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Influence of Ti doping on SnO2 thin films properties prepared by ultrasonic spray technique
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Influence of Ti doping on SnO2 thin films properties prepared by ultrasonic spray technique Chafia Khelifi , Abdallah Attaf PII: DOI: Reference:
S2468-0230(19)30764-3 https://doi.org/10.1016/j.surfin.2020.100449 SURFIN 100449
To appear in:
Surfaces and Interfaces
Received date: Revised date: Accepted date:
28 December 2019 15 January 2020 20 January 2020
Please cite this article as: Chafia Khelifi , Abdallah Attaf , Influence of Ti doping on SnO2 thin films properties prepared by ultrasonic spray technique, Surfaces and Interfaces (2020), doi: https://doi.org/10.1016/j.surfin.2020.100449
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Influence of Ti doping on SnO2 thin films properties prepared by ultrasonic spray technique Chafia Khelifi a, Abdallah Attaf a a
Physics Laboratory of Thin Films and Applications LPCMA, University of Biskra,
B.P. 145, Biskra R.P. 07000, Algeria. Corresponding author: Abdallah Attaf E-mail : [email protected] Tel : +213770384492, Fax : +21333543190 Abstract Titanium doped tin dioxide (Ti-SnO2) thin films have been deposited using ultrasonic spray technique to minimize the poor properties of the pure SnO2. The process has been carried out by doping SnO2 films with the appropriate percentage of titanium.
X Ray
Diffraction shows that the rutile structure remains the same of pure tin dioxide but with better crystallization. The titanium doping enhances the transmittance up to 83 % and the conductivity about 1.4 ×102(Ω.cm) -1 approximately, with high figure of merit values about 7, 64.10-3 Ω-1.these results are required specially for bio-sensors and optoelectronics Keywords: Ti-SnO2, thin film, ultrasonic spray, structural properties, optical properties, electrical properties.
1. Introduction Transparent conductive oxides (TCOs) are one of the most important group of material with numerous applications in modern human life. The most attractive TCOs are ZnO, TiO2, In2O3 and SnO2, [1]that last produce high quality of thin films can resist high temperature. Several metal oxides like IGZO, ITO have been demonstrated as bio-sensor membranes owing to their excellent sensing characteristics and SnO2 is one of them, it can detect various gas concentrations by changing the surface conductivity owing to the absorption and desorption process[2] In recent years, research has targeted to improve the physical properties of tin oxide films by doping them with different dopants like fluorine (F-SnO2)[3] [4], antimony (SbSnO2)[5], Indium (ITO)[6] and titanium (Ti-SnO2). Doping can enhance the conductivity of tin oxide films [7] more that, the addition of transition metals like titanium (Ti) improve in 1
performance that observed in several bio-sensing materials by passivation of the defects in the bulk of the sensing material and fixing the dangling bond at the oxide interface. [2] In this paper, titanium doped tin dioxide was prepared by a very simple processing ultrasonic spray onto glass substrates. The method has been employed for the deposition of porous films, dense films, and for powder production [8]. The purpose of this study is explore how the Ti concentration affect the properties of SnO2 films especially the structural, optical and the electrical properties to explore the most suitable application of these films. 2. Experimental methodology For deposit Ti-SnO2 thin films by easy and low cost technique the ultrasonic spray, we used an alcoholic precursor solution consisting of stannic chloride SnCl4, dissolving in methanol that served as a starting solution. The Titanium (IV) isopropoxide (C 12H28O4Ti) was added to the starting solution as source of titanium. Spray pyrolysis was done at different Ti concentrations from 0 to 5 at% onto glass substrates heated at 400 °C for 5 min, when the nozzle-substrate distance is 5cm. 3. Results and discussion 3.1 Scanning electron microscopy The scanning microscopy was used to investigate the surface topography of the various ti-sno2 samples. This sample had done by using Zeiss-SMT LEO 1540 XB scanning electron microscopy. Fig.1 shows the surface morphology of spray deposited Ti-SnO2 film at 5 at %:
Fig.1. Scanning electron micrograph of nanocrystalline Ti-SnO2 sample at 5 at %Ti doping. 2
It is clear in the SEM image that the 5 at% Ti doped SnO2 film has big homogenous with middle size grains of the order 50 nm approximately. In addition, the incorporation of Ti made a very smooth surface with less roughness which giving rise to increase the transmittance to 80% in the visible region. The SnO2 doping with titanium at 5 at% represents a very smooth and uniform film, thus enhance both of transmittance, and of conductivity. 3.2 Growth Velocity Thickness of Ti-SnO2 samples prepared at different Ti doping was calculated using gravimetric method which based on the coating mass m, the density ρ of SnO2 and the area A on which the material is deposited [9]: t=
⁄
(1)
1200
1100
Thickness (nm)
1000
900
800
700
600 0
1
2
3
4
5
Ti concentration (at %)
Fig. 2. Ti-SnO2 films thicknesses variations at different Ti concentration. Fig.2 shows proportion between decreasing in thickness as increasing Ti concentration uniformly, maybe that is due to Ti atom size that is smaller than Sn and O atoms. With augmentation of the quantity of titanium, they occupy substitution sites of Sn, as the rayon of Ti is smaller than Sn so, the size of the mesh decreases and consequently the thickness decreases too. More that, the growth orientation was changed with doping thus made small grains. 3.3 Structural characteristics The crystal structure was determined by XRD of spray deposited SnO 2 and Ti-SnO2 thin films with different titanium doping concentration are showing in Fig.3: 3
800
(200)
(110)
700 (211)
(310) (301)
5%
600
Intensity (a,u)
500 400
4% 300
3% 200 100
2%
0
0% 10
20
30
40
50
60
70
80
2 (°)
Fig. 3. XRD spectrum of Ti doped SnO2 thin films at different Ti doping. It is interesting to note that from a general view, all X-ray diffraction patterns look like the same as the pure SnO2 sample [10]. These peaks found are in good agreement with the ASTM data, which confirm the tetragonal (cassiterite) structure of our deposits as Batzill et al [11] found it. The diagram in Fig.3 shows that several peaks are present with a preferential orientation according to (200) which determines a strong crystallographic texture or low energy. There are minority peaks according to (110), (101), (211), (310) and (301) reflecting the poly crystallinity of the pure structure. More that, by comparing our results with the ASTM files, we notice a slight shift of the peaks that reflects the creation of the constraints. 3.4 Figure of merit In order to evaluate the efficiency of Ti-SnO2 films and when they 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 [12]: ⁄
(3)
4
0% 1% 2% 3% 4% 5%
16 14
.10-3(ohm-1)
12 10 8 6 4 2 0 500
600
700
800
900
Wavelenght (nm)
Fig. 4. Figure of merit values of Ti-SnO2 at various Ti concentrations. It is clear in Fig.4 that figure of merit values decrease when the Ti concentration increase, and the best values are at the pure SnO2. When the 4 at% Ti-SnO2 seem better than the others doping values. 3.5 Crystallite size and dislocation density Generally, the average crystallites size evaluated from XRD results according to the Scherrer formula [13]: ⁄
D=
(4)
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 Ti doping are presenting in the Fig.5 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 [14] : δ= ⁄
(5)
5
0,0014 55 0,0012
0,0008
40
35
0,0006
30
0,0004
25
2
0,0010
45
dislocation (line/nm )
Crystallite size (nm)
50
0,0002 0
1
2
3
4
5
Ti concentration (at %)
Fig. 5. Variation of crystallite size and dislocation of Ti-SnO2 at various Ti doping. The variation in grain size shows an increase in the beginning then after it was decrease when the dopant concentration augment. It is observed that the grain size increases to the value 47.48 nm for a percentage of 1 and 2% when the larger grains are required for the fabrication of high efficiency solar cells [15]. After this value, it was decreases and stabilized at 40.69 nm. We can interpret the increase of the grains size by the reconstruction nucleation sites through the increase of titanium atoms so the quality of crystalline improves. In addition, A. panimaya see that larger grains were formed because of the higher oxygen affinity of Ti [2]. The decrease in the grain size is due to the decrease in thickness because we have increase dislocation in our films or less density of grains that responsible to do a complete nucleation [16]. 3.6 Optical properties The transmittance spectra obtained by UV-visible spectroscopy as function of wavelength over spectral range 350-1200 nm are showing in Fig 6:
6
100
Transmittance %
80
60
40
0% 1% 2% 3% 4% 5%
20
0 300
400
500
600
700
800
900
1000
1100
1200
Wavelenght (nm)
Fig. 6. Optical transmittance spectra of Ti-SnO2 thin films at different Ti concentration. Fig.6. shows that the optical transmission spectra of pure and Ti-doped SnO2 thin films deposited at different Ti concentrations. It is clear that the films show high transparency in the range of [400 - 1100 nm], where the highest transmittance values up to 83% approximately. At first, the transmittance decrease with the augmentation of Ti then after it started to increase but with less fringes, however 4% represents high transparency with more fringes, which confirm that the film prepared under these conditions is very smooth and uniform due to the multiple reflections on the different interfaces [17]. The decrease is due to the existence of defects in the lattice, which made deformation and no homogeneity structure thus affect the morphology and consequently decrease the transmittance by scattering of light. However, the increase of transmittance is due to the decrease both of the defects and the film thickness, vice versa is happened with the decrease. 3.7 Photoluminescence We used photoluminescence (PL) spectroscopy technique for investigating the structure, defect, impurity levels and quality of thin films. Fig.7 represents the photoluminescence spectra of the pure and Ti-SnO2 films:
7
500
0% 1% 2% 3% 4% 5%
486 463
400
PL Intensity (a.u )
399 300
531
200
100
0
400
450
500
550
600
650
700
Wavelenght (nm)
Fig. 7. Photoluminescence spectra of Ti doped SnO2 thin films. A broad dominant peak is observed in the range 390-540 nm shows three emissions bands, as presented in Fig.7; this no homogeneity broadened luminescence might be due to all the luminescent centers, such as Nano crystals and defects in the Ti-SnO2 films [18]. The first peak at 399 nm corresponding to the UV emissions band due to oxygen vacancies that forms the donor levels and were the responsible for available electrons in the CB [19]. The second peak positioned at 486 nm with shoulder peak at 463 nm, corresponding to the blue emissions signal that a new defect level presented into the band gap by the Ti doping and that peak is absence in the pure, which confirm that a new defect in the others is caused by Ti doping. A very low intensity pic at 531 agreeing to the green, which created from the electron-hole recombination at the defect sites due to electron transition from the oxygen vacancy [20]. The increase in PL intensity with the increasing Ti doping concentration maybe due to the decrease of band gap energy of Ti-SnO2 samples [21]. 3.8 Band gap Energy The optical band gap energy Eg was calculated according to Tauc’s formula [22]: (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 obtained values variation are showing in the Fig.8: 8
3,97 3,96
Band Gap (eV)
3,95 3,94 3,93 3,92 3,91 3,90 0
1
2
3
4
5
Ti concentration (at %)
Fig. 8. The optical band gap variation of Ti-SnO2 films at different Ti concentrations. The blue emission of PL spectroscopy confirms that the injection of Ti ions in the crystal lattice, created an energy level presented into the band gap so that level maybe near the valence band of SnO2, then it makes a decrease in the band gap. The compressive stress and the decrease of thickness too can be related and responsible to this shift of band gap. 3.9 Electrical properties Using 4-point measurements that permit to indicate how Ti-SnO2 films are resistive, the obtained results of electrical resistivity (ρ) and the conductivity (σ) as function of Ti doping are showing in Fig.9: 220 0,018 200
-1
Conductivity (ohm.cm)
0,014
160 140
0,012
120
0,010
100
0,008
80
Resistivity (ohm.cm)
0,016
180
0,006
60 0,004 40 0
1
2
3
4
5
Ti concentration (at %)
Fig. 9. Variation of conductivity and resistivity of Ti-SnO2 with different Ti doping.
9
Fig.9 shows that the electrical conductivity started to decrease until 2 at% of Ti doping then after it started to increase up to 140 (Ω.cm) -1 but the augmentation was slowly and less than the speed of decrease. The decrease of conductivity is due to the incorporation of Ti in the beginning that caused no stoichiometry in SnO2 lattice with some defects. By the other hand the existence of O2 transferred from substrate to the films by temperature effect are muchness. Also in the beginning Takumi Tomita [23] confirm that some cases, the oxygen vacancies cannot act as a native donor that is due to the its defect level which is much lower than the conductive band. The increase of conductivity is due to the decrease of the band gap. More that, the existence of the oxygen vacancy at these concentrations may act the role of donor, so that increase the carrier concentration due to more free electrons transferred from the defect level of O, which became located near the conductive band in the lattice. 4. Conclusion Many researches have done on Ti-SnO2 thin films prepared by ultrasonic spray to enhance its properties. The characterizations techniques show that all Ti-SnO2 films are Polycrystalline with strong orientation along (200) which confirm the highest texture along these orientations. The best values in figure of merit are seen at 4 at% reached to 7, 64.10 -3Ω-1 at 800 wavelength. Photoluminescence spectroscopy confirm that we have UV emission which due to the existence of the oxygen vacancies in one hand, and the presence of new defect level in the band gap due to the incorporation of the titanium. Few changes were noted in the optical band gap between 3.91-3.97 eV that seem constant approximately. The Ti-SnO2 films revealed the maximum transmittance up 83% in the visible region with maximum conductivity about 1.4 ×102 (Ω-cm)-1 at 5 at %. Acknowledgments 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. References [1]
C. Method et al., “Studies on SnO 2 Nanoparticles Prepared by Co-Precipitation Method,” vol. 18, no. 5, pp. 3511–3517, 2018.
[2]
A. P. Selvi et al., “Sensing and structural properties of Ti-doped tin oxide ( SnO2 ) 10
membrane for bio-sensor applications,” Ceram. Int., no. March, pp. 0–1, 2017. [3]
M. D. P. Chafia Khelifi, Abdallah Attaf, Hanane saidi, Anouar Yahia, “Investigation of F doped SnO 2 thin fi lms properties deposited via ultrasonic spray technique for several applications,” Surfaces and Interfaces, vol. 15, no. April, pp. 244–249, 2019.
[4]
B. Benhaoua, S. Abbas, A. Rahal, A. Benhaoua, and 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., vol. 83, pp. 78–88, 2015.
[5]
S. Myint and P. Kaung, “Optical and electrical properties of polycrystalline SnO 2 : Sb thin films,” Yangon Inst. Educ., vol. 4, no. 1, pp. 1–9, 2012.
[6]
M. Maaza, O. Nemraoui, A. C. Beye, C. Sella, and T. Derry, “Induced structural damages by He + irradiation in conducting transparent indium – tin oxide thin films,” vol. 90, pp. 111–119, 2006.
[7]
C. Wang, W. Zeng, L. Luo, P. Zhang, and Z. Wang, “Gas-sensing properties and mechanisms of Cu-doped SnO2 spheres towards H2S,” Ceram. Int., vol. 42, no. 8, pp. 10006–10013, 2016.
[8]
D. Perednis and L. J. Gauckler, “Thin Film Deposition Using Spray Pyrolysis,” Electroceramics, vol. 14, pp. 103–111, 2005.
[9]
C. Khelifi, A. Attaf, H. Saidi, A. Yahia, and 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., vol. 127, no. 23, pp. 11055–11062, 2016.
[10] C. Khelifi, A. Attaf, H. Saidi, A. Yahia, and A. Saadi, “Optik Effect of solution flow on the properties of tin dioxide SnO 2 thin films deposited by spray pyrolysis technique,” Opt. - Int. J. Light Electron Opt., vol. 127, no. 23, pp. 11055–11062, 2016. [11] U. D. M. Batzill, “The surface and materials science of tin oxide,” Prog. Surf. Sci., vol. 79, pp. 47–154, 2005. [12] B. Thangaraju, “Structural and electrical studies on highly conducting spray deposited fluorine and antimony doped SnO2 thin films from SnCl2 precursor,” Thin Solid Films, vol. 402, no. 1–2, pp. 71–78, 2002. [13] N. H. Touidjen, B. Bendahmane, M. L. Zeggar, F. Mansour, and M. S. Aida, “SnO2 thin film synthesis for organic vapors sensing at ambient temperature,” Sens. BioSensing Res., vol. 11, no. December, pp. 52–57, 2016. [14] Güven and U. T. Sönmez, Erdal S. Aydınb, R. Dilbera, “Synthesis and characterization of Mo doped SnO2 thin films with spray pyrolysis,” Superlattices Microstruct., vol. 69, 11
no. November, pp. 175–186, 2014. [15] H. R. W. Daranfed, M.S. Aida, N. Attaf, J. Bougdira, Properties and Characterization of Modern Materials, vol. 33, no. January. springer, 2017. [16] C. Nacereddine et al., “Structural, electrical and magnetic properties of evaporated Ni/Cu and Ni/glass thin films,” Mater. Sci. Eng. B Solid-State Mater. Adv. Technol., vol. 136, no. 2–3, pp. 197–202, 2007. [17] S. Benzitouni et al., “High Sensitivity of Porous Cu-Doped SnO 2 Thin Films to Methanol,” Adv. Nanoparticles, vol. 5, no. 5, pp. 140–148, 2016. [18] T. W. Kim, D. U. Lee, and Y. S. Yoon, “Microstructural, electrical, and optical properties of SnO[sub 2] nanocrystalline thin films grown on InP (100) substrates for applications as gas sensor devices,” J. Appl. Phys., vol. 88, no. 6, p. 3759, 2000. [19] S. Bansal, D. K. Pandya, S. C. Kashyap, and D. Haranath, “Growth ambient dependence of defects, structural disorder and photoluminescence in SnO2 films deposited by reactive magnetron sputtering,” J. Alloys Compd., vol. 583, pp. 186–190, 2014. [20] Y. Cao et al., “Improved photocatalytic activity of Sn 4,” New J. Chem., vol. 28, pp. 218–222, 2004. [21] R. K. Mishra, A. Kushwaha, and P. P. Sahay, “Influence of Cu doping on the structural, photoluminescence and formaldehyde sensing properties of SnO2 nanoparticles,” RSC Adv., vol. 4, no. 8, pp. 3904–3912, 2014. [22] P. Malik, M. Srivastava, R. Verma, M. Kumar, D. Kumar, and J. Singh, “Nanostructured SnO2 encapsulated guar-gum hybrid nanocomposites for electrocatalytic determination of hydrazine,” Mater. Sci. Eng. C, vol. 58, pp. 432–441, 2016. [23] T. Tomita, K. Yamashita, Y. Hayafuji, and H. Adachi, “The origin of n-type conductivity in undoped In2O3,” Appl. Phys. Lett., vol. 87, no. 5, pp. 5–8, 2005.
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Conflict of Interest and Authorship Conformation Form
All authors have participated in drafting the article, as well as analysis and interpretation of the data. This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue. The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
13
Ethics Statement
I affirm the Author Representations noted, and confirm that I have reviewed and complied with the relevant Instructions to Authors, Ethics in Publishing policy This paper entitled “Influence of Ti doping on SnO2 thin films properties prepared by ultrasonic spray technique” by Chafia Khelifi is original work and has not be published or accepted for publication. It is not under consideration at any other journal. Corresponding author: Attaf Abdallah Material Sciences Department, Faculty of Sciences, University of Biskra, Biskra 07000,145 B. P, Algeria. +213770384492, Fax: +21333543190 E-mail : [email protected] Signature Attaf Abdallah
14
Influence of Ti doping on SnO2 thin films properties prepared by ultrasonic spray technique Chafia Khelifi , Abdallah Attaf PII: DOI: Reference:
S2468-0230(19)30764-3 https://doi.org/10.1016/j.surfin.2020.100449 SURFIN 100449
To appear in:
Surfaces and Interfaces
Received date: Revised date: Accepted date:
28 December 2019 15 January 2020 20 January 2020
Please cite this article as: Chafia Khelifi , Abdallah Attaf , Influence of Ti doping on SnO2 thin films properties prepared by ultrasonic spray technique, Surfaces and Interfaces (2020), doi: https://doi.org/10.1016/j.surfin.2020.100449
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Influence of Ti doping on SnO2 thin films properties prepared by ultrasonic spray technique Chafia Khelifi a, Abdallah Attaf a a
Physics Laboratory of Thin Films and Applications LPCMA, University of Biskra,
B.P. 145, Biskra R.P. 07000, Algeria. Corresponding author: Abdallah Attaf E-mail : [email protected] Tel : +213770384492, Fax : +21333543190 Abstract Titanium doped tin dioxide (Ti-SnO2) thin films have been deposited using ultrasonic spray technique to minimize the poor properties of the pure SnO2. The process has been carried out by doping SnO2 films with the appropriate percentage of titanium.
X Ray
Diffraction shows that the rutile structure remains the same of pure tin dioxide but with better crystallization. The titanium doping enhances the transmittance up to 83 % and the conductivity about 1.4 ×102(Ω.cm) -1 approximately, with high figure of merit values about 7, 64.10-3 Ω-1.these results are required specially for bio-sensors and optoelectronics Keywords: Ti-SnO2, thin film, ultrasonic spray, structural properties, optical properties, electrical properties.
1. Introduction Transparent conductive oxides (TCOs) are one of the most important group of material with numerous applications in modern human life. The most attractive TCOs are ZnO, TiO2, In2O3 and SnO2, [1]that last produce high quality of thin films can resist high temperature. Several metal oxides like IGZO, ITO have been demonstrated as bio-sensor membranes owing to their excellent sensing characteristics and SnO2 is one of them, it can detect various gas concentrations by changing the surface conductivity owing to the absorption and desorption process[2] In recent years, research has targeted to improve the physical properties of tin oxide films by doping them with different dopants like fluorine (F-SnO2)[3] [4], antimony (SbSnO2)[5], Indium (ITO)[6] and titanium (Ti-SnO2). Doping can enhance the conductivity of tin oxide films [7] more that, the addition of transition metals like titanium (Ti) improve in 1
performance that observed in several bio-sensing materials by passivation of the defects in the bulk of the sensing material and fixing the dangling bond at the oxide interface. [2] In this paper, titanium doped tin dioxide was prepared by a very simple processing ultrasonic spray onto glass substrates. The method has been employed for the deposition of porous films, dense films, and for powder production [8]. The purpose of this study is explore how the Ti concentration affect the properties of SnO2 films especially the structural, optical and the electrical properties to explore the most suitable application of these films. 2. Experimental methodology For deposit Ti-SnO2 thin films by easy and low cost technique the ultrasonic spray, we used an alcoholic precursor solution consisting of stannic chloride SnCl4, dissolving in methanol that served as a starting solution. The Titanium (IV) isopropoxide (C 12H28O4Ti) was added to the starting solution as source of titanium. Spray pyrolysis was done at different Ti concentrations from 0 to 5 at% onto glass substrates heated at 400 °C for 5 min, when the nozzle-substrate distance is 5cm. 3. Results and discussion 3.1 Scanning electron microscopy The scanning microscopy was used to investigate the surface topography of the various ti-sno2 samples. This sample had done by using Zeiss-SMT LEO 1540 XB scanning electron microscopy. Fig.1 shows the surface morphology of spray deposited Ti-SnO2 film at 5 at %:
Fig.1. Scanning electron micrograph of nanocrystalline Ti-SnO2 sample at 5 at %Ti doping. 2
It is clear in the SEM image that the 5 at% Ti doped SnO2 film has big homogenous with middle size grains of the order 50 nm approximately. In addition, the incorporation of Ti made a very smooth surface with less roughness which giving rise to increase the transmittance to 80% in the visible region. The SnO2 doping with titanium at 5 at% represents a very smooth and uniform film, thus enhance both of transmittance, and of conductivity. 3.2 Growth Velocity Thickness of Ti-SnO2 samples prepared at different Ti doping was calculated using gravimetric method which based on the coating mass m, the density ρ of SnO2 and the area A on which the material is deposited [9]: t=
⁄
(1)
1200
1100
Thickness (nm)
1000
900
800
700
600 0
1
2
3
4
5
Ti concentration (at %)
Fig. 2. Ti-SnO2 films thicknesses variations at different Ti concentration. Fig.2 shows proportion between decreasing in thickness as increasing Ti concentration uniformly, maybe that is due to Ti atom size that is smaller than Sn and O atoms. With augmentation of the quantity of titanium, they occupy substitution sites of Sn, as the rayon of Ti is smaller than Sn so, the size of the mesh decreases and consequently the thickness decreases too. More that, the growth orientation was changed with doping thus made small grains. 3.3 Structural characteristics The crystal structure was determined by XRD of spray deposited SnO 2 and Ti-SnO2 thin films with different titanium doping concentration are showing in Fig.3: 3
800
(200)
(110)
700 (211)
(310) (301)
5%
600
Intensity (a,u)
500 400
4% 300
3% 200 100
2%
0
0% 10
20
30
40
50
60
70
80
2 (°)
Fig. 3. XRD spectrum of Ti doped SnO2 thin films at different Ti doping. It is interesting to note that from a general view, all X-ray diffraction patterns look like the same as the pure SnO2 sample [10]. These peaks found are in good agreement with the ASTM data, which confirm the tetragonal (cassiterite) structure of our deposits as Batzill et al [11] found it. The diagram in Fig.3 shows that several peaks are present with a preferential orientation according to (200) which determines a strong crystallographic texture or low energy. There are minority peaks according to (110), (101), (211), (310) and (301) reflecting the poly crystallinity of the pure structure. More that, by comparing our results with the ASTM files, we notice a slight shift of the peaks that reflects the creation of the constraints. 3.4 Figure of merit In order to evaluate the efficiency of Ti-SnO2 films and when they 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 [12]: ⁄
(3)
4
0% 1% 2% 3% 4% 5%
16 14
.10-3(ohm-1)
12 10 8 6 4 2 0 500
600
700
800
900
Wavelenght (nm)
Fig. 4. Figure of merit values of Ti-SnO2 at various Ti concentrations. It is clear in Fig.4 that figure of merit values decrease when the Ti concentration increase, and the best values are at the pure SnO2. When the 4 at% Ti-SnO2 seem better than the others doping values. 3.5 Crystallite size and dislocation density Generally, the average crystallites size evaluated from XRD results according to the Scherrer formula [13]: ⁄
D=
(4)
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 Ti doping are presenting in the Fig.5 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 [14] : δ= ⁄
(5)
5
0,0014 55 0,0012
0,0008
40
35
0,0006
30
0,0004
25
2
0,0010
45
dislocation (line/nm )
Crystallite size (nm)
50
0,0002 0
1
2
3
4
5
Ti concentration (at %)
Fig. 5. Variation of crystallite size and dislocation of Ti-SnO2 at various Ti doping. The variation in grain size shows an increase in the beginning then after it was decrease when the dopant concentration augment. It is observed that the grain size increases to the value 47.48 nm for a percentage of 1 and 2% when the larger grains are required for the fabrication of high efficiency solar cells [15]. After this value, it was decreases and stabilized at 40.69 nm. We can interpret the increase of the grains size by the reconstruction nucleation sites through the increase of titanium atoms so the quality of crystalline improves. In addition, A. panimaya see that larger grains were formed because of the higher oxygen affinity of Ti [2]. The decrease in the grain size is due to the decrease in thickness because we have increase dislocation in our films or less density of grains that responsible to do a complete nucleation [16]. 3.6 Optical properties The transmittance spectra obtained by UV-visible spectroscopy as function of wavelength over spectral range 350-1200 nm are showing in Fig 6:
6
100
Transmittance %
80
60
40
0% 1% 2% 3% 4% 5%
20
0 300
400
500
600
700
800
900
1000
1100
1200
Wavelenght (nm)
Fig. 6. Optical transmittance spectra of Ti-SnO2 thin films at different Ti concentration. Fig.6. shows that the optical transmission spectra of pure and Ti-doped SnO2 thin films deposited at different Ti concentrations. It is clear that the films show high transparency in the range of [400 - 1100 nm], where the highest transmittance values up to 83% approximately. At first, the transmittance decrease with the augmentation of Ti then after it started to increase but with less fringes, however 4% represents high transparency with more fringes, which confirm that the film prepared under these conditions is very smooth and uniform due to the multiple reflections on the different interfaces [17]. The decrease is due to the existence of defects in the lattice, which made deformation and no homogeneity structure thus affect the morphology and consequently decrease the transmittance by scattering of light. However, the increase of transmittance is due to the decrease both of the defects and the film thickness, vice versa is happened with the decrease. 3.7 Photoluminescence We used photoluminescence (PL) spectroscopy technique for investigating the structure, defect, impurity levels and quality of thin films. Fig.7 represents the photoluminescence spectra of the pure and Ti-SnO2 films:
7
500
0% 1% 2% 3% 4% 5%
486 463
400
PL Intensity (a.u )
399 300
531
200
100
0
400
450
500
550
600
650
700
Wavelenght (nm)
Fig. 7. Photoluminescence spectra of Ti doped SnO2 thin films. A broad dominant peak is observed in the range 390-540 nm shows three emissions bands, as presented in Fig.7; this no homogeneity broadened luminescence might be due to all the luminescent centers, such as Nano crystals and defects in the Ti-SnO2 films [18]. The first peak at 399 nm corresponding to the UV emissions band due to oxygen vacancies that forms the donor levels and were the responsible for available electrons in the CB [19]. The second peak positioned at 486 nm with shoulder peak at 463 nm, corresponding to the blue emissions signal that a new defect level presented into the band gap by the Ti doping and that peak is absence in the pure, which confirm that a new defect in the others is caused by Ti doping. A very low intensity pic at 531 agreeing to the green, which created from the electron-hole recombination at the defect sites due to electron transition from the oxygen vacancy [20]. The increase in PL intensity with the increasing Ti doping concentration maybe due to the decrease of band gap energy of Ti-SnO2 samples [21]. 3.8 Band gap Energy The optical band gap energy Eg was calculated according to Tauc’s formula [22]: (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 obtained values variation are showing in the Fig.8: 8
3,97 3,96
Band Gap (eV)
3,95 3,94 3,93 3,92 3,91 3,90 0
1
2
3
4
5
Ti concentration (at %)
Fig. 8. The optical band gap variation of Ti-SnO2 films at different Ti concentrations. The blue emission of PL spectroscopy confirms that the injection of Ti ions in the crystal lattice, created an energy level presented into the band gap so that level maybe near the valence band of SnO2, then it makes a decrease in the band gap. The compressive stress and the decrease of thickness too can be related and responsible to this shift of band gap. 3.9 Electrical properties Using 4-point measurements that permit to indicate how Ti-SnO2 films are resistive, the obtained results of electrical resistivity (ρ) and the conductivity (σ) as function of Ti doping are showing in Fig.9: 220 0,018 200
-1
Conductivity (ohm.cm)
0,014
160 140
0,012
120
0,010
100
0,008
80
Resistivity (ohm.cm)
0,016
180
0,006
60 0,004 40 0
1
2
3
4
5
Ti concentration (at %)
Fig. 9. Variation of conductivity and resistivity of Ti-SnO2 with different Ti doping.
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Fig.9 shows that the electrical conductivity started to decrease until 2 at% of Ti doping then after it started to increase up to 140 (Ω.cm) -1 but the augmentation was slowly and less than the speed of decrease. The decrease of conductivity is due to the incorporation of Ti in the beginning that caused no stoichiometry in SnO2 lattice with some defects. By the other hand the existence of O2 transferred from substrate to the films by temperature effect are muchness. Also in the beginning Takumi Tomita [23] confirm that some cases, the oxygen vacancies cannot act as a native donor that is due to the its defect level which is much lower than the conductive band. The increase of conductivity is due to the decrease of the band gap. More that, the existence of the oxygen vacancy at these concentrations may act the role of donor, so that increase the carrier concentration due to more free electrons transferred from the defect level of O, which became located near the conductive band in the lattice. 4. Conclusion Many researches have done on Ti-SnO2 thin films prepared by ultrasonic spray to enhance its properties. The characterizations techniques show that all Ti-SnO2 films are Polycrystalline with strong orientation along (200) which confirm the highest texture along these orientations. The best values in figure of merit are seen at 4 at% reached to 7, 64.10 -3Ω-1 at 800 wavelength. Photoluminescence spectroscopy confirm that we have UV emission which due to the existence of the oxygen vacancies in one hand, and the presence of new defect level in the band gap due to the incorporation of the titanium. Few changes were noted in the optical band gap between 3.91-3.97 eV that seem constant approximately. The Ti-SnO2 films revealed the maximum transmittance up 83% in the visible region with maximum conductivity about 1.4 ×102 (Ω-cm)-1 at 5 at %. Acknowledgments 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. References [1]
C. Method et al., “Studies on SnO 2 Nanoparticles Prepared by Co-Precipitation Method,” vol. 18, no. 5, pp. 3511–3517, 2018.
[2]
A. P. Selvi et al., “Sensing and structural properties of Ti-doped tin oxide ( SnO2 ) 10
membrane for bio-sensor applications,” Ceram. Int., no. March, pp. 0–1, 2017. [3]
M. D. P. Chafia Khelifi, Abdallah Attaf, Hanane saidi, Anouar Yahia, “Investigation of F doped SnO 2 thin fi lms properties deposited via ultrasonic spray technique for several applications,” Surfaces and Interfaces, vol. 15, no. April, pp. 244–249, 2019.
[4]
B. Benhaoua, S. Abbas, A. Rahal, A. Benhaoua, and 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., vol. 83, pp. 78–88, 2015.
[5]
S. Myint and P. Kaung, “Optical and electrical properties of polycrystalline SnO 2 : Sb thin films,” Yangon Inst. Educ., vol. 4, no. 1, pp. 1–9, 2012.
[6]
M. Maaza, O. Nemraoui, A. C. Beye, C. Sella, and T. Derry, “Induced structural damages by He + irradiation in conducting transparent indium – tin oxide thin films,” vol. 90, pp. 111–119, 2006.
[7]
C. Wang, W. Zeng, L. Luo, P. Zhang, and Z. Wang, “Gas-sensing properties and mechanisms of Cu-doped SnO2 spheres towards H2S,” Ceram. Int., vol. 42, no. 8, pp. 10006–10013, 2016.
[8]
D. Perednis and L. J. Gauckler, “Thin Film Deposition Using Spray Pyrolysis,” Electroceramics, vol. 14, pp. 103–111, 2005.
[9]
C. Khelifi, A. Attaf, H. Saidi, A. Yahia, and 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., vol. 127, no. 23, pp. 11055–11062, 2016.
[10] C. Khelifi, A. Attaf, H. Saidi, A. Yahia, and A. Saadi, “Optik Effect of solution flow on the properties of tin dioxide SnO 2 thin films deposited by spray pyrolysis technique,” Opt. - Int. J. Light Electron Opt., vol. 127, no. 23, pp. 11055–11062, 2016. [11] U. D. M. Batzill, “The surface and materials science of tin oxide,” Prog. Surf. Sci., vol. 79, pp. 47–154, 2005. [12] B. Thangaraju, “Structural and electrical studies on highly conducting spray deposited fluorine and antimony doped SnO2 thin films from SnCl2 precursor,” Thin Solid Films, vol. 402, no. 1–2, pp. 71–78, 2002. [13] N. H. Touidjen, B. Bendahmane, M. L. Zeggar, F. Mansour, and M. S. Aida, “SnO2 thin film synthesis for organic vapors sensing at ambient temperature,” Sens. BioSensing Res., vol. 11, no. December, pp. 52–57, 2016. [14] Güven and U. T. Sönmez, Erdal S. Aydınb, R. Dilbera, “Synthesis and characterization of Mo doped SnO2 thin films with spray pyrolysis,” Superlattices Microstruct., vol. 69, 11
no. November, pp. 175–186, 2014. [15] H. R. W. Daranfed, M.S. Aida, N. Attaf, J. Bougdira, Properties and Characterization of Modern Materials, vol. 33, no. January. springer, 2017. [16] C. Nacereddine et al., “Structural, electrical and magnetic properties of evaporated Ni/Cu and Ni/glass thin films,” Mater. Sci. Eng. B Solid-State Mater. Adv. Technol., vol. 136, no. 2–3, pp. 197–202, 2007. [17] S. Benzitouni et al., “High Sensitivity of Porous Cu-Doped SnO 2 Thin Films to Methanol,” Adv. Nanoparticles, vol. 5, no. 5, pp. 140–148, 2016. [18] T. W. Kim, D. U. Lee, and Y. S. Yoon, “Microstructural, electrical, and optical properties of SnO[sub 2] nanocrystalline thin films grown on InP (100) substrates for applications as gas sensor devices,” J. Appl. Phys., vol. 88, no. 6, p. 3759, 2000. [19] S. Bansal, D. K. Pandya, S. C. Kashyap, and D. Haranath, “Growth ambient dependence of defects, structural disorder and photoluminescence in SnO2 films deposited by reactive magnetron sputtering,” J. Alloys Compd., vol. 583, pp. 186–190, 2014. [20] Y. Cao et al., “Improved photocatalytic activity of Sn 4,” New J. Chem., vol. 28, pp. 218–222, 2004. [21] R. K. Mishra, A. Kushwaha, and P. P. Sahay, “Influence of Cu doping on the structural, photoluminescence and formaldehyde sensing properties of SnO2 nanoparticles,” RSC Adv., vol. 4, no. 8, pp. 3904–3912, 2014. [22] P. Malik, M. Srivastava, R. Verma, M. Kumar, D. Kumar, and J. Singh, “Nanostructured SnO2 encapsulated guar-gum hybrid nanocomposites for electrocatalytic determination of hydrazine,” Mater. Sci. Eng. C, vol. 58, pp. 432–441, 2016. [23] T. Tomita, K. Yamashita, Y. Hayafuji, and H. Adachi, “The origin of n-type conductivity in undoped In2O3,” Appl. Phys. Lett., vol. 87, no. 5, pp. 5–8, 2005.
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Conflict of Interest and Authorship Conformation Form
All authors have participated in drafting the article, as well as analysis and interpretation of the data. This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue. The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
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Ethics Statement
I affirm the Author Representations noted, and confirm that I have reviewed and complied with the relevant Instructions to Authors, Ethics in Publishing policy This paper entitled “Influence of Ti doping on SnO2 thin films properties prepared by ultrasonic spray technique” by Chafia Khelifi is original work and has not be published or accepted for publication. It is not under consideration at any other journal. Corresponding author: Attaf Abdallah Material Sciences Department, Faculty of Sciences, University of Biskra, Biskra 07000,145 B. P, Algeria. +213770384492, Fax: +21333543190 E-mail : [email protected] Signature Attaf Abdallah
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