Journal of Materials Science and Engineering A 3 (3) (2013) 182-186
Thickness Effect on the Structural and Optical Properties of SnS2 Films Grown by CBD Process
Kotte Tulasi Ramakrishna Reddy1, 2, Gedi Sreedevi1 and Robert Wilson Miles2
1. Department of Physics, Sri Venkateswara University, Tirupati 517 502, India
2. School of Engineering and Environment, Northumbria University, Newcastle, NE1 8ST, UK
Received: November 27, 2012 / Accepted: December 19, 2012 / Published: March 10, 2013.
Abstract: Tin disulphide has been ud in the fabrication of photovoltaic solar cells and optoelectronic devices such as thin film transistors (TFT). Tin sulphide films were chemically deposited on to the soda-lime glass substrates from the bath containing 0.8 M of stannous chloride (SnCl2·2H2O), 0.5 M of thioacetamide (C2H5NS), 12 mL of ammonia (NH3), 3.75 M of triethanolamine (C6H15NO3), and double distilled water. The influence of film thickness on the structural, morphological, and optical properties was studied from X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical transmittance data. XRD patterns revealed that the deposited films have hexagonal crystal structure and the crystal quality of films was improved with the increa of film thickness. The as-grown films sho
wed various phas of tin and sulphur. The surface morphology showed that the films had good surface coverage by equally sized grains without any cracks and the optical studies revealed direct band gap nature of the films. The energy band gap values varied in the range of 2.8-3.0 eV. The optical transmittance spectra of the films was ud to calculate the refractive index (n)and the extinction coefficient (k). The results will be prented and discusd.
Key words: Chemical bath deposition, morphology, optical studies, structure, tin disulphide.
Nomenclature
JCPDS Joint Committee on Powder Diffraction Standards Greek Letters
λWavelength of X-rays
β2θFull width at half maximum of the (001) peak
θ Diffraction
angle
αAbsorption coefficient
νPhoton frequency
1. Introduction
The metal chalcogenide, tin disulphide (SnS2) has promising applications in thin film transistors as well as solar photovoltaic cells becau of its higher electrical mobility and larger energy band gap [1, 2]. SnS2 is an n-type compound miconductor with hexagonal CdI2/PbI2 structure. It consists of two hexagonal slabs of cloly packed sulphur anions with Corresponding author:Kotte Tulasi Ramakrishna Reddy, Ph.D., professor, rearch fields: material science and thin film solarcells.E-mail:*******************.sandwiched tin cations in octahedral coordination [3, 4]. The constituent elements of this compound are abundant in nature and environmental safe to handle. Thin layers of SnS2 have been grown by different methods [5-7]. However, preparation of SnS2 layers by chemical bath deposition has attracted great deal of attention since it is a simple and low cost method that has minimum material wastage without any sophisticated instrumentation or vacuum involved. Further, large area deposition of layers is possible using this method. In this work, SnS2 films were formed by this wet chemical technique; chemical bath deposition (CBD) and the properties of the layers grown with different thickness were reported and discusd for their possible application in solar cells.
笔字草书2. Experiments
2.1 Reagents
The chemical reagents ud in this work for
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Thickness Effect on the Structural and Optical Properties of SnS2 Films Grown by CBD Process 183
preparation of the experimental films were of analytical grade stannous chloride (SnCl2·2H2O), thioacetimide (C2H5NS), triethanol-amine (C6H15NO3), ammonia (NH3) and concentrated Hydrochloric acid (HCl).
2.2 Sample Preparation
The tin sulphide films were obtained using a simple chemical bath deposition technique on cleaned s
oda-lime glass substrates. The precursor solution was prepared by 0.8 M of stannous chloride, 0.5 M of thioacetamide. Few drops of concentrated hydrochloric acid was added to get a clear solution and 12 mL of ammonia, 3.75 M of triethanolamine were ud as complexing agents. Finally sufficient quantity of double distilled water is added to the mixture in order to obtained 100 mL of the reaction bath. The bath temperature was maintained constant at 60 °C.
2.3 Characterization
The crystal structure of the as-grown films was determined by using a Seifert X-ray diffractrometer (XRD) using Cu Kα (λ = 1.5418 Å) radiation in the scanning angle range, 10°-70°. The film composition and morphology were evaluated by energy dispersive X-ray analysis (EDAX) with a Inca Penta X-ray energy analyr attached to the Zeiss scanning electron microscope (SEM). Optical measurements were carried out with a Hitachi UV-Vis-NIR double beam spectrophotometer.
3. Results and Discussion
The visual obrvations of the as-grown films revealed that the layers were reddish brown in color and pinhole free. The scratch tape test indicated that the layers were well adherent to the substrate surface. The thickness of the layers was determined by using Manifacier’s formula [8].
3.1 Structural Analysis
The X-ray diffraction studies revealed that all the as-grown films were polycrystalline in nature. Fig. 1 shows the XRD spectra of the films formed with different film thickness. It reveals that the crystallinity of the films depends significantly on the film thickness. The XRD spectra indicated various peaks such as (001), (100), (101), (113) and (110) that mainly correspond to the SnS2 pha.
翟小宁
萝卜肉丸子
Fig. 1 The XRD profiles of tin disulphide films.
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Thickness Effect on the Structural and Optical Properties of SnS 2 Films Grown by CBD Process
184Minor peaks related to the (111) and (113) planes of SnS and (130) plane of Sn 2S 3 were also f
ound the XRD spectra. The layers had hexagonal crystal structure and the evaluated lattice constants were a = 3.648 Å, c = 5.899 Å, The values are comparable with the reported JCPDS Data File No. 23-0677. The SnS pha is in the form of orthorhombic (File No. 75-0925) and Sn 2S 3 is also in the form of orthorhombic (File No. 14-0619). When the thickness of the film increas from 350 nm to 580 nm, the intensity of SnS and Sn 2S 3 phas were relatively weak and the intensity of the peaks related to SnS 2 became stronger. This indicates the layers were completely transformed into the hexagonal SnS 2 pha with improved crystallinity [9]. The crystallite size, D of the films was calculated using the Scherrer’s formula [10]:
s D θλβθ
=
(1) where, λ is the wavelength of X-rays ud, β2θ is the full width at half maximum of the (001) peak and θ is the diffraction angle. Fig. 2 shows the variation of
crystallite size with the thickness. From the figure it
was obrved that the crystallite size incread continuously with the increa of film thickness and its value varied in the range, 26-38 nm.
3.2 Compositional Analysis
The elemental composition of the films was determined using the energy dispersive X-ray analysis. Fig. 3 shows the EDAX spectrum of the film of thickness 580 nm. The spectrum showed Sn and S peaks that confirms their prence in the films. The composition of both the elements were determined to be approximately clo to the stoichiometric values and were Sn = 34.6 and S = 65.4 respectively and agreed with the reported value [11]. 3.3 Surface Morphology
Fig. 4 shows the scanning electron micrographs of SnS 2 films deposited at different thickness formed in the range, 350-580 nm on glass substrate. It can be
obrved that the grains were approximately spherical in shape and were uniformly distributed over the substrate surface. The evaluated grain size incread with the increa of film thickness. 3.4 Optical Properties
The optical transmittance versus wavelength spectrum of the SnS 2 films was measured in the wavelength range of 300-2,500 nm and is shown in Fig. 5a. It can be obrved that the transmittance of SnS 2 layers decread sharply near the absorption edge, indicating a direct optical transition in the material. Also the transmittance decread from 85% to 60% with the increa of fil
m thickness from 350 nm to 580 nm. The evaluated optical absorption coefficient of the films was ~103 cm -1. The change of optical transmittance with wavelength was ud to evaluate the
Fig. 2 Variation of crystallite size with thickness.
Fig. 3 EDAX profile of SnS 2 film of thickness 580 nm.
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Thickness Effect on the Structural and Optical Properties of SnS 2 Films Grown by CBD Process
185
Fig. 4 SEM images of SnS 2 films grown with different thickness: (a) 350 nm, (b) 460 nm and (c) 580 nm.
(a) (b)
Fig. 5 (a) Transmittance vs wavelength spectra of SnS 2 films. (b). Plots of (αhv )2 vers photon energy, h ν.烟雨遥
optical band gap of the films. Fig. 5(b) shows the plot of (αh ν)2 versus h ν. The extrapolation of the plot onto the energy (h ν) axis directly gives the energy band gap of the material. The evaluated optical energy band gap of the films decread with the increa of layer thickness and varied in the range, 2.8-3.0 eV and comparable with the reported data [12, 13].
The optical constants such as refractive index (n) and extinction coefficient (k) were evaluated for SnS 2 films by using the relations,
2
/12222).(s a n n N N n -+= (2)
where
T
n n n n N s a s a 22)(2
2++=
(3) here, n a and n s are the refractive indices of air and substrate.
π
αλ
4=k (4)
where, α, λ are the absorption coefficient and wavelength.
Fig. 6. shows the variation of both n and k with wavelength for the layers grown with different film thickness. The refractive index of the films varied in the range 2.57-2.63 with the change of film thickness. However, average extinction coefficient of the films decread from 0.69 to 0.61 with the increa of film thickness.
4. Conclusions
The tin disulphide films were grown by chemical bath deposition on glass substrates. The layers were formed with different thickness that varied from
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Thickness Effect on the Structural and Optical Properties of SnS 2 Films Grown by CBD Process
186
Fig. 6 Variation of refractive index and extinction coefficient of SnS 2 films with thickness.
350 nm to 580 nm. The as-deposited films were polycrystalline in nature with an inten (001) peak
as the preferred orientation. The crystallinity of the layers incread with thickness and varied in the range, 26 - 38 nm. Both the optical transmittance and the energy band gap of the layers decread with the increa of film thickness. The films had a direct band gap that
varied in the range of 2.8-3.0 eV.
Acknowledgments
五行属水的颜色The authors would like to thank Mr. P.A. Nwofe,
School of Engineering and Environment, Northumbria University, UK for recording the X-ray diffraction data.
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