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Crystallization of as-deposited amorphous silicon films on glass prepared by magnetron sputtering with different substrate biases and temperatures
Journal of Crystal Growth 321 (2011) 50–54
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Crystallization of as-deposited amorphous silicon films on glass prepared by magnetron sputtering with different substrate biases and temperatures Weiyan Wang a,b, Jinhua Huang a, Xianpeng Zhang a, Ye Yang a, Weijie Song a,n, Fuqiang Huang b a b
Division of Functional Materials and Nanodevices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2010 Received in revised form 17 February 2011 Accepted 17 February 2011 Communicated by D.W. Shaw Available online 23 February 2011
The rapid thermal annealing (RTA) crystallization of sputtered amorphous silicon (a-Si) films on quartz glass deposited with different substrate biases (0–150 W) and at different substrate temperatures (100–400 1C) has been investigated in detail by an X-ray diffractometer, and Raman and transmission electron microscopes. It was found that only the a-Si film deposited under the optimal condition (substrate bias: 100 W, substrate temperature: 300 1C) attained noticeable degrees of crystallization during the post-deposition RTA at 750 1C. The RTA crystallized a-Si film deposited under optimal condition possessed crystalline fraction of 94.1%, and was proved to be polycrystalline in nature. Furthermore, it was revealed that the structural property of Si film improved with post-deposition RTA time or temperature. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Crystallization A3. Physical vapor deposition processes B2. Semiconducting silicon B3. Solar cells
1. Introduction Polycrystalline silicon (Si) films are considered as one of the most promising materials for thin-film solar cells [1–3] because of their superior electrical properties and stability compared with amorphous silicon (a-Si) materials. Solid-phase crystallization using a-Si films as precursor is the commonly used technique to obtain polycrystalline Si films [4–6]. It is well known that the crystallization kinetics and the properties of polycrystalline Si films strongly depend on the properties of the as-deposited a-Si films [7], such as pre-existing nuclei for crystallization [8], impurities [9,10], defects, and stress state [11]. Magnetron sputter deposition is a particularly attractive method for Si film preparation because of its capability for large area deposition and process controllability [12,13]. Apparently, the deposition condition can affect the structure of as-deposited a-Si films, leading to difference in the crystalline Si film properties. Among these conditions, substrate bias [14–18] and substrate temperature [19–21] are thought to be the predominant parameters, both of which are capable of influencing the behavior of Si ad-atoms on the film surface through ion bombardment or heating, which cause various a-Si film networks. Jun et al. [17] found that substrate bias made a-Si films dense with few defects and, moreover, enhanced crystallization during the subsequent
n
Corresponding author. Fax: + 86 574 86685163. E-mail address: [email protected] (W. Song).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.02.031
furnace annealing and the grain size in the resulting polycrystalline Si film [18]; however, these studies did not investigate how the a-Si films crystallization was influenced by different substrate biases. Substrate temperature has been reported to have an effect on the electrical properties and microstructures of a-Si films [19,20]; however, its effect on the subsequent crystallization process has not been reported. Therefore, an extensive investigation into the effect of different substrate biases and temperatures on the a-Si films crystallization is needed for a better understanding of the crystallization mechanism in a-Si films. In this work, rapid thermal annealing (RTA) is chosen as the crystallization technique for its small thermal budget, high throughput, and process automation [22,23]. The a-Si films were deposited by magnetron sputtering with different substrate biases (0 150 W) and at different substrate temperatures (100 400 1C), followed by RTA crystallization. The effects of a-Si film deposition condition, in terms of substrate bias and temperature, as well as the post-deposition RTA time and temperature, on the RTA crystallization of sputtered a-Si films were intensively investigated.
2. Experiment A J-sputter8000 magnetron sputtering system, equipped with six magnetron sources and a heated and biased substrate holder, was used for a-Si films deposition on quartz glass substrate. Firstly, the substrates were immersed in the ultrasonic baths of
W. Wang et al. / Journal of Crystal Growth 321 (2011) 50–54
acetone and distilled water for about 15 min each. They were then blow-dried with nitrogen gas, after which the substrates were immediately transferred to an ultrahigh vacuum chamber for a-Si films deposition. The a-Si films were deposited under the following fixed conditions: RF power of 120 W, base pressure of 5 10 5 Pa, argon gas pressure of 0.21 Pa, and deposition time of 3 h, while the substrate temperature was varied in the range of 100 400 1C, and the substrate bias in the range of 0 150 W (corresponding to 0 125 V). To investigate the effect of a-Si film deposition condition on the annealed Si film structural property, all the a-Si films were subjected to a five-step RTA at 750 1C for 60 s in Ar atmosphere with the cooling rate of 20 1C/s; to investigate the RTA crystallization process of sputtered a-Si film, the a-Si film deposited under optimal condition was subjected to different steps of RTA at 750 1C for 60 s in an Ar atmosphere at the cooling rate of 20 1C/s, and two-step RTA at 900 1C for 30 s in Ar at the cooling rate of 20 1C/s. The structural properties of the annealed Si films were characterized using a Bruker AXS D8 Advanced X-ray diffractometer (XRD) with the Cu Ka radiation (l ¼0.154 nm; electron beam condition: 40 kV, 40 mA); the reflectance of the annealed Si films was measured using a J.A. Woollam M-2000DI spectroscopic ellipsometer; the Raman spectra of the annealed Si film was measured using a Jobin Yvon Labor HR-800 Raman excited by Ar laser with wavelength of l ¼514.5 nm; the surface and crosssection morphologies of the annealed Si films were observed using a Veeco Dimension 3100V atomic force microscope (AFM) in contact mode and an FEI Tecnai F20 transmission electron microscope (TEM) operating at 200 kV.
51
3. Results and discussion Fig. 1 shows the XRD patterns of the a-Si films deposited with different substrate biases or at different substrate temperatures subjected to five-step RTA 750 1C/60 s. As shown in Fig. 1(a), although the substrate bias during deposition significantly influenced the RTA crystallization, only the film deposited with a substrate bias of 100 W (100 V) showed Si characteristic peaks at 2y ¼28.51, 47.51, and 56.31, corresponding to (1 1 1), (2 2 0), and (3 1 1), respectively; in contrast, the films deposited with no substrate bias, a low substrate bias of 50 W (60 V), or a high substrate bias of 150 W (125 V) showed no characteristic peaks. Enhanced crystallization of substrate-biased a-Si film is in accordance with the previous result [18]; how a particular substrate bias value was required in our experiment will be discussed below. The effects of different substrate temperatures, as shown in Fig. 1(b), were as follows: at a low temperature of 100 1C, no Si characteristic peaks existed; at an increased temperature of 200 1C, the Si characteristic peaks with a low intensity appeared; further increasing the temperature to 300 1C resulted in increased intensities of XRD peaks; however, when the substrate temperature is increased to 400 1C, the XRD peaks suddenly disappeared. These observations suggest that the precursor a-Si film structure, which depends on the deposition conditions, significantly influences the subsequent RTA crystallization. Fig. 2 shows the UV reflectance spectra for the same samples presented in Fig. 1. The change in the profile and the shift in the peaks in the UV spectra indicates the modification of the electronic density of states as a result of the long-range order. For
Fig. 1. XRD patterns of the Si films deposited under different substrate biases (a) or substrate temperatures (b), followed by five-step RTA 750 1C/60 s in Ar.
Fig. 2. UV reflectance spectra of the Si films deposited under different substrate biases (a) or substrate temperatures (b), followed by five-step RTA 750 1C/60 s in Ar.
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crystalline Si, two main optical transition peaks are expected: direct transition at 273 nm and indirect transitions at 360 nm [24,25]. For our substrate-biased samples, only the sample deposited with a substrate bias of 100 W showed the characteristic peaks at 273 nm (E1) and 360 nm (E2) after annealing, as shown in Fig. 2(a). Among the samples prepared at different substrate temperatures, those deposited at 200 and 300 1C exhibited the peaks at 273 nm (E1) and 360 nm (E2) after annealing, as shown in Fig. 2(b). The existence of E1 and E2 peaks is clear evidence of crystallization occurring in the a-Si films. The UV reflectance spectra results are in accordance with the XRD results. Based on the above results, it can be concluded that when the a-Si films deposited under various conditions were subjected to RTA, the films underwent different changes, resulting in the different states of ultimate Si films, which were either crystalline or amorphous. Therefore, the microstructures of both the asdeposited a-Si films and the RTA-annealed a-Si films were investigated by AFM. Fig. 3 shows the roughness differences of the Si films before and after RTA. It can be clearly observed that the roughness difference of the Si film deposited at 300 1C with a substrate bias of 100 W (0.95 nm) was larger than those of the Si films deposited under other conditions (0.04–0.33 nm), indicating a greater change in surface microstructure occurring during the RTA crystallization process. It should be pointed out that the roughness difference for the Si film deposited at 200 1C with a substrate bias of 100 W was not very pronounced (0.21 nm), probably because of insufficient crystallization in that case. Fig. 4 shows representative AFM images of the Si films before and after RTA; all of the Si films were deposited at a substrate temperature of 300 1C. In the case of no substrate bias, the AFM images of the Si films before and after RTA were nearly the same, as shown in Fig. 4(a) and (b). It is supposed that the Si atoms could not be transferred easily on applying RTA at 750 1C, resulting in a very low value of roughness difference (0.04 nm). In contrast, for the samples prepared with a substrate bias of 100 W, the images of the surface before and after RTA were markedly different, as shown in Fig. 4(c) and (d). In this case, Si atoms moved easily when RTA was applied at 750 1C, leading to the large roughness difference (0.95 nm). Therefore, it is revealed that the as-deposited a-Si film structure, depending on substrate bias and temperature, has a strong influence on the RTA crystallization of a-Si films: that is, only the a-Si film deposited with a certain substrate bias (100 W) and at a certain temperature (300 1C) undergoes a noticeable degree of crystallization by post-deposition RTA at 750 1C. Biasing the substrate is known to induce Ar ions bombardment of the film surface [14–18], which possibly enhances crystallization by inducing nuclei for Si crystallites [18] and by reducing oxygen incorporation in the film. Oxygen and other impurities, such as carbon and fluorine, are known to retard the Si crystallization process [9,10]. However, at a relatively high substrate bias of 150 W, the increased level of Ar ion bombardment creates an excessive disorder in the film [14], which cannot be easily crystallized through RTA. Only the a-Si films are deposited with the particular substrate bias of 100 W, therefore, can attain a noticeable level of crystallization during the subsequent RTA at 750 1C, as verified by the characteristic peaks in XRD (Fig. 1(a)) and UV reflectance spectra (Fig. 2(a)). Previous studies have established an optimal range of substrate temperature based on defect density: outside this temperature range, the defect density increases [19,20]; however, this temperature range itself may vary under different experimental conditions. The optimal substrate temperature range in the case presented here appears to be approximately 300 1C; in other words, the a-Si films deposited at 300 1C possess a relatively well-developed amorphous network.
Fig. 3. Roughness differences of the Si films before and after five-step RTA 750 1C/ 60 s in Ar.
Fig. 4. Representative AFM images of the Si films before and after RTA.
As a result, these films can be crystallized easily during RTA at 750 1C, as verified by the existence of the characteristic peaks in XRD and UV reflectance data shown earlier in Figs. 1(b) and 2(b), respectively. Therefore, it can be concluded that the optimal condition for a-Si film deposition was as follows: substrate temperature of 300 1C and substrate bias of 100 W. Subsequently, the crystalline fraction and microstructure of RTA crystallized a-Si films deposited under optimal condition were investigated. Fig. 5 shows the Raman spectra of the a-Si film deposited under optimal condition subjected to five-step RTA 750 1C/60 s; the Raman spectra for single crystal Si wafer is also shown as reference; the inset in Fig. 5 shows the deconvolution of the Raman spectrum, which is used to give qualitative information about the crystalline fraction of the Si films. As seen from the inset of Fig. 5, the Raman spectrum can be decomposed into three components: the crystalline component peaked at 514 cm 1, the amorphous component peaked at 480 cm 1, and an intermediate
W. Wang et al. / Journal of Crystal Growth 321 (2011) 50–54
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Fig. 5. Raman spectrum of the Si film deposited with a substrate bias of 100 W and a substrate temperature of 300 1C, followed by five-step RTA 750 1C/60 s in Ar; the Raman spectrum of single crystal Si wafer is also shown as reference. The inset is the deconvolution of the Raman spectrum of crystalline Si film.
component peaked at 500 cm 1, which is associated with band dilation at grain boundaries [26,27]. Accordingly, the crystalline volume fraction can be calculated as follows:
Fig. 6. Cross-sectional TEM image of the Si film deposited with a substrate bias of 100 W at a substrate temperature of 300 1C, followed by five-step RTA 750 1C/60 s in Ar. The inset is the electron diffraction pattern of a film region.
Xc ¼ ðI514 þI500 Þ=ðI514 þI500 þ yI480 Þ where I514, I480, and I500 denote the integrated intensities of the crystalline, amorphous, and intermediate peaks, respectively; y is the ratio of the cross section for amorphous to crystalline phase, which was estimated to be 0.21 using Si crystallite size of 55 nm. Therefore the crystalline volume fraction for Si film on quartz glass was 94.1%, indicating that many a-Si films were crystallized by five-step RTA 750 1C/60 s. Moreover, as shown in Fig. 5, the peak position of Raman TO-mode shifted to lower wave-numbers ( 514 cm 1) for the crystalline Si film herein. Therefore with respect to the crystalline counterpart ( 520 cm 1) for single crystal Si wafer the peak position of the TO-mode was 6 cm 1 lower, which was likely due to a large tensile strain [28] or small grain size [26] in the crystalline Si film. Fig. 6 shows the cross-sectional TEM image of the a-Si film deposited under optimal condition subjected to five-step RTA 750 1C/60 s; the selective area electron diffraction (SAED) image is in the inset of Fig. 6. It was observed that the RTA-annealed a-Si film on glass was polycrystalline in nature, as shown from the SAED image; moreover, the polycrystalline Si film with thickness of 325 nm sharply formed on the glass substrate, leaved no amorphous transition region between the amorphous glass substrate and the polycrystalline Si film. Finally, the RTA crystallization process of sputtered a-Si films was investigated. Fig. 7 shows the XRD patterns of the a-Si film deposited under optimal condition subjected to different steps of RTA 750 1C/60 s. It was observed that when the a-Si film is subjected to one-step RTA 750 1C/60 s no Si characteristic peaks appeared, the broad peak at around 2y ¼221 was related to amorphous glass; as soon as the a-Si film was subjected to twostep RTA 750 1C/60 s, the Si film started to show Si characteristic peaks at 2y ¼28.51 and 47.51, corresponding to (1 1 1) and (2 2 0), respectively, indicating the initiation of crystallization of a-Si films; on increasing annealing time to five-step RTA 750 1C/60 s, the intensities of Si characteristic peaks were significantly enhanced, while with further increasing time of ten-step RTA 750 1C/60 s the intensities of Si characteristic peaks were almost unchanged. It was suggested that the structural property of Si film
Fig. 7. XRD patterns of the Si films deposited with a substrate bias of 100 W at a substrate temperature of 300 1C, followed by different steps of RTA 750 1C/60 s in Ar.
improved with the increase of 750 1C-RTA time from 1 to 5 min, but it was almost invariable with the increase of 750 1C-RTA time from 5 to 10 min. Fig. 8 shows the XRD patterns of a-Si film deposited under optimal condition subjected to RTA at temperatures of 750 and 900 1C for 2 min. It was observed that both Si films gave Si characteristic peaks at 2y ¼28.51 and 47.51, corresponding to (1 1 1) and (2 2 0), respectively, indicating the crystallization of a-Si films; moreover, the Si characteristic peaks intensities in the a-Si film subjected to RTA at 900 1C were higher than that in the a-Si film subjected to RTA at 750 1C, indicating that structural property of crystalline Si film improved with increase of RTA temperature. This is because the radiation energy
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W. Wang et al. / Journal of Crystal Growth 321 (2011) 50–54
Acknowledgement The authors would like to acknowledge the financial supports from the National Natural Science Foundation of China (Grant no. 20975107); the Chinese Academy of Sciences under the ‘‘Hundred Talents Program’’ and the ‘‘Solar Action Program’’; Zhejiang Natural Science Foundation (Grant no. Y4100169); Ningbo Natural Science Foundation (Grant no. 2010A610166); and China Postdoctoral Science Foundation (Grant no. 20100470735), ‘‘Seeding Project’’ of Ningbo Institute of Material Technology and Engineering of Chinese Academy of Sciences. References
Fig. 8. XRD patterns of the Si films deposited with a substrate bias of 100 W at a substrate temperature of 300 1C, followed by RTA at 750 or 900 1C for 2 min in Ar.
supply for Si films increases with RTA time or temperature [29,30], leading to better structural property of Si film.
4. Conclusion This work investigated the effect of substrate bias (0–150 W) and temperature (100–400 1C) during precursor a-Si films deposition, as well as the post-deposition RTA time (1–10 min) and temperature (750, 900 1C) on the RTA crystallization of a-Si films. Only the a-Si film deposited under optimal condition: substrate bias of 100 W and substrate temperature of 300 1C, attained noticeable levels of crystallization during the subsequent RTA at 750 1C. On one hand, under a certain substrate bias (100 W), the appropriate level of ion bombardment is supposed to introduce nuclei for Si crystallites and reduce oxygen impurities in the amorphous film; on the other hand, at a certain substrate temperature (300 1C), the appropriate degree of heating is supposed to reduce defect density in the film. As a result, RTA crystallized a-Si film deposited under optimal condition showed crystalline fraction of 94.1%, and was proved to be polycrystalline Si films formed directly on the quartz glass substrate with no intervening amorphous transition region. Furthermore, it was found that the structural property of Si films enhanced with the increase of 750 1C-RTA time from 1 to 5 min, and also enhanced with the increase of RTA temperature from 750 to 900 1C.
[1] M.A. Green, Appl. Phys. A 96 (2009) 153. [2] A.G. Aberle, J. Cryst. Growth 287 (2006) 386. [3] S. Gall, C. Becker, E. Conrad, P Dogan, F. Fenske, B. Gorka, K.Y. Lee, B. Rau, F. Ruske, B. Rech, Sol. Energy Mater. Sol. Cells 93 (2009) 1004. [4] M.K. Hatalis, D.W. Greve, J. Appl. Phys. 63 (1988) 2260. [5] A. Mimura, N. Konishi, K. Ono, J.-I. Ohwada, Y. Hosokawa, Y.A. Ono, T. Suzuki, K. Miyata, H. Kawakami, IEEE Trans. Electron Device 36 (1989) 351. [6] G. Farhi, M. Aoucher, T. Mohammed-Brahim, Sol. Energy Mater. Sol. Cells 72 (2002) 551. [7] C. Spinellaa, S. Lombardo, F. Priolo, J. Appl. Phys. 84 (1998) 5383. [8] T. Matsuyama, N. Terada, T. Baba, T. Sawada, S. Tsuge, K. Wakisaka, S. Tsuda, J. Non-Cryst. Solids 198-200 (1996) 940. ¨ [9] I. Suni, G. Goltz, M.G. Grimaldi, M-.A. Nicolet, S.S. Lau, Appl. Phys. Lett. 40 (1982) 269. [10] M. Wittmer, J. Roth, P. Revesz, J.W. Mayer, J. Appl. Phys. 49 (1978) 5207. [11] Y. Kimura, M. Kishi, T. Katoda, J. Appl. Phys. 86 (1999) 2278. [12] Y. Mishima, M. Takei, T. Uematsu, N. Matsumoto, T. Kakehi, U. Wakino, M. Okabe, J. Appl. Phys. 78 (1995) 217. [13] C.S. McCormick, C.E. Weber, J.R. Abelson, G.A. Davis, R.E. Weiss, V. Aebi, J. Vac. Sci. Technol. A 15 (1997) 2770. [14] P. Reinig, F. Fenske, W. Fuhs, B. Selle, J. Non-Cryst. Solids 299-302 (2002) 128. [15] J.E. Gerbi, J.R. Abelson, J. Appl. Phys. 101 (2007) 063508. [16] M.Y. Yeh, C.C. Lee, G.Y. Tzeng, Y.F. Luo, Jpn. J. Appl. Phys. 48 (2009) 056505. [17] S.-I. Jun, P.D. Rack, T.E. McKnight, A.V. Melechko, M.L. Simpson, Appl. Phys. Lett. 87 (2005) 132108. [18] S.-I. Jun, P.D. Rack, T.E. McKnight, A.V. Melechko, M.L. Simpson, Appl. Phys. Lett. 89 (2006) 022104. [19] M. Pinarbasi, N. Maley, A. Myers, J.R. Abelson, Thin Solid Films 171 (1989) 217. [20] Y.H. Liang, N. Maley, J.R. Abelson, J. Appl. Phys. 75 (7) (1994) 3704. [21] J. Kondo, A. Tabata, T. Kawamura, T. Mizutani, Vacuum 66 (2002) 409. [22] R. Kakkad, J. Smith, W.S. Lau, S.J. Fonash, J. Appl. Phys. 65 (1989) 2069. [23] Y. Wang, S. Liao, Z. Ma, G Yue, H. Diao, J. He, G. Kong, Y. Zhao, Z. Li, F. Yun, Appl. Surf. Sci. 135 (1998) 205. [24] C.-H. Kuo, I.-C. Hsieh, D.K. Sohroder, G.N. Maraoas, S. Chen, T.W. Sigmon, Appl. Phys. Lett. 71 (1997) 359. [25] A. Straub, P.I. Widenborg, A. Sproul, Y. Huang, N.-P. Harder, A.G. Aberle, J. Cryst. Growth 265 (2004) 168. [26] S. Veprek, F.A. Sarott, Z. Iqbal, Phys. Rev. B 36 (1987) 3344. [27] G. Yue, J.D. Lorentzen, J. Lin, D. Han, Q. Wang, Appl. Phys. Lett. 75 (1999) 492. + [28] I De Wolf, J. Vanhellemont, A. Romano-Rodrigues, H. Nortstrom, H.E. Maes, J. Appl. Phys. 71 (1992) 898. [29] R. Singh, J. Appl. Phys. 63 (1988) R59. [30] R. Singh, M. Fakhruddin, K.F. Poole, Appl. Surf. Sci. 168 (2000) 198.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Crystallization of as-deposited amorphous silicon films on glass prepared by magnetron sputtering with different substrate biases and temperatures Weiyan Wang a,b, Jinhua Huang a, Xianpeng Zhang a, Ye Yang a, Weijie Song a,n, Fuqiang Huang b a b
Division of Functional Materials and Nanodevices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2010 Received in revised form 17 February 2011 Accepted 17 February 2011 Communicated by D.W. Shaw Available online 23 February 2011
The rapid thermal annealing (RTA) crystallization of sputtered amorphous silicon (a-Si) films on quartz glass deposited with different substrate biases (0–150 W) and at different substrate temperatures (100–400 1C) has been investigated in detail by an X-ray diffractometer, and Raman and transmission electron microscopes. It was found that only the a-Si film deposited under the optimal condition (substrate bias: 100 W, substrate temperature: 300 1C) attained noticeable degrees of crystallization during the post-deposition RTA at 750 1C. The RTA crystallized a-Si film deposited under optimal condition possessed crystalline fraction of 94.1%, and was proved to be polycrystalline in nature. Furthermore, it was revealed that the structural property of Si film improved with post-deposition RTA time or temperature. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Crystallization A3. Physical vapor deposition processes B2. Semiconducting silicon B3. Solar cells
1. Introduction Polycrystalline silicon (Si) films are considered as one of the most promising materials for thin-film solar cells [1–3] because of their superior electrical properties and stability compared with amorphous silicon (a-Si) materials. Solid-phase crystallization using a-Si films as precursor is the commonly used technique to obtain polycrystalline Si films [4–6]. It is well known that the crystallization kinetics and the properties of polycrystalline Si films strongly depend on the properties of the as-deposited a-Si films [7], such as pre-existing nuclei for crystallization [8], impurities [9,10], defects, and stress state [11]. Magnetron sputter deposition is a particularly attractive method for Si film preparation because of its capability for large area deposition and process controllability [12,13]. Apparently, the deposition condition can affect the structure of as-deposited a-Si films, leading to difference in the crystalline Si film properties. Among these conditions, substrate bias [14–18] and substrate temperature [19–21] are thought to be the predominant parameters, both of which are capable of influencing the behavior of Si ad-atoms on the film surface through ion bombardment or heating, which cause various a-Si film networks. Jun et al. [17] found that substrate bias made a-Si films dense with few defects and, moreover, enhanced crystallization during the subsequent
n
Corresponding author. Fax: + 86 574 86685163. E-mail address: [email protected] (W. Song).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.02.031
furnace annealing and the grain size in the resulting polycrystalline Si film [18]; however, these studies did not investigate how the a-Si films crystallization was influenced by different substrate biases. Substrate temperature has been reported to have an effect on the electrical properties and microstructures of a-Si films [19,20]; however, its effect on the subsequent crystallization process has not been reported. Therefore, an extensive investigation into the effect of different substrate biases and temperatures on the a-Si films crystallization is needed for a better understanding of the crystallization mechanism in a-Si films. In this work, rapid thermal annealing (RTA) is chosen as the crystallization technique for its small thermal budget, high throughput, and process automation [22,23]. The a-Si films were deposited by magnetron sputtering with different substrate biases (0 150 W) and at different substrate temperatures (100 400 1C), followed by RTA crystallization. The effects of a-Si film deposition condition, in terms of substrate bias and temperature, as well as the post-deposition RTA time and temperature, on the RTA crystallization of sputtered a-Si films were intensively investigated.
2. Experiment A J-sputter8000 magnetron sputtering system, equipped with six magnetron sources and a heated and biased substrate holder, was used for a-Si films deposition on quartz glass substrate. Firstly, the substrates were immersed in the ultrasonic baths of
W. Wang et al. / Journal of Crystal Growth 321 (2011) 50–54
acetone and distilled water for about 15 min each. They were then blow-dried with nitrogen gas, after which the substrates were immediately transferred to an ultrahigh vacuum chamber for a-Si films deposition. The a-Si films were deposited under the following fixed conditions: RF power of 120 W, base pressure of 5 10 5 Pa, argon gas pressure of 0.21 Pa, and deposition time of 3 h, while the substrate temperature was varied in the range of 100 400 1C, and the substrate bias in the range of 0 150 W (corresponding to 0 125 V). To investigate the effect of a-Si film deposition condition on the annealed Si film structural property, all the a-Si films were subjected to a five-step RTA at 750 1C for 60 s in Ar atmosphere with the cooling rate of 20 1C/s; to investigate the RTA crystallization process of sputtered a-Si film, the a-Si film deposited under optimal condition was subjected to different steps of RTA at 750 1C for 60 s in an Ar atmosphere at the cooling rate of 20 1C/s, and two-step RTA at 900 1C for 30 s in Ar at the cooling rate of 20 1C/s. The structural properties of the annealed Si films were characterized using a Bruker AXS D8 Advanced X-ray diffractometer (XRD) with the Cu Ka radiation (l ¼0.154 nm; electron beam condition: 40 kV, 40 mA); the reflectance of the annealed Si films was measured using a J.A. Woollam M-2000DI spectroscopic ellipsometer; the Raman spectra of the annealed Si film was measured using a Jobin Yvon Labor HR-800 Raman excited by Ar laser with wavelength of l ¼514.5 nm; the surface and crosssection morphologies of the annealed Si films were observed using a Veeco Dimension 3100V atomic force microscope (AFM) in contact mode and an FEI Tecnai F20 transmission electron microscope (TEM) operating at 200 kV.
51
3. Results and discussion Fig. 1 shows the XRD patterns of the a-Si films deposited with different substrate biases or at different substrate temperatures subjected to five-step RTA 750 1C/60 s. As shown in Fig. 1(a), although the substrate bias during deposition significantly influenced the RTA crystallization, only the film deposited with a substrate bias of 100 W (100 V) showed Si characteristic peaks at 2y ¼28.51, 47.51, and 56.31, corresponding to (1 1 1), (2 2 0), and (3 1 1), respectively; in contrast, the films deposited with no substrate bias, a low substrate bias of 50 W (60 V), or a high substrate bias of 150 W (125 V) showed no characteristic peaks. Enhanced crystallization of substrate-biased a-Si film is in accordance with the previous result [18]; how a particular substrate bias value was required in our experiment will be discussed below. The effects of different substrate temperatures, as shown in Fig. 1(b), were as follows: at a low temperature of 100 1C, no Si characteristic peaks existed; at an increased temperature of 200 1C, the Si characteristic peaks with a low intensity appeared; further increasing the temperature to 300 1C resulted in increased intensities of XRD peaks; however, when the substrate temperature is increased to 400 1C, the XRD peaks suddenly disappeared. These observations suggest that the precursor a-Si film structure, which depends on the deposition conditions, significantly influences the subsequent RTA crystallization. Fig. 2 shows the UV reflectance spectra for the same samples presented in Fig. 1. The change in the profile and the shift in the peaks in the UV spectra indicates the modification of the electronic density of states as a result of the long-range order. For
Fig. 1. XRD patterns of the Si films deposited under different substrate biases (a) or substrate temperatures (b), followed by five-step RTA 750 1C/60 s in Ar.
Fig. 2. UV reflectance spectra of the Si films deposited under different substrate biases (a) or substrate temperatures (b), followed by five-step RTA 750 1C/60 s in Ar.
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crystalline Si, two main optical transition peaks are expected: direct transition at 273 nm and indirect transitions at 360 nm [24,25]. For our substrate-biased samples, only the sample deposited with a substrate bias of 100 W showed the characteristic peaks at 273 nm (E1) and 360 nm (E2) after annealing, as shown in Fig. 2(a). Among the samples prepared at different substrate temperatures, those deposited at 200 and 300 1C exhibited the peaks at 273 nm (E1) and 360 nm (E2) after annealing, as shown in Fig. 2(b). The existence of E1 and E2 peaks is clear evidence of crystallization occurring in the a-Si films. The UV reflectance spectra results are in accordance with the XRD results. Based on the above results, it can be concluded that when the a-Si films deposited under various conditions were subjected to RTA, the films underwent different changes, resulting in the different states of ultimate Si films, which were either crystalline or amorphous. Therefore, the microstructures of both the asdeposited a-Si films and the RTA-annealed a-Si films were investigated by AFM. Fig. 3 shows the roughness differences of the Si films before and after RTA. It can be clearly observed that the roughness difference of the Si film deposited at 300 1C with a substrate bias of 100 W (0.95 nm) was larger than those of the Si films deposited under other conditions (0.04–0.33 nm), indicating a greater change in surface microstructure occurring during the RTA crystallization process. It should be pointed out that the roughness difference for the Si film deposited at 200 1C with a substrate bias of 100 W was not very pronounced (0.21 nm), probably because of insufficient crystallization in that case. Fig. 4 shows representative AFM images of the Si films before and after RTA; all of the Si films were deposited at a substrate temperature of 300 1C. In the case of no substrate bias, the AFM images of the Si films before and after RTA were nearly the same, as shown in Fig. 4(a) and (b). It is supposed that the Si atoms could not be transferred easily on applying RTA at 750 1C, resulting in a very low value of roughness difference (0.04 nm). In contrast, for the samples prepared with a substrate bias of 100 W, the images of the surface before and after RTA were markedly different, as shown in Fig. 4(c) and (d). In this case, Si atoms moved easily when RTA was applied at 750 1C, leading to the large roughness difference (0.95 nm). Therefore, it is revealed that the as-deposited a-Si film structure, depending on substrate bias and temperature, has a strong influence on the RTA crystallization of a-Si films: that is, only the a-Si film deposited with a certain substrate bias (100 W) and at a certain temperature (300 1C) undergoes a noticeable degree of crystallization by post-deposition RTA at 750 1C. Biasing the substrate is known to induce Ar ions bombardment of the film surface [14–18], which possibly enhances crystallization by inducing nuclei for Si crystallites [18] and by reducing oxygen incorporation in the film. Oxygen and other impurities, such as carbon and fluorine, are known to retard the Si crystallization process [9,10]. However, at a relatively high substrate bias of 150 W, the increased level of Ar ion bombardment creates an excessive disorder in the film [14], which cannot be easily crystallized through RTA. Only the a-Si films are deposited with the particular substrate bias of 100 W, therefore, can attain a noticeable level of crystallization during the subsequent RTA at 750 1C, as verified by the characteristic peaks in XRD (Fig. 1(a)) and UV reflectance spectra (Fig. 2(a)). Previous studies have established an optimal range of substrate temperature based on defect density: outside this temperature range, the defect density increases [19,20]; however, this temperature range itself may vary under different experimental conditions. The optimal substrate temperature range in the case presented here appears to be approximately 300 1C; in other words, the a-Si films deposited at 300 1C possess a relatively well-developed amorphous network.
Fig. 3. Roughness differences of the Si films before and after five-step RTA 750 1C/ 60 s in Ar.
Fig. 4. Representative AFM images of the Si films before and after RTA.
As a result, these films can be crystallized easily during RTA at 750 1C, as verified by the existence of the characteristic peaks in XRD and UV reflectance data shown earlier in Figs. 1(b) and 2(b), respectively. Therefore, it can be concluded that the optimal condition for a-Si film deposition was as follows: substrate temperature of 300 1C and substrate bias of 100 W. Subsequently, the crystalline fraction and microstructure of RTA crystallized a-Si films deposited under optimal condition were investigated. Fig. 5 shows the Raman spectra of the a-Si film deposited under optimal condition subjected to five-step RTA 750 1C/60 s; the Raman spectra for single crystal Si wafer is also shown as reference; the inset in Fig. 5 shows the deconvolution of the Raman spectrum, which is used to give qualitative information about the crystalline fraction of the Si films. As seen from the inset of Fig. 5, the Raman spectrum can be decomposed into three components: the crystalline component peaked at 514 cm 1, the amorphous component peaked at 480 cm 1, and an intermediate
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Fig. 5. Raman spectrum of the Si film deposited with a substrate bias of 100 W and a substrate temperature of 300 1C, followed by five-step RTA 750 1C/60 s in Ar; the Raman spectrum of single crystal Si wafer is also shown as reference. The inset is the deconvolution of the Raman spectrum of crystalline Si film.
component peaked at 500 cm 1, which is associated with band dilation at grain boundaries [26,27]. Accordingly, the crystalline volume fraction can be calculated as follows:
Fig. 6. Cross-sectional TEM image of the Si film deposited with a substrate bias of 100 W at a substrate temperature of 300 1C, followed by five-step RTA 750 1C/60 s in Ar. The inset is the electron diffraction pattern of a film region.
Xc ¼ ðI514 þI500 Þ=ðI514 þI500 þ yI480 Þ where I514, I480, and I500 denote the integrated intensities of the crystalline, amorphous, and intermediate peaks, respectively; y is the ratio of the cross section for amorphous to crystalline phase, which was estimated to be 0.21 using Si crystallite size of 55 nm. Therefore the crystalline volume fraction for Si film on quartz glass was 94.1%, indicating that many a-Si films were crystallized by five-step RTA 750 1C/60 s. Moreover, as shown in Fig. 5, the peak position of Raman TO-mode shifted to lower wave-numbers ( 514 cm 1) for the crystalline Si film herein. Therefore with respect to the crystalline counterpart ( 520 cm 1) for single crystal Si wafer the peak position of the TO-mode was 6 cm 1 lower, which was likely due to a large tensile strain [28] or small grain size [26] in the crystalline Si film. Fig. 6 shows the cross-sectional TEM image of the a-Si film deposited under optimal condition subjected to five-step RTA 750 1C/60 s; the selective area electron diffraction (SAED) image is in the inset of Fig. 6. It was observed that the RTA-annealed a-Si film on glass was polycrystalline in nature, as shown from the SAED image; moreover, the polycrystalline Si film with thickness of 325 nm sharply formed on the glass substrate, leaved no amorphous transition region between the amorphous glass substrate and the polycrystalline Si film. Finally, the RTA crystallization process of sputtered a-Si films was investigated. Fig. 7 shows the XRD patterns of the a-Si film deposited under optimal condition subjected to different steps of RTA 750 1C/60 s. It was observed that when the a-Si film is subjected to one-step RTA 750 1C/60 s no Si characteristic peaks appeared, the broad peak at around 2y ¼221 was related to amorphous glass; as soon as the a-Si film was subjected to twostep RTA 750 1C/60 s, the Si film started to show Si characteristic peaks at 2y ¼28.51 and 47.51, corresponding to (1 1 1) and (2 2 0), respectively, indicating the initiation of crystallization of a-Si films; on increasing annealing time to five-step RTA 750 1C/60 s, the intensities of Si characteristic peaks were significantly enhanced, while with further increasing time of ten-step RTA 750 1C/60 s the intensities of Si characteristic peaks were almost unchanged. It was suggested that the structural property of Si film
Fig. 7. XRD patterns of the Si films deposited with a substrate bias of 100 W at a substrate temperature of 300 1C, followed by different steps of RTA 750 1C/60 s in Ar.
improved with the increase of 750 1C-RTA time from 1 to 5 min, but it was almost invariable with the increase of 750 1C-RTA time from 5 to 10 min. Fig. 8 shows the XRD patterns of a-Si film deposited under optimal condition subjected to RTA at temperatures of 750 and 900 1C for 2 min. It was observed that both Si films gave Si characteristic peaks at 2y ¼28.51 and 47.51, corresponding to (1 1 1) and (2 2 0), respectively, indicating the crystallization of a-Si films; moreover, the Si characteristic peaks intensities in the a-Si film subjected to RTA at 900 1C were higher than that in the a-Si film subjected to RTA at 750 1C, indicating that structural property of crystalline Si film improved with increase of RTA temperature. This is because the radiation energy
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Acknowledgement The authors would like to acknowledge the financial supports from the National Natural Science Foundation of China (Grant no. 20975107); the Chinese Academy of Sciences under the ‘‘Hundred Talents Program’’ and the ‘‘Solar Action Program’’; Zhejiang Natural Science Foundation (Grant no. Y4100169); Ningbo Natural Science Foundation (Grant no. 2010A610166); and China Postdoctoral Science Foundation (Grant no. 20100470735), ‘‘Seeding Project’’ of Ningbo Institute of Material Technology and Engineering of Chinese Academy of Sciences. References
Fig. 8. XRD patterns of the Si films deposited with a substrate bias of 100 W at a substrate temperature of 300 1C, followed by RTA at 750 or 900 1C for 2 min in Ar.
supply for Si films increases with RTA time or temperature [29,30], leading to better structural property of Si film.
4. Conclusion This work investigated the effect of substrate bias (0–150 W) and temperature (100–400 1C) during precursor a-Si films deposition, as well as the post-deposition RTA time (1–10 min) and temperature (750, 900 1C) on the RTA crystallization of a-Si films. Only the a-Si film deposited under optimal condition: substrate bias of 100 W and substrate temperature of 300 1C, attained noticeable levels of crystallization during the subsequent RTA at 750 1C. On one hand, under a certain substrate bias (100 W), the appropriate level of ion bombardment is supposed to introduce nuclei for Si crystallites and reduce oxygen impurities in the amorphous film; on the other hand, at a certain substrate temperature (300 1C), the appropriate degree of heating is supposed to reduce defect density in the film. As a result, RTA crystallized a-Si film deposited under optimal condition showed crystalline fraction of 94.1%, and was proved to be polycrystalline Si films formed directly on the quartz glass substrate with no intervening amorphous transition region. Furthermore, it was found that the structural property of Si films enhanced with the increase of 750 1C-RTA time from 1 to 5 min, and also enhanced with the increase of RTA temperature from 750 to 900 1C.
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