Synthesis and characterization of spin-coated ZnS thin films M. Burhanuz Zaman, Tarun Chandel, Kshetramohan Dehury, and P. Rajaram
Citation: AIP Conference Proceedings 1953, 100066 (2018); doi: 10.1063/1.5033002 View online: https://doi.org/10.1063/1.5033002 View Table of Contents: http://aip.scitation.org/toc/apc/1953/1 Published by the American Institute of Physics
Synthesis and Characterization of Spin-coated ZnS Thin Films M Burhanuz Zaman b), Tarun Chandel, Kshetramohan Dehury and P. Rajaram a) School of Studies in Physics, Jiwaji University, Gwalior (M.P.) INDIA-474011 a)
[email protected], b)
[email protected]
Abstract. In this paper, we report synthesis of ZnS thin films using a sol-gel method. A unique aprotic solvent, dimethlysulphoxide (DMSO) has been used to obtain a homogeneous ZnS gel. Zinc acetate and thiourea were used as the precursor sources for Zn and S, respectively, to deposit nanocrystalline ZnS thin films. Optical, structural and morphological properties of the films were studied. Optical studies reveal high transmittance of the samples over the entire visible region. The energy band gap (Eg) for the ZnS thin films is found to be about 3.6 eV which matches with that of bulk ZnS. The interference fringes in transmissions spectrum show the high quality of synthesized samples. Strong photoluminescence peak in the UV region makes the films suitable for optoelectronic applications. X-ray diffraction studies reveal that sol-gel derived ZnS thin films are polycrystalline in nature with hexagonal structure. SEM studies confirmed that the ZnS films show smooth and uniform grains morphology having size in 20-25 nm range. The EDAX studies confirmed that the films are nearly stoichiometric.
INTRODUCTION Metal sulfides include diverse class of compounds that exhibit a wide range of useful properties. Zinc sulphide (ZnS) is II–VI n-type semiconducting material with a wide direct band gap of 3.65 eV [1]. It has potential applications in optoelectronic devices such as light emitting diodes [2], electroluminescent devices and photovoltaic cells [3]. As buffer layer, it is an important part of thin film solar cells. ZnS has a wider energy band gap than CdS, hence results in the transmission of more high-energy photons to the junction, and results in enhancement of blue response of the photovoltaic cells [4]. Furthermore and most importantly, it replaces the toxic cadmium with sulpur. To fabricate ZnS thin films, several techniques such as thermal evaporation [5], molecular beam epitaxy [6], metal-organic vapor phase epitaxy [7], chemical vapor deposition [8], spray pyrolysis [9], and chemical bath deposition (CBD) [10] have been used. Choosing a technique to deposit or synthesize zinc sulfide or any related material requires the consideration of certain factors, such as quality of the material being prepared, cost and ease of preparation. The aim of this research is to synthesize ZnS thin films at relatively low cost using simple methods and operating devices, while maintaining high film quality. Based on these considered factors, spin coating process was selected as the technique for the successful deposition of ZnS thin films, given its simplicity, cost-effectiveness and lower material losses. Moreover, it is a low temperature technique, simplest and the economical one.
EXPERIMENTAL DETAILS ZnS thin films were prepared using a sol–gel spin coating process. Zinc acetate and thio-urea were used as precursor sources of Zn and S, respectively. To prepare the sols, zinc acetate and thio-urea were dissolved in DMSO. The concentration of zinc acetate and thio-urea was fixed at 1M in the present study. After dissolving the precursors in 10 ml DMSO, the solution was refluxed under constant stirring at 60 °C. A transparent pale yellow homogeneous sol thus obtained was kept overnight at 20 °C for gelation. The gel was spin coated on cleaned glass substrates at 2000 rpm for 60 seconds. The coated samples were dried at a temperature of 80 °C. The coating and drying processes were repeated several times to get the films of desired thickness. After repeating the coating procedure for several times,
2nd International Conference on Condensed Matter and Applied Physics (ICC 2017) AIP Conf. Proc. 1953, 100066-1–100066-4; https://doi.org/10.1063/1.5033002 Published by AIP Publishing. 978-0-7354-1648-2/$30.00
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the films were finally annealed at a temperature of 300 °C for 45 minutes to ensure the complete evaporation of solvent and to provide good crystallinity. The as-grown and annealed samples were named as A and B, respectively. The films were studied for their optical, structural and morphological properties. The optical studies of the prepared samples were carried out using a Shimadzu UV-2450 UV-VIS spectrophotometer. The crystal structure was studied using Xray diffraction (XRD) (RIKAGU 600 Miniflex) employing Cu-Kα radiation of wavelength 1.5406 Å. Morphological studies of the films were done using a Field Effect Scanning Electron Microscope (FE-SEM) (Philips, Model-Quanta 200 FEG) equipped with EDAX instrument.
RESULTS AND DISCUSSION Fig. 1 shows the transmittance spectra of ZnS thin films within the range 300 to 950 nm. The transmission spectrum shows that the films are highly transparent (about 80%) in the visible region. A strong absorption around 345 nm corresponding to band edge is observed. The sharp absorption determines the good crystalline nature with fewer defects, demonstrating the high quality ZnS thin films [11]. The energy band gap (Eg) calculated from the transmission spectra using Tauc relationship is found to be 3.6 eV. The value of Eg was decreased in case of sample B, which could be possible due to increase in grain size on annealing. Interference fringes show the high quality of the samples. The thickness of film is calculated using the Swanepoel method [12]. In this method, the interference fringes obtained from the transmittance data were plotted against wavelength of incident light in the U-V region. The refractive index is given by the equation: (1) Where, (2) ‘s’ is the refractive index of the substrate (for glass s=1.5), TM is the maxima of the interference fringe and Tm is the corresponding minima. The thickness thus calculated was found to be about 525 nm. Finally the thickness was calculated using the relation: =
1 2
( 1 �2 − 2 �1 ) PL spectrum of ZnS gel was carried out at room temperature using a wavelength of 250 nm for excitation. From the spectrum it is clearly seen that a strong emission peak is found at 328 nm (3.7 eV) and a weak one at 659 nm (1.8 eV). The emission observed at 3.7 eV corresponds to near band emission which is attributed to bound excitons and presence of near band emission.
(a)
(b)
FIGURE 1. a) Transmission spectra (insets: (αhν)2 vs photon energy) and b) PL spectrum of ZnS thin films.
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Fig. 2 (a) shows the XRD pattern of as grown and annealed ZnS films. The XRD peaks at 32.87°, 35.49° and 37.32° corresponds to (102), (104) and (105) planes, respectively, confirm the formation of hexagonal ZnS. The diffraction peaks matched with the standard data reported for hexagonal structure of ZnS (JCPDS: 39-1363). No extraneous peaks can be observed confirming that single phase ZnS is formed by the method. With annealing, the XRD peaks get sharpened and more intense as illustrated from XRD spectrum. Furthermore, the main peak width (FWHM) gets narrower describing the increase in crystallite size on annealing. The crystallite size is calculated for the main peak using the Scherrer’s formula [13] and is found to be about 70 nm for the annealed sample. The FWHM of the XRD peaks may also include contributions from the lattice strains. The strain is calculated using the Williamson-Hall plot for different β values (Fig. 1b). The negative slope indicates that the compressive strain is present and the annealing of films resulted in decrease of strain.
(a)
(b)
FIGURE 2. a) Xrd spectrum (b) Williamson-Hall plots of ZnS thin films.
Fig. 3(a) shows the SEM micrograph of ZnS (annealed) film. The SEM studies clearly show that ZnS thin films have uniform morphology with well covered to the substrate. The average particle size is around 20-30 nm, along with a slightly bigger particles spread over the surface representing the agglomeration of smaller particles with annealing. Compositional studies of annealed sample shown in Fig. 3 (b), show that the sample has stochiometric growth.
(a)
(b)
FIGURE 3: (a) SEM micrograph (b) EDAX spectrum of annealed ZnS thin film.
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CONCLUSIONS Zinc sulfide thin films were deposited on glass substrate using a sol gel technique. XRD analysis shows that the deposited ZnS films are polycrystalline with hexagonal structure. It was observed that the prepared films are highly transparent (above 85%) in the visible region. Optical and PL studies confirm the good quality of the ZnS films. From transmission spectrum, the energy band gap of ZnS thin film is 3.6 eV, which is close to the reported value. SEM micrograph show the particle size of the films is around 20-30 nm. EDAX spectra show that the ZnS films grown have uniform composition throughout the area of the films.
ACKNOWLEDGMENTS The authors are thankful to CIF, Jiwaji University, Gwalior and Mr. Shiv Kumar, IIC, IIT Roorkee, India for providing the XRD and SEM/EDAX facilities, respectively.
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