Structural and optical properties of ZnS thin films deposited by RF magnetron sputtering
© Hwang et al; licensee Springer. 2012
Received: 8 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
Zinc sulfide [ZnS] thin films were deposited on glass substrates using radio frequency magnetron sputtering. The substrate temperature was varied in the range of 100°C to 400°C. The structural and optical properties of ZnS thin films were characterized with X-ray diffraction [XRD], field emission scanning electron microscopy [FESEM], energy dispersive analysis of X-rays and UV-visible transmission spectra. The XRD analyses indicate that ZnS films have zinc blende structures with (111) preferential orientation, whereas the diffraction patterns sharpen with the increase in substrate temperatures. The FESEM data also reveal that the films have nano-size grains with a grain size of approximately 69 nm. The films grown at 350°C exhibit a relatively high transmittance of 80% in the visible region, with an energy band gap of 3.79 eV. These results show that ZnS films are suitable for use as the buffer layer of the Cu(In, Ga)Se2 solar cells.
KeywordsZnS film RF magnetron sputtering solar cell Cd-free buffer layer
Generally, Cu(In, Ga)Se2 [CIGS] solar cells are fabricated using a cadmium sulfide [CdS] buffer layer in order to protect the junction region from sputtering damage during subsequent n-type zinc oxide deposition and to modify the surface of p-type CIGS absorber . CdS is the most promising buffer layer for thin film hetero-junction solar cells, and the highest conversion efficiencies have been achieved with the chemical bath-deposited CdS buffer layer in CIGS solar cells. The chemical bath deposition [CBD] technique, which is also known as solution growth or chemical deposition, has emerged as a rather efficient method for the deposition of metal chalcogenide thin films. This method is attractive largely because the technique possesses many advantages over other thin film deposition methods, such as low cost, low deposition temperature, and easy coating of large surfaces, making it appropriate for large area industrial applications. Over the years, many studies have been conducted to grow a buffer layer material (such as the CdS thin film) by this method [2–4]. However, the CdS layer fabricated by CBD causes serious environmental problems due to the large amount of cadmium-containing waste during the deposition process. Therefore, the development of a Cd-free buffer layer is one of the major objectives in the field of CIGS solar cells.
Today, zinc sulfide [ZnS] is considered one of the best materials for the CIGS solar cells among possible alternative buffer layers. In comparison with CdS, the advantages of ZnS include its non-toxic and environmentally safe handling as well as its ability to provide better lattice matching to CIGS absorbers having energy band gaps in the range of 1.3 to 1.5 eV compared with CdS and having a wider energy band gap compared with CdS, which transmits even higher energy photons and increases the light absorption in the absorber layer [5–7]. Several growth techniques, such as CBD , metal organic chemical vapor deposition , molecular beam epitaxy , and atomic layer epitaxy , have been applied to grow high quality ZnS films for device applications in electroluminescent displays and solar cells. Among these, radio frequency [RF] magnetron sputtering, a relatively cost-effective deposition technique compared with those listed above, has sufficient control over the stoichiometry and uniformity of the film employed to produce ZnS thin films [12–14].
In this study, we prepared ZnS thin films using RF magnetron sputtering. The influence of different substrate temperatures on the structural properties of the films has been investigated, and the optical properties of the films have also been analyzed.
Sputtering conditions of ZnS films
ZnS (99.99% pure)
Corning E2000 glass
Pure argon (55 sccm)
3 × 10-2 Torr
100°C, 200°C, 250°C, 300°C, 350°C, 400°C
Target to substrate distance
The crystalline phase of the films was studied with an X-ray diffractometer [XRD] (Bruker D8 Advance; Bruker, Billerica, MA, USA) using Cu Kα radiation (λ = 0.15406 nm) operated at 40 kV and 40 mA. The surface morphology and grain size of the films were determined by FESEM (Hitachi S-4800; Hitachi, Ltd., Tokyo, Japan). The thickness of the films was estimated using the cross-sectional FESEM image. The composition of the films on glass substrates was investigated by energy dispersive analysis of X-ray [EDAX] (Horiba 7593-H; Horiba, Ltd., Kyoto, Japan). The optical properties of the films were characterized by a UV-Visible spectrometer (Shimadzu UV-1800; Shimadzu Corp., Kyoto, Japan) with a wavelength range from 200 to 1,100 nm.
Results and discussion
Estimated FWHM and crystallite sizes of ZnS films grown at various substrate temperatures
by XRD (nm)
by FESEM (nm)
Chemical composition of ZnS films deposited at various substrate temperatures
Substrate temperature (°C)
Zn (atomic %)
S (atomic %)
where D is a constant, and Eg is estimated by extrapolating the straight-line portion of the spectrum to a zero absorption coefficient value. The optical band gap of the film deposited at 100°C was 3.45 eV. As the growth temperature increased from 200°C to 350°C, the optical band gap red-shifted from 3.57 to 3.79 eV. The band gaps between the film formed at 250°C (Eg = 3.72 eV) and those grown at 300°C (Eg = 3.73 eV) were slightly changed along with the deposition temperatures. The band gap of the film also decreased with the temperature up to 400°C (Eg = 3.76 eV). These results indicate that an increase in the substrate temperature improves the band gap energy of the films.
ZnS thin films have been successfully grown on glass substrates using RF magnetron sputtering at various substrate temperatures ranging from 100°C to 400°C. The influence of substrate temperature on the structural and optical properties of ZnS films prepared in the experiment has been characterized. The XRD measurements reveal that the films deposited at 350°C have a strongly (111) preferred orientation and are parallel to the substrate surface. The smallest FWHM value of 0.141° has also been observed for these films, indicating that the crystallinity of the films can be improved by increasing the substrate temperatures. All of the ZnS films deposited at different substrate temperatures are Zn-rich and S-deficient in terms of EDAX results. However, the Zn/S ratio of the films formed at 350°C is 0.99, indicating an ideal stoichiometric proportion of ZnS. The surface morphology studied by FESEM has shown that the grain sizes of ZnS films are influenced by the substrate temperatures. The films formed at 350°C exhibited good optical properties with a relatively high transmittance of 80% in the visible region, and the energy band gap is estimated to be 3.79 eV.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0024830).
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