Post-annealing Effects on ZnS Thin Films Grown by ... - Springer Link

2 downloads 0 Views 2MB Size Report
Post-annealing Effects on ZnS Thin Films Grown by Using the CBD Method. Heejin Ahn and Youngho Um. ∗. Department of Physics, University of Ulsan, Ulsan ...
Journal of the Korean Physical Society, Vol. 67, No. 6, September 2015, pp. 1045∼1050

Post-annealing Effects on ZnS Thin Films Grown by Using the CBD Method Heejin Ahn and Youngho Um∗ Department of Physics, University of Ulsan, Ulsan 680-749, Korea (Received 31 December 2014, in final form 25 April 2015) Herein, the structural, morphological, and optical properties of zinc sulfide (ZnS) thin films deposited via the chemical bath deposition method are reported. These films were deposited on soda-lime glass (SLG) substrates by using ZnSO4 , thiourea, and 25% ammonia at 90 ◦ C. The effect of changing the annealing temperature from 100 ◦ C to 300 ◦ C on the properties of the ZnS thin films was investigated. X-ray diffraction (XRD) patterns showed that the ZnS thin film annealed at 100 ◦ C had an amorphous structure; however, as the annealing temperature was increased, the crystalline quality of the thin film was enhanced. Moreover, transmission measurements showed that the optical transmittance was about 80% for wavelengths above 500 nm. The band gap energy (Eg ) value of the film annealed at 300 ◦ C was decreased to about 3.82 eV. PACS numbers: 81.10.Dn, 81.70.Jb Keywords: Chemical bath deposition, CIGS buffer layer, ZnS, Annealing effect, XPS DOI: 10.3938/jkps.67.1045

I. INTRODUCTION Solar energy is considered to be the cleanest, most sustainable renewable energy. The effective use of solar energy has become an important research topic. Recently, a supply shortage in the market for metallurgical-grade silicon has been identified; as such, it is hoped that low-cost thin film [1] replacements for Si solar cells, which currently comprise approximately 90% of the market share, will be developed. Cu(In,Ga)Se2 (CIGS)-based thin-film solar cells with high efficiency have been reported on both the laboratory scale and in large-area devices [2]. Although the theoretical conversion efficiency of CIGSbased thin- film solar cells is ∼30%, the best efficiency achieved thus far has been approximately 20% (for smallarea solar cells) [3]. The CIGS layer has an excellent light absorption coefficient (105 cm−1 ) and is a better layer choice than many other materials. In CIGS thin film solar cells, a chemically-deposited CdS buffer layer with high resistivity is generally used between the absorber and the transparent conducting oxide layers. However, CdS causes serious environmental problems due to the large amount of Cd-containing waste that is generated during the deposition process [4]. In addition, the efficiency of solar cells with a CdS buffer layer is decreased due to the reduced absorption resulting from the presence of the CdS buffer layer [5]. ZnS exhibits a wider band gap (3.8 eV) compared to CdS. This implies that replacing CdS buffer layers with ZnS buffer layers can decrease absorption losses ∗ E-mail:

[email protected]; Fax: +82-52-259-1693

and improve the short-circuit current (Jsc ) in solar cells [6]. ZnS thin films can be synthesized by using different methods such as thermal evaporation [7], spray pyrolysis [8], sputtering [9], chemical bath deposition (CBD) [10–13], improved CBD [14, 15], chemical vapor deposition (CVD) [16], successive ionic layer adsorption and reaction (SILAR) [17], and metal-organic vapor phase epitaxy (MOVPE) [18]. Among these deposition methods, the CBD method is highly attractive because the technique possesses a number of advantages over conventional thin film deposition methods. The main advantages of the CBD methods are low cost, low evaporation temperature and easy coating of large surfaces [19]. However, generally, the films obtained by using the CBD method are either amorphous or poorly crystallized. Therefore, an annealing process is needed to improve the crystallinity of the films [20]. In this paper, ZnS thin films have been deposited on soda-lime glass substrates by using the CBD method. The structural, morphological, and optical properties of these ZnS thin films post-annealed at different temperatures were analyzed. This knowledge may be useful for choosing the appropriate processing variables during the preparation of ZnS layers for optoelectronic, semiconductor, and photovoltaic devices.

II. EXPERIMENTS AND DISCUSSION ZnS thin films were produced by using the CBD method on soda-lime glass (SLG). The substrates used for the deposition of ZnS thin films were SLG with the

-1045-

-1046-

Journal of the Korean Physical Society, Vol. 67, No. 6, September 2015

Fig. 2. (Color online) X-ray diffraction patterns of the ZnS thin films post-annealed at three different temperatures.

Fig. 1. (Color online) (a) Preparation apparatus for ZnS thin films via the CBD method (1 - thermocouple, 2 - stirrer, 3 - hot plate, 4 - substrate, and 5 - reactive aqueous solution). (b) Temperature profile for the preparation of ZnS thin films.

size of 25 × 25 mm2 . Before the deposition, the substrate was cleaned in deionized (D.I.) water, then cleaned with methanol and acetone, again washed with deionized water and finally dried with N2 gas. The ZnS thin films were chemically grown on SLG substrates by using an aqueous solution of ZnSO4 (0.010 M), ammonia (0.07 M), and thiourea (0.8 M) at 90 ◦ C. The mixture was poured into a beaker and heated to 90 ◦ C. The process consisted of an aqueous bath whose temperature was controlled with a hot plate. When the temperature reached the deposition temperature, three precleaned SLG were introduced into the solution. After deposition, the thin films were rinsed with D.I. water and dried using N2 gas. Both sides of the substrates were coated with films; one side of the coated film was removed by using a swab with acetone. The samples were heated at 100, 200, and 300 ◦ C for 45 minutes on the hot plate to study how that would affect the structural and optical properties. Figure 1 shows a schematic of the preparation equipment used to make the ZnS thin films via the CBD method as well as the temperature profile. The crystalline structure of the film was characterized using an

X-ray diffractometer (Rigaku RAD-3D, Japan) with CuKα radiation within the 2θ range of 20◦ ∼ 80◦ . The morphologies of the ZnS thin films were obtained via fieldemission scanning electron microscopy (JSM-820 JEOL, Japan) after a Pt coating had been applied onto the specimen’s surface. The optical properties between the wavelengths of 200 and 1100 nm were recorded by using an ultraviolet-visible (UV-VIS) spectrophotometer (HP UV-VIS 8453, U.S.A). The elemental composition of the thin film was obtained with X-ray photoelectron spectroscopy (K-alpha thermo, UK). ZnS crystals typically existed in two phases: cubic (zinc blende) and hexagonal (wurtzite). The cubic phase is stable at room temperature while the wurtzite (a less dense hexagonal phase) is stable at higher temperatures [21]. Figure 2 shows the XRD patterns of ZnS thin films post-annealed at different temperatures. No peaks, with the exception of a broad diffraction peak between 20◦ and 40◦ , are seen indicating that the film is amorphous. As the substrate temperature was increased, the ZnS film showed better crystallinity. The annealed film showed a diffraction peak at 29.45◦ , which was identified as a reflection peak from the (111) plane of the cubic phase. Figure 3 shows FE-SEM images of ZnS films postannealed at various temperatures. As can be seen from the figure, the film annealed at 100 ◦ C showed large grains and many pinholes on the surface while the samples annealed at higher temperatures had a uniform, compact morphology. ZnS films were formed in the quasi-linear phase, showing a compact, granular structure with grain sizes of about 100 nm. The average grain size of the annealed film is much larger than that of the as-deposited thin film, which implies that the post-annealing technique enables the production of highquality ZnS thin films. Each grain in the FE-SEM image of the ZnS thin film is clearly a coalescence composed of nanometer-sized crystallites [15].

Post-annealing Effects on ZnS Thin Films Grown · · · – Heejin Ahn and Youngho Um

-1047-

Fig. 3. (Color online) FE-SEM images of the ZnS thin films post-annealed at temperature of (a) 100 ◦ C, (b) 200 ◦ C, and (c) 300 ◦ C.

Fig. 4. (Color online) AFM images (5 μm × 5 μm) of the surfaces of ZnS films annealed at temperature of (a) 100 ◦ C, (b) 200 ◦ C, and (c) 300 ◦ C.

Fig. 5. (Color online) Transmittance spectra of the ZnS thin films grown on SLG substrates and post-annealed at three temperatures.

The surface states strongly influence the optical properties (transmittance and absorbance) of the ZnS films. The influence of the post-annealing temperature on the surface morphology of the ZnS film, as revealed by AFM images, is shown in Fig. 4. Not only are the annealed films composed of clusters, but they also possess more compact, smoother surfaces as the annealing tempera-

ture increases from 100 ◦ C to 300 ◦ C. The optical properties of these ZnS thin films were determined from transmission and absorption measurements taken between 200 and 1010 nm. The transmittance values of the films with different annealing temperatures were taken with a UV/VIS spectrophotometer, and the results are shown in Fig. 5. The observed transmittances were 80% and 70% for films annealed at 100 ◦ C and 300 ◦ C, respectively. The different optical performances between 300 and 600 nm is caused by the different annealing temperatures; this is believed to be caused by the structural change from the amorphous phase to the cubic phase. With these transmittance values, our ZnS films are suitable for use as buffer layers replacing the CdS films in CIGS solar cells. The energy band gaps were calculated with the help of the optical absorption spectra. To determine the energy band gap, we plotted (αhν)2 against hν. The absorption coefficient (α), where is associated with the strong absorption region of the films, was calculated from absorbance (A) and the film’s thickness (t) by using the relation, α = 2.303A/t.

(1)

The theory of inter-band absorption shows that at the optical absorption edge, the absorption coefficient α

-1048-

Journal of the Korean Physical Society, Vol. 67, No. 6, September 2015

2

Fig. 6. (Color online) Plots of (αhν) versus photon energy (hν) for the ZnS thin films annealed at three temperatures. Table 1. Calculated band gaps, grain sizes, and surface roughness of the post-annealed CBD-ZnS thin films for three annealing temperatures. Annealing Temperature (◦ C) 100 200 300

Eg (eV) 3.89 3.85 3.82

Grain Size (nm) 134.1 137.8 178.5

Roughness (nm) 28.49 22.45 19.91

function of the annealing temperature. The band gap energy decreases with the increasing grain size. XPS measurements were performed to determine the bonding of Zn and S. XPS spectra revealed that, in addition to Zn and S, O and C were also present in the CBD-ZnS films. A large amount of oxygen likely existed in the ZnS layer via the formation of ZnO and Zn(OH)2 by the following chemical reactions: Zn2+ + 2OH − ↔ Zn(OH)2 , Zn(OH)2 ↔ ZnO + H2 O.

varies with the photon energy hν according to α(hν) = K(hν − Eg )n ,

Fig. 7. (Color online) The band gap energy and grain size of the ZnS films as a function of annealing temperature.

(2)

where Eg is the optical band gap, K is a constant and the exponent n = 12 , 1, 2, 3, depending on the type of electronic transition in k-space. For the determination of, the optical band gap of the ZnS thin film, taking n = 1 2 gives the best fit for the films. Figure 6 shows a plot of (αhν)2 versus hν, where α is the optical absorption coefficient and hν is the energy of the incident photon. The band gap energy (Eg ) is determined by extrapolating the straight line portion of the spectrum to αhν = 0. The calculated Eg values for the annealed films are summarized in Table 1. Based on Fig. 6, the calculated direct energy gap decreased as the annealing temperature was increased. This decrease can be attributed to the increased crystallite size, which induced a decrease in the dislocation density. These band gap values (which are in good agreement with those found in earlier reports) are higher than that of bulk ZnS, which is likely due to quantum size effects [11]. Such effects are dominant in polycrystalline thin films with very small grains. This is the reason the film annealed at 100 ◦ C did not show any characteristic XRD peaks, suggesting poor crystallinity for that film. Figure 7 shows the band gap energy and the grain size of the ZnS films as

(3) (4)

The C1s, O1s, Zn2p3/2 , and S2p XPS spectra were recorded for all of the thin films. All of the XPS spectra of the ZnS films were well-fitted with Gaussian functions, as shown in Fig. 8(a). The fitting results revealed that the O1s peaks were centered between 531 and 535 eV, which were deconvoluted into three Gaussian peaks centered at 530.3 eV (Zn(OH)2 ), 531.4 eV (ZnO), and 533.3 eV (C−O bond), respectively. The 535.3 eV peak originated from the cluster OH; this cluster OH peak decreases with increasing annealing temperature. This implies that the post-annealing treatment enhances the crystalline quality of the CBD-ZnS thin films. Figure 8(b) shows that the XPS spectrum can be fitted with three bands corresponding to C−C, C=O, and C−O bonds at 284.6 eV, 286.1 eV, and 288.1 eV, respectively. As the annealing temperature was increased, the intensity of the C=O peak decreased. Figure 8(c) shows that with increasing annealing temperature, the Zn 2p3/2 peaks shifted from binding energies assigned to Zn (1021.2 eV) to higher energies assigned to ZnS (1021.3 eV). Moreover, Fig. 8(d) shows the peaks of binding energy for S 2p in the ZnS thin films for different annealing temperatures. The S 2p peaks are observed at 161.2 eV, and are attributed to the Zn-S bond.

Post-annealing Effects on ZnS Thin Films Grown · · · – Heejin Ahn and Youngho Um

-1049-

Fig. 8. (Color online) (a) O 1s, (b) C 1s, (c) Zn 2p, (d) S 2p XPS spectra of ZnS thin films annealed at 100, 200, and 300 C. The insect in (b) presents the XPS spectrum fitted with three bonds and showing the C 1s peak, the intensity of the C=O peak decreased with increasing annealing temperature. The Zn 2p3/2 peak in (c) is shifted to higher binding energy. ◦

III. CONCLUSION

ACKNOWLEDGMENTS

In this work, ZnS films were deposited on soda-lime glass substrates by using the CBD method and postannealed at different temperatures. The deposited CBDZnS thin films had a cubic phase and were preferentially aligned perpendicular to the (111) plane. The grain sizes were estimated to be in the range of 134 ∼ 178 nm. Optical measurements showed that the film’s transmittance were 70% ∼ 80% in the visible region. Also, all of the films have a direct band gap, which decreased from 3.89 to 3.82 eV with increasing annealing temperature. FESEM images showed that the surface of the thin films became compact and uniform as the annealing temperature increased. The characteristic peak of the Zn-S bond appeared clearly in the XPS spectrum of the CBD-ZnS film post-annealed at 300 ◦ C. This material has potential for use in electroluminescent devices and photovoltaic cells.

This work was supported by the 2013 Research Fund of the University of Ulsan.

REFERENCES [1] A. Rochett and R.W. Birkmire, J. Appl. phys. 70, R81 (1991). [2] L. Qj and G. Mao, Appl. Surf. Sci 254, 5711 (2008). [3] S. N. Kundu, S. Johnston and L.C. Olsen, Thin Solid Films. 515, 2625 (2006). [4] T. Nakada, M. Hongo and E. Hayashi, Thin Solid Films. 431, 242 (2003). [5] L. O. Oladeji, L. Chow, J. R. Liu, W. K. Chu, A. N. P. Bustamante, C. Fredricksen and A. F. Schulte, Thin Solid Films. 359, 154 (2000). [6] R. N. Bhattacharya and M. A. Contreras, J. Appl. Phys. 43, L1475 (2004).

-1050-

Journal of the Korean Physical Society, Vol. 67, No. 6, September 2015

[7] V. Dimitrova and J. Tate, Thin Solid Films 365, 134 (2000). [8] O. Mustafa, M. Bedir, S. Ocak and R. G. Yildirim, J. Mater. Electron. 18, 505 (2007). [9] L. X. Shao, K. H. Chang and H. L. Hwang, Appl. Surf. Sci. 212, 305 (2003). [10] F. Gode, C. Gumus and M. Zor, J. Cryst. Growth 299, 136 (2007). [11] L. V. Makhova, I. Konovalov, R. Szargan, N. Aschkenov, M. Schubert and T. Chass´e, Phys. Stat. Sol (c) 2, 1206 (2005). [12] F. Long, W. M. Wang, Z. K. Cui, L. Z. Fan, Z. G. Zou and T. K. Jia, Chem. Phys. Lett. 462, 84 (2008). [13] R. N.Bhattacharya, K. Ramanathan, L. Gedvilas and B. Ketes, J. Phys. Chem. Solids 66, 1862 (2005). [14] F. Zhenyi, C. Yichao, H. Yongliang, Y. Yaoyuan, D. Yanping, Y. Zewu, T. Hongchang, X. Hongtao and W. Hem-

ing, J, Cryst. Growth 237, 1707 (2002). [15] R. Sahraei, G. M. Aval and A. Goudarzi, J. Alloys Compd. 466, 488 (2008). [16] M. Ichimura, F. Goto, Y. One and E. Arai, J. Cryst. Growth 198, 308 (1999). [17] R. Nomura, T. Murai, T. Toyosaki and H. Matsuda, Thin Solid Films 271, 4 (1995). [18] P. Roy, J. R. Ota and S. K. Srivastava, Thin Solid Films 515, 1912 (2006). [19] R. S. Mane and C. D. Lokhande, Mater. Chem. Phys. 65, 1 (2000). [20] J. Vidal, O. Vigil, O. D. Melo, N. Lopez and O. Z. Angel, Mater. Chem. Phys. 61, 139 (1999). [21] L. I. Berger, Semiconductor Materials (CRC press, Boca Raton, 1997).