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Thin Solid Films
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Preparation of SnS2 thin films by close-spaced sublimation at differentsource temperatures
Chengwu Shi ⁎, Pengfei Yang, Min Yao, Xiaoyan Dai, Zhu ChenSchool of Chemical Engineering, Hefei University of Technology, Hefei 230009, ChinaKey Lab of Novel Thin Film Solar Cells, Chinese Academy of Sciences, Hefei 230031, China
⁎ Corresponding author at: School of Chemical EngTechnology, Hefei 230009, China. Tel./fax: +86 551
Article history:Received 8 July 2012Received in revised form 14 January 2013Accepted 14 January 2013Available online 31 January 2013
Keywords:Tin sulfideThin filmClose-spaced sublimationSource temperature
In the recent years, a number of methods had been reported for preparing tin disulfide (SnS2) films.Compared with the other available methods, close-spaced sublimation (CSS) was reported as a simple, cost-effective, and non-wet thin film deposition technique. The present research aimed to demonstrate a CSSapproach to prepare SnS2 thin films using SnS2 powder as a source. The influence of the source temperatureon the chemical composition, crystal structure, surface morphology, and optical band gap of tin sulfide thinfilmswas systemically investigated by energy dispersive X-ray spectroscopy, X-ray diffraction, scanning electronmicroscope, and ultraviolet–visible absorption spectra, respectively. By the CSS technique, SnS2 thin films wereprepared at the source temperature of 580 °C, and SnS2 crystals showed a characteristic of a preferred orientationalong (001) plane, possessing hexagonal phase and sheet appearance. The optical band gap of SnS2 thin filmswas calculated to be 2.08 eV. At the source temperature of 650 °C, the tin sulfide thin films mainly consistedof Sn2S3 along with other phases and exhibited rod appearance. Therefore, it was found that the optimal sourcetemperature for the preparation of SnS2 thin films was 580 °C using the CSS method. Further studies arerecommended to optimize and apply this thin film in solar cells.
Because of its suitable band gap, optical absorption, and goodstability, cadmium sulfide (CdS) an n-type direct gap semiconductingcompound (e.g., ≅2.43 eV) has extensively been used as the windowlayer in a number of compound thin film solar cells. But, CdS possessesa serious threat to environment. Tin disulfide (SnS2) may set to replaceCdS as an alternative material due to its important properties liken-type electrical conductivity, wide optical band gap (2.12–2.44 eV),and strong photo-conducting behaviors [1–5]. In the last few years, var-ious methods to prepare SnS2 thin films were reported, which mainlyincluded the following: chemical vapor deposition [6], chemical bathdeposition [7], dip deposition [8], spray pyrolysis [9,10], successiveionic layer adsorption and reaction [11], solvothermal synthesis [12],reactive evaporation [13], and physical vapor transport [14]. However,the preparation of SnS2 thin films by close-spaced sublimation (CSS)method using SnS2 powder as a source has rarely been studied. Com-pared with the other methods, CSS was reported as a simple, cost-effective, and non-wet thin film deposition technique. Highly uniform
ineering, Hefei University of2901450.
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and pore-free films that are strongly adherent to the substrate surfacecould be prepared using CSS due to the high kinetic energy of theincoming atoms [15–19].
The vaporization process of SnS2(s) occurred through the followingthree equilibriums [20]:
4SnS2 sð Þ → Sn2S3 sð Þ þ S2 gð Þ ð1Þ
2Sn2S3 sð Þ → 4SnS sð Þ þ S2 gð Þ ð2Þ
SnS sð Þ → SnS gð Þ: ð3Þ
The sublimation or chemical decomposition of SnS2 occurs atdifferent source temperatures. Therefore, the source temperaturebecomes a key factor in the preparation of SnS2 thin films by CSS. Inthe present work, SnS2 thin films were deposited on soda–lime glass(SLG) substrates by CSS using SnS2 powder as a source. Meanwhile,the influence of the source temperature on the chemical composition,crystal structure, surface morphology, and optical band gap of tinsulfide thin films was systemically investigated by energy dispersiveX-ray (EDX) spectroscopy, X-ray diffraction (XRD), scanning electronmicroscope (SEM), and ultraviolet (UV)–visible absorption spectra,respectively.
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2. Experimental details
2.1. Chemicals and measurement
All chemicals used were of analytical grade, and they were com-mercially available and required no further purification. The chemicalcomposition of SnS2 powder was determined by X-ray fluorescence(Model XRF-1800, Shimadzu, IN, Japan). The thermogravimetric anal-ysis (TGA) of SnS2 powder was determined by a thermogravimetricanalyzer (Q5000IR, TA, USA) used under nitrogen (N2, 99.9%, flowrate: 20 mL/min, and heating rate: 10 °C/min). The chemical compo-sition and crystal structure of tin sulfide thin films were determined
Fig. 2. EDX spectra of the tin sulfide films prepared at diffe
using a Japan EDX spectrometer (JSM6490/LV, Japan) and XRD,respectively. XRD patterns were measured using CuKα radiation(λ=0.154056 nm, 40 kV, and 50 mA) (Philips X' Pert PRO SUPER,Netherlands). A scanning rate of 0.017°s−1 was applied to record thepatterns in the 2θ range of 10–80°. The surface morphology of the tinsulfide thin films was observed by a SEM (Sirion 200, USA) under theoperating voltage of 5 kV. UV–visible absorption spectra were recordedby a double beam UV–visible spectrophotometer (U-3900H, China) inthe wavelength range 200–900 nm at a resolution of 0.5 nm.
2.2. Deposition of SnS2 thin films using CSS
The preparation of SnS2 powder followed the proceduresrecommended in an early report [1]. The chemical composition ofSnS2 powder was demonstrated to be SnS2.26. The molar ratio ofSn-to-S exceeded 2.0, as the ammonium sulfide (NH4)2S solutioncontained a little amount of (NH4)2Sx(X=2–6). SnS2 thin films weredeposited byCSS on SLG substrates. The SLGwere ultrasonically cleanedby detergent solution for 10 min and were subsequently rinsed withdeionized water for three times. The parameters of CSS procedureincluded the following: the source temperatures were 580 °C and650 °C, distance between the source and substrate was 5 mm, pressurein the reactor of CSS furnace before deposition was 133 Pa, and deposi-tion timewas 1 h. The SLG substrate temperatureswere in the ranges of360–420 °C and 390–490 °C at the source temperatures of 580 °C and650 °C, respectively. Before the deposition, the air in the reactor ofCSS furnacewas removed by high pure N2 (99.999%) to avoid the oxida-tion of the resulting tin sulfide thin films.
rent source temperatures: (a) 580 °C and (b) 650 °C.
Table 1Chemical composition of the tin sulfide films prepared at different source temperatures.
The source temperature 580 °C 650 °CThe atomic ratio of Sn to S 1:2.07 1:1.54
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3. Results and discussion
3.1. Thermogravimetric analysis of SnS2 powder
Fig. 1 shows the TGA curve of SnS2 powder. Aweight loss of 4.5 wt.%in the temperature range of 30–270 °C was attributed to the evapora-tion of the absorbed water and residual ammonium chloride in thesample of SnS2 powder. A weight loss of 4.3 wt.%, corresponding tothe decrease in sulfur element during the decomposition process[SnS2.26(s)→SnS2(s)+0.26S(g)], was observed in the temperaturerange of 270–450 °C. There was no obvious weight loss observed be-tween 450 °C and 550 °C. Subsequently, a weight loss of 5.3 wt.% wasobserved in the temperature range of 550–630 °C due to the sublimationprocess of SnS2(s) to SnS2(g). Above 630 °C, the following decomposi-tion reactions occurred: 4SnS2(s)→2Sn2S3(s)+S2(g) and 2Sn2S3(s)→4SnS(s)+S2(g). Therefore, two different source temperatures of580 °C and 650 °C were selected to explore the influence of the sourcetemperature on the chemical composition, crystal structure, surfacemorphology, and optical band gap of tin sulfide thin films.
3.2. Chemical composition of tin sulfide thin films
Fig. 2 shows the EDX spectra of tin sulfide thin films prepared atthe two different source temperatures. The variation of Sn to S atomicratio was calculated and presented in Table 1. The results revealedthat Sn to S atomic ratios in tin sulfide thin films were 1:2.07 and1:1.54 at the source temperatures of 580 °C and 650 °C, respectively.
3.3. Crystal structure of tin sulfide thin films
Fig. 3 shows the XRD patterns of the tin sulfide thin films. At thesource temperature of 580 °C, the peaks at 2θ such as 14.8°, 30.1°,46.0°, and 62.8° corresponded to (001), (002), (003), and (004) planesof SnS2 with hexagonal phase; and a preferred orientation along (001)plane (JCPDS 23-0677) was appeared. The sharp peaks also indicatedthat the tin sulfide thin films were well crystallized. Combined withthe XRD and EDX spectra results, it was concluded that the tin sulfidethin films consisted only of the pure SnS2 phase at the source
Fig. 3. XRD patterns of the tin sulfide films prepared at different source temperatures:(a) 580 °C and (b) 650 °C.
temperature of 580 °C and did not have other tin sulfide phases. Theresult was in accordance with that of TGA, as the sublimation processof SnS2(s) to SnS2(g) occurred in the temperature range of 550–630 °C.
When increasing the source temperature to 650 °C, the peaks at 2θsuch as 12.6°, 16.0°, 19.9°, 20.7°, 23.6°, 30.8°, 31.9°, 32.4°, 40.6°, and62.8° represented (002), (102), (200), (201), (202), (301), (211),(204), (400), and (600) planes of Sn2S3 with orthorhombic phase; anda preferred orientation along (600) plane (JCPDS 75-2183) wasappeared. In addition, the peaks of other phases such as 7.6°, 9.8°,10.3°, and 15.1° also appeared at 2θ. The results revealed that the tinsulfide thin films prepared at the source temperature of 650 °C mainlyconsisted of Sn2S3 along with other phases. This was incurred by thedecomposition reactions of SnS2 occurring at the source temperatureof 650 °C. Therefore, it was suggested that the source temperature of580 °C was suitable for the preparation of SnS2 thin films by CSS tech-nique using SnS2 powder as a source.
3.4. Surface morphology of tin sulfide thin films
Fig. 4(a) shows the SEM image of SnS2 thin film prepared at thesource temperature of 580 °C. This film had a sheet appearance,because SnS2 was a layered compound semiconductor with cadmiumiodide structure. This resulted in the holding together of SnS2 layersby van der Waals forces, which allowed the crystals to get easilycleaved perpendicular to the c-axis producing atomically smooth sur-faces [3]. Fig. 4(b) shows the SEM images of the tin sulfide thin filmsprepared at the source temperature of 650 °C. This film had a rodappearance mainly consisting of orthorhombic Sn2S3. The thicknesses
Fig. 4. SEM images of the tin sulfide films prepared at different source temperatures:(a) 580 °C and (b) 650 °C.
Fig. 5. (αhν)2 vshνplots for the tin sulfidefilms prepared at different source temperatures:(a) 580 °C and (b) 650 °C.
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of the tin sulfide thin films prepared at different source temperatureof 580 °C and 650 °C were 0.54 and 2.26 μm, respectively.
3.5. Optical band gap of tin sulfide thin films
The optical band gaps of the tin sulfide thin films were estimatedby extrapolating the linear region of a plot of the absorbance squared(αhν)2 versus energy (hν), as shown in Fig. 5. The optical band gapsof tin sulfide thin films prepared at source temperatures of 580 °Cand 650 °C were 2.08 and 1.53 eV, respectively. This deduced valueof 2.08 eV was very similar to those values of SnS2 thin films reportedin the literature and quite close to the optimum band gap required forthe window layer in the thin film solar cells [9,21]. At the source tem-perature of 650 °C, the optical band gap of tin sulfide thin filmsconsisting of Sn2S3 with other phases was 1.53 eV. This was attributedto the decomposition reactions of SnS2.
4. Conclusion
At the source temperature of 580 °C, SnS2 thin films were depositedby CSS technique using the SnS2 powder as a source. The SnS2 thin filmwas nearly stoichiometric, well crystallized, and had an optical band
gap of 2.08 eV with sheet appearance. When the source temperatureincreased to 650 °C, the tin sulfide thin film mainly consisted of Sn2S3with rod appearance. Therefore, the source temperature of 580 °C wasconsidered suitable for the preparation of SnS2 thin films by CSS tech-nique using SnS2 powder as a source. Further studies are recommendedto optimize and apply this thin film in solar cells.
Acknowledgments
This work is financially supported by the National Natural ScienceFoundation of China (51072043), the National Basic Research Programof China (2011CBA00700), the Anhui Province Science and TechnologyPlan Project of China (2010AKND0794), and the College Natural ScienceFoundation of Anhui Province (KJ2010A266).
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