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Effect of (Co, Fe, Ni) doping on structural, optical and electrical properties of sprayed SnO2 thin film
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Effect of (Co, Fe, Ni) doping on structural, optical and electrical properties of sprayed SnO2 thin film L. Soussi , T. Garmim , O. Karzazi , A. Rmili , A. El Bachiri Writing - original draf , A. Louardi , H. Erguig PII: DOI: Reference:
S2468-0230(19)30748-5 https://doi.org/10.1016/j.surfin.2020.100467 SURFIN 100467
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Surfaces and Interfaces
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19 December 2019 21 January 2020 5 February 2020
Please cite this article as: L. Soussi , T. Garmim , O. Karzazi , A. Rmili , A. El Bachiri Writing - original draf , A. Louardi , H. Erguig , Effect of (Co, Fe, Ni) doping on structural, optical and electrical properties of sprayed SnO2 thin film, Surfaces and Interfaces (2020), doi: https://doi.org/10.1016/j.surfin.2020.100467
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Effect of (Co, Fe, Ni) doping on structural, optical and electrical properties of sprayed SnO2 thin film. L. Soussi1*, T. Garmim4, O. Karzazi2, A. Rmili1, A. El Bachiri1,3, A. Louardi4, H. Erguig1 1
Laboratoire d’Ingénieries des Systèmes Electriques et des Télécommunications, Ecole Nationale des Sciences Appliquées de Kénitra (ENSAK), Morocco. 2
Laboratoire d’Ingénierie d’Electrochimie de Modélisation et Environnement Faculté des Sciences, Université Sidi Mohammed Ben Abdellah, Fès, Morocco. 3Laboratoire Bio-Géosciences et Ingénierie des Matériaux (LBGIM), Ecole normale supérieure, Université Hassan II, BP 50069 Casablanca, Morocco. 4
Laboratoire de Physique de la Matière Condensée (LPMC), Département de physique, Faculté des sciences, Université Chouaib-Doukkali, El Jadida, Morocco.
*Corresponding author: Email: [email protected]
Abstract: Pure and (Co, Fe, Ni)-doped tin oxide (SnO2) thin film were deposited on glass substrates kept at 400° C by spray pyrolysis technique. The effect of (Co, Fe, Ni) doping on the structural, morphological, optical and electrical properties of SnO2 thin films was investigated. First, X-ray diffraction (XRD) study revealed that all the pure and (Co, Fe, Ni)doped SnO2 films were polycrystalline with the tetragonal rutile structure. The scanning electron microscopy (SEM) showed a smooth and dense surface with large grain sizes for doped films. Second, the optical analysis by means of the transmittance revealed the films depend on the type of the dopant and the doping promotes the decrease of the estimated band gap. Finally, the I-V curves revealed that the undoped and (Co, Fe, and Ni)-doped SnO2 thin films obey ohms law and the resistivity decreased with doping.
Keywords: (Co, Fe, Ni)-dopedSnO2, spray pyrolysis, optical properties, electrical properties
Introduction Transparent conductive oxide thin films such as ZnO, TiO2 and SnO2 have attracted wide research interests owing to their unique physical and chemical properties and diverse potential applications in gas sensing, solar energy conversion, ultraviolet light emitting diodes, and high power electronic devices [1,2]. Among these, tin oxide (SnO2) with the rutile structure is a promising functional n-type semiconductor material with a wide band gap (Eg = 3.65 eV at 300 K) and high chemical and mechanical stability, has been used extensively in energy storage and solar cells, photocatalysis, gas sensors, transparent conducting electrodes and optoelectronic devices [3,4]. However, pure SnO2 thin film has low electrical conductivity due to its intrinsically low carrier density and mobility [5]. Hence, many elements were used as a dopant source to improve the electrical and optical characteristics of SnO2 thin films. According to the literature survey, many research groups have doped transition metal ions such as Co, Al, Fe, Zn, Mn and Sb in SnO2 films [6, 7]. Incorporation of transition metal atoms and alkali metals into SnO2 thin films can influence their optoelectrical and magnetic properties as reported for equivalent TCOs films such as ZnO [8] TiO2 [9] and CeO2 [10]. Only a few research studies has compared the effect of different dopant as Fe, Co, and Ni on the electrical and optical properties of SnO2 thin films prepared by spray pyrolysis method using simple perfume atomizer. Pure and doped SnO2 thin films can be prepared using several methods, including pulsed laser deposition [11], thermal evaporation [12], sputtering [13], co-precipitation [14] and spray pyrolysis sol-gel deposition [15]. Spray pyrolysis is an effective technique adapted for production because of its low cost, controllable thickness, as well as high uniformity and flexibility, and easy control of the chemical composition of the thin film [16]. Hence, in this work, spray pyrolysis was employed to obtain pure and 4 at. % (Co, Fe, Ni)-doped SnO2 thin films on the glass substrates kept at 400° C. The effect of adding Co, Fe, Ni ions on the structural, optical and electrical properties of the elaborated thin films was investigated.
Experimental procedure SnO2 thin films were deposited by spray pyrolysis method from 0.15M of SnCl2 dehydrate dissolved in distilled water, few drops of chlorhydric acid were added to obtain a transparent and homogeneous solution. The doping elements (Co, Fe, and Ni) were added to the precursor solution from there chlorides, with the concentration equal to 4 at. %. The pure and doped films were deposited on glass substrates heated at 400°C. Before the deposition process, the glass substrate was cleaned with acetone and chlorhydric acid, then rinsed with deionized water and dried in air. The as-deposited films were transparent uniform and strongly adherent to the glass substrate. The structure crystallinity and phase of undoped and doped SnO2 thin films were determined with a diffractometer “ XPERT-PRO” model using CuKα radiation ( λ = 1.542±0.002Å) with 2θ ranging from 10° to 80°. The surface morphology of the films was characterized by scanning electron microscopy (SEM) (Model: Quattro). Optical transmittance of the deposited films was carried out in the wavelength range of 300-2500nm using a SHIMADZU 3101PC, UV-Vis-NIR spectrophotometer. Electrical resistivity measurements were carried out at dark and at room temperature with the four probe method using Keighley 2400. The film thickness was estimated at about 500 nm using the gravimetric method. Result and discussion Structural properties The effect of (Co, Fe, and Ni) doping on the structural properties of SnO2 thin films was investigated by X-ray diffraction analysis. Fig. 1 shows the XRD patterns of undoped and (Co, Fe, and Ni)-doped SnO2 thin films. These spectra indicate that all the films are polycrystalline and exhibit well defined tetragonal SnO2 rutile structure. All the observed XRD peaks (110), (101), (200), (211), (220), and (301) of the undoped and doped films are well matched to the corresponding JCPDS card data, JCPDS (41-1445) and no other secondary phases related to the doping elements were found. That could confirm that (Co, Fe, Ni) were substituted into the host SnO2 lattice. Moreover, it can be seen a slight shift in the Bragg angle of the (110) peak toward higher value of θ, which can be due to a decrease in the lattice parameters due to the substitution of Sn4+ (0.71Å) by smaller ionic radius of doping elements Co2+ (0.58Å), Fe2+ (0.64Å) and Ni2+ (0.69Å) [17]. It could be observed also from the
XRD pattern that the undoped SnO2 films are highly oriented along (110) direction, which is the same preferred direction for films doped by Fe and Ni but for those doped by Co, a shift in the preferred orientation toward (101) peak was observed. Similar behavior for Co-doped SnO2 thin films was also observed in other studies [18,19].
The lattice parameters “a” and “c” and the cell volume, calculated from the diffraction peak position, are listed in table1. It was found a slight decrease related to Co, Fe, and Ni doing confirming the substitution of the larger ionic radius Sn4+ by the smaller ionic radius of Co2+, Fe2+, and Ni2+ in SnO2 lattice. Table 1: The calculated lattice parameters and unit cell volume for pure and Co, Fe, and Ni)doped SnO2 thin films Sample SnO2 undoped Co-SnO2 Fe-SnO2 Ni-SnO2
a (Å) 4.744 4.730 4.736 4.734
c (Å) 3.191 3.193 3.188 3.183
Cell volume V (Å3) 71.82 71.47 71.51 71.35
Table 2 summarizes the calculated values of some microstructural parameters of pure and (Co, Fe, and Ni)-doped SnO2 thin films. The average crystallite size of the films was calculated using the Scherrer formula based on the XRD patterns. Dhkl 0.9
hkl .cos( hkl )
(1)
Where D is the average crystallite size, λ is the X-ray wavelength (1.542Å), β the full width at half maximum (FWHM) of the particular peak and θ is the Bragg’s angle. From the calculations, it was found that the average crystallite size increase with adding the dopant elements and it takes higher values for films doped by Co and Ni. It can be concluded that there is an improvement of the crystalline structure of the SnO2 films by adding the Co, Fe, and Ni dopants The texture coefficient Tc(hkl) of the film was calculated using the following equation [20]:
I (hkl ) TC (hkl )
1 N
I 0 (hkl ) hkl I (hkl ) I0 (hkl )
(2)
Where Tc (hkl)is the texture coefficient of the hkl plane, Ihkl is the measured intensity of peak (hkl) deduced from XRD pattern. I0(hkl) is the standard peaks intensity according to JCPDS card (41-1445) and N is the number of peaks in XRD pattern. From these values, it can be noticed that the Tc (hkl) corresponding to (110) plane is high for pure and for (Fe, Ni) doped SnO2 thin films. However, for Co doped SnO2 films, the (101) direction takes higher value due to the change in the preferential orientation for SnO2 films doped with Co. The microstrain (ε) is one of the most important factors in nanostructured thin films, and adversely affecting the structural properties. The microstrain (ε) of pure SnO2 and (Co, Fe, and Ni)-doped SnO2 films for the (110), (101) and (211) peaks, was calculated using the equation below [21]:
1 cos sin D
Where
(3)
is the full-width at half-maximum of the peak, and D is the average grain size. As
shown in Table 2, doping SnO2 lattice with Co, Ni and Fe lead to a decrease of the microstrain, which indicates the improvement of the crystalline quality of SnO2 films with doping. The increase in the average crystallite size and slight decrease of the lattice parameters can reduce the number of defects such as grain boundaries and dislocation density in the films. The dislocation density (δ) defined as the length of dislocation lines per unit volume of the crystal is given by the following equation [22]:
1 D2
(4)
Where D is the average crystallite size, the mean calculated values are presented in table 2, it has been found that the dislocation density decrease with incorporation of dopants and takes minimum value equal to 0.32.10-3 lines/nm2.
Table 2: Microstructural parameters of pure and (Co, Fe, Ni)-doped SnO2 thin films calculated from XRD patterns Sample Plan 2θ dspacing Dhkl Tc (hkl) Microstrain δ (10-3 (hkl) ε nm-2)
SnO2 undoped Co-SnO2
Fe-SnO2
Ni-SnO2
(110) (101) (211) (110) (101) (211) (110) (101) (211) (110) (101) (211)
26.56 33.84 51.70 26.65 33.86 51.78 26.61 33.88 51.65 26.62 33.92 51.71
3.354 2.648 1.767 3.345 2.647 1.765 3.349 2.645 1.769 3.347 2.642 1.767
40.41 32.46 34.53 57.55 47.60 32.82 46.82 19.58 81.61 57.55 58.49 50.66
3.31 0.49 0.31 0.85 1.46 0.68 2.07 0.78 0.56 2.80 0.43 0.55
0.414 0.407 0.255 0.290 0.278 0.269 0.357 0.675 0.108 0.291 0.225 0.175
0.78
0.47
0.41
0.32
Morphological characterization Morphological characteristics of pure and (Co, Fe, Ni)-doped SnO2 thin films were carried out by the scanning electron microscopy (SEM). It displays that Fe, Co, and Ni-doped SnO2 films have a smooth and dense surface with large grain sizes compared to pure SnO2 films, suggesting that there was uniform nucleation throughout the substrate surface during the growth of SnO2 films doped with Co, Fe, and Ni. This indicates the crystalline quality of SnO2 thin film is greatly improved by (Co, Fe, and Ni)-doping. SEM micrographs show also that the surface morphology depends on the dopant element confirming the substitution of Sn4+ with the transition metal ions into SnO2 lattice. That is in a good agreement with our XRD results. Based on this structural study, we point out that the increase in crystallite size (D) and the decrease in the microstrain (ε) calculated values are attributed to doping SnO2 lattice by (Co, Ni, and Fe) element, which improves the structure of SnO2 thin films. This phenomenon is consistent with the effect of doping on the interplanar spacing dspacing, lattice parameters “a” and “c” for tetragonal SnO2 rutile structure. Moreover, XRD studies revealed that doping elements were successfully incorporated into SnO2 lattice without the formation of additional phases, such that the rutile structure of SnO2 was preserved.
Optical properties The optical properties of pure and (Co, Fe, Ni)-doped SnO2thin films were determined from the transmission measurement in the range of 300–900 nm (Fig. 3). Obviously, all of these films exhibit high transmission within the visible range (60 and 90%), indicating the good optical quality of the undoped and doped films. In addition, it can be seen that the optical transmission depends substantially on the nature of dopant. Thus, doping with Fe increases the optical transmission compared to pure SnO2 films. This increase can be due to the good crystalline quality and homogeneity of those films according to the X-ray diffraction and SEM results. However, the Co and Ni-doping decrease optical transmission. This decrease may be attributed to the scattering increase caused by the rise in the surface morphology roughness and to the crystalline state as was shown from XRD analysis.
The absorption coefficient data was used to determine the band gap energy Eg using Tauc’s model equation [23]:
( h ) A(h Eg )2
(5)
Where hν is the photon energy, A is a constant, thus, a plot of (αhν)2 against hν is a curve line whose intercept on the energy axis gives the band gap energy. Therefore, the band energy gap of pure and (Co, Fe, and Ni)-doped SnO2 films was then determined by extrapolating the linear regions on the energy axis (Fig. 4). Table 3: Band gap values of pure and (Co, Fe, and Ni)-doped SnO2 films. Sample
Pure SnO2
Co-SnO2
Fe-SnO2
Ni-SnO2
Eg (eV)
3.8
3.5
3.4
3.2
Table 3 summarizes the calculated band gap values of pure and (Co, Fe, and Ni)-doped SnO2 thin films. The band gap energy of undoped SnO2 films is 3.8 eV. The obtained band gap energy is in good agreement with the reported one found by other researchers groups for SnO2 films prepared with different methods [24-28]. A decrease of the gap energy with the insertion of Co, Fe and Ni atoms as a dopant has been observed, this can be due to the presence of impurity concentration and disorder in SnO2 lattice, and/or it can be due to grain size increase
and shape effect as reported in other publications [29,30]. Adding dopant elements to SnO2 films can increase the carriers charge concentration as a result the band gap decrease due to Moss-Burstein effect. This behavior may be attributed also to defects introduced after Co, Fe and Ni ions substitute Sn4+ into SnO2 host lattice, due to the difference of the electronegativity and the ionic radius between Sn4+ and the other doping ions.
Electrical properties The voltage-current characteristics at room temperature of (Co, Fe, and Ni)-doped SnO2 thin films are presented in Fig. 5. The results show a linear I–V relations under both forward and reverse bias indicate a good Ohmic contact behavior of the studied films. The resistivity is calculated from the slope of each curve and tabulated in Table 4. Table 4: resistivity of pure and (Co, Fe, and Ni)-doped SnO2 thin films Sample
Pure SnO2
Fe-SnO2
Co-SnO2
Ni-SnO2
Resistivity (Ω.cm)
2.03×103
6.08×103
1.14×102
2.83×101
The measured resistivity value found to decrease from 2,03 kΩ.cm for pure SnO2 film to 28,3 Ω.cm for Ni-doped SnO2 films. This indicates the improvement of the electrical property often oxide thin films by including (Co, Ni, and Fe) ions as a dopant in the lattice due to the increase in carrier charge mobility. The decrease in the electrical conductivity for Fe-doped SnO2 films may be due to the presence of some crystalline defect. These observations are in good agreement with the results obtained by T. Serin and al. [31], and N.Salah and al [32].
Conclusion
Pure and (Co, Fe, Ni)-doped SnO2 thin films with the doping concentration of 4 at. % was successfully deposited on glass substrates using a spray pyrolysis technique. The effect of adding (Co, Fe, and Ni) on the structural, electrical, and optical properties of SnO2 thin films was investigated.
The XRD pattern revealed that all deposited films exhibited a
polycrystalline tetragonal rutile structure. The lattice parameters and the microstrain decreased with adding the transition metal dopant.SnO2 thin films showed a more uniform grain size and a homogeneous surface by adding (Co, Fe, and Ni). The optical analysis revealed the films depend on the type of the dopant and the (Co, Fe, Ni) doping promotes the decrease of the estimated band gap. The I-V curves revealed that all the films obey ohms law and the resistivity decreased with doping, which suggests that (Co, Fe, Ni) doped SnO2thin films are suitable for application in optoelectronic devices.
Authors contributions (CRediT): 1- L.Soussi: Writing-original draft, Investigation, Writing-review and editing Analysis, Conceptualization. 2- T.Garmim: Writing-original draft, Investigation 3- O.Karzazi: Writing-original draft, Analysis. 4- A.Rmili: Writing-original draft, Investigation. 5- A.ElBachiri: Writing-original draft and revising, Writing-review and editing, Validation. 6- A.Louardi: Writing-original draft and revising, Writing-review and editing, Validation. 7- H.Erguig: Conceptualization, Methodology, Validation, Resources, Supervision. Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
o All the authors have participated in (i) conception and design or analysis and interpretation of the data; (ii) drafting the article or revising it critically for important intellectual content; and (iii) approval the final version. o The authors have no affiliation with any organization with direct or indirect financial interest in the subject matter discussed in the manuscript.
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Figure captions:
Fig. 1: XRD spectra of pure SnO2 and (Co, Fe, Ni)-doped SnO2 thin films. Fig. 2: SEM micrographs of pure and (Co, Fe, Ni)-doped SnO2 thin films. Fig. 3: Transmission spectra of pure and (Co, Fe, Ni)-doped SnO2 thin films Fig.4: (αhν)2 versus (hν) plots of pure and (Co, Fe, Ni)-doped SnO2 films. Fig 5: I-V curves for the pure and (Co, Fe, Ni)-doped SnO2 thin films.
Fig. 1
Pure SnO2
Co-doped SnO2
Fig2:
Fe-doped SnO2
Ni-doped SnO2 Fig 2:
Fig 3:
Fig 4:
Fig 5:
Effect of (Co, Fe, Ni) doping on structural, optical and electrical properties of sprayed SnO2 thin film L. Soussi , T. Garmim , O. Karzazi , A. Rmili , A. El Bachiri Writing - original draf , A. Louardi , H. Erguig PII: DOI: Reference:
S2468-0230(19)30748-5 https://doi.org/10.1016/j.surfin.2020.100467 SURFIN 100467
To appear in:
Surfaces and Interfaces
Received date: Revised date: Accepted date:
19 December 2019 21 January 2020 5 February 2020
Please cite this article as: L. Soussi , T. Garmim , O. Karzazi , A. Rmili , A. El Bachiri Writing - original draf , A. Louardi , H. Erguig , Effect of (Co, Fe, Ni) doping on structural, optical and electrical properties of sprayed SnO2 thin film, Surfaces and Interfaces (2020), doi: https://doi.org/10.1016/j.surfin.2020.100467
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Effect of (Co, Fe, Ni) doping on structural, optical and electrical properties of sprayed SnO2 thin film. L. Soussi1*, T. Garmim4, O. Karzazi2, A. Rmili1, A. El Bachiri1,3, A. Louardi4, H. Erguig1 1
Laboratoire d’Ingénieries des Systèmes Electriques et des Télécommunications, Ecole Nationale des Sciences Appliquées de Kénitra (ENSAK), Morocco. 2
Laboratoire d’Ingénierie d’Electrochimie de Modélisation et Environnement Faculté des Sciences, Université Sidi Mohammed Ben Abdellah, Fès, Morocco. 3Laboratoire Bio-Géosciences et Ingénierie des Matériaux (LBGIM), Ecole normale supérieure, Université Hassan II, BP 50069 Casablanca, Morocco. 4
Laboratoire de Physique de la Matière Condensée (LPMC), Département de physique, Faculté des sciences, Université Chouaib-Doukkali, El Jadida, Morocco.
*Corresponding author: Email: [email protected]
Abstract: Pure and (Co, Fe, Ni)-doped tin oxide (SnO2) thin film were deposited on glass substrates kept at 400° C by spray pyrolysis technique. The effect of (Co, Fe, Ni) doping on the structural, morphological, optical and electrical properties of SnO2 thin films was investigated. First, X-ray diffraction (XRD) study revealed that all the pure and (Co, Fe, Ni)doped SnO2 films were polycrystalline with the tetragonal rutile structure. The scanning electron microscopy (SEM) showed a smooth and dense surface with large grain sizes for doped films. Second, the optical analysis by means of the transmittance revealed the films depend on the type of the dopant and the doping promotes the decrease of the estimated band gap. Finally, the I-V curves revealed that the undoped and (Co, Fe, and Ni)-doped SnO2 thin films obey ohms law and the resistivity decreased with doping.
Keywords: (Co, Fe, Ni)-dopedSnO2, spray pyrolysis, optical properties, electrical properties
Introduction Transparent conductive oxide thin films such as ZnO, TiO2 and SnO2 have attracted wide research interests owing to their unique physical and chemical properties and diverse potential applications in gas sensing, solar energy conversion, ultraviolet light emitting diodes, and high power electronic devices [1,2]. Among these, tin oxide (SnO2) with the rutile structure is a promising functional n-type semiconductor material with a wide band gap (Eg = 3.65 eV at 300 K) and high chemical and mechanical stability, has been used extensively in energy storage and solar cells, photocatalysis, gas sensors, transparent conducting electrodes and optoelectronic devices [3,4]. However, pure SnO2 thin film has low electrical conductivity due to its intrinsically low carrier density and mobility [5]. Hence, many elements were used as a dopant source to improve the electrical and optical characteristics of SnO2 thin films. According to the literature survey, many research groups have doped transition metal ions such as Co, Al, Fe, Zn, Mn and Sb in SnO2 films [6, 7]. Incorporation of transition metal atoms and alkali metals into SnO2 thin films can influence their optoelectrical and magnetic properties as reported for equivalent TCOs films such as ZnO [8] TiO2 [9] and CeO2 [10]. Only a few research studies has compared the effect of different dopant as Fe, Co, and Ni on the electrical and optical properties of SnO2 thin films prepared by spray pyrolysis method using simple perfume atomizer. Pure and doped SnO2 thin films can be prepared using several methods, including pulsed laser deposition [11], thermal evaporation [12], sputtering [13], co-precipitation [14] and spray pyrolysis sol-gel deposition [15]. Spray pyrolysis is an effective technique adapted for production because of its low cost, controllable thickness, as well as high uniformity and flexibility, and easy control of the chemical composition of the thin film [16]. Hence, in this work, spray pyrolysis was employed to obtain pure and 4 at. % (Co, Fe, Ni)-doped SnO2 thin films on the glass substrates kept at 400° C. The effect of adding Co, Fe, Ni ions on the structural, optical and electrical properties of the elaborated thin films was investigated.
Experimental procedure SnO2 thin films were deposited by spray pyrolysis method from 0.15M of SnCl2 dehydrate dissolved in distilled water, few drops of chlorhydric acid were added to obtain a transparent and homogeneous solution. The doping elements (Co, Fe, and Ni) were added to the precursor solution from there chlorides, with the concentration equal to 4 at. %. The pure and doped films were deposited on glass substrates heated at 400°C. Before the deposition process, the glass substrate was cleaned with acetone and chlorhydric acid, then rinsed with deionized water and dried in air. The as-deposited films were transparent uniform and strongly adherent to the glass substrate. The structure crystallinity and phase of undoped and doped SnO2 thin films were determined with a diffractometer “ XPERT-PRO” model using CuKα radiation ( λ = 1.542±0.002Å) with 2θ ranging from 10° to 80°. The surface morphology of the films was characterized by scanning electron microscopy (SEM) (Model: Quattro). Optical transmittance of the deposited films was carried out in the wavelength range of 300-2500nm using a SHIMADZU 3101PC, UV-Vis-NIR spectrophotometer. Electrical resistivity measurements were carried out at dark and at room temperature with the four probe method using Keighley 2400. The film thickness was estimated at about 500 nm using the gravimetric method. Result and discussion Structural properties The effect of (Co, Fe, and Ni) doping on the structural properties of SnO2 thin films was investigated by X-ray diffraction analysis. Fig. 1 shows the XRD patterns of undoped and (Co, Fe, and Ni)-doped SnO2 thin films. These spectra indicate that all the films are polycrystalline and exhibit well defined tetragonal SnO2 rutile structure. All the observed XRD peaks (110), (101), (200), (211), (220), and (301) of the undoped and doped films are well matched to the corresponding JCPDS card data, JCPDS (41-1445) and no other secondary phases related to the doping elements were found. That could confirm that (Co, Fe, Ni) were substituted into the host SnO2 lattice. Moreover, it can be seen a slight shift in the Bragg angle of the (110) peak toward higher value of θ, which can be due to a decrease in the lattice parameters due to the substitution of Sn4+ (0.71Å) by smaller ionic radius of doping elements Co2+ (0.58Å), Fe2+ (0.64Å) and Ni2+ (0.69Å) [17]. It could be observed also from the
XRD pattern that the undoped SnO2 films are highly oriented along (110) direction, which is the same preferred direction for films doped by Fe and Ni but for those doped by Co, a shift in the preferred orientation toward (101) peak was observed. Similar behavior for Co-doped SnO2 thin films was also observed in other studies [18,19].
The lattice parameters “a” and “c” and the cell volume, calculated from the diffraction peak position, are listed in table1. It was found a slight decrease related to Co, Fe, and Ni doing confirming the substitution of the larger ionic radius Sn4+ by the smaller ionic radius of Co2+, Fe2+, and Ni2+ in SnO2 lattice. Table 1: The calculated lattice parameters and unit cell volume for pure and Co, Fe, and Ni)doped SnO2 thin films Sample SnO2 undoped Co-SnO2 Fe-SnO2 Ni-SnO2
a (Å) 4.744 4.730 4.736 4.734
c (Å) 3.191 3.193 3.188 3.183
Cell volume V (Å3) 71.82 71.47 71.51 71.35
Table 2 summarizes the calculated values of some microstructural parameters of pure and (Co, Fe, and Ni)-doped SnO2 thin films. The average crystallite size of the films was calculated using the Scherrer formula based on the XRD patterns. Dhkl 0.9
hkl .cos( hkl )
(1)
Where D is the average crystallite size, λ is the X-ray wavelength (1.542Å), β the full width at half maximum (FWHM) of the particular peak and θ is the Bragg’s angle. From the calculations, it was found that the average crystallite size increase with adding the dopant elements and it takes higher values for films doped by Co and Ni. It can be concluded that there is an improvement of the crystalline structure of the SnO2 films by adding the Co, Fe, and Ni dopants The texture coefficient Tc(hkl) of the film was calculated using the following equation [20]:
I (hkl ) TC (hkl )
1 N
I 0 (hkl ) hkl I (hkl ) I0 (hkl )
(2)
Where Tc (hkl)is the texture coefficient of the hkl plane, Ihkl is the measured intensity of peak (hkl) deduced from XRD pattern. I0(hkl) is the standard peaks intensity according to JCPDS card (41-1445) and N is the number of peaks in XRD pattern. From these values, it can be noticed that the Tc (hkl) corresponding to (110) plane is high for pure and for (Fe, Ni) doped SnO2 thin films. However, for Co doped SnO2 films, the (101) direction takes higher value due to the change in the preferential orientation for SnO2 films doped with Co. The microstrain (ε) is one of the most important factors in nanostructured thin films, and adversely affecting the structural properties. The microstrain (ε) of pure SnO2 and (Co, Fe, and Ni)-doped SnO2 films for the (110), (101) and (211) peaks, was calculated using the equation below [21]:
1 cos sin D
Where
(3)
is the full-width at half-maximum of the peak, and D is the average grain size. As
shown in Table 2, doping SnO2 lattice with Co, Ni and Fe lead to a decrease of the microstrain, which indicates the improvement of the crystalline quality of SnO2 films with doping. The increase in the average crystallite size and slight decrease of the lattice parameters can reduce the number of defects such as grain boundaries and dislocation density in the films. The dislocation density (δ) defined as the length of dislocation lines per unit volume of the crystal is given by the following equation [22]:
1 D2
(4)
Where D is the average crystallite size, the mean calculated values are presented in table 2, it has been found that the dislocation density decrease with incorporation of dopants and takes minimum value equal to 0.32.10-3 lines/nm2.
Table 2: Microstructural parameters of pure and (Co, Fe, Ni)-doped SnO2 thin films calculated from XRD patterns Sample Plan 2θ dspacing Dhkl Tc (hkl) Microstrain δ (10-3 (hkl) ε nm-2)
SnO2 undoped Co-SnO2
Fe-SnO2
Ni-SnO2
(110) (101) (211) (110) (101) (211) (110) (101) (211) (110) (101) (211)
26.56 33.84 51.70 26.65 33.86 51.78 26.61 33.88 51.65 26.62 33.92 51.71
3.354 2.648 1.767 3.345 2.647 1.765 3.349 2.645 1.769 3.347 2.642 1.767
40.41 32.46 34.53 57.55 47.60 32.82 46.82 19.58 81.61 57.55 58.49 50.66
3.31 0.49 0.31 0.85 1.46 0.68 2.07 0.78 0.56 2.80 0.43 0.55
0.414 0.407 0.255 0.290 0.278 0.269 0.357 0.675 0.108 0.291 0.225 0.175
0.78
0.47
0.41
0.32
Morphological characterization Morphological characteristics of pure and (Co, Fe, Ni)-doped SnO2 thin films were carried out by the scanning electron microscopy (SEM). It displays that Fe, Co, and Ni-doped SnO2 films have a smooth and dense surface with large grain sizes compared to pure SnO2 films, suggesting that there was uniform nucleation throughout the substrate surface during the growth of SnO2 films doped with Co, Fe, and Ni. This indicates the crystalline quality of SnO2 thin film is greatly improved by (Co, Fe, and Ni)-doping. SEM micrographs show also that the surface morphology depends on the dopant element confirming the substitution of Sn4+ with the transition metal ions into SnO2 lattice. That is in a good agreement with our XRD results. Based on this structural study, we point out that the increase in crystallite size (D) and the decrease in the microstrain (ε) calculated values are attributed to doping SnO2 lattice by (Co, Ni, and Fe) element, which improves the structure of SnO2 thin films. This phenomenon is consistent with the effect of doping on the interplanar spacing dspacing, lattice parameters “a” and “c” for tetragonal SnO2 rutile structure. Moreover, XRD studies revealed that doping elements were successfully incorporated into SnO2 lattice without the formation of additional phases, such that the rutile structure of SnO2 was preserved.
Optical properties The optical properties of pure and (Co, Fe, Ni)-doped SnO2thin films were determined from the transmission measurement in the range of 300–900 nm (Fig. 3). Obviously, all of these films exhibit high transmission within the visible range (60 and 90%), indicating the good optical quality of the undoped and doped films. In addition, it can be seen that the optical transmission depends substantially on the nature of dopant. Thus, doping with Fe increases the optical transmission compared to pure SnO2 films. This increase can be due to the good crystalline quality and homogeneity of those films according to the X-ray diffraction and SEM results. However, the Co and Ni-doping decrease optical transmission. This decrease may be attributed to the scattering increase caused by the rise in the surface morphology roughness and to the crystalline state as was shown from XRD analysis.
The absorption coefficient data was used to determine the band gap energy Eg using Tauc’s model equation [23]:
( h ) A(h Eg )2
(5)
Where hν is the photon energy, A is a constant, thus, a plot of (αhν)2 against hν is a curve line whose intercept on the energy axis gives the band gap energy. Therefore, the band energy gap of pure and (Co, Fe, and Ni)-doped SnO2 films was then determined by extrapolating the linear regions on the energy axis (Fig. 4). Table 3: Band gap values of pure and (Co, Fe, and Ni)-doped SnO2 films. Sample
Pure SnO2
Co-SnO2
Fe-SnO2
Ni-SnO2
Eg (eV)
3.8
3.5
3.4
3.2
Table 3 summarizes the calculated band gap values of pure and (Co, Fe, and Ni)-doped SnO2 thin films. The band gap energy of undoped SnO2 films is 3.8 eV. The obtained band gap energy is in good agreement with the reported one found by other researchers groups for SnO2 films prepared with different methods [24-28]. A decrease of the gap energy with the insertion of Co, Fe and Ni atoms as a dopant has been observed, this can be due to the presence of impurity concentration and disorder in SnO2 lattice, and/or it can be due to grain size increase
and shape effect as reported in other publications [29,30]. Adding dopant elements to SnO2 films can increase the carriers charge concentration as a result the band gap decrease due to Moss-Burstein effect. This behavior may be attributed also to defects introduced after Co, Fe and Ni ions substitute Sn4+ into SnO2 host lattice, due to the difference of the electronegativity and the ionic radius between Sn4+ and the other doping ions.
Electrical properties The voltage-current characteristics at room temperature of (Co, Fe, and Ni)-doped SnO2 thin films are presented in Fig. 5. The results show a linear I–V relations under both forward and reverse bias indicate a good Ohmic contact behavior of the studied films. The resistivity is calculated from the slope of each curve and tabulated in Table 4. Table 4: resistivity of pure and (Co, Fe, and Ni)-doped SnO2 thin films Sample
Pure SnO2
Fe-SnO2
Co-SnO2
Ni-SnO2
Resistivity (Ω.cm)
2.03×103
6.08×103
1.14×102
2.83×101
The measured resistivity value found to decrease from 2,03 kΩ.cm for pure SnO2 film to 28,3 Ω.cm for Ni-doped SnO2 films. This indicates the improvement of the electrical property often oxide thin films by including (Co, Ni, and Fe) ions as a dopant in the lattice due to the increase in carrier charge mobility. The decrease in the electrical conductivity for Fe-doped SnO2 films may be due to the presence of some crystalline defect. These observations are in good agreement with the results obtained by T. Serin and al. [31], and N.Salah and al [32].
Conclusion
Pure and (Co, Fe, Ni)-doped SnO2 thin films with the doping concentration of 4 at. % was successfully deposited on glass substrates using a spray pyrolysis technique. The effect of adding (Co, Fe, and Ni) on the structural, electrical, and optical properties of SnO2 thin films was investigated.
The XRD pattern revealed that all deposited films exhibited a
polycrystalline tetragonal rutile structure. The lattice parameters and the microstrain decreased with adding the transition metal dopant.SnO2 thin films showed a more uniform grain size and a homogeneous surface by adding (Co, Fe, and Ni). The optical analysis revealed the films depend on the type of the dopant and the (Co, Fe, Ni) doping promotes the decrease of the estimated band gap. The I-V curves revealed that all the films obey ohms law and the resistivity decreased with doping, which suggests that (Co, Fe, Ni) doped SnO2thin films are suitable for application in optoelectronic devices.
Authors contributions (CRediT): 1- L.Soussi: Writing-original draft, Investigation, Writing-review and editing Analysis, Conceptualization. 2- T.Garmim: Writing-original draft, Investigation 3- O.Karzazi: Writing-original draft, Analysis. 4- A.Rmili: Writing-original draft, Investigation. 5- A.ElBachiri: Writing-original draft and revising, Writing-review and editing, Validation. 6- A.Louardi: Writing-original draft and revising, Writing-review and editing, Validation. 7- H.Erguig: Conceptualization, Methodology, Validation, Resources, Supervision. Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
o All the authors have participated in (i) conception and design or analysis and interpretation of the data; (ii) drafting the article or revising it critically for important intellectual content; and (iii) approval the final version. o The authors have no affiliation with any organization with direct or indirect financial interest in the subject matter discussed in the manuscript.
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Figure captions:
Fig. 1: XRD spectra of pure SnO2 and (Co, Fe, Ni)-doped SnO2 thin films. Fig. 2: SEM micrographs of pure and (Co, Fe, Ni)-doped SnO2 thin films. Fig. 3: Transmission spectra of pure and (Co, Fe, Ni)-doped SnO2 thin films Fig.4: (αhν)2 versus (hν) plots of pure and (Co, Fe, Ni)-doped SnO2 films. Fig 5: I-V curves for the pure and (Co, Fe, Ni)-doped SnO2 thin films.
Fig. 1
Pure SnO2
Co-doped SnO2
Fig2:
Fe-doped SnO2
Ni-doped SnO2 Fig 2:
Fig 3:
Fig 4:
Fig 5: