Investigation of F doped SnO2 thin films properties deposited via ultrasonic spray technique for several applications
T
⁎
Chafia Khelifi, Abdallah Attaf , Hanane saidi, Anouar Yahia, Mohamed Dahnoun Physics Laboratory of Thin Films and Applications LPCMA, University of Biskra, B.P. 145, Biskra R.P. 07000, Algeria
A R T I C LE I N FO
A B S T R A C T
Keywords: FTO Thin film Ultrasonic spray Structural properties Optical properties Electrical properties
Previous studies have shown that pure SnO2 thin films have low electrical and optical properties. These poor properties of the SnO2 can be enhanced by adding the appropriate percentage of fluorine. In this work, F-SnO2 thin films of high conductivity and transmittance have been prepared in the University of Biskra using chemical ultrasonic spray technique. Many characterization techniques such as XRD, SEM, UV-visible, PL and 4-point, have been used to determinate the morphology, texture coefficient, grain size, dislocation, transmittance and the conductivity of the prepared F-SnO2 films. Our XRD results show that the F-SnO2 still have the same rutile structure as the pure SnO2 but with better crystallization. The injection of fluorine increases the transmittance up to 84%, and the conductivity to about 8.11 × 102 Ω.cm−1, with high figure of merit value about 9 × 10−3 Ω−1.
1. Introduction
have obtained were compared with those of many previous research
For many years ago, researches have been carried out aiming to obtain high quality thin films for solar cells. Transparent conductive oxides (TCOs) are interesting materials because of their unique characteristics such as high electrical conductivity and high transparency in the visible spectrum; which makes them ideal candidates for many uses like optoelectronics, photovoltaic and catalytic applications [1]. Among several TCOs like tin dioxide (SnO2) [2], zinc oxide (ZnO) [3], Titanium dioxide (TiO2) [4], the fluorine doped SnO2 films (F-SnO2) seem to be the most appropriate for the aforementioned applications owing to its better properties [5]. Moreover, the fluorine is commercially low-cost and available. In this paper, undoped tin dioxide and fluorine doped tin dioxide were prepared by ultrasonic spray technique onto glass substrates. This technique represents a very simple processing method, especially the equipment costs. It is easy for preparing films of any composition and does not require high quality chemicals or substrates. The method has been employed for the deposition of porous films, dense films, and for powder production [6]. The illustration of the deposition process used is shown in Fig. 1. The purpose of this work is to study the effect of the F concentration on the properties of SnO2 films such as transmittance, thickness, figure of merit, texture coefficient, energy gap and conductivity using many characterization techniques like XRD, SEM, UV–Visible, PL and 4-points in order to optimize these properties as much as possible. The results we
2. Experimental methodology
⁎
F-SnO2 thin films have been prepared with easy and low cost spray pyrolysis technique using an alcoholic precursor solution consisting of stannic chloride SnCl4, dissolving in methanol, which served as a starting solution, with 0.1 M concentration. The ammonium fluoride (NH4F) was added to the starting solution as source of fluorine. Spray pyrolysis was done at different F concentrations varied from 0 to 5% onto glass substrate heated at 450 °C with 5 cm nozzle-substrate distance and deposited for 5 min. The ultrasonic spray technique and equipment used to obtain FSnO2 films are illustrated in Fig. 1. The setup consists of a syringe pump that contains the precursor, an ultrasonic generator to generate the ultrasonic waves, and an atomizer that uses these latter to decompose the solution to fine droplets. 3. Results and discussion 3.1. Scanning electron microscopy The scanning electron microscopy facilitates the observation of the surface topography of bulk samples by sweeping these surfaces. The film was studied using Zeiss-SMT LEO 1540 XB scanning electron microscope.
Corresponding author. E-mail address: [email protected] (A. Attaf).
https://doi.org/10.1016/j.surfin.2019.04.001 Received 23 December 2018; Received in revised form 24 March 2019; Accepted 2 April 2019 Available online 03 April 2019 2468-0230/ © 2019 Elsevier B.V. All rights reserved.
Surfaces and Interfaces 15 (2019) 244–249
C. Khelifi, et al.
Fig. 1. Illustration of ultrasonic spray pyrolysis deposition.
Fig. 2. Scanning electron micrograph of nanocrystalline pure SnO2 sample at different magnifications.
Fig. 2 shows the surface morphology of spray deposited pure SnO2 thin films by SEM. It is clear in the SEM image that the film is uniform and rough without cracks. It shows that the sample has grains with different sizes while the intergranular regions appear dark. The presence of big and faceting grains observed in this image presents the fact that the crystallites are formed by coalescence type of growth. The grains are randomly distributed with some porosity giving rise to a scattering effect, thereby reducing transmittance [7, 8] . The doping with fluorine is in order to have uniform and smooth films which result in high transmittance and conductivity. Choi et al. [9] have observed that nanosheets have a high surface-to-volume ratio as well as specific exposed crystal facets, and the tuning of these properties can improve the gas-sensing properties of sensors. The same SEM image of F:SnO2 was found by Ida et al. [10]. 3.2. Growth velocity The film thickness of FTO is measured using gravimetric method which is based on the coating mass m, the density of SnO2 ρ, and the area A on which the material is deposited [11]:
t = m/A. ρ
Fig. 3. F-SnO2 films thicknesses and growth rate variations at different F concentration.
(1)
In the figure insert, the growth rate (thickness/5 min) varies slowly with F concentration up to 3%, and then the trend increases asymptotically to a linear variation after outsize concentration. This is because at the beginning the growth of the film was onto surface area (2D) then
For all the films thicknesses of FTO that have been measured, they are found to be in the range 500–1100 nm. Fig. 3 shows proportionality between thickness and F concentration. 245
Surfaces and Interfaces 15 (2019) 244–249
C. Khelifi, et al.
intensities. It is evident from the XRD spectra there is no diffraction peaks of any other impurity phases are detected in the prepared samples such as Sn and SnF2, indicating the O atoms were replaced by F atoms in the F-SnO2 films [12]. The inserting of fluorine does not affect the structural properties of the films, however, the increase in the intensities of the main XRD peaks with increasing fluorine doping levels, is probably attributed to the change in the growth rate which leads to an increase in the thickness of the films[11] or the change of solution concentration . These explications were obtained well in other papers [13] and confirmed by Demet Tatar [14]. 3.4. Texture coefficient (TC) Texture coefficient measures the relative degree of preferred orientation of the SnO2 films among crystal planes, which is obtained from the X-ray data using the following expression [5]:
TC(hkl) = [I(hkl)/I0(hkl) ]/⎡N−1 ∑ (I(hkl)/ I0(hkl) ) ⎤ ⎥ ⎢ N ⎦ ⎣
Fig. 4. XRD spectrum of F doped SnO2 thin films at various F doping.
(2)
Where I(hkl) is the measured intensity of the plane (hkl), I0 (hkl) is the standard intensity of the plane (hkl) according to the JCPDS data then N is the number of diffraction peaks. The texture coefficient was calculated for (110), (200) and (310) peaks, shown in Table1: The TC results plotted as a function of F concentration in Fig. 5: The higher values in TC(200) mean that the preferential orientation is along (200) lattice plane, and this is good agreement with XRD results which confirmed that the (200) plane shows highly textured growth than (110) plane. The relation between the energy minimization and the morphology is of great importance in texture development; Yaqin Wang and all see that the value of TC decreases as the film thickness or average crystalline size goes up [15].
after it will be perpendicular to the substrate (3D) [11]. On the other hand, with the increase in solution amount, the fluorine atoms will take oxygen vacancy, by way of F size is large thereby, enhancement quantity of F atoms caused an increase in thickness.
3.3. Structural characteristics The crystal structure was determined by XRD of spray deposited SnO2 and F-SnO2 thin films with different fluorine doping concentration at 450 °C shown in Fig. 4: The results clearly indicate that all the films are tetragonal Rutile structure with a polycrystalline nature. The major peaks located at 26.75, 38.09, 51.64, 61.88 and 65.81 corresponding to (110), (200), (211), (310) and (301) diffraction planes respectively. A strong (110) orientation is observed at 0, 2 and 3% which indicate that the films have a strong crystallographic texture along (110) where the formation energy was lower. Above these values at 4, 5%, we can see that the orientation was changed to (200) which has high texture growth confirmed by TC in Fig. 5. With the increasing of fluorine doping concentration the rutile structure remains the same. Other main planes of cassiterite have also been detected but with substantially lower
3.5. Figure of merit Calculation of figure of merit is to define at which thickness the films present the best condition for their applications as widow and collector in solar cells. Figure of merit φ was obtained by using Haacke's formula [16]:
φ = T10/ Rsh
(3)
It is clear in Fig. 6 that the augmentation in figure of merit values in the visible region was well observed when the F concentration increase as well, and the best values are at 4%. M.N. Yusnidar and all [17] see that ultrasonic spray pyrolysis method may produce thin films with high figure of merit due to their high electrical conductivities, which result from their large grain and crystallite sizes with enhanced oxygen vacancies. 3.6. Crystallite size and dislocation density Generally, the average crystallites size were evaluated from XRD results according to Scherrer formula [18]: Table.1 Texture coefficient results at different F-SnO2 concentration.
Fig. 5. Texture coefficient plot of F-SnO2 as function of F doping. 246
F-SnO2%
TC
0 2 3 4 5
0.5820 0.6145 0.7658 0.0635 0.2878
(110)
TC(200)
TC(310)
1.1645 1.6803 1.6187 2.336 1.9871
1.2534 0.7050 0.6154 0.5996 0.7249
Surfaces and Interfaces 15 (2019) 244–249
C. Khelifi, et al.
Fig. 8. Optical transmittance spectra of F-SnO2 thin films at different F concentration.
Fig. 6. Figure of merit values of F-SnO2 at different F concentrations.
D = 0.9λ /β cos θ
(4)
3.7. Optical properties
Where λ is the X-ray wavelength of 1.5406 A°, θ is the Bragg's angle and β is the full width at half maximum (FWHM). The results of the crystallite size D and dislocation δ as a function of F concentrations are presented in the Fig. 7 when δ is defined as the length of dislocations lines per unit of volume of the crystal and was estimated using Williamson and Smallman's relation [19]:
δ = 1/ D 2
The transmittance spectra obtained by UV-visible spectroscopy as function of wavelength over spectral range 350–1200 nm are showed in Fig. 8: It is clearly that an average optical transparency is about 84% in the visible range. As the doping concentration increases, the transmittance increase too which indicate the good crystallization also well structural homogeneity [21]. At 5% the transmittance decreases, but it remains bigger than the pure SnO2, this decrease probably due to the scattering effect [22]or due to the diffusion photon by crystal defects or also due to free carrier as A. Arunachalam and all supposed [23].
(5)
In the beginning until 4% the FWHM increase with increasing F concentrations this indicates a decrease of grain size due to the incorporation of F ions into the SnO2 lattice which take interstitial sites then pressed there on one hand, and the augmentation of dislocation density by the other hand. At 5% the grain size is increase when dislocation decrease due to the O atoms which replaced by F in the F-SnO2 lattice [12]. Also, above 4% as the fluorine injected increase, the grain size increases too and this is probably due to the increasing in fluorine atoms arriving to the substrate which have big atomic rayon, thereby causing an increase in the nucleation numbers which combine together to form larger grains [20]. Therefore, the crystalline quality improves and that is clear in XRD results.
3.8. Photoluminescence photoluminescence (PL) spectroscopy technique uses to investigate the structure, defect, impurity levels and quality of thin films [24]. Fig. 9 represents the photoluminescence spectra of the pure and F-SnO2 films: All the F-SnO2 samples in the range 350–550 nm show four emissions bands. The first peak at 386 nm corresponding to the UV emissions band due to oxygen vacancies [25] which form the donor levels and they were the responsible for available electrons in the CB. The
Fig. 7. Variation of crystallite size and dislocation of F-SnO2 with different fluorine doping.
Fig. 9. Photoluminescence spectra of F doped SnO2 thin films. 247
Surfaces and Interfaces 15 (2019) 244–249
C. Khelifi, et al.
second peak positioned at 430 nm corresponding to the violet emissions band. The third low intensity pic centered at 477 nm corresponding to the blue emissions signal that a new defect level presented into the band gap by the F doping [26]. A very low intensity pic at 520 agreeing to the green, which created from the electron-hole recombination at the defect sites due to electron transition from the oxygen vacancy [26]. The decrease in PL intensity with the increase of F doping concentration maybe due to the increase of band gap energy of F-SnO2 samples or could be attributed to the energy exchange between a pair of F ions [27]. 3.9. Band gap energy The optical band gap energy Eg was calculated according to Tauc's formula [28]:
(αhν ) = D (hν − Eg )n
(6)
Where α is absorption coefficient, hυ is the photon energy, D is constant and n is an exponent depending on the nature of the electronic transition (equal 1/2 for direct or 2 for indirect band gap semiconductor respectively). The plot of (αhν)2 as function of hν illustrated in the Fig. 10 which determine how we calculate Eg. The obtained values variation are in the Fig. 11: The injection of F ions in the crystal lattice and the oxygen vacancies induced during the film growth increases the electrons near the conduction band edge, which created donor levels in the conductive band of SnO2, then will increases the band gap [29]. The widening effect in band gap can be attributed to Burstein-Moss effect which states that: carrier concentration is due to the high doping levels, fills blank states belonging to CB of the thin films so increase the energy magnitude necessary for the valence band to conduction band transitions [30]. In other hand, the changes in crystallite size or structural phase and carrier concentration [31] with the increase of F doping, make large band gap and caused the increase of the crystal defects existed in the band gap of the F-SnO2 films [32].
Fig. 11. The optical band gap variation of F-SnO2 films at different F concentrations.
Fig. 12. Variation of conductivity and resistivity of F-SnO2 with different fluorine doping.
3.10. Electrical properties decrease. The conductivity increases due to the oxygen vacancies, which caused no stoichiometry in SnO2 lattice in one hand and the increase of carrier concentration by the other hand. The increase of the carrier concentration maybe due to more free electrons provided by substitution of O − 2 by F− ion in the lattice [13]. The fluorine too was created donor level below the conduction band, allowed an increase in the number of electrons transferred from the donor level to the CB, which caused the increase of the conductivity [33]. Yufei shi too find that doping concentration has an important influence on both morphology and conductivity [34]
Fig. 12 shows plot of electrical resistivity (ρ) and the conductivity (σ) as function of F concentrations. When the resistivity is opposite to the conductivity it is found that, the conductivity increase with the increase of fluorine concentration and reaches saturate value 8.11×102 (Ω-cm−1) at 4%. However, at 5% the conductivity is started to
4. Conclusion Major research has been performed on F-SnO2 thin films prepared by easy and low cost chemical technique, ultrasonic spray at 450 °C. Several characterizations techniques were done to the prepared films such as XRD, UV-visible, PL, 4-point. The Polycrystalline F-SnO2 films showed strong orientation along (110) at 0, 2 and 3%, which has been found to changed to (200) at 4 and 5% when they have higher values of TC(200),that indicate a strong crystallographic texture along these orientations. The best values in figure of merit are seen at 4% reached to 9.10−3Ω−1. The F-SnO2 films revealed the maximum transmittance of 86% in the visible range and maximum conductivity about 8.11 × 102 (Ωcm)−1 at 4% .This good characteristics like crystalline structure, high
Fig. 10. Typical Tauc plot used for optical band gap determination of F-SnO2 films at different F concentrations. 248
Surfaces and Interfaces 15 (2019) 244–249
C. Khelifi, et al.
transparency and great conductivity propose that these films likely to be useful as electrical contacts in various devices or for gas sensor applications due to the high conductivity [10] [35].
[18]
Acknowledgments
[19]
This work was partially supported by Algiers Research Center of Nuclear(CRNA), Algeria and by Hungarian Academy of Sciences Center for Energy Research(MTA EK). Special thanks goes to the reviewers for their comments and useful suggestions.
[20]
[21]
References [22] [1] S.S. Lekshmy, G.P. Daniel, K. Joy, Microstructure and physical properties of sol gel derived SnO2: Sb thin films for optoelectronic applications, Appl. Surf. Sci. 274 (2013) 95–100, https://doi.org/10.1016/j.apsusc.2013.02.109. [2] Y.B.H. Bendjedidi, A. Attaf, H. Saidi, M.S. Aida, S. Semmari, A. Bouhdjar, Properties of n-type SnO2 semiconductor prepared by spray ultrasonic technique for photovoltaic applications, J. Semicond. 36 (2015) 123002, , https://doi.org/10.1088/ 1674-4926/36/12/123002. [3] N. Lehraki, M.S. Aida, S. Abed, N. Attaf, A. Attaf, M. Poulain, ZnO thin films deposition by spray pyrolysis: influence of precursor solution properties, Curr. Appl. Phys. 12 (2012) 1283–1287, https://doi.org/10.1016/j.cap.2012.03.012. [4] Z.P. Tshabalala, D.E. Motaung, G.H. Mhlongo, O.M. Ntwaeaborwa, Facile synthesis of improved room temperature gas sensing properties of TiO2 nanostructures: effect of acid treatment, Sensors Actuators B Chem. 224 (2016) 841–856, https://doi.org/ 10.1016/j.snb.2015.10.079. [5] B. Benhaoua, S. Abbas, A. Rahal, A. Benhaoua, M.S. Aida, Effect of film thickness on the structural, optical and electrical properties of SnO2: f thin films prepared by spray ultrasonic for solar cells applications, Superlattices Microstruct. 83 (2015) 78–88, https://doi.org/10.1016/j.spmi.2015.03.017. [6] D. Perednis, L.J. Gauckler, Thin film deposition using spray pyrolysis, Electroceramics 14 (2005) 103–111. [7] A.A. Yadav, Influence of film thickness on structural, optical, and electrical properties of spray deposited antimony doped SnO2 thin films, Thin Solid Films 591 (2015) 18–24, https://doi.org/10.1016/j.tsf.2015.08.013. [8] N. Lehraki, M.S. Aida, S. Abed, N. Attaf, A. Attaf, M. Poulain, ZnO thin films deposition by spray pyrolysis: influence of precursor solution properties, Curr. Appl. Phys. 12 (2012) 1283–1287, https://doi.org/10.1016/j.cap.2012.03.012. [9] P.G. Choi, N. Izu, N. Shirahata, Y. Masuda, Fabrication and H 2 - Sensing properties of SnO 2 Nanosheet gas sensors, ACS Omega 3 (2018) 14592–14596, https://doi. org/10.1021/acsomega.8b01635. [10] L.I.M. Ballesteros, C.J.T. Vergara, A.S. Somintac, Fluorine-doped SnO 2 thin film as chemiresistive gas sensor for room temperature monitoring of fish freshness, Pisika 1 (2018) 3–6, https://doi.org/10.20526/pisika.01a18.10. [11] C. Khelifi, A. Attaf, H. Saidi, A. Yahia, A. Saadi, Effect of solution flow on the properties of tin dioxide SnO2 thin films deposited by spray pyrolysis technique, Opt. - Int. J. Light Electron Opt. 127 (2016) 11055–11062, https://doi.org/10. 1016/j.ijleo.2016.09.060. [12] A.H. Omran Alkhayatt, S.K. Hussian, Fluorine highly doped nanocrystalline SnO2 thin films prepared by SPD technique, Mater. Lett 155 (2015) 109–113, https://doi. org/10.1016/j.matlet.2015.04.130. [13] M. Thirumoorthi, J.T.J. Prakash, Effect of F doping on physical properties of (211) oriented SnO2 thin films prepared by jet nebulizer spray pyrolysis technique, Superlattices Microstruct. 89 (2016) 378–389, https://doi.org/10.1016/j.spmi. 2015.11.023. [14] B.D. DEMET TATAR, TURGUT GÜVEN, Effect of substrate temperature on the crystal growth, Rom. Journ. Phys. 58 (2013) 143–158. [15] Y. Wang, W. Tang, L. Zhang, Crystalline ize effects on texture coefficient, electrical and optical properties of sputter-deposited Ga-doped ZnO Thin Films, J. Mater. Sci. Technol. 31 (2015) 175–181, https://doi.org/10.1016/j.jmst.2014.11.009. [16] B. Thangaraju, Structural and electrical studies on highly conducting spray deposited fluorine and antimony doped SnO2 thin films from SnCl2 precursor, Thin Solid Films 402 (2002) 71–78, https://doi.org/10.1016/S0040-6090 (01)01667-4. [17] V. Fauzia, M.N. Yusnidar, L.H. Lalasari, A. Subhan, A.A. Umar, High figure of merit
[23]
[24]
[25]
[26] [27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
249
transparent conducting Sb-doped SnO2thin films prepared via ultrasonic spray pyrolysis, J. Alloys Compd 720 (2017) 79–85, https://doi.org/10.1016/j.jallcom. 2017.05.243. N.H. Touidjen, B. Bendahmane, M.L. Zeggar, F. Mansour, M.S. Aida, SnO2 thin film synthesis for organic vapors sensing at ambient temperature, Sens. Bio-Sensing Res. 11 (2016) 52–57, https://doi.org/10.1016/j.sbsr.2016.11.001. U.T. Güven, E. Sönmez, S. Aydınb, R. Dilbera, Synthesis and characterization of Mo doped SnO2 thin films with spray pyrolysis, Superlattices Microstruct 69 (2014) 175–186, https://doi.org/10.1016/j.spmi.2014.02.009. A. Bouhdjer, A. Attaf, H. Saidi, H. Bendjedidi, Y. Benkhetta, I. Bouhaf, Correlation between the structural, morphological, optical, and electrical properties of In 2 O 3 thin films obtained by an ultrasonic spray CVD process, J. Semicond. 36 (2015) 82002, https://doi.org/10.1088/1674-4926/36/8/082002. S.S. Shinde, A.P. Korade, C.H. Bhosale, K.Y. Rajpure, Influence of tin doping onto structural, morphological, optoelectronic and impedance properties of sprayed ZnO thin films, J. Alloys Compd. 551 (2013) 688–693, https://doi.org/10.1016/j. jallcom.2012.11.057. M.S. Bannur, A. Antony, K.I. Maddani, P. Poornesh, A. Rao, K.S. Choudhari, Tailoring the nonlinear optical susceptibility χ(3), photoluminescence and optical band gap of nanostructured SnO2 thin films by Zn doping for photonic device applications, Phys. E Low-Dimensional Syst. Nanostructures. (2018), https://doi.org/ 10.1016/j.physe.2018.06.025. A. Arunachalam, S. Dhanapandian, C. Manoharan, Effect of Sn doping on the structural, optical and electrical properties of TiO2 films prepared by spray pyrolysis, Phys. E Low-Dimensional Syst. Nanostructures. 76 (2015) 35–46, https:// doi.org/10.1016/j.physe.2015.09.048. M.S. Bannur, A. Antony, K.I. Maddani, P. Poornesh, K.B. Manjunatha, S.D. Kulkarni, K.S. Choudhari, Role of Ba in engineering band gap, photoluminescence and nonlinear optical properties of SnO2 nanostructures for photovoltaic and photocatalytic applications, Superlattices Microstruct (2018), https://doi.org/10.1016/j.spmi. 2018.08.012. M. Thirumoorthi, J.T.J. Prakash, Effect of F doping on physical properties of (211) oriented SnO2 thin films prepared by jet nebulizer spray pyrolysis technique, Superlattices Microstruct 89 (2016) 378–389, https://doi.org/10.1016/j.spmi. 2015.11.023. Y. Cao, W. Yang, W. Zhang, G. Liu, P. Yue, C.W. Bay, H. Kong, Improved photocatalytic activity of Sn 4, New J. Chem. 28 (2004) 218–222. R.K. Mishra, A. Kushwaha, P.P. Sahay, Influence of Cu doping on the structural, photoluminescence and formaldehyde sensing properties of SnO2 nanoparticles, RSC Adv. 4 (2014) 3904–3912, https://doi.org/10.1039/C3RA43709D. P. Malik, M. Srivastava, R. Verma, M. Kumar, D. Kumar, J. Singh, Nanostructured SnO2 encapsulated guar-gum hybrid nanocomposites for electrocatalytic determination of hydrazine, Mater. Sci. Eng. C. 58 (2016) 432–441, https://doi.org/10. 1016/j.msec.2015.08.035. A.R. Paloly, M. Satheesh, M.C. Martinez-Tomas, V. Munoz-Sanjose, S. Rajappan Achary, M.J. Bushiri, Growth of tin oxide thin films composed of nanoparticles on hydrophilic and hydrophobic glass substrates by spray pyrolysis technique, Appl. Surf. Sci 357 (2015) 915–921, https://doi.org/10.1016/j.apsusc.2015.09.112. P.M. Mwathe, Influence of Surface Passivation on Optical Properties of Spray Pyrolysis Deposited Pd-F: SnO2, Int. J. Mater. Sci. Appl. 3 (2014) 137, https://doi. org/10.11648/j.ijmsa.20140305.11. F. Moharrami, M.M. Bagheri-Mohagheghi, H. Azimi-Juybari, Study of structural, electrical, optical, thermoelectric and photoconductive properties of S and Al codoped SnO2 semiconductor thin films prepared by spray pyrolysis, Thin Solid Films 520 (2012) 6503–6509, https://doi.org/10.1016/j.tsf.2012.06.075. Y.M. Lu, J. Jiang, M. Becker, B. Kramm, L. Chen, a. Polity, Y.B. He, P.J. Klar, B.K. Meyer, Polycrystalline SnO2 films grown by chemical vapor deposition on quartz glass, Vacuum 122 (2015) 3–8, https://doi.org/10.1016/j.vacuum.2015.03. 018. S.S. Roy, J. Podder, Studies on tin oxide (SnO2) and CU doped SnO2 Tthin films deposited by spray pyrolysis technique for window materials in solar cells, Proc. Int. Conf. Mech. Eng. 2009 (2009) 26–28. Y. Shi, M. Xu, X. Sun, S. Zhang, S. Jia, Z. Deng, Effect of Sb-doping on the morphology and the infrared emissivity of peony-like SnO 2 microspheres, Integr. Ferroelectr. 191 (2019) 1–7, https://doi.org/10.1080/10584587.2018.1457319. X.Y. Ying Wang, Hongbin Yu, Hui Suo, Xinhong Wang, Ying Lu, Suiyi Zhu, Facile synthesis of nano-structured SnO 2 fi lms and their properties in ethanol gas sensing, Micro Nano Lett 13 (2018) 1101–1104, https://doi.org/10.1049/mnl.2017. 0806.
Contents lists available at ScienceDirect
Surfaces and Interfaces journal homepage: www.elsevier.com/locate/surfin
Investigation of F doped SnO2 thin films properties deposited via ultrasonic spray technique for several applications
T
⁎
Chafia Khelifi, Abdallah Attaf , Hanane saidi, Anouar Yahia, Mohamed Dahnoun Physics Laboratory of Thin Films and Applications LPCMA, University of Biskra, B.P. 145, Biskra R.P. 07000, Algeria
A R T I C LE I N FO
A B S T R A C T
Keywords: FTO Thin film Ultrasonic spray Structural properties Optical properties Electrical properties
Previous studies have shown that pure SnO2 thin films have low electrical and optical properties. These poor properties of the SnO2 can be enhanced by adding the appropriate percentage of fluorine. In this work, F-SnO2 thin films of high conductivity and transmittance have been prepared in the University of Biskra using chemical ultrasonic spray technique. Many characterization techniques such as XRD, SEM, UV-visible, PL and 4-point, have been used to determinate the morphology, texture coefficient, grain size, dislocation, transmittance and the conductivity of the prepared F-SnO2 films. Our XRD results show that the F-SnO2 still have the same rutile structure as the pure SnO2 but with better crystallization. The injection of fluorine increases the transmittance up to 84%, and the conductivity to about 8.11 × 102 Ω.cm−1, with high figure of merit value about 9 × 10−3 Ω−1.
1. Introduction
have obtained were compared with those of many previous research
For many years ago, researches have been carried out aiming to obtain high quality thin films for solar cells. Transparent conductive oxides (TCOs) are interesting materials because of their unique characteristics such as high electrical conductivity and high transparency in the visible spectrum; which makes them ideal candidates for many uses like optoelectronics, photovoltaic and catalytic applications [1]. Among several TCOs like tin dioxide (SnO2) [2], zinc oxide (ZnO) [3], Titanium dioxide (TiO2) [4], the fluorine doped SnO2 films (F-SnO2) seem to be the most appropriate for the aforementioned applications owing to its better properties [5]. Moreover, the fluorine is commercially low-cost and available. In this paper, undoped tin dioxide and fluorine doped tin dioxide were prepared by ultrasonic spray technique onto glass substrates. This technique represents a very simple processing method, especially the equipment costs. It is easy for preparing films of any composition and does not require high quality chemicals or substrates. The method has been employed for the deposition of porous films, dense films, and for powder production [6]. The illustration of the deposition process used is shown in Fig. 1. The purpose of this work is to study the effect of the F concentration on the properties of SnO2 films such as transmittance, thickness, figure of merit, texture coefficient, energy gap and conductivity using many characterization techniques like XRD, SEM, UV–Visible, PL and 4-points in order to optimize these properties as much as possible. The results we
2. Experimental methodology
⁎
F-SnO2 thin films have been prepared with easy and low cost spray pyrolysis technique using an alcoholic precursor solution consisting of stannic chloride SnCl4, dissolving in methanol, which served as a starting solution, with 0.1 M concentration. The ammonium fluoride (NH4F) was added to the starting solution as source of fluorine. Spray pyrolysis was done at different F concentrations varied from 0 to 5% onto glass substrate heated at 450 °C with 5 cm nozzle-substrate distance and deposited for 5 min. The ultrasonic spray technique and equipment used to obtain FSnO2 films are illustrated in Fig. 1. The setup consists of a syringe pump that contains the precursor, an ultrasonic generator to generate the ultrasonic waves, and an atomizer that uses these latter to decompose the solution to fine droplets. 3. Results and discussion 3.1. Scanning electron microscopy The scanning electron microscopy facilitates the observation of the surface topography of bulk samples by sweeping these surfaces. The film was studied using Zeiss-SMT LEO 1540 XB scanning electron microscope.
Corresponding author. E-mail address: [email protected] (A. Attaf).
https://doi.org/10.1016/j.surfin.2019.04.001 Received 23 December 2018; Received in revised form 24 March 2019; Accepted 2 April 2019 Available online 03 April 2019 2468-0230/ © 2019 Elsevier B.V. All rights reserved.
Surfaces and Interfaces 15 (2019) 244–249
C. Khelifi, et al.
Fig. 1. Illustration of ultrasonic spray pyrolysis deposition.
Fig. 2. Scanning electron micrograph of nanocrystalline pure SnO2 sample at different magnifications.
Fig. 2 shows the surface morphology of spray deposited pure SnO2 thin films by SEM. It is clear in the SEM image that the film is uniform and rough without cracks. It shows that the sample has grains with different sizes while the intergranular regions appear dark. The presence of big and faceting grains observed in this image presents the fact that the crystallites are formed by coalescence type of growth. The grains are randomly distributed with some porosity giving rise to a scattering effect, thereby reducing transmittance [7, 8] . The doping with fluorine is in order to have uniform and smooth films which result in high transmittance and conductivity. Choi et al. [9] have observed that nanosheets have a high surface-to-volume ratio as well as specific exposed crystal facets, and the tuning of these properties can improve the gas-sensing properties of sensors. The same SEM image of F:SnO2 was found by Ida et al. [10]. 3.2. Growth velocity The film thickness of FTO is measured using gravimetric method which is based on the coating mass m, the density of SnO2 ρ, and the area A on which the material is deposited [11]:
t = m/A. ρ
Fig. 3. F-SnO2 films thicknesses and growth rate variations at different F concentration.
(1)
In the figure insert, the growth rate (thickness/5 min) varies slowly with F concentration up to 3%, and then the trend increases asymptotically to a linear variation after outsize concentration. This is because at the beginning the growth of the film was onto surface area (2D) then
For all the films thicknesses of FTO that have been measured, they are found to be in the range 500–1100 nm. Fig. 3 shows proportionality between thickness and F concentration. 245
Surfaces and Interfaces 15 (2019) 244–249
C. Khelifi, et al.
intensities. It is evident from the XRD spectra there is no diffraction peaks of any other impurity phases are detected in the prepared samples such as Sn and SnF2, indicating the O atoms were replaced by F atoms in the F-SnO2 films [12]. The inserting of fluorine does not affect the structural properties of the films, however, the increase in the intensities of the main XRD peaks with increasing fluorine doping levels, is probably attributed to the change in the growth rate which leads to an increase in the thickness of the films[11] or the change of solution concentration . These explications were obtained well in other papers [13] and confirmed by Demet Tatar [14]. 3.4. Texture coefficient (TC) Texture coefficient measures the relative degree of preferred orientation of the SnO2 films among crystal planes, which is obtained from the X-ray data using the following expression [5]:
TC(hkl) = [I(hkl)/I0(hkl) ]/⎡N−1 ∑ (I(hkl)/ I0(hkl) ) ⎤ ⎥ ⎢ N ⎦ ⎣
Fig. 4. XRD spectrum of F doped SnO2 thin films at various F doping.
(2)
Where I(hkl) is the measured intensity of the plane (hkl), I0 (hkl) is the standard intensity of the plane (hkl) according to the JCPDS data then N is the number of diffraction peaks. The texture coefficient was calculated for (110), (200) and (310) peaks, shown in Table1: The TC results plotted as a function of F concentration in Fig. 5: The higher values in TC(200) mean that the preferential orientation is along (200) lattice plane, and this is good agreement with XRD results which confirmed that the (200) plane shows highly textured growth than (110) plane. The relation between the energy minimization and the morphology is of great importance in texture development; Yaqin Wang and all see that the value of TC decreases as the film thickness or average crystalline size goes up [15].
after it will be perpendicular to the substrate (3D) [11]. On the other hand, with the increase in solution amount, the fluorine atoms will take oxygen vacancy, by way of F size is large thereby, enhancement quantity of F atoms caused an increase in thickness.
3.3. Structural characteristics The crystal structure was determined by XRD of spray deposited SnO2 and F-SnO2 thin films with different fluorine doping concentration at 450 °C shown in Fig. 4: The results clearly indicate that all the films are tetragonal Rutile structure with a polycrystalline nature. The major peaks located at 26.75, 38.09, 51.64, 61.88 and 65.81 corresponding to (110), (200), (211), (310) and (301) diffraction planes respectively. A strong (110) orientation is observed at 0, 2 and 3% which indicate that the films have a strong crystallographic texture along (110) where the formation energy was lower. Above these values at 4, 5%, we can see that the orientation was changed to (200) which has high texture growth confirmed by TC in Fig. 5. With the increasing of fluorine doping concentration the rutile structure remains the same. Other main planes of cassiterite have also been detected but with substantially lower
3.5. Figure of merit Calculation of figure of merit is to define at which thickness the films present the best condition for their applications as widow and collector in solar cells. Figure of merit φ was obtained by using Haacke's formula [16]:
φ = T10/ Rsh
(3)
It is clear in Fig. 6 that the augmentation in figure of merit values in the visible region was well observed when the F concentration increase as well, and the best values are at 4%. M.N. Yusnidar and all [17] see that ultrasonic spray pyrolysis method may produce thin films with high figure of merit due to their high electrical conductivities, which result from their large grain and crystallite sizes with enhanced oxygen vacancies. 3.6. Crystallite size and dislocation density Generally, the average crystallites size were evaluated from XRD results according to Scherrer formula [18]: Table.1 Texture coefficient results at different F-SnO2 concentration.
Fig. 5. Texture coefficient plot of F-SnO2 as function of F doping. 246
F-SnO2%
TC
0 2 3 4 5
0.5820 0.6145 0.7658 0.0635 0.2878
(110)
TC(200)
TC(310)
1.1645 1.6803 1.6187 2.336 1.9871
1.2534 0.7050 0.6154 0.5996 0.7249
Surfaces and Interfaces 15 (2019) 244–249
C. Khelifi, et al.
Fig. 8. Optical transmittance spectra of F-SnO2 thin films at different F concentration.
Fig. 6. Figure of merit values of F-SnO2 at different F concentrations.
D = 0.9λ /β cos θ
(4)
3.7. Optical properties
Where λ is the X-ray wavelength of 1.5406 A°, θ is the Bragg's angle and β is the full width at half maximum (FWHM). The results of the crystallite size D and dislocation δ as a function of F concentrations are presented in the Fig. 7 when δ is defined as the length of dislocations lines per unit of volume of the crystal and was estimated using Williamson and Smallman's relation [19]:
δ = 1/ D 2
The transmittance spectra obtained by UV-visible spectroscopy as function of wavelength over spectral range 350–1200 nm are showed in Fig. 8: It is clearly that an average optical transparency is about 84% in the visible range. As the doping concentration increases, the transmittance increase too which indicate the good crystallization also well structural homogeneity [21]. At 5% the transmittance decreases, but it remains bigger than the pure SnO2, this decrease probably due to the scattering effect [22]or due to the diffusion photon by crystal defects or also due to free carrier as A. Arunachalam and all supposed [23].
(5)
In the beginning until 4% the FWHM increase with increasing F concentrations this indicates a decrease of grain size due to the incorporation of F ions into the SnO2 lattice which take interstitial sites then pressed there on one hand, and the augmentation of dislocation density by the other hand. At 5% the grain size is increase when dislocation decrease due to the O atoms which replaced by F in the F-SnO2 lattice [12]. Also, above 4% as the fluorine injected increase, the grain size increases too and this is probably due to the increasing in fluorine atoms arriving to the substrate which have big atomic rayon, thereby causing an increase in the nucleation numbers which combine together to form larger grains [20]. Therefore, the crystalline quality improves and that is clear in XRD results.
3.8. Photoluminescence photoluminescence (PL) spectroscopy technique uses to investigate the structure, defect, impurity levels and quality of thin films [24]. Fig. 9 represents the photoluminescence spectra of the pure and F-SnO2 films: All the F-SnO2 samples in the range 350–550 nm show four emissions bands. The first peak at 386 nm corresponding to the UV emissions band due to oxygen vacancies [25] which form the donor levels and they were the responsible for available electrons in the CB. The
Fig. 7. Variation of crystallite size and dislocation of F-SnO2 with different fluorine doping.
Fig. 9. Photoluminescence spectra of F doped SnO2 thin films. 247
Surfaces and Interfaces 15 (2019) 244–249
C. Khelifi, et al.
second peak positioned at 430 nm corresponding to the violet emissions band. The third low intensity pic centered at 477 nm corresponding to the blue emissions signal that a new defect level presented into the band gap by the F doping [26]. A very low intensity pic at 520 agreeing to the green, which created from the electron-hole recombination at the defect sites due to electron transition from the oxygen vacancy [26]. The decrease in PL intensity with the increase of F doping concentration maybe due to the increase of band gap energy of F-SnO2 samples or could be attributed to the energy exchange between a pair of F ions [27]. 3.9. Band gap energy The optical band gap energy Eg was calculated according to Tauc's formula [28]:
(αhν ) = D (hν − Eg )n
(6)
Where α is absorption coefficient, hυ is the photon energy, D is constant and n is an exponent depending on the nature of the electronic transition (equal 1/2 for direct or 2 for indirect band gap semiconductor respectively). The plot of (αhν)2 as function of hν illustrated in the Fig. 10 which determine how we calculate Eg. The obtained values variation are in the Fig. 11: The injection of F ions in the crystal lattice and the oxygen vacancies induced during the film growth increases the electrons near the conduction band edge, which created donor levels in the conductive band of SnO2, then will increases the band gap [29]. The widening effect in band gap can be attributed to Burstein-Moss effect which states that: carrier concentration is due to the high doping levels, fills blank states belonging to CB of the thin films so increase the energy magnitude necessary for the valence band to conduction band transitions [30]. In other hand, the changes in crystallite size or structural phase and carrier concentration [31] with the increase of F doping, make large band gap and caused the increase of the crystal defects existed in the band gap of the F-SnO2 films [32].
Fig. 11. The optical band gap variation of F-SnO2 films at different F concentrations.
Fig. 12. Variation of conductivity and resistivity of F-SnO2 with different fluorine doping.
3.10. Electrical properties decrease. The conductivity increases due to the oxygen vacancies, which caused no stoichiometry in SnO2 lattice in one hand and the increase of carrier concentration by the other hand. The increase of the carrier concentration maybe due to more free electrons provided by substitution of O − 2 by F− ion in the lattice [13]. The fluorine too was created donor level below the conduction band, allowed an increase in the number of electrons transferred from the donor level to the CB, which caused the increase of the conductivity [33]. Yufei shi too find that doping concentration has an important influence on both morphology and conductivity [34]
Fig. 12 shows plot of electrical resistivity (ρ) and the conductivity (σ) as function of F concentrations. When the resistivity is opposite to the conductivity it is found that, the conductivity increase with the increase of fluorine concentration and reaches saturate value 8.11×102 (Ω-cm−1) at 4%. However, at 5% the conductivity is started to
4. Conclusion Major research has been performed on F-SnO2 thin films prepared by easy and low cost chemical technique, ultrasonic spray at 450 °C. Several characterizations techniques were done to the prepared films such as XRD, UV-visible, PL, 4-point. The Polycrystalline F-SnO2 films showed strong orientation along (110) at 0, 2 and 3%, which has been found to changed to (200) at 4 and 5% when they have higher values of TC(200),that indicate a strong crystallographic texture along these orientations. The best values in figure of merit are seen at 4% reached to 9.10−3Ω−1. The F-SnO2 films revealed the maximum transmittance of 86% in the visible range and maximum conductivity about 8.11 × 102 (Ωcm)−1 at 4% .This good characteristics like crystalline structure, high
Fig. 10. Typical Tauc plot used for optical band gap determination of F-SnO2 films at different F concentrations. 248
Surfaces and Interfaces 15 (2019) 244–249
C. Khelifi, et al.
transparency and great conductivity propose that these films likely to be useful as electrical contacts in various devices or for gas sensor applications due to the high conductivity [10] [35].
[18]
Acknowledgments
[19]
This work was partially supported by Algiers Research Center of Nuclear(CRNA), Algeria and by Hungarian Academy of Sciences Center for Energy Research(MTA EK). Special thanks goes to the reviewers for their comments and useful suggestions.
[20]
[21]
References [22] [1] S.S. Lekshmy, G.P. Daniel, K. Joy, Microstructure and physical properties of sol gel derived SnO2: Sb thin films for optoelectronic applications, Appl. Surf. Sci. 274 (2013) 95–100, https://doi.org/10.1016/j.apsusc.2013.02.109. [2] Y.B.H. Bendjedidi, A. Attaf, H. Saidi, M.S. Aida, S. Semmari, A. Bouhdjar, Properties of n-type SnO2 semiconductor prepared by spray ultrasonic technique for photovoltaic applications, J. Semicond. 36 (2015) 123002, , https://doi.org/10.1088/ 1674-4926/36/12/123002. [3] N. Lehraki, M.S. Aida, S. Abed, N. Attaf, A. Attaf, M. Poulain, ZnO thin films deposition by spray pyrolysis: influence of precursor solution properties, Curr. Appl. Phys. 12 (2012) 1283–1287, https://doi.org/10.1016/j.cap.2012.03.012. [4] Z.P. Tshabalala, D.E. Motaung, G.H. Mhlongo, O.M. Ntwaeaborwa, Facile synthesis of improved room temperature gas sensing properties of TiO2 nanostructures: effect of acid treatment, Sensors Actuators B Chem. 224 (2016) 841–856, https://doi.org/ 10.1016/j.snb.2015.10.079. [5] B. Benhaoua, S. Abbas, A. Rahal, A. Benhaoua, M.S. Aida, Effect of film thickness on the structural, optical and electrical properties of SnO2: f thin films prepared by spray ultrasonic for solar cells applications, Superlattices Microstruct. 83 (2015) 78–88, https://doi.org/10.1016/j.spmi.2015.03.017. [6] D. Perednis, L.J. Gauckler, Thin film deposition using spray pyrolysis, Electroceramics 14 (2005) 103–111. [7] A.A. Yadav, Influence of film thickness on structural, optical, and electrical properties of spray deposited antimony doped SnO2 thin films, Thin Solid Films 591 (2015) 18–24, https://doi.org/10.1016/j.tsf.2015.08.013. [8] N. Lehraki, M.S. Aida, S. Abed, N. Attaf, A. Attaf, M. Poulain, ZnO thin films deposition by spray pyrolysis: influence of precursor solution properties, Curr. Appl. Phys. 12 (2012) 1283–1287, https://doi.org/10.1016/j.cap.2012.03.012. [9] P.G. Choi, N. Izu, N. Shirahata, Y. Masuda, Fabrication and H 2 - Sensing properties of SnO 2 Nanosheet gas sensors, ACS Omega 3 (2018) 14592–14596, https://doi. org/10.1021/acsomega.8b01635. [10] L.I.M. Ballesteros, C.J.T. Vergara, A.S. Somintac, Fluorine-doped SnO 2 thin film as chemiresistive gas sensor for room temperature monitoring of fish freshness, Pisika 1 (2018) 3–6, https://doi.org/10.20526/pisika.01a18.10. [11] C. Khelifi, A. Attaf, H. Saidi, A. Yahia, A. Saadi, Effect of solution flow on the properties of tin dioxide SnO2 thin films deposited by spray pyrolysis technique, Opt. - Int. J. Light Electron Opt. 127 (2016) 11055–11062, https://doi.org/10. 1016/j.ijleo.2016.09.060. [12] A.H. Omran Alkhayatt, S.K. Hussian, Fluorine highly doped nanocrystalline SnO2 thin films prepared by SPD technique, Mater. Lett 155 (2015) 109–113, https://doi. org/10.1016/j.matlet.2015.04.130. [13] M. Thirumoorthi, J.T.J. Prakash, Effect of F doping on physical properties of (211) oriented SnO2 thin films prepared by jet nebulizer spray pyrolysis technique, Superlattices Microstruct. 89 (2016) 378–389, https://doi.org/10.1016/j.spmi. 2015.11.023. [14] B.D. DEMET TATAR, TURGUT GÜVEN, Effect of substrate temperature on the crystal growth, Rom. Journ. Phys. 58 (2013) 143–158. [15] Y. Wang, W. Tang, L. Zhang, Crystalline ize effects on texture coefficient, electrical and optical properties of sputter-deposited Ga-doped ZnO Thin Films, J. Mater. Sci. Technol. 31 (2015) 175–181, https://doi.org/10.1016/j.jmst.2014.11.009. [16] B. Thangaraju, Structural and electrical studies on highly conducting spray deposited fluorine and antimony doped SnO2 thin films from SnCl2 precursor, Thin Solid Films 402 (2002) 71–78, https://doi.org/10.1016/S0040-6090 (01)01667-4. [17] V. Fauzia, M.N. Yusnidar, L.H. Lalasari, A. Subhan, A.A. Umar, High figure of merit
[23]
[24]
[25]
[26] [27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
249
transparent conducting Sb-doped SnO2thin films prepared via ultrasonic spray pyrolysis, J. Alloys Compd 720 (2017) 79–85, https://doi.org/10.1016/j.jallcom. 2017.05.243. N.H. Touidjen, B. Bendahmane, M.L. Zeggar, F. Mansour, M.S. Aida, SnO2 thin film synthesis for organic vapors sensing at ambient temperature, Sens. Bio-Sensing Res. 11 (2016) 52–57, https://doi.org/10.1016/j.sbsr.2016.11.001. U.T. Güven, E. Sönmez, S. Aydınb, R. Dilbera, Synthesis and characterization of Mo doped SnO2 thin films with spray pyrolysis, Superlattices Microstruct 69 (2014) 175–186, https://doi.org/10.1016/j.spmi.2014.02.009. A. Bouhdjer, A. Attaf, H. Saidi, H. Bendjedidi, Y. Benkhetta, I. Bouhaf, Correlation between the structural, morphological, optical, and electrical properties of In 2 O 3 thin films obtained by an ultrasonic spray CVD process, J. Semicond. 36 (2015) 82002, https://doi.org/10.1088/1674-4926/36/8/082002. S.S. Shinde, A.P. Korade, C.H. Bhosale, K.Y. Rajpure, Influence of tin doping onto structural, morphological, optoelectronic and impedance properties of sprayed ZnO thin films, J. Alloys Compd. 551 (2013) 688–693, https://doi.org/10.1016/j. jallcom.2012.11.057. M.S. Bannur, A. Antony, K.I. Maddani, P. Poornesh, A. Rao, K.S. Choudhari, Tailoring the nonlinear optical susceptibility χ(3), photoluminescence and optical band gap of nanostructured SnO2 thin films by Zn doping for photonic device applications, Phys. E Low-Dimensional Syst. Nanostructures. (2018), https://doi.org/ 10.1016/j.physe.2018.06.025. A. Arunachalam, S. Dhanapandian, C. Manoharan, Effect of Sn doping on the structural, optical and electrical properties of TiO2 films prepared by spray pyrolysis, Phys. E Low-Dimensional Syst. Nanostructures. 76 (2015) 35–46, https:// doi.org/10.1016/j.physe.2015.09.048. M.S. Bannur, A. Antony, K.I. Maddani, P. Poornesh, K.B. Manjunatha, S.D. Kulkarni, K.S. Choudhari, Role of Ba in engineering band gap, photoluminescence and nonlinear optical properties of SnO2 nanostructures for photovoltaic and photocatalytic applications, Superlattices Microstruct (2018), https://doi.org/10.1016/j.spmi. 2018.08.012. M. Thirumoorthi, J.T.J. Prakash, Effect of F doping on physical properties of (211) oriented SnO2 thin films prepared by jet nebulizer spray pyrolysis technique, Superlattices Microstruct 89 (2016) 378–389, https://doi.org/10.1016/j.spmi. 2015.11.023. Y. Cao, W. Yang, W. Zhang, G. Liu, P. Yue, C.W. Bay, H. Kong, Improved photocatalytic activity of Sn 4, New J. Chem. 28 (2004) 218–222. R.K. Mishra, A. Kushwaha, P.P. Sahay, Influence of Cu doping on the structural, photoluminescence and formaldehyde sensing properties of SnO2 nanoparticles, RSC Adv. 4 (2014) 3904–3912, https://doi.org/10.1039/C3RA43709D. P. Malik, M. Srivastava, R. Verma, M. Kumar, D. Kumar, J. Singh, Nanostructured SnO2 encapsulated guar-gum hybrid nanocomposites for electrocatalytic determination of hydrazine, Mater. Sci. Eng. C. 58 (2016) 432–441, https://doi.org/10. 1016/j.msec.2015.08.035. A.R. Paloly, M. Satheesh, M.C. Martinez-Tomas, V. Munoz-Sanjose, S. Rajappan Achary, M.J. Bushiri, Growth of tin oxide thin films composed of nanoparticles on hydrophilic and hydrophobic glass substrates by spray pyrolysis technique, Appl. Surf. Sci 357 (2015) 915–921, https://doi.org/10.1016/j.apsusc.2015.09.112. P.M. Mwathe, Influence of Surface Passivation on Optical Properties of Spray Pyrolysis Deposited Pd-F: SnO2, Int. J. Mater. Sci. Appl. 3 (2014) 137, https://doi. org/10.11648/j.ijmsa.20140305.11. F. Moharrami, M.M. Bagheri-Mohagheghi, H. Azimi-Juybari, Study of structural, electrical, optical, thermoelectric and photoconductive properties of S and Al codoped SnO2 semiconductor thin films prepared by spray pyrolysis, Thin Solid Films 520 (2012) 6503–6509, https://doi.org/10.1016/j.tsf.2012.06.075. Y.M. Lu, J. Jiang, M. Becker, B. Kramm, L. Chen, a. Polity, Y.B. He, P.J. Klar, B.K. Meyer, Polycrystalline SnO2 films grown by chemical vapor deposition on quartz glass, Vacuum 122 (2015) 3–8, https://doi.org/10.1016/j.vacuum.2015.03. 018. S.S. Roy, J. Podder, Studies on tin oxide (SnO2) and CU doped SnO2 Tthin films deposited by spray pyrolysis technique for window materials in solar cells, Proc. Int. Conf. Mech. Eng. 2009 (2009) 26–28. Y. Shi, M. Xu, X. Sun, S. Zhang, S. Jia, Z. Deng, Effect of Sb-doping on the morphology and the infrared emissivity of peony-like SnO 2 microspheres, Integr. Ferroelectr. 191 (2019) 1–7, https://doi.org/10.1080/10584587.2018.1457319. X.Y. Ying Wang, Hongbin Yu, Hui Suo, Xinhong Wang, Ying Lu, Suiyi Zhu, Facile synthesis of nano-structured SnO 2 fi lms and their properties in ethanol gas sensing, Micro Nano Lett 13 (2018) 1101–1104, https://doi.org/10.1049/mnl.2017. 0806.