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Growth and characterization of RF-sputtered ZnS thin film deposited at various substrate temperatures for photovoltaic application
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Growth and characterization of RF-sputtered ZnS thin film deposited at various substrate temperatures for photovoltaic application P. Chelvanathan a , Y. Yusoff a , F. Haque a , M. Akhtaruzzaman a , M.M. Alam c , M.J. Rashid a , K. Sopian a , N. Amin a,b,c,∗ a
Solar Energy Research Institute, The National University of Malaysia, 43600 Bangi, Selangor, Malaysia Department of Electrical, Electronic and Systems Engineering, FKAB, The National University of Malaysia, 43600 Bangi, Selangor, Malaysia c Chemistry Department, College of Sciences, King Saud University, Riyadh 11451, Saudi Arabia b
a r t i c l e
i n f o
Article history: Received 15 June 2014 Received in revised form 23 August 2014 Accepted 26 August 2014 Available online xxx Keywords: Thin films ZnS Sputtering growth Structural properties Optical properties
a b s t r a c t RF-sputtered ZnS thin film was grown under various substrate temperatures with the aim of investigating its effects on the structural, surface morphology and optical properties. Investigated substrate temperature in this study was in the range of 25 ◦ C–300 ◦ C and the structural and optical properties were investigated in order to elucidate the changes induced by the varying thermal energy during the growth process. Structural determination by XRD method indicates all sputtered films have cubic structure with (1 1 1) as the preferential orientation. However, higher substrate temperature up to 200 ◦ C increases the film’s crystallinity and grain size evident by the increase in peak intensity. Slight peak shift indicates ZnS lattice undergoes strain relaxation process mediated through the increase in the lattice constant ˚ SEM image of surface morphology clearly shows the evolution of grain growth in from 5.32 A˚ to 5.40 A. which sputtered film at 200 ◦ C has the largest grains with distinct grain boundaries. Calculation from the obtained transmission spectra indicates optical band gap is in the range of 3.6–3.9 eV. Theoretical analysis in terms of lattice parameter between ZnS with several upcoming photovoltaic absorber layers shows that lattice matched ZnS buffer layer can be grown by varying the substrate temperature. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Cadmium sulfide (CdS) has been used extensively as the buffer layer deposited on top of Cu(In,Ga)Se2 (CIGSe) absorbers for thin film solar cell applications till date and cell efficiency of 20.3% has been reported by using CdS for CIGS solar cells [1]. But further enhancement of efficiency is limited as the CdS has a bandgap of 2.45 eV causing high photon absorption at shorter wavelengths [2]. Among various alternative candidates, ZnS is considered as one of the most promising materials [3]. ZnS is non-toxic, abundant and cheap. It is a direct wide bandgap n-type compound semiconductor having a bandgap of 3.2–3.9 eV at room temperature which is higher in comparison with CdS having a bandgap of 2.45 eV. The wider bandgap of ZnS enables high energy incident photons to reach the window-absorber junction, enhancing the blue response of the photovoltaic cells and thus contributes to a better cell performance [4]. The efficiency of heterojunction solar cells depends
∗ Corresponding author at: Solar Energy Research Institute, The National University of Malaysia, 43600 Bangi, Selangor, Malaysia. Tel.: +60 193296750; fax: +60 389118359. E-mail addresses: [email protected], [email protected] (N. Amin).
largely on the interfacial properties between absorber and buffer layer. ZnS has the ability to provide better lattice matching with CIGSe as well as CZTS absorbers [5]. Several growth techniques for preparing ZnS thin films have been reported in the literature. These are chemical bath deposition (CBD) [6], metal organic chemical vapour deposition (MOCVD) [7], molecular beam epitaxy (MBE) [8], RF magnetron sputtering [3] and thermal evaporation [9]. Magnetron sputtering technique has some advantages compared to the other methods such as easier controllability of the deposition parameters, high film growth rate, relatively cost-effective and especially for the compatibility with the sputtering depositions of CIGSe2 absorbers and window layers, leading to a development of a full in-line sputtering technique [3]. In this work, we have deposited ZnS thin films by RF magnetron sputtering with different substrate temperature the effects on the structural, topographical and optical is studied. Theoretical lattice mismatch between ZnS deposited in this study and several new photovoltaic absorbers is calculated and presented. 2. Experimental ZnS thin films were deposited on soda lime glass substrates of 75 mm × 25 mm via radio frequency (RF) magnetron
http://dx.doi.org/10.1016/j.apsusc.2014.08.155 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: P. Chelvanathan, et al., Growth and characterization of RF-sputtered ZnS thin film deposited at various substrate temperatures for photovoltaic application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.08.155
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sputtering system. Prior to deposition, all the substrates were cleaned ultrasonically in methanol–acetone–methanol–deionised water sequence and finally dried by N2 gas flow. Source material used in this study is a 50.8 mm diameter ZnS (99.99%) sputtering target (Kurt.J.Lesker) which was pre-sputtered for 15 min to remove contamination on the target’s surface. Deposition chamber base pressure was brought down close to 10−6 Torr by turbomolecular pump and the working pressure during all the deposition run was maintained at 10 mTorr by flowing 5 sccm of purified Ar (99.99%) as the working gas into the chamber. RF power, substrate holder rotation and target to substrate distance were fixed at 50 W (≈2.5 W/cm2 ), 10 rpm and 80 mm, respectively. RF-sputtered ZnS films were grown at 4 different substrate deposition temperatures; room temperature (RT), 100 ◦ C, 200 ◦ C and 300 ◦ C and the changes on the structural, surface morphology and optical properties were observed. ZnS films sputtered at elevated temperature were left in the sputtering chamber under high vacuum for natural cooling process to avoid surface oxidation. Structural and crystallinity properties as well as the orientation along the film’s surface normal were examined by BRUKER aXS-D8 Advance Cu-K␣ diffractometer. XRD patterns were recorded in the 2 range from 20◦ to 60◦ using ˚ Grain size and surface Cu K␣ radiation wavelength, = 1.5408 A. morphology were observed by using LEO 1450 Variable Pressure Scanning Electron Microscope (VPSEM) while surface topography and roughness were analyzed by using Integra Prima, NT-MDT Scanning Probe Microscope (SPM) with non-contact mode settings. The mean crystallite sizes (D) of the films was calculated using Scherrer formula D = 0.9/ˇ cos
(1)
whereby, is the X-ray wavelength (0.15406 nm), and ˇ is the full width at half maximum [FWHM] of the film diffraction peak at 2 in radian and is the Bragg diffraction angle in degree. The microstrain, ε and dislocation density, ı developed in the thin films are calculated from Eqs. (2) and (3), respectively. ε = ˇ/4 tan ı = n/D
2
(2) (3)
whereby, n is a factor, which is almost equal to unity for minimum dislocation density and D is the grain size. Optical transmission measurements were performed at room temperature by using Perkin Elmer Lambda 35 UV/Vis spectrophotometer from wavelength range of 300–900 nm. Energy band gap values were calculated from the obtained transmission spectra. 3. Results and discussion 3.1. Structural properties of ZnS films Fig. 1 shows the XRD pattern for ZnS films deposited with various substrate temperatures from 20◦ to 60◦ . All films exhibited most intense peak at 2 = 28.5◦ which corresponds to (1 1 1) preferred orientation of cubic phase of -ZnS while films sputtered at 100 ◦ C and 200 ◦ C have a slight peak centred at 244 = 47.3◦ which can be attributed to (2 2 0) plane of -ZnS phase as well. However due to the polymorphism nature of ZnS, the peaks assigned initially as cubic phase of zincblende structure could also belong to (0 0 0 2) plane of hexagonal phase which corresponds to wurtzite structure [4]. The obtained XRD pattern from this study is in agreement with the indexed standard spectrum documented in JCPDS card No. 80-0020 Microstructural evolution based on structural zone model (SZM) for thin film growth for ratio of substrate temperature (Ts ) to melting temperature, (Tm ) Ts /Tm < 0.1 is expected to yield amorphous or random polycrystalline due to lack of surface mobility during nucleation [10]. However, ZnS thin film deposited
Fig. 1. XRD patterns ZnS thin films deposited at different substrate temperatures.
at room temperature of 25 ◦ C in this study yields ZnS film with preferred orientation as shown in the XRD spectra. Kinetic energy of incident sputtered particles is believed to facilitate ZnS crystallization process in the absence of intentional heating. Ein which is the kinetic energy of incident particles depends on initial kinetic energy of the sputter ejected particles Eout , distance between substrate and target l, cross section for momentum transfer collision with background gas atom , Boltzmann constant k, working gas pressure p and temperature T in the following relation [11]. Ein = Eout exp(−pl/kT )
(4)
The variable T in this relation is the temperature of incident particles which are ejected out due to bombardment of working gas and should not be confused for substrate temperature Ts . Hence, it is deduced that RF sputtering configuration in this study (details are in Section 2) produces incident particles with sufficient kinetic energy Ein which in return induces crystallization mechanism. Increase in the peak intensity is observed for (1 1 1) orientation as the substrate temperature is increased from room temperature to 200 ◦ C indicating improvement in film crystallinity due to higher thermal stress and increase in surface diffusivity of incoming sputtered particles. Slight decrease in the intensity and broadening of the same peak is observed for films sputtered at substrate temperature of 300 ◦ C. Subbaiah et al. [13] has reported the similar observation for ZnS thin film deposited by close space evaporation for temperatures >325 ◦ C whereby the deterioration on degree of crystallinity is linked to sulphur re-evaporation due to high vapour pressure. The difference in the observed temperature onset of the decrement of crystallinity from this study and reported as in [12] can be due to the difference in the kinetic energy of particles. Incoming sputtered particles have higher kinetic energy than evaporated particles hence sulphur re-evaporation in sputter deposition method may have occurred at lower temperature (higher kinetic energy is translated to thermal energy upon collision) as observed in this study. Improvement in crystallinity of ZnS film is accompanied by increase in grain size calculated from Scherrer’s equation as shown in Fig. 2. Enhancement of grain size can be attributed to the increased mobility of sputtered species at higher substrate temperature which induces better grain coalescence. Highest grain size of ∼14 nm for sputtered film at 200 ◦ C is obtained which is substantially lower than ∼31 nm reported in [13] observed in RF-sputtered ZnS film at 180 ◦ C. Discrepancy in the reported grain size values is mainly due to the different thicknesses of sputtered ZnS films. Film as thick as 1.76 m has reported in [13] meanwhile film thickness in this study
Please cite this article in press as: P. Chelvanathan, et al., Growth and characterization of RF-sputtered ZnS thin film deposited at various substrate temperatures for photovoltaic application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.08.155
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Fig. 2. Grain size and FWHM variations for ZnS thin films deposited at different substrate temperatures.
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Fig. 4. Lattice parameter and strain variations for ZnS thin films deposited at different substrate temperatures.
Fig. 5. Microstrain and dislocation density along (1 1 1) orientation.
Fig. 3. XRD peak shift of (1 1 1) orientation of ZnS thin films deposited at different substrate temperatures.
was restricted to 50–80 nm in the aim of studying ultra thin ZnS structural evolution. However, further increase in substrate temperature to 300 ◦ C results in decrease in the ZnS grain size. Since the decrement in the grain size is concomitant with the observed lower degree of crystallinity for ZnS film sputtered at 300 ◦ C, the aforesaid sulphur re-evaporation mechanism at elevated temperature could have suppressed the grain growth process. Similar findings were reported by Hwang et al. [14] whereby decrease in grain size for RF sputtered ZnS thin films was observed at substrate temperature of 400 ◦ C and was accompanied by decreasing Full Width Half Maximum (FWHM) value in the corresponding XRD peak. Differences in geometrical configuration of sputter system (substrate to target distance, target surface area) and process variables (operating pressure and RF power) between in this study and as reported in [14] could be the reason in the variation of substrate temperature at which decrement of grain size occurs. Closer scrutiny on the XRD spectra (Fig. 3) reveals gradual peak shift towards lower 2 occurs for the preferred orientation indicating substrate temperature influences the stress distribution in ZnS film. Variation in lattice parameter and strain (percentages of changes in lattice constant) as a function of substrate temperature is given in Fig. 4. Increasing substrate temperature promotes lattice relaxation with film sputtered at 300 ◦ C possessing equilibrium lattice parameter with no strain. Film sputtered at lower
temperature is found to exhibit higher compressive strain (smaller lattice parameter). However, bare observation by naked eye did not show any film cracking despite possessing compressive strain which would lead to peeling off. Good adherence of ZnS film to the SLG substrate specifically for films sputtered at lower temperatures indicates the resulting range of compressive strain in this study is not detrimental from adhesion perspective. Fig. 5 shows the calculated microstrain and dislocation density along (1 1 1) crystallographic plane for different substrate temperature. Maximum value of dislocation density is obtained for film sputtered at RT due to inverse dependence of dislocation density to the grain size as shown in Eq. (3). Physically, grain size enlargement through grain coalescence at higher substrate temperature decreases the lattice point mismatch of two neighbouring crystals in the following manner. During grain coalescence process, merging of two neighbouring crystallites eliminates the grain boundary which initially separates them. Since lattice point mismatch originates from misregistry of lattice (dislocation cores) in one part of the crystallite with respect to another part along the interconnecting grain boundary, the elimination of this particular grain boundary subsequently vanishes the existing lattice point mismatch. Furthermore, variation in microstrain parameter shown in Fig. 5 coincides with the dislocation density values indicating microstrain is caused predominately by dislocation density as described in Eq. (5) [15]. < ε2hkl >= 2b2 M 2 Chkl Pdislocation
(5)
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Fig. 6. FESEM images of ZnS thin film deposited at different substrate temperatures. (a) RT, (b) 100 ◦ C, (c) 200 ◦ C and (d) 300 ◦ C.
whereby, <hkl 2 > is the mean squared microstrain in along (hkl) plane, Burgers vector b, the contrast factor of dislocation Chkl , Wilkens factor M, and Pdislocation is the dislocation density along the same (hkl) plane. Table 1 shows the expected lattice mismatch of several photovoltaic semiconductor materials with ZnS films deposited in this study. It can be seen that ZnS films deposited at different substrate temperature by RF-sputtering in this study can be used as lattice matched buffer layer (numerical values in bold indicates lowest lattice mismatch percentage) with various photovoltaic material such as rigorously researched CZTS and the upcoming novel absorber materials of Cu2 ZnGeS4 and FeS2 . 3.2. Surface morphology VPSEM was used to view and analyze the surface morphology meanwhile atomic force microscopy was used to determine the root mean square (RMS) surface roughness of all ZnS films. Fig. 6 shows the VPSEM images of ZnS films deposited at different substrate temperatures. Grain size observed form the surface morphology progressively increases for film sputtered at room temperature up to 200 ◦ C in agreement with the calculated value
(Fig. 2). Large grains are visible distinctly for film sputtered at 200 ◦ C. Thick dark fringes encompassing the grains are likely to be voids rather that grain boundary due to ultra thin ZnS layer. Hence, although ZnS film (200 ◦ C) showed highest degree of crystallinity from XRD data, VPSEM reveals undesirable voids which are detrimental if it is incorporated (as a n-type buffer layer) in full working device of thin film solar cell. This is due to possibility of shunting effect of transparent conducting oxide penetrating all the way towards the Mo back contact. In addition, non-uniform buffer layer of ZnS creates non-homogeneous and localized heterojunction with p-type absorber layer (CIGS, CZTSSe) affecting the built-in potential spatial distribution which is crucial to drift the photo-generated carrier towards front and back terminals. Hence, films sputtered at 100 ◦ C and 300 ◦ C are better options for buffer layer deposition as lesser voids resembling pinholes are observed for these films. Surface topography probed by AFM is shown if Fig. 7(a) and (b) for film sputtered as room temperature and 200 ◦ C respectively. Particles with greater variation in terms of height which forms rougher surface are observed in the former images than the latter. Quantitative variation of RMS roughness is plotted Fig. 8 and it can be seen clearly that film sputtered at higher temperature has
Table 1 Lattice mismatch of several photovoltaic materials with ZnS film from this study. Sample identification
ZnS lattice parameter (nm)
ZnS-RT ZnS-100 ZnS-200 ZnS-300
0.5326 0.5355 0.5370 0.5406
a b
Lattice mismatch % JCPDS-CZTS (a = 0.5427 nm) [16]
a
1.8610 1.3267 1.0503 0.3869
Lattice mismatch % b PLD-CZTS (a = 0.5365 nm) [17]
Lattice mismatch % Cu2 ZnGeS4 (a = 0.5341 nm) [18]
Lattice mismatch % FeS2 (a = 0.5266 nm) [19]
0.7269 0.1864 0.0932 0.7642
0.2808 0.2621 0.5430 1.2170
1.1393 1.6900 1.9749 2.6585
Lattice constant from JCPDS-26-0575. Pulsed Laser Deposited (PLD) CZTS.
Please cite this article in press as: P. Chelvanathan, et al., Growth and characterization of RF-sputtered ZnS thin film deposited at various substrate temperatures for photovoltaic application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.08.155
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Fig. 7. (a) AFM surface topography of ZnS thin film deposited at room temperature (RT). (b) AFM surface topography of ZnS thin film deposited at room temperature 200 ◦ C.
smoother topography than film sputtered with no intentional heating. This is due to higher mobility and surface diffusion of sputtered atoms which leads to homogeneous surface. 3.3. Optical properties Optical properties were characterized by measuring the transmittance of deposited ZnS films on glass substrates in the wavelength region of 300–900 nm. Transmittance of the reference glass is subtracted to offset the influence of underlying substrate. The transmittance values of the films at temperature have been measured by a UV/VIS spectrophotometer and given in Fig. 9. Transmittance values in the wavelength range of 300–500 nm are higher for films deposited with lesser thermal stress. On the other hand, the opposite trend is observed as transmittance values in the wavelength region of 500–900 nm for higher for films deposited with higher substrate temperature (for 100 ◦ C and 300 ◦ C samples only). Transmission of ZnS film sputtered at room temperature and 200 ◦ C is the lowest in this study. The absorption coefficient, ˛ is related to the optical energy band gap, Eg of the semiconductors strong absorption region by the relation, ˛h = A(h − Eg )
n/2
where A is constant, Eg is the energy gap and n is constant equal to 1 for direct gap compound and 3 for indirect gap compound. Direct band gap values can be estimated by extrapolating the straight-line portion in the graph of (˛h)2 versus h as shown in Fig. 10. Sputtered films (100 ◦ C and 300 ◦ C) exhibit steep increase in (˛h)2 value meanwhile films deposited at RT and 200 ◦ C manifest gradual increase for the same value of photon energy indicating these films (100 ◦ C and 300 ◦ C) have higher homogeneity in the shape and size of the grains and lower defects density near the band edge than films sputtered at RT and 200 ◦ C [20]. Band gap of ZnS in this study was found to be in the range of 3.68–3.95 eV. Identical band gap of 3.68 eV was obtained for sputtered films at 100 ◦ C and 300 ◦ C which exactly the same value for bulk ZnS [21]. It was found out that the variation in the band gap values is not systematic with the variation in the substrate temperature in this study. This could be due to the inhomogeneity broadening effect which leads to different absorption behaviour which is manifested through different absorption peak corresponding to band to band transition (energy band gap) [22]. Inhomogeneity broadening effect originates from the fluctuation of grain shape and size especially in thin film featuring nanostructures as reported in this study (see Fig. 6). On the other hand, absorption behaviour of ZnS thin film is also susceptible
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to surface optical loss due to scattering by ultra small crystallites [22] which alters the refractive index and extinction coefficient values of the film [23]. Hence, due to the aforesaid inhomogeneity broadening effects and surface optical loss, non-systematic variation in optical properties especially the energy band gap values are observed in this study. 4. Conclusion
Fig. 8. RMS surface roughness variations for ZnS thin films deposited at different substrate temperatures.
Microstructural properties such as orientation, grain size, lattice parameter, microstrain and dislocation density have been studied for RF-sputtered ZnS thin films with different substrate temperature. ZnS thin film sputtered without intentional heating showed preferred orientation growth due to the sufficient kinetic energy of incident particles which facilitated crystallization. ZnS thin film preferred orientation parallel to the SLG substrate as well as degree of crystallinity and grain size were found to increase as the substrate temperature is increased to 200 ◦ C. Relaxation of ZnS lattice parameter due to increasing thermal stress could be used to obtain minimum lattice mismatch structure (nZnS/p-absorber layer) with CZTS, Cu2 ZnGeS4 and FeS2 photovoltaic material. However, increase in degree of crystallinity did not grant uniform enlarged grains as significant amount of voids in between grains are observed by AFM indicating further surface morphology optimization is needed to avoid possible shunting mechanism. Higher degree of homogeneity in grain size as well as uniformity improves the optical transmission of ZnS film deposited at higher temperature and optical band gap in between 3.6 eV and 3.9 eV was observed which is well in the range of standard reported value. Acknowledgements
Fig. 9. Transmission spectra for ZnS thin films deposited at different substrate temperatures.
The authors would like to acknowledge and appreciate the contribution of the Ministry of Higher Education of Malaysia (MOHE) through its research grant with code FRGS/1/2013/TK07/UKM/01/3. Appreciations are also extended to the Solar Research Institute (SERI), The National University of Malaysia (Universiti Kebangsaan Malaysia (UKM)), Centre of Research and Instrumentation Management CRIM, UKM and the College of Science Research Center, Deanship of Scientific Research, King Saud University for their support. References
Fig. 10. Extrapolated band gap values of ZnS thin films deposited at different substrate temperatures.
[1] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, New world record efficiency for Cu(In,Ga)Se2 thin film solar cells beyond 20%, Prog. Photovolt. Res. Appl. 19 (2011) 894. [2] T. Nakada, M. Mizutani, Y. Hagiwara, A. Kunioka, High-efficiency Cu(In,Ga)Se2 thin-film solar cells with a CBD–ZnS buffer layer, Sol. Energy Mater. Sol. Cells 67 (2001) 255. [3] L.-X. Shao, K.-H. Chang, H.-L. Hwang, Zinc sulfide thin films deposited by RF reactive sputtering for photovoltaic applications, App. Surf. Sci. 212 (213) (2003) 305–310. [4] M.A. Islam, M.S. Hossain, M.M. Aliyu, Y. Sulaiman, M.R. Karim, K. Sopian, N. Amin, Comparative study of ZnS thin films grown by chemical bath deposition and magnetron sputtering, in: 7th International Conference on Electrical and Computer Engineering, Dhaka, Bangladesh, 2012. [5] S.W. Shin, S.R. Kang, J.H. Yun, A.V. Moholkar, J.-H. Moon, J.Y. Lee, J.H. Kim, Effect of different annealing conditions on the properties of chemically deposited ZnS thin films on ITO coated glass substartes, Sol. Energy Mater. Sol. Cells 95 (2009) 970–972. [6] S. Tec-Yam, J. Rojas, V. Rejon, A.I. Oliva, High quality antireflective ZnS thin films prepared by chemical bath deposition, Mater. Chem. Phys. 136 (2012) 386–393. [7] S. Takata, T. Minami, T. Miyata, H. Nanto, Growth of hexagonal ZnS thin films by MOCVD CS2 gas as a sulfur source, J. Cryst. Growth 86 (1988) 257–262. [8] M.M. Islam, S. Ishizuka, A. Yamada, K. Sakurai, S. Niki, T. Sakurai, K. Akimoto, CIGS solar cell with MBE–grown ZnS buffer layer, Sol. Energy Mater. Sol. Cells 93 (2011) 856–863. [9] R.A. Bhuiyan, M.M. Alam, M.A. Momin, Effect of substrate temperature on the optical properties of thermally evaporated ZnS thin films, Turk. J. Phys. 34 (2010) 43–49.
Please cite this article in press as: P. Chelvanathan, et al., Growth and characterization of RF-sputtered ZnS thin film deposited at various substrate temperatures for photovoltaic application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.08.155
G Model APSUSC-28610; No. of Pages 7
ARTICLE IN PRESS P. Chelvanathan et al. / Applied Surface Science xxx (2014) xxx–xxx
[10] J.A. Thornton, Influence of apparatus geometry and deposition conditions on structure and topography of thick sputtered coatings, J. Vac. Sci. Technol. 11 (1974) 666–670. [11] S.A. Pethe, E. Takahashi, A. Kaul, N.G. Dhere, Effect of sputtering process parameters on film properties of molybdenum back contact, Sol. Energy Mater. Sol. Cells 100 (2012) 1–5. [12] Y.P.V. Subbaiah, P. Prathap, K.T.R. Reddy, Structural, electrical and optical properties of ZnS films deposited by close-spaced evaporation, App. Surf. Sci. 253 (2006) 2409–2415. [13] V.L. Gayou, B. Salazar-Hernandez, M.E. Constantino, E.R. Andrés, T. Díaz, R.D. Macuil, M.R. López, Structural studies of ZnS thin films grown on GaAs by RF magnetron sputtering, Vacuum 84 (2010) 1191–1194. [14] D.H. Hwang, J.H. Ahn, K.N. Hui, K.S. Hui, Y.G. Son, Structural and optical properties of ZnS thin films deposited by RF magnetron sputtering, Nanoscale Res. Lett. 26 (2012) 1–7. [15] D. Rafaja, H. Kostenbauer, U. Muhle, C. Loffler, G. Schreiber, M. Kathrein, J. Winkler, Effect of the deposition process and substrate temperature on the microstructure defects and electrical conductivity of molybdenum thin films, Thin Solid Films 528 (2013) 42–48.
7
[16] Joint Committee on Powder Diffraction Standards (JCPDS), Card No. 26-0575. [17] L. Sun, J. He, H. Kong, F. Yue, P. Yang, J. Chu, Structure, composition and optical properties of Cu2 ZnSnS4 thin films deposited by pulsed laser deposition method, Sol. Energy Mater. Sol. Cells 95 (2011) 2907–2913. [18] M. Himmrich, H. Haeuseler, Far infrared studies on stannite and wurtzstannite type compounds, Spectrochim. Acta 47A (1991) 933–942. [19] D.J. Vaughan, J.R. Craig, Mineral Chemistry of Metal Sulfide, Univ. Press, London, Cambridge, 1978. [20] P.P. Hankare, P.A. Chate, D.J. Sathe, A.A. Patil, Structure, surface morphological and opto-electronic properties of zinc sulphide thin films deposited by dip method, App. Surf. Sci. 256 (2009) 81–84. [21] C. Kittel, Introduction in Solid State Physics, 8th ed., Wiley, Hoboken NJ, 2005. [22] S.K. Mandal, S. Chaudhuri, A.K. Pal, Optical properties of nanocrystalline ZnS films prepared by high pressure magnetron sputtering, Thin Solid Films 350 (1999) 209–213. [23] X. Wu, F. Lai, L. Lin, J. Lv, B. Zhuang, Q. Yan, Z. Huang, Optical in homogeneity of ZnS film deposited by thermal evaporation, Appl. Surf. Sci. 254 (2008) 6455–6460.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Growth and characterization of RF-sputtered ZnS thin film deposited at various substrate temperatures for photovoltaic application P. Chelvanathan a , Y. Yusoff a , F. Haque a , M. Akhtaruzzaman a , M.M. Alam c , M.J. Rashid a , K. Sopian a , N. Amin a,b,c,∗ a
Solar Energy Research Institute, The National University of Malaysia, 43600 Bangi, Selangor, Malaysia Department of Electrical, Electronic and Systems Engineering, FKAB, The National University of Malaysia, 43600 Bangi, Selangor, Malaysia c Chemistry Department, College of Sciences, King Saud University, Riyadh 11451, Saudi Arabia b
a r t i c l e
i n f o
Article history: Received 15 June 2014 Received in revised form 23 August 2014 Accepted 26 August 2014 Available online xxx Keywords: Thin films ZnS Sputtering growth Structural properties Optical properties
a b s t r a c t RF-sputtered ZnS thin film was grown under various substrate temperatures with the aim of investigating its effects on the structural, surface morphology and optical properties. Investigated substrate temperature in this study was in the range of 25 ◦ C–300 ◦ C and the structural and optical properties were investigated in order to elucidate the changes induced by the varying thermal energy during the growth process. Structural determination by XRD method indicates all sputtered films have cubic structure with (1 1 1) as the preferential orientation. However, higher substrate temperature up to 200 ◦ C increases the film’s crystallinity and grain size evident by the increase in peak intensity. Slight peak shift indicates ZnS lattice undergoes strain relaxation process mediated through the increase in the lattice constant ˚ SEM image of surface morphology clearly shows the evolution of grain growth in from 5.32 A˚ to 5.40 A. which sputtered film at 200 ◦ C has the largest grains with distinct grain boundaries. Calculation from the obtained transmission spectra indicates optical band gap is in the range of 3.6–3.9 eV. Theoretical analysis in terms of lattice parameter between ZnS with several upcoming photovoltaic absorber layers shows that lattice matched ZnS buffer layer can be grown by varying the substrate temperature. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Cadmium sulfide (CdS) has been used extensively as the buffer layer deposited on top of Cu(In,Ga)Se2 (CIGSe) absorbers for thin film solar cell applications till date and cell efficiency of 20.3% has been reported by using CdS for CIGS solar cells [1]. But further enhancement of efficiency is limited as the CdS has a bandgap of 2.45 eV causing high photon absorption at shorter wavelengths [2]. Among various alternative candidates, ZnS is considered as one of the most promising materials [3]. ZnS is non-toxic, abundant and cheap. It is a direct wide bandgap n-type compound semiconductor having a bandgap of 3.2–3.9 eV at room temperature which is higher in comparison with CdS having a bandgap of 2.45 eV. The wider bandgap of ZnS enables high energy incident photons to reach the window-absorber junction, enhancing the blue response of the photovoltaic cells and thus contributes to a better cell performance [4]. The efficiency of heterojunction solar cells depends
∗ Corresponding author at: Solar Energy Research Institute, The National University of Malaysia, 43600 Bangi, Selangor, Malaysia. Tel.: +60 193296750; fax: +60 389118359. E-mail addresses: [email protected], [email protected] (N. Amin).
largely on the interfacial properties between absorber and buffer layer. ZnS has the ability to provide better lattice matching with CIGSe as well as CZTS absorbers [5]. Several growth techniques for preparing ZnS thin films have been reported in the literature. These are chemical bath deposition (CBD) [6], metal organic chemical vapour deposition (MOCVD) [7], molecular beam epitaxy (MBE) [8], RF magnetron sputtering [3] and thermal evaporation [9]. Magnetron sputtering technique has some advantages compared to the other methods such as easier controllability of the deposition parameters, high film growth rate, relatively cost-effective and especially for the compatibility with the sputtering depositions of CIGSe2 absorbers and window layers, leading to a development of a full in-line sputtering technique [3]. In this work, we have deposited ZnS thin films by RF magnetron sputtering with different substrate temperature the effects on the structural, topographical and optical is studied. Theoretical lattice mismatch between ZnS deposited in this study and several new photovoltaic absorbers is calculated and presented. 2. Experimental ZnS thin films were deposited on soda lime glass substrates of 75 mm × 25 mm via radio frequency (RF) magnetron
http://dx.doi.org/10.1016/j.apsusc.2014.08.155 0169-4332/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: P. Chelvanathan, et al., Growth and characterization of RF-sputtered ZnS thin film deposited at various substrate temperatures for photovoltaic application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.08.155
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sputtering system. Prior to deposition, all the substrates were cleaned ultrasonically in methanol–acetone–methanol–deionised water sequence and finally dried by N2 gas flow. Source material used in this study is a 50.8 mm diameter ZnS (99.99%) sputtering target (Kurt.J.Lesker) which was pre-sputtered for 15 min to remove contamination on the target’s surface. Deposition chamber base pressure was brought down close to 10−6 Torr by turbomolecular pump and the working pressure during all the deposition run was maintained at 10 mTorr by flowing 5 sccm of purified Ar (99.99%) as the working gas into the chamber. RF power, substrate holder rotation and target to substrate distance were fixed at 50 W (≈2.5 W/cm2 ), 10 rpm and 80 mm, respectively. RF-sputtered ZnS films were grown at 4 different substrate deposition temperatures; room temperature (RT), 100 ◦ C, 200 ◦ C and 300 ◦ C and the changes on the structural, surface morphology and optical properties were observed. ZnS films sputtered at elevated temperature were left in the sputtering chamber under high vacuum for natural cooling process to avoid surface oxidation. Structural and crystallinity properties as well as the orientation along the film’s surface normal were examined by BRUKER aXS-D8 Advance Cu-K␣ diffractometer. XRD patterns were recorded in the 2 range from 20◦ to 60◦ using ˚ Grain size and surface Cu K␣ radiation wavelength, = 1.5408 A. morphology were observed by using LEO 1450 Variable Pressure Scanning Electron Microscope (VPSEM) while surface topography and roughness were analyzed by using Integra Prima, NT-MDT Scanning Probe Microscope (SPM) with non-contact mode settings. The mean crystallite sizes (D) of the films was calculated using Scherrer formula D = 0.9/ˇ cos
(1)
whereby, is the X-ray wavelength (0.15406 nm), and ˇ is the full width at half maximum [FWHM] of the film diffraction peak at 2 in radian and is the Bragg diffraction angle in degree. The microstrain, ε and dislocation density, ı developed in the thin films are calculated from Eqs. (2) and (3), respectively. ε = ˇ/4 tan ı = n/D
2
(2) (3)
whereby, n is a factor, which is almost equal to unity for minimum dislocation density and D is the grain size. Optical transmission measurements were performed at room temperature by using Perkin Elmer Lambda 35 UV/Vis spectrophotometer from wavelength range of 300–900 nm. Energy band gap values were calculated from the obtained transmission spectra. 3. Results and discussion 3.1. Structural properties of ZnS films Fig. 1 shows the XRD pattern for ZnS films deposited with various substrate temperatures from 20◦ to 60◦ . All films exhibited most intense peak at 2 = 28.5◦ which corresponds to (1 1 1) preferred orientation of cubic phase of -ZnS while films sputtered at 100 ◦ C and 200 ◦ C have a slight peak centred at 244 = 47.3◦ which can be attributed to (2 2 0) plane of -ZnS phase as well. However due to the polymorphism nature of ZnS, the peaks assigned initially as cubic phase of zincblende structure could also belong to (0 0 0 2) plane of hexagonal phase which corresponds to wurtzite structure [4]. The obtained XRD pattern from this study is in agreement with the indexed standard spectrum documented in JCPDS card No. 80-0020 Microstructural evolution based on structural zone model (SZM) for thin film growth for ratio of substrate temperature (Ts ) to melting temperature, (Tm ) Ts /Tm < 0.1 is expected to yield amorphous or random polycrystalline due to lack of surface mobility during nucleation [10]. However, ZnS thin film deposited
Fig. 1. XRD patterns ZnS thin films deposited at different substrate temperatures.
at room temperature of 25 ◦ C in this study yields ZnS film with preferred orientation as shown in the XRD spectra. Kinetic energy of incident sputtered particles is believed to facilitate ZnS crystallization process in the absence of intentional heating. Ein which is the kinetic energy of incident particles depends on initial kinetic energy of the sputter ejected particles Eout , distance between substrate and target l, cross section for momentum transfer collision with background gas atom , Boltzmann constant k, working gas pressure p and temperature T in the following relation [11]. Ein = Eout exp(−pl/kT )
(4)
The variable T in this relation is the temperature of incident particles which are ejected out due to bombardment of working gas and should not be confused for substrate temperature Ts . Hence, it is deduced that RF sputtering configuration in this study (details are in Section 2) produces incident particles with sufficient kinetic energy Ein which in return induces crystallization mechanism. Increase in the peak intensity is observed for (1 1 1) orientation as the substrate temperature is increased from room temperature to 200 ◦ C indicating improvement in film crystallinity due to higher thermal stress and increase in surface diffusivity of incoming sputtered particles. Slight decrease in the intensity and broadening of the same peak is observed for films sputtered at substrate temperature of 300 ◦ C. Subbaiah et al. [13] has reported the similar observation for ZnS thin film deposited by close space evaporation for temperatures >325 ◦ C whereby the deterioration on degree of crystallinity is linked to sulphur re-evaporation due to high vapour pressure. The difference in the observed temperature onset of the decrement of crystallinity from this study and reported as in [12] can be due to the difference in the kinetic energy of particles. Incoming sputtered particles have higher kinetic energy than evaporated particles hence sulphur re-evaporation in sputter deposition method may have occurred at lower temperature (higher kinetic energy is translated to thermal energy upon collision) as observed in this study. Improvement in crystallinity of ZnS film is accompanied by increase in grain size calculated from Scherrer’s equation as shown in Fig. 2. Enhancement of grain size can be attributed to the increased mobility of sputtered species at higher substrate temperature which induces better grain coalescence. Highest grain size of ∼14 nm for sputtered film at 200 ◦ C is obtained which is substantially lower than ∼31 nm reported in [13] observed in RF-sputtered ZnS film at 180 ◦ C. Discrepancy in the reported grain size values is mainly due to the different thicknesses of sputtered ZnS films. Film as thick as 1.76 m has reported in [13] meanwhile film thickness in this study
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Fig. 2. Grain size and FWHM variations for ZnS thin films deposited at different substrate temperatures.
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Fig. 4. Lattice parameter and strain variations for ZnS thin films deposited at different substrate temperatures.
Fig. 5. Microstrain and dislocation density along (1 1 1) orientation.
Fig. 3. XRD peak shift of (1 1 1) orientation of ZnS thin films deposited at different substrate temperatures.
was restricted to 50–80 nm in the aim of studying ultra thin ZnS structural evolution. However, further increase in substrate temperature to 300 ◦ C results in decrease in the ZnS grain size. Since the decrement in the grain size is concomitant with the observed lower degree of crystallinity for ZnS film sputtered at 300 ◦ C, the aforesaid sulphur re-evaporation mechanism at elevated temperature could have suppressed the grain growth process. Similar findings were reported by Hwang et al. [14] whereby decrease in grain size for RF sputtered ZnS thin films was observed at substrate temperature of 400 ◦ C and was accompanied by decreasing Full Width Half Maximum (FWHM) value in the corresponding XRD peak. Differences in geometrical configuration of sputter system (substrate to target distance, target surface area) and process variables (operating pressure and RF power) between in this study and as reported in [14] could be the reason in the variation of substrate temperature at which decrement of grain size occurs. Closer scrutiny on the XRD spectra (Fig. 3) reveals gradual peak shift towards lower 2 occurs for the preferred orientation indicating substrate temperature influences the stress distribution in ZnS film. Variation in lattice parameter and strain (percentages of changes in lattice constant) as a function of substrate temperature is given in Fig. 4. Increasing substrate temperature promotes lattice relaxation with film sputtered at 300 ◦ C possessing equilibrium lattice parameter with no strain. Film sputtered at lower
temperature is found to exhibit higher compressive strain (smaller lattice parameter). However, bare observation by naked eye did not show any film cracking despite possessing compressive strain which would lead to peeling off. Good adherence of ZnS film to the SLG substrate specifically for films sputtered at lower temperatures indicates the resulting range of compressive strain in this study is not detrimental from adhesion perspective. Fig. 5 shows the calculated microstrain and dislocation density along (1 1 1) crystallographic plane for different substrate temperature. Maximum value of dislocation density is obtained for film sputtered at RT due to inverse dependence of dislocation density to the grain size as shown in Eq. (3). Physically, grain size enlargement through grain coalescence at higher substrate temperature decreases the lattice point mismatch of two neighbouring crystals in the following manner. During grain coalescence process, merging of two neighbouring crystallites eliminates the grain boundary which initially separates them. Since lattice point mismatch originates from misregistry of lattice (dislocation cores) in one part of the crystallite with respect to another part along the interconnecting grain boundary, the elimination of this particular grain boundary subsequently vanishes the existing lattice point mismatch. Furthermore, variation in microstrain parameter shown in Fig. 5 coincides with the dislocation density values indicating microstrain is caused predominately by dislocation density as described in Eq. (5) [15]. < ε2hkl >= 2b2 M 2 Chkl Pdislocation
(5)
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Fig. 6. FESEM images of ZnS thin film deposited at different substrate temperatures. (a) RT, (b) 100 ◦ C, (c) 200 ◦ C and (d) 300 ◦ C.
whereby, <hkl 2 > is the mean squared microstrain in along (hkl) plane, Burgers vector b, the contrast factor of dislocation Chkl , Wilkens factor M, and Pdislocation is the dislocation density along the same (hkl) plane. Table 1 shows the expected lattice mismatch of several photovoltaic semiconductor materials with ZnS films deposited in this study. It can be seen that ZnS films deposited at different substrate temperature by RF-sputtering in this study can be used as lattice matched buffer layer (numerical values in bold indicates lowest lattice mismatch percentage) with various photovoltaic material such as rigorously researched CZTS and the upcoming novel absorber materials of Cu2 ZnGeS4 and FeS2 . 3.2. Surface morphology VPSEM was used to view and analyze the surface morphology meanwhile atomic force microscopy was used to determine the root mean square (RMS) surface roughness of all ZnS films. Fig. 6 shows the VPSEM images of ZnS films deposited at different substrate temperatures. Grain size observed form the surface morphology progressively increases for film sputtered at room temperature up to 200 ◦ C in agreement with the calculated value
(Fig. 2). Large grains are visible distinctly for film sputtered at 200 ◦ C. Thick dark fringes encompassing the grains are likely to be voids rather that grain boundary due to ultra thin ZnS layer. Hence, although ZnS film (200 ◦ C) showed highest degree of crystallinity from XRD data, VPSEM reveals undesirable voids which are detrimental if it is incorporated (as a n-type buffer layer) in full working device of thin film solar cell. This is due to possibility of shunting effect of transparent conducting oxide penetrating all the way towards the Mo back contact. In addition, non-uniform buffer layer of ZnS creates non-homogeneous and localized heterojunction with p-type absorber layer (CIGS, CZTSSe) affecting the built-in potential spatial distribution which is crucial to drift the photo-generated carrier towards front and back terminals. Hence, films sputtered at 100 ◦ C and 300 ◦ C are better options for buffer layer deposition as lesser voids resembling pinholes are observed for these films. Surface topography probed by AFM is shown if Fig. 7(a) and (b) for film sputtered as room temperature and 200 ◦ C respectively. Particles with greater variation in terms of height which forms rougher surface are observed in the former images than the latter. Quantitative variation of RMS roughness is plotted Fig. 8 and it can be seen clearly that film sputtered at higher temperature has
Table 1 Lattice mismatch of several photovoltaic materials with ZnS film from this study. Sample identification
ZnS lattice parameter (nm)
ZnS-RT ZnS-100 ZnS-200 ZnS-300
0.5326 0.5355 0.5370 0.5406
a b
Lattice mismatch % JCPDS-CZTS (a = 0.5427 nm) [16]
a
1.8610 1.3267 1.0503 0.3869
Lattice mismatch % b PLD-CZTS (a = 0.5365 nm) [17]
Lattice mismatch % Cu2 ZnGeS4 (a = 0.5341 nm) [18]
Lattice mismatch % FeS2 (a = 0.5266 nm) [19]
0.7269 0.1864 0.0932 0.7642
0.2808 0.2621 0.5430 1.2170
1.1393 1.6900 1.9749 2.6585
Lattice constant from JCPDS-26-0575. Pulsed Laser Deposited (PLD) CZTS.
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Fig. 7. (a) AFM surface topography of ZnS thin film deposited at room temperature (RT). (b) AFM surface topography of ZnS thin film deposited at room temperature 200 ◦ C.
smoother topography than film sputtered with no intentional heating. This is due to higher mobility and surface diffusion of sputtered atoms which leads to homogeneous surface. 3.3. Optical properties Optical properties were characterized by measuring the transmittance of deposited ZnS films on glass substrates in the wavelength region of 300–900 nm. Transmittance of the reference glass is subtracted to offset the influence of underlying substrate. The transmittance values of the films at temperature have been measured by a UV/VIS spectrophotometer and given in Fig. 9. Transmittance values in the wavelength range of 300–500 nm are higher for films deposited with lesser thermal stress. On the other hand, the opposite trend is observed as transmittance values in the wavelength region of 500–900 nm for higher for films deposited with higher substrate temperature (for 100 ◦ C and 300 ◦ C samples only). Transmission of ZnS film sputtered at room temperature and 200 ◦ C is the lowest in this study. The absorption coefficient, ˛ is related to the optical energy band gap, Eg of the semiconductors strong absorption region by the relation, ˛h = A(h − Eg )
n/2
where A is constant, Eg is the energy gap and n is constant equal to 1 for direct gap compound and 3 for indirect gap compound. Direct band gap values can be estimated by extrapolating the straight-line portion in the graph of (˛h)2 versus h as shown in Fig. 10. Sputtered films (100 ◦ C and 300 ◦ C) exhibit steep increase in (˛h)2 value meanwhile films deposited at RT and 200 ◦ C manifest gradual increase for the same value of photon energy indicating these films (100 ◦ C and 300 ◦ C) have higher homogeneity in the shape and size of the grains and lower defects density near the band edge than films sputtered at RT and 200 ◦ C [20]. Band gap of ZnS in this study was found to be in the range of 3.68–3.95 eV. Identical band gap of 3.68 eV was obtained for sputtered films at 100 ◦ C and 300 ◦ C which exactly the same value for bulk ZnS [21]. It was found out that the variation in the band gap values is not systematic with the variation in the substrate temperature in this study. This could be due to the inhomogeneity broadening effect which leads to different absorption behaviour which is manifested through different absorption peak corresponding to band to band transition (energy band gap) [22]. Inhomogeneity broadening effect originates from the fluctuation of grain shape and size especially in thin film featuring nanostructures as reported in this study (see Fig. 6). On the other hand, absorption behaviour of ZnS thin film is also susceptible
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to surface optical loss due to scattering by ultra small crystallites [22] which alters the refractive index and extinction coefficient values of the film [23]. Hence, due to the aforesaid inhomogeneity broadening effects and surface optical loss, non-systematic variation in optical properties especially the energy band gap values are observed in this study. 4. Conclusion
Fig. 8. RMS surface roughness variations for ZnS thin films deposited at different substrate temperatures.
Microstructural properties such as orientation, grain size, lattice parameter, microstrain and dislocation density have been studied for RF-sputtered ZnS thin films with different substrate temperature. ZnS thin film sputtered without intentional heating showed preferred orientation growth due to the sufficient kinetic energy of incident particles which facilitated crystallization. ZnS thin film preferred orientation parallel to the SLG substrate as well as degree of crystallinity and grain size were found to increase as the substrate temperature is increased to 200 ◦ C. Relaxation of ZnS lattice parameter due to increasing thermal stress could be used to obtain minimum lattice mismatch structure (nZnS/p-absorber layer) with CZTS, Cu2 ZnGeS4 and FeS2 photovoltaic material. However, increase in degree of crystallinity did not grant uniform enlarged grains as significant amount of voids in between grains are observed by AFM indicating further surface morphology optimization is needed to avoid possible shunting mechanism. Higher degree of homogeneity in grain size as well as uniformity improves the optical transmission of ZnS film deposited at higher temperature and optical band gap in between 3.6 eV and 3.9 eV was observed which is well in the range of standard reported value. Acknowledgements
Fig. 9. Transmission spectra for ZnS thin films deposited at different substrate temperatures.
The authors would like to acknowledge and appreciate the contribution of the Ministry of Higher Education of Malaysia (MOHE) through its research grant with code FRGS/1/2013/TK07/UKM/01/3. Appreciations are also extended to the Solar Research Institute (SERI), The National University of Malaysia (Universiti Kebangsaan Malaysia (UKM)), Centre of Research and Instrumentation Management CRIM, UKM and the College of Science Research Center, Deanship of Scientific Research, King Saud University for their support. References
Fig. 10. Extrapolated band gap values of ZnS thin films deposited at different substrate temperatures.
[1] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, New world record efficiency for Cu(In,Ga)Se2 thin film solar cells beyond 20%, Prog. Photovolt. Res. Appl. 19 (2011) 894. [2] T. Nakada, M. Mizutani, Y. Hagiwara, A. Kunioka, High-efficiency Cu(In,Ga)Se2 thin-film solar cells with a CBD–ZnS buffer layer, Sol. Energy Mater. Sol. Cells 67 (2001) 255. [3] L.-X. Shao, K.-H. Chang, H.-L. Hwang, Zinc sulfide thin films deposited by RF reactive sputtering for photovoltaic applications, App. Surf. Sci. 212 (213) (2003) 305–310. [4] M.A. Islam, M.S. Hossain, M.M. Aliyu, Y. Sulaiman, M.R. Karim, K. Sopian, N. Amin, Comparative study of ZnS thin films grown by chemical bath deposition and magnetron sputtering, in: 7th International Conference on Electrical and Computer Engineering, Dhaka, Bangladesh, 2012. [5] S.W. Shin, S.R. Kang, J.H. Yun, A.V. Moholkar, J.-H. Moon, J.Y. Lee, J.H. Kim, Effect of different annealing conditions on the properties of chemically deposited ZnS thin films on ITO coated glass substartes, Sol. Energy Mater. Sol. Cells 95 (2009) 970–972. [6] S. Tec-Yam, J. Rojas, V. Rejon, A.I. Oliva, High quality antireflective ZnS thin films prepared by chemical bath deposition, Mater. Chem. Phys. 136 (2012) 386–393. [7] S. Takata, T. Minami, T. Miyata, H. Nanto, Growth of hexagonal ZnS thin films by MOCVD CS2 gas as a sulfur source, J. Cryst. Growth 86 (1988) 257–262. [8] M.M. Islam, S. Ishizuka, A. Yamada, K. Sakurai, S. Niki, T. Sakurai, K. Akimoto, CIGS solar cell with MBE–grown ZnS buffer layer, Sol. Energy Mater. Sol. Cells 93 (2011) 856–863. [9] R.A. Bhuiyan, M.M. Alam, M.A. Momin, Effect of substrate temperature on the optical properties of thermally evaporated ZnS thin films, Turk. J. Phys. 34 (2010) 43–49.
Please cite this article in press as: P. Chelvanathan, et al., Growth and characterization of RF-sputtered ZnS thin film deposited at various substrate temperatures for photovoltaic application, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.08.155
G Model APSUSC-28610; No. of Pages 7
ARTICLE IN PRESS P. Chelvanathan et al. / Applied Surface Science xxx (2014) xxx–xxx
[10] J.A. Thornton, Influence of apparatus geometry and deposition conditions on structure and topography of thick sputtered coatings, J. Vac. Sci. Technol. 11 (1974) 666–670. [11] S.A. Pethe, E. Takahashi, A. Kaul, N.G. Dhere, Effect of sputtering process parameters on film properties of molybdenum back contact, Sol. Energy Mater. Sol. Cells 100 (2012) 1–5. [12] Y.P.V. Subbaiah, P. Prathap, K.T.R. Reddy, Structural, electrical and optical properties of ZnS films deposited by close-spaced evaporation, App. Surf. Sci. 253 (2006) 2409–2415. [13] V.L. Gayou, B. Salazar-Hernandez, M.E. Constantino, E.R. Andrés, T. Díaz, R.D. Macuil, M.R. López, Structural studies of ZnS thin films grown on GaAs by RF magnetron sputtering, Vacuum 84 (2010) 1191–1194. [14] D.H. Hwang, J.H. Ahn, K.N. Hui, K.S. Hui, Y.G. Son, Structural and optical properties of ZnS thin films deposited by RF magnetron sputtering, Nanoscale Res. Lett. 26 (2012) 1–7. [15] D. Rafaja, H. Kostenbauer, U. Muhle, C. Loffler, G. Schreiber, M. Kathrein, J. Winkler, Effect of the deposition process and substrate temperature on the microstructure defects and electrical conductivity of molybdenum thin films, Thin Solid Films 528 (2013) 42–48.
7
[16] Joint Committee on Powder Diffraction Standards (JCPDS), Card No. 26-0575. [17] L. Sun, J. He, H. Kong, F. Yue, P. Yang, J. Chu, Structure, composition and optical properties of Cu2 ZnSnS4 thin films deposited by pulsed laser deposition method, Sol. Energy Mater. Sol. Cells 95 (2011) 2907–2913. [18] M. Himmrich, H. Haeuseler, Far infrared studies on stannite and wurtzstannite type compounds, Spectrochim. Acta 47A (1991) 933–942. [19] D.J. Vaughan, J.R. Craig, Mineral Chemistry of Metal Sulfide, Univ. Press, London, Cambridge, 1978. [20] P.P. Hankare, P.A. Chate, D.J. Sathe, A.A. Patil, Structure, surface morphological and opto-electronic properties of zinc sulphide thin films deposited by dip method, App. Surf. Sci. 256 (2009) 81–84. [21] C. Kittel, Introduction in Solid State Physics, 8th ed., Wiley, Hoboken NJ, 2005. [22] S.K. Mandal, S. Chaudhuri, A.K. Pal, Optical properties of nanocrystalline ZnS films prepared by high pressure magnetron sputtering, Thin Solid Films 350 (1999) 209–213. [23] X. Wu, F. Lai, L. Lin, J. Lv, B. Zhuang, Q. Yan, Z. Huang, Optical in homogeneity of ZnS film deposited by thermal evaporation, Appl. Surf. Sci. 254 (2008) 6455–6460.
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