Effect of Fe Doping on the Surface Morphological

4th Intl. Conf. on Structure, Processing and Properties of Materials, SPPM2018; 1 – 3 March 2018, BUET, Dhaka

Effect of Fe Doping on the Surface Morphological, Structural, Optical and Electrical Properties of Nano-Crystalline SnO2 Thin Films Deposited by Thermal Spray Pyrolysis 1

1

1

2

M H Babu , J Podder , B C Dev , M A Zubair 1 Department of Physics, Bangladesh University of Engineering and Technology, Dhaka-1000 2 Department of Glass and Ceramic Engineering, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh

Abstract Tin Oxide (SnO2) and Fe-doped SnO2 (SnO2: Fe) thin films have been synthesized by spray pyrolysis technique (SPT) on plane glass substrates in four different concentrations (viz. 2.0, 4.0, 6.0 and 8.0 wt. %). The surface morphology, structural, optical and electrical properties are studied by FE-SEM, XRD, UV-vis spectroscopy and four point probe method. The Scanning Electron Microscopic (SEM) images of asdeposited films show uniform surface with comprised of dense nanoparticles. In the undoped SnO2 films, large polyhedron-like grains are distributed over small round grains. From EDX data, the atomic weight percentage of tin (Sn) and oxygen (O2) in the SnO2 is found to be 66.25% and 33.75% respectively. It is observed that the SnO2 thin films are highly stoichiometric. The X-ray diffraction (XRD) analysis has revealed that the deposited films are polycrystalline in nature with mixed faces of tetragonal rutile structure and the undoped SnO2 thin films have preferred (110), (200), (211) orientation. The crystallite size is found to decrease with increase in stacking fault density resulting from increasing Fe content in the SnO 2 films. For pure SnO2 the crystallite size is found to be 47.33 nm and minimum crystallite size is about 23.40 nm for 8 -4 2 wt. % Fe doping concentration. The dislocation density is found to be 4.46 × 10 line/nm for pure SnO2 and -4 2 after Fe doping up to 8.0 wt. %, the dislocation density is increased to 18.26 × 10 line/nm . The optical transparency is found to decrease from 79 % to 55 % in visible region for Fe doping in the variation of 0.0 to 8.0 wt. % and the direct optical band gap is decreased from 3.92 eV to 3.46 eV. SnO2: Fe thin films show 3 high electrical resistivity of about 2.860  10 -m. The high roughness surface and high resistivity of as deposited SnO2: Fe thin films might be suitable candidate for gas sensing devices as well as for solar cells. Keywords: Fe doped SnO2; XRD; Band gap energy; Electrical resistivity.

1. Introduction: SnO2 is a member of the transparent conducting oxides (TCOs) family and have vast applications-in gas sensing devices [1], solar cells [2], display devices [3], hybrid microelectronics [4] optoelectronic devices and photo-catalysts [5]. Rutile type SnO2 is an important n-type semiconductor material with wide band gap of about 3.6-4.0 eV [6]. It has incomparable appealing features such as high transmittance in the visible region and reflectance in the infrared region, low electrical resistivity, harmless to the environment, low cost, stability under atmospheric conditions and mechanic durability [7]. It has a degenerate semiconductor nature in the non-stoichiometric form due to O vacancies or Sn interstitials [8]. These properties make sure that it a very commodious material for gas sensor devices. Further, the properties of SnO2 material can be improved by pertinent dopant elements such as antimony (Sb), vanadium (V), iron (Fe), niobium (Nb), aluminum (Al), 4+ 2fluorine (F) and so on. Doped SnO2 can be accomplished by replacing Sn and O atoms with dopant atoms. The efficiency of the dopant element rely on difference between its ionic radius and ionic radius of 3+ host atoms. Fe is among the most doped elements into SnO2 lattice structure since the radii of Fe (0.64 Å) 4+ and Sn (0.71 Å) are relatively close to each other [9]. If Fe dopant is supplanted with Sn host atoms, it might be expected to improve electrical conductivity along with optical transparency. The variety of deposition techniques of film like chemical vapor deposition, thermal evaporation, PLD, spray pyrolysis, sol– gel coating have been used to prepare doped SnO2 film [10-11]. Among these techniques, the spray pyrolysis is a beneficial method to get intended thin films according to aim of the research. The simple and

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economic experimental arrangement, ease of adding doping materials, reproducibility and mass production capability for uniform large area deposition are among the advantages of this technique. It is our aim to deposit Fe doped SnO2 thin films onto glass substrates by thermal spray pyrolysis and to investigate Fe doping effect on surface morphological, microstructural, optical and electrical properties. 2. Material and Methods The working solution was prepared by taking tin (II) chloride dihydrate [SnCl 2.2H2O] as a source of Sn dissolved into ethanol (CH3CH2OH) 5 mL, hydrochloric acid (HCl) 5mL and water (H2O) 90 mL. Aqueous solutions were prepared by mixing 0.20 M of SnCl 2.2H2O, CH3CH2OH and HCl for pure SnO2 and addition of (2, 4, 6, 8 wt. %) iron hexahydrate (FeCl3.6H2O) for Fe doped SnO2 thin films. The solutions were stirred for 2 hours for obtaining more homogenous solution. The glass substrates were firstly kept in HCl acid for half an hours and then they were rinsed with deionized water. Finally, the substrate were cleaned with acetone 0 and dried. Before the growth process, the substrates were preheated to the 350 C temperature. Other deposition condition such as the normalized distance between the spray nozzle and the substrates (30 cm), the flow rate (0.5 ml/min) were kept fixed during the growth. The main endothermic reaction that leads to the formation of SnO2 is SnCl2 + H2O

SnO +

 SnO + HCl

1 O2  2

SnO2 + Fe



(1)

SnO2

(2)

SnO2: Fe

(3)

The deposited films were characterized with Field Emission Scanning Electron Microscope (FE-SEM), X-ray Diffraction (XRD), four point probe electrical measurements, UV–VIS spectrophotometer. The film thickness was measured by the Fizeau fringes Interferometer method. 3. Results and Discussion The surface morphology of Fe doped SnO2 films were investigated by FESEM as shown in Fig. 1. It is observed that the surface morphology of the films depends on Fe doping concentration. The surface nature of the film changes with Fe doping concentrations and the surface morphology becomes inhomogeneous. The size of the particles initially increased with Fe concentration. After 4 wt. % Fe doping, polyhedron like grains reduce and the smaller grains are observed in the film surface up to 6 wt. % Fe doping. Further for 8 wt. % Fe doping, the surface roughness is observed. This tendency may be caused by the agglomeration of crystallites due to the migration of Fe on the growing surface. These inhomogeneous and rough surfaces are the prerequisite for gas sensing application [12].

(a)

(b)

(d)

(c)

(e)

Fig. 1. FE-SEM micrograph of SnO2 and Fe doped SnO2 thin films: (a) 0 wt. %, (b) 2 wt. % (c) 4 wt. %, (d) 6 wt. % (e) 8 wt. % of Fe.

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The X-ray diffractograms of SnO2 samples with different concentration of Fe are shown in Fig. 2. The XRD pattern of Fig. 2 reveals that all films are cassiterite tetragonal rutile type structure in accordance with JCPDS card number 41-1445. The secondary structure peaks such as SnO, Fe2O3 are identified. The (110) peak is the most intense peak which is observed for all samples. Other peaks are assigned as (101) and (210) also observed. It is also observed that FWHM increases with Fe doping, which signifies decrease of crystallite size crystallite size of the prepared undoped SnO2 and doped SnO2 thin film are determined from the Uniform Deformation Model (UDM) formula [13].

 hkl Cos 

K  4 sin  D

(4)

Where hkl is the full width at half maximum (FWHM) of the peak in radian and D is the particle size. It is observed that the crystallite sizes of deposited thin films have been obtained in the range of 47.33 nm to 23.40 nm. It is seen that the crystallite size decreases with Fe concentrations. The decrease in crystallite 4+ 3+ size is basically due to the replacement of Sn ions with Fe ions in the lattice of the SnO2 film. This 4+ process leads to the movement of Sn ions in interstitial sites, thus, increasing the amorphous phase. The maximum value of microstrain is about 0.83367 for 8.0 wt. % of Fe concentration and minimum is 0.00254 -4 2 for pure SnO2. The maximum value of dislocation density is about 18.26 × 10 line/nm for 8.0 wt. % of Fe concentration. Larger crystallite size indicates the less dislocation per unit area and minimum strain inside the crystallite. Increase of strain and dislocation density with Fe concentration indicates more deformation of the crystallites at higher Fe concentration.

0

Fig. 2. XRD patterns of Fe doped SnO2 thin films at Ts = 450 C

Table 1. Structural parameters of SnO2 and SnO2: Fe thin films Fe Concentration Wt. %

Crystallite size, D (nm)

Microstrain,



Dislocation -4 Density, , 10 2 Line/nm

Lattice parameter a

Lattice parameter c

0

47.33

0.0064

4.46

4.726

3.284

2

68.93

0.0032

2.10

4.702

3.291

4

34.47

0.0472

8.42

4.731

3.295

6

30.15

0.0756

11.00

4.756

3.274

8

23.40

0.1051

18.26

4.789

3.234

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Fig. 3. Plot of hkl cos vs 4sin of SnO2: Fe thin films. Optical properties of pure and Fe doped SnO2 thin films such as transmittance, absorbance, direct and indirect band gap, refractive index, etc. are calculated using UV-vis spectroscopy. Transmission spectra are taken within range between 300 nm to 1100 nm. Fig. 4(a) shows the variation of transmittance with wavelength for pure and Fe doped SnO2 thin films having thickness 200 ±15 nm. The transmittance spectrum as shown in Fig. 4(a) gives the sharp rise in transmittance, which indicate that the transition is direct. It is seen that the transmittance decreases with the increase of Fe doping concentration. This result may be due to the reduction of crystallite size and increase of light scattering. This result may also be due to the damaging of lattice structure by Fe doping concentration. A sharp increase in transmittance is observed at 380 nm and attributed to the band edge absorption and transition is direct. The optical band gap energy of the films has been obtained from the intercept on the energy axis after extrapolation of the straight line section from the energy curve. The optical band gap energy of thin film has been calculated using Tauc relationship: (αhν) = A (hν – Eg)

n

(5)

Where, α is the absorption coefficient, A is the constant, hν is the photon energy, Eg is the optical band gap of the semiconductor and n is the index related to the density of states for the energy band. As it is seen from Fig. 4 the transition is very sharp and there is no trace of kink in the curve therefore it is wise to consider direct transition for band gap calculation and the value of exponent n = ½ is taken. From Fig. 4(b), it is observed that the band gap energy for direct transition decreases with the increase of Fe doping concentration. The optical band gap reduction might be due to appearance of the Fe-Sn metallic compounds, decrease of light scattering and small crystallite size.

(a)

(b)

Fig. 4. Variation of (a) optical transmittance (T%) and (b) Tauc plots for determination of direct band gap Eg (eV) of Fe doped SnO2 thin films deposited at various Fe concentrations.

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The resistivity of undoped and SnO2:Fe thin films is measured by Four Point Probe Method and data are taken as a function of temperature. Fig. 5 shows that the resistivity decreases with the increase of temperature that confirms the semiconducting nature of the samples. It is also seen that the resistivity of the samples increase with increasing the doping concentration of Fe. This tendency might be caused by the 4+ 3+ decrease of carrier concentration or carrier mobility. With the substitution of Sn ions by Fe , one broken bond (hole) is produced, which acts as an acceptor energy level near the valence band that accepts an electron from the valence band, thus increasing the hole concentration or the p-conductivity [18]. For doping level of 8 wt. % of Fe in the films the majority carriers are converted from electrons to holes and then the film 3 reveals p-type. At 303 K temperature, resistivity of all the samples is found to be in the order 10 Ω- m. The resistivity is congruous for gas sensing application [14].

Fig. 5. Variation of resistivity with temperature for undoped SnO2 and SnO2: Fe thin films.

4. Conclusion Present study reported Fe doping effect on morphological, microstructural, optical and electrical properties of SnO2 films deposited by spray pyrolysis. The films were characterized by FE-SEM, XRD, UV–VIS spectrophotometer and Four Point Probe. From FE-SEM micrographs, it is observed that the deposited SnO2: Fe thin films are inhomogeneous and the surface is comprised of dense nanoparticles. It shows that the glass substrates are entirely covered with grains of different sizes. The surface roughness increases with increase of Fe concentrations. From XRD data, it is clear that the deposited SnO2: Fe thin films represent polycrystalline tetragonal rutile structure with the presence of secondary phase of SnO. Crystallite size decreases with increasing Fe concentrations. The minimum crystallite size is found to be 23.40 ± 5 nm. The 2 dislocation density is found to be 4.46× 10-4 line/nm for pure SnO2 and after Fe doping up to 8.0 wt. %, the 2 dislocation density is increased to 18.26 × 10-4 line/nm . From the UV-vis data, the transmittance of the deposited thin films decreases with Fe concentrations. The maximum transmittance is found 79% for pure SnO2 and minimum 55 % for 8.0 wt. % Fe doped SnO 2. The optical band gap for direct transition of the SnO2 is found to be 3.92 eV. Band gap declines with the Fe concentration, the minimum band gap is found to be 3.46 eV for 8.0 wt. % SnO2: Fe thin film. SnO2: Fe thin films show higher electrical resistivity of about 3 2.860 × 10 -m for 8.0 wt. % of Fe concentration. The results obtained from surface morphological, structural, optical and electrical characteristics of Fe doped SnO 2 thin films are found to be in good agreement with the previous reported data by other researchers. The Surface of the SnO 2: Fe thin films show roughness characteristics and the films are polycrystalline in nature. The SnO 2: Fe thin films show very high resistivity. The higher resistivity and rough surface of as deposited SnO2: Fe thin films could be suitable candidate for gas sensing devices as well as for solar cells. References [1]

Goetzberger, A. and Hebling, C, 2000. Photovoltaic materials, past, present, future. Solar Energy Materials and Solar Cells, 62 (1-2), 1-19.

[2]

Cohen, S. 1981. Low emissivity coatings for the improvement of the insulation properties of doubleglazing units. Thin Solid Films, 77(1-3), 127.

372


[3]

Güven, T. Adem, K. and Erdal, S. 2015. Influences of Pr and Ta doping concentration on the characteristic features of FTO thin film deposited by spray pyrolysis. Chinese Physics B, 24 (10), 233.

[4]

Yang, W., Yu, S., Zhang, Y., and Zhang, W. 2013. Thin Solid Films 542, 285.

[5]

Ahmed, Sk. F., Khan, S., Ghosh, P. K., Mitra, M. K., Chattopadhyay, K. K. 2006. Effect of Al doping on the conductivity type inversion and electro-optical properties of SnO2 thin films synthesized by sol-gel technique. J Sol-Gel Sci Techn., 39, 241–247.

[6]

Bagheri-Mohagheghi, M. M. and Saremi, M. S. 2004. The influence of Al doping on the electrical, optical and structural properties of SnO2 transparent conducting films deposited by spray pyrolysis technique. J. Phys. D: Appl. Phys., 37, 1248-1253.

[7]

Serin, T., Serin, N., Karadeniz, S., Sari, H., Tugluoglu, N. and Pakma, O. 2006. Electrical, structural and optical properties of SnO2 thin films prepared by spray pyrolysis. J. of Non-Crys. Sol., 352, 209-215.

[8]

Guven, T., Erdal, S., Mehmet, Y., Cogenli, M. S., Yilmaz, M., Turgut, U. and Dilber, R. 2014. The variation of the features of SnO2 and SnO2:F thin films as a function of V dopant. J Mater Sci: Mater Electron, 25, 2808–2828.

[9]

Bagheri-Mohagheghi, M. M., Shahtahmasebi, N., Alinejad, M. R., Youssefi, A. and Saremi, M. S. 2009. Fe-doped SnO2 transparent semi-conducting thin films deposited by spray pyrolysis technique: Thermoelectric and p-type conductivity properties. Solid State Sciences, 11, 233-239.

[10]

Thanachayanont, C., Yordsri,V. and Boothroyd, C. 2011. Microstructural investigation and SnO 2 nano defects in Spray-pyrolyzed SnO2 thin films. Mater. Lett. 65, 2610.

[11]

Tripathi, A. and Shukla, R. K. 2013. Blue shift in optical band gap of sol–gel derived Sn1-xZnxO2 polycrystalline thin films. J Mater Sci: Mater Electron, 24, 4014–4022.

[12]

Patil, G. E., Kajale, D. D., Shinde, S. D., Wagh, V. G., Gaikwad, V. B. and Jain, G. H. 2013. Synthesis of Cu-Doped SnO2 Thin Films by Spray Pyrolysis for Gas Sensor Application. Advancement in Sensing Technology, SSMI 1, 299-311.

[13]

Mote, V. D., Purushotham, Y. and Dole, B. N. 2012. Williamson-Hall analysis in estimation of lattice strain in nanometer-sized ZnO particles. J. Theo. and App. Phys. 6 (6), 1-8.

[14]

Lalauze, R., Breuil, P. and Pijolat, C. 1991. Thin films for gas sensors. Sensors and Actuators B: Chemical, 3(3), 175-182.

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