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Study on the initial growth process of crystalline silicon films on aluminum-coated polyethylene napthalate by Raman spectroscopy
ARTICLE IN PRESS
Journal of Crystal Growth 308 (2007) 330–333 www.elsevier.com/locate/jcrysgro
Study on the initial growth process of crystalline silicon films on aluminum-coated polyethylene napthalate by Raman spectroscopy Junshuai Lia,b, Jinxiao Wanga, Min Yina, Pingqi Gaoa, Deyan Hea,, Qiang Chenb, Yali Lib, Hajime Shiraib a Department of Physics, Lanzhou University, 730000 Lanzhou, China Faculty of Engineering, Saitama University, 255 Shimo-Okubo, Sakura, Saitama 338-8570, Japan
b
Received 25 June 2007; received in revised form 8 August 2007; accepted 28 August 2007 Communicated by D.W. Shaw Available online 4 September 2007
Abstract The authors report on the study of the initial growth process of crystalline silicon films deposited on aluminum-coated polyethylene napthalate (PEN) by inductively coupled plasma (ICP-) chemical vapor deposition (CVD) at room temperature. By using micro-Raman spectroscopy and transmission electron microscopy, microstructures of silicon films on bare and aluminum-coated substrates were studied with different film thicknesses. Compared to the films deposited on bare PEN, the phase transition from amorphous to crystalline was observed when increasing the thickness of the resultant films on aluminum-coated substrates. It indicates that the amorphous silicon layer formed under the assistance of the aluminum layer is very important to obtain highly crystallized silicon films on PEN substrates in our experiment. Therefore, the selective deposition of highly crystallized silicon films on plastic substrates can be performed by adjusting the distribution of aluminum on plastic substrates at room temperature in ICP-CVD process. r 2007 Elsevier B.V. All rights reserved. PACS: 81.10.Bk Keywords: A1. Micro-structural characterization; A1. Plastic substrates; A1. Raman spectroscopy; A1. Transmission electron microscopy; A3. Chemical vapor deposition processes; B1. Crystalline silicon films
1. Introduction In the last two decades, low-temperature crystalline silicon films have been extensively studied. This is due to the possibility of depositing this material in large areas at low costs and to its good optical and electronic properties [1–7]. And it is exciting that some successful models have been proposed to explain the formation process of crystalline silicon films: (1) surface diffusion model [8,9], (2) etching model [10] and (3) chemical annealing model [11]. Meanwhile, because of the light weight and flexibility demanded in modern solar cells and personal information displays, such as e-books, cellular phones, the preparation of high-quality crystalline silicon thin films on plastic has Corresponding author. Tel.: +86 931 8912172; fax: +86 931 8913554.
E-mail address: [email protected] (D. He). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.08.027
been attracting much attention the recent years [12–15]. And some approaches have been proposed to manufacture crystalline silicon on polymer substrates, such as laser annealing crystallization of amorphous silicon films [14,16] and the deposition of crystalline silicon films assisted with the appropriate substrate temperature, about 100 1C or more, in plasma chemical vapor deposition (CVD) processes [12,14,17,18]. However, some drawbacks in the above-mentioned approaches severely block the preparation of high-quality crystalline silicon films on plastic substrates. In LAC process, the fabrication cost is high and the annealing procedure would prolong the period of fabrication. It should be concerned that the sampletransfer process from the deposition chamber to the annealing instrument could bring some contaminations to the silicon films. And in reports concerning the deposition of crystalline silicon films on plastic in plasma CVD
ARTICLE IN PRESS J. Li et al. / Journal of Crystal Growth 308 (2007) 330–333
The plasma was generated by a built-in one-turn inductance coil (in diameter 10 cm) made of a copper tube installed in the reactor. Water was fed in to cool the copper tube in the deposition process. The input power of 13.56 MHz was 300 W. The flow ratio of SiH4 to H2 was fixed at 4:16 sccm and the working pressure was about 20 Pa. In order to create high-density and homogeneous plasma, the inductance coil was coated with a layer of 0.1cm-thick fiberglass. The base vacuum was 1 10 3 Pa. To demonstrate the effect of aluminum layers on the crystallization of silicon films, two sets of samples were prepared on different substrates, bare and aluminum-coated PEN. The thickness of the evaporated aluminum layers is about 100 nm. And silicon films with the same thickness on different substrates were simultaneously deposited. Considering the difficulty in determining the accurate thickness of the silicon films on plastic substrates, the silicon films with different thicknesses were prepared by adjusting the deposition duration. The deposition rate was determined by depositing silicon films on silicon substrates at the same plasma conditions and its value was about 6.2 nm/min. The samples were characterized by micro-Raman spectroscopy (Jobin-Yvon HR 800, excitation wavelength: 532 nm) and TEM (JEM1200EX). 3. Results and discussion Fig. 1 shows the Raman spectra of the silicon samples on bare PEN substrates. The thicknesses are about 20, 40, 60, 80 and 140 nm, as marked in the figure. The position of the characteristic peaks located in the range 480–520 cm 1, depicting the physical phase of silicon materials: amorphous phase (centered at 480 cm 1), nano-crystalline phase (centered around 510 cm 1) and poly- or single-crystalline phase (centered at 520 cm 1) [20–22], and the full-width at half-maximum (FWHM) are summarized in Fig. 2. As the thickness of silicon films increases from 40 to 80 nm, the
Intensity (arb.units)
80 nm 60 nm 40 nm 20 nm
200
300
400
500
600
700
800
Raman shift (cm-1) Fig. 1. Raman spectra of the samples on bare PEN substrates, the thicknesses of these samples are 20, 40, 60, 80 and 140 nm.
512 20 504 16 496 12
FWHM (cm-1)
2. Experimental procedure
140 nm
Peak position (cm-1)
process, it is necessary to choose the suitable plastic, which cannot deform in the deposition process. To realize the low-cost manufacture and the use of ordinary plastic substrates, we have successfully prepared highly crystallized silicon films on aluminum-coated polyethylene napthalate (PEN) by inductively coupled plasma chemical vapor deposition (ICP-CVD) at room temperature by optimizing the deposition parameters [19]. In this paper, the initial growth process of highly crystallized silicon films was investigated by using microRaman spectroscopy and transmission electron microscopy (TEM) in detail. By comparing the crystallization process of the films with different thicknesses on bare and aluminum-coated PEN substrates, it was found that the appropriate-thickness amorphous layer formed with the assistance of the evaporated aluminum layer is important to deposit highly crystallized silicon films.
331
488 8 40
60
80 100 120 Film thickness (nm)
140
Fig. 2. Peak position and FWHM of the Raman spectra of the samples on bare PEN substrates as a function of the film thickness. The thicknesses of these samples are 40, 60, 80 and 140 nm.
peak position shifts towards low wavenumbers and the FWHM increases gradually. It demonstrates, in the case of bare PEN substrates, as the film thickness increases, the crystallinity of the resultant films degrades and the grain size diminishes. Thus, it indicates that there exists large tensile stress in the silicon films deposited in the initial growth process (less than 60 nm) due to the lattice mismatch between silicon films and plastic substrates [23,24]. To release the stress, while prolonging the deposition period, the crystallinity of the silicon films decreases. And then, as depicted in Fig. 2, the Raman peak shifts to high wave numbers again with increasing the film thickness to 140 nm after the formation of more amorphous phase. As demonstrated by deconvoluting the Raman spectra [22], the calculated volume fraction of the nano-crystalline phase is boosted from 3% to 38% with increase in the film thickness from 80 to 140 nm. However, the crystallization quality cannot be improved to the level of the initial stage. The characteristic peak maintains at
ARTICLE IN PRESS J. Li et al. / Journal of Crystal Growth 308 (2007) 330–333
332
Intensity (arb.units)
500 cm 1 until the thickness of the films increases to 1000 nm [19]. It is worth noting that except for the first order Raman scattering of silicon films, the other peaks come from the substrate material because the silicon films are so thin in the initial growth stage. However, there was a significantly different initial growth process when aluminum-coated PEN was used as a substrate. As shown in Fig. 3, when the thickness of the resultant silicon films is less than 80 nm, the films are only composed of amorphous phase. As the thickness was increased to about 80 nm, the shoulder peak marked by an arrow indicates the formation of crystalline silicon nucleus. With further increase of the film thickness to 140 nm, the sharp peak with the FWHM of 6.3 cm 1 appears at 520 cm 1. Although the amorphous bump in Raman spectra is that evident, the sharp peak of 520 cm 1 demonstrates the formation of the silicon film with excellent crystallinity and big crystalline grains after the appropriate thickness amorphous incubation layer was formed. To obtain more information about the microstructure, we performed the TEM measurement on the films of 80 and 140 nm on the aluminum-coated PEN substrates. As depicted in Fig. 4(a), the selected-region electron diffraction pattern of the 80-nm film is full of halolike background. It shows a phase information similar to the corresponding Raman spectrum. Meanwhile, the weak non-ununiformity of the diffraction pattern in the radial directions still indicates the generation of crystalline silicon nucleus. To the sample with the thickness of 140 nm (as shown in Fig. 4(b)), the strong diffraction pattern beset in the weak amorphous background demonstrates the formation of highly crystallized silicon film. The calculated interplanar distance is about 0.313 nm and this is consistent with that of single-crystalline silicon (1 1 1). So the preferred orientation of the present silicon film is (1 1 1). It is well known that effective release of the film stress, originating from the mismatched crystal lattices between
140 nm 80 nm 60 nm 40 nm 20 nm
200
300
400
500
600
700
800
Raman shift (cm-1) Fig. 3. Raman spectra of the samples on aluminum-coated PEN substrates. The thicknesses of these samples are 20, 40, 60, 80 and 140 nm.
Fig. 4. Selected-region electron diffraction patterns of the samples on aluminum-coated PEN. The thicknesses of the resultant silicon films are 80 nm (a) and 140 nm (b).
film materials and substrate materials, is a key to obtain highly crystallized films on heterogenous substrates at low temperatures [25–27]. In our experiment, when bare PEN severs as substrates, the silicon films consisting of nanocrystalline phase could only be deposited in the initial growth process (the film thickness is less than 60 nm). With increase in film thickness, the stress was continuously accumulated, leading to the phase transition from nanocrystalline to amorphous. However, in the case of aluminum-coated PEN substrates, one amorphous silicon layer was formed prior to the formation of crystalline silicon and this served as a buffer layer to release the stress existing in the crystalline layer. Here, it should be pointed out that the accurate formation mechanism of the amorphous silicon layer with the help of the aluminum is under veil and more study is needed. 4. Conclusions In summary, based on the successful preparation of highly crystallized silicon thin films on aluminum-coated plastic at room temperature by ICP-CVD, we studied the initial growth process of the silicon films on bare and aluminum-coated PEN substrates. By analyzing the microstructure of the silicon films using micro-Raman spectroscopy, it was found different crystallization processes of the silicon films on different substrates are closely correlated with the formation of the amorphous incubation layer. In the case of bare PEN substrates, the formation of the nano-crystalline-phase silicon film at the initial stages creates a large stress and deteriorates the final crystallinity of the resultant silicon film. Different from the initial growth process on bare PEN, the amorphous silicon layer firstly forms prior to the incubation of crystalline silicon. It indicates that the amorphous incubation layer formed under the assistance of the aluminum layer is very important to obtain highly crystallized silicon films in our experiment. Thus, the selective deposition of highly crystallized silicon films on plastic substrates can be
ARTICLE IN PRESS J. Li et al. / Journal of Crystal Growth 308 (2007) 330–333
performed by modifying the distribution of aluminum layers at room temperature in ICP-CVD process. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 10175030) and the Natural Science Foundation of Gansu Province under grant no. 4WS035-A72-134. References [1] J. Jang, J. Oh, S. Kim, Y. Choi, S. Yoon, C. Kim, Nature 395 (1998) 481. [2] A. Shah, P. Torres, R. Tscharner, N. Wyrsch, H. Keppner, Science 285 (1999) 692. [3] R.B. Bergmann, Appl. Phys. A 69 (1999) 187. [4] K. Yamamoto, M. Yoshimi, Y. Tawada, Y. Okamoto, A. Nakajima, S. Igari, Appl. Phys. A 69 (1999) 179. [5] L. Guo, Y. Toyoshima, M. Kondo, A. Matsuda, Appl. Phys. Lett. 75 (1999) 3515. [6] H. Liu, S. Jung, Y. Fujimura, C. Fukai, H. Shirai, Y. Toyoshima, Jpn. J. Appl. Phys. 40 (2001) 44. [7] H. Fujiwara, M. Kondo, A. Mastuda, J. Appl. Phys. 93 (2003) 2400. [8] A. Matsuda, Thin Solid Films 337 (1999) 1. [9] A. Matsuda, J. Non-Cryst. Solids 59/60 (1983) 767. [10] C.C. Tsai, G.B. Anderson, R. Thompson, B. Wacker, J. Non-Cryst. Solids 114 (1989) 151.
333
[11] K. Nakamura, K. Yoshida, S. Takeoka, I. Shimizu, Jpn. J. Appl. Phys. 34 (1995) 442. [12] A.K. Kalkan, S.H. Bae, H.D. Li, D.J. Hayes, S.J. Fonash, J. Appl. Phys. 88 (2000) 555. [13] S. Wagner, S.J. Fonash, T.N. Jakson, J.C. Sturm, Proc. SPIE 4362 (2001) 226. [14] D.Y. Kim, M.S. Shim, C.H. Kim, J. Yi, Thin Solid Films 453/454 (2004) 100. [15] E. Menard, R.G. Nuzzo, J.A. Rogers, Appl. Phys. Lett. 86 (2005) 093507. [16] J.Y. Kwon, D.Y. Kim, H.S. Cho, K.B. Park, J.S. Jung, J.M. Kim, Y.S. Park, T. Noguchi, IEICE Trans. Electron. E88-C (4) (2005) 667. [17] P. Alpuim, V. Chu, J.P. Conde, J. Non-Cryst. Solids 266–269 (2000) 110. [18] M. Losurdo, M. Giangregorio, A. Grimaldi, P. Capezzuto, G. Bruno, Eur. Phys. J. Appl. Phys. 26 (2004) 187. [19] J. Li, J. Wang, M. Yin, P. Gao, D. He, Q. Chen, H. Shirai, J. Crystal Growth 306 (2007) 1. [20] H. Campbell, P.M. Fauchet, Solid State Commun. 58 (1986) 739. [21] H. Richter, Z.P. Wang, L. Ley, Solid State Commun. 39 (1981) 625. [22] J. Li, M. Yin, J. Wang, D. He, Chin. Phys. Lett. 22 (2005) 3130. [23] V. Paillard, P. Puech, R. Sirvin, S. Hamma, P. Roca, I. Cabarrocas, J. Appl. Phys. 90 (2001) 3276. [24] E. Amanatides, E. Katsia, D. Mataras, A. Soto, G.A. Voyiatzis, Thin Solid Films 511/512 (2006) 603. [25] J. Rath, Sol. Energy Mater. Sol. Cells 76 (2003) 431. [26] A. Matsuda, Jpn. J. Appl. Phys. 43 (2004) 7909. [27] H. Shirai, C. Fukai, Y. Sakuma, Y. Moriya, J. Non-Cryst. Solids 266–269 (2000) 131.
Journal of Crystal Growth 308 (2007) 330–333 www.elsevier.com/locate/jcrysgro
Study on the initial growth process of crystalline silicon films on aluminum-coated polyethylene napthalate by Raman spectroscopy Junshuai Lia,b, Jinxiao Wanga, Min Yina, Pingqi Gaoa, Deyan Hea,, Qiang Chenb, Yali Lib, Hajime Shiraib a Department of Physics, Lanzhou University, 730000 Lanzhou, China Faculty of Engineering, Saitama University, 255 Shimo-Okubo, Sakura, Saitama 338-8570, Japan
b
Received 25 June 2007; received in revised form 8 August 2007; accepted 28 August 2007 Communicated by D.W. Shaw Available online 4 September 2007
Abstract The authors report on the study of the initial growth process of crystalline silicon films deposited on aluminum-coated polyethylene napthalate (PEN) by inductively coupled plasma (ICP-) chemical vapor deposition (CVD) at room temperature. By using micro-Raman spectroscopy and transmission electron microscopy, microstructures of silicon films on bare and aluminum-coated substrates were studied with different film thicknesses. Compared to the films deposited on bare PEN, the phase transition from amorphous to crystalline was observed when increasing the thickness of the resultant films on aluminum-coated substrates. It indicates that the amorphous silicon layer formed under the assistance of the aluminum layer is very important to obtain highly crystallized silicon films on PEN substrates in our experiment. Therefore, the selective deposition of highly crystallized silicon films on plastic substrates can be performed by adjusting the distribution of aluminum on plastic substrates at room temperature in ICP-CVD process. r 2007 Elsevier B.V. All rights reserved. PACS: 81.10.Bk Keywords: A1. Micro-structural characterization; A1. Plastic substrates; A1. Raman spectroscopy; A1. Transmission electron microscopy; A3. Chemical vapor deposition processes; B1. Crystalline silicon films
1. Introduction In the last two decades, low-temperature crystalline silicon films have been extensively studied. This is due to the possibility of depositing this material in large areas at low costs and to its good optical and electronic properties [1–7]. And it is exciting that some successful models have been proposed to explain the formation process of crystalline silicon films: (1) surface diffusion model [8,9], (2) etching model [10] and (3) chemical annealing model [11]. Meanwhile, because of the light weight and flexibility demanded in modern solar cells and personal information displays, such as e-books, cellular phones, the preparation of high-quality crystalline silicon thin films on plastic has Corresponding author. Tel.: +86 931 8912172; fax: +86 931 8913554.
E-mail address: [email protected] (D. He). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.08.027
been attracting much attention the recent years [12–15]. And some approaches have been proposed to manufacture crystalline silicon on polymer substrates, such as laser annealing crystallization of amorphous silicon films [14,16] and the deposition of crystalline silicon films assisted with the appropriate substrate temperature, about 100 1C or more, in plasma chemical vapor deposition (CVD) processes [12,14,17,18]. However, some drawbacks in the above-mentioned approaches severely block the preparation of high-quality crystalline silicon films on plastic substrates. In LAC process, the fabrication cost is high and the annealing procedure would prolong the period of fabrication. It should be concerned that the sampletransfer process from the deposition chamber to the annealing instrument could bring some contaminations to the silicon films. And in reports concerning the deposition of crystalline silicon films on plastic in plasma CVD
ARTICLE IN PRESS J. Li et al. / Journal of Crystal Growth 308 (2007) 330–333
The plasma was generated by a built-in one-turn inductance coil (in diameter 10 cm) made of a copper tube installed in the reactor. Water was fed in to cool the copper tube in the deposition process. The input power of 13.56 MHz was 300 W. The flow ratio of SiH4 to H2 was fixed at 4:16 sccm and the working pressure was about 20 Pa. In order to create high-density and homogeneous plasma, the inductance coil was coated with a layer of 0.1cm-thick fiberglass. The base vacuum was 1 10 3 Pa. To demonstrate the effect of aluminum layers on the crystallization of silicon films, two sets of samples were prepared on different substrates, bare and aluminum-coated PEN. The thickness of the evaporated aluminum layers is about 100 nm. And silicon films with the same thickness on different substrates were simultaneously deposited. Considering the difficulty in determining the accurate thickness of the silicon films on plastic substrates, the silicon films with different thicknesses were prepared by adjusting the deposition duration. The deposition rate was determined by depositing silicon films on silicon substrates at the same plasma conditions and its value was about 6.2 nm/min. The samples were characterized by micro-Raman spectroscopy (Jobin-Yvon HR 800, excitation wavelength: 532 nm) and TEM (JEM1200EX). 3. Results and discussion Fig. 1 shows the Raman spectra of the silicon samples on bare PEN substrates. The thicknesses are about 20, 40, 60, 80 and 140 nm, as marked in the figure. The position of the characteristic peaks located in the range 480–520 cm 1, depicting the physical phase of silicon materials: amorphous phase (centered at 480 cm 1), nano-crystalline phase (centered around 510 cm 1) and poly- or single-crystalline phase (centered at 520 cm 1) [20–22], and the full-width at half-maximum (FWHM) are summarized in Fig. 2. As the thickness of silicon films increases from 40 to 80 nm, the
Intensity (arb.units)
80 nm 60 nm 40 nm 20 nm
200
300
400
500
600
700
800
Raman shift (cm-1) Fig. 1. Raman spectra of the samples on bare PEN substrates, the thicknesses of these samples are 20, 40, 60, 80 and 140 nm.
512 20 504 16 496 12
FWHM (cm-1)
2. Experimental procedure
140 nm
Peak position (cm-1)
process, it is necessary to choose the suitable plastic, which cannot deform in the deposition process. To realize the low-cost manufacture and the use of ordinary plastic substrates, we have successfully prepared highly crystallized silicon films on aluminum-coated polyethylene napthalate (PEN) by inductively coupled plasma chemical vapor deposition (ICP-CVD) at room temperature by optimizing the deposition parameters [19]. In this paper, the initial growth process of highly crystallized silicon films was investigated by using microRaman spectroscopy and transmission electron microscopy (TEM) in detail. By comparing the crystallization process of the films with different thicknesses on bare and aluminum-coated PEN substrates, it was found that the appropriate-thickness amorphous layer formed with the assistance of the evaporated aluminum layer is important to deposit highly crystallized silicon films.
331
488 8 40
60
80 100 120 Film thickness (nm)
140
Fig. 2. Peak position and FWHM of the Raman spectra of the samples on bare PEN substrates as a function of the film thickness. The thicknesses of these samples are 40, 60, 80 and 140 nm.
peak position shifts towards low wavenumbers and the FWHM increases gradually. It demonstrates, in the case of bare PEN substrates, as the film thickness increases, the crystallinity of the resultant films degrades and the grain size diminishes. Thus, it indicates that there exists large tensile stress in the silicon films deposited in the initial growth process (less than 60 nm) due to the lattice mismatch between silicon films and plastic substrates [23,24]. To release the stress, while prolonging the deposition period, the crystallinity of the silicon films decreases. And then, as depicted in Fig. 2, the Raman peak shifts to high wave numbers again with increasing the film thickness to 140 nm after the formation of more amorphous phase. As demonstrated by deconvoluting the Raman spectra [22], the calculated volume fraction of the nano-crystalline phase is boosted from 3% to 38% with increase in the film thickness from 80 to 140 nm. However, the crystallization quality cannot be improved to the level of the initial stage. The characteristic peak maintains at
ARTICLE IN PRESS J. Li et al. / Journal of Crystal Growth 308 (2007) 330–333
332
Intensity (arb.units)
500 cm 1 until the thickness of the films increases to 1000 nm [19]. It is worth noting that except for the first order Raman scattering of silicon films, the other peaks come from the substrate material because the silicon films are so thin in the initial growth stage. However, there was a significantly different initial growth process when aluminum-coated PEN was used as a substrate. As shown in Fig. 3, when the thickness of the resultant silicon films is less than 80 nm, the films are only composed of amorphous phase. As the thickness was increased to about 80 nm, the shoulder peak marked by an arrow indicates the formation of crystalline silicon nucleus. With further increase of the film thickness to 140 nm, the sharp peak with the FWHM of 6.3 cm 1 appears at 520 cm 1. Although the amorphous bump in Raman spectra is that evident, the sharp peak of 520 cm 1 demonstrates the formation of the silicon film with excellent crystallinity and big crystalline grains after the appropriate thickness amorphous incubation layer was formed. To obtain more information about the microstructure, we performed the TEM measurement on the films of 80 and 140 nm on the aluminum-coated PEN substrates. As depicted in Fig. 4(a), the selected-region electron diffraction pattern of the 80-nm film is full of halolike background. It shows a phase information similar to the corresponding Raman spectrum. Meanwhile, the weak non-ununiformity of the diffraction pattern in the radial directions still indicates the generation of crystalline silicon nucleus. To the sample with the thickness of 140 nm (as shown in Fig. 4(b)), the strong diffraction pattern beset in the weak amorphous background demonstrates the formation of highly crystallized silicon film. The calculated interplanar distance is about 0.313 nm and this is consistent with that of single-crystalline silicon (1 1 1). So the preferred orientation of the present silicon film is (1 1 1). It is well known that effective release of the film stress, originating from the mismatched crystal lattices between
140 nm 80 nm 60 nm 40 nm 20 nm
200
300
400
500
600
700
800
Raman shift (cm-1) Fig. 3. Raman spectra of the samples on aluminum-coated PEN substrates. The thicknesses of these samples are 20, 40, 60, 80 and 140 nm.
Fig. 4. Selected-region electron diffraction patterns of the samples on aluminum-coated PEN. The thicknesses of the resultant silicon films are 80 nm (a) and 140 nm (b).
film materials and substrate materials, is a key to obtain highly crystallized films on heterogenous substrates at low temperatures [25–27]. In our experiment, when bare PEN severs as substrates, the silicon films consisting of nanocrystalline phase could only be deposited in the initial growth process (the film thickness is less than 60 nm). With increase in film thickness, the stress was continuously accumulated, leading to the phase transition from nanocrystalline to amorphous. However, in the case of aluminum-coated PEN substrates, one amorphous silicon layer was formed prior to the formation of crystalline silicon and this served as a buffer layer to release the stress existing in the crystalline layer. Here, it should be pointed out that the accurate formation mechanism of the amorphous silicon layer with the help of the aluminum is under veil and more study is needed. 4. Conclusions In summary, based on the successful preparation of highly crystallized silicon thin films on aluminum-coated plastic at room temperature by ICP-CVD, we studied the initial growth process of the silicon films on bare and aluminum-coated PEN substrates. By analyzing the microstructure of the silicon films using micro-Raman spectroscopy, it was found different crystallization processes of the silicon films on different substrates are closely correlated with the formation of the amorphous incubation layer. In the case of bare PEN substrates, the formation of the nano-crystalline-phase silicon film at the initial stages creates a large stress and deteriorates the final crystallinity of the resultant silicon film. Different from the initial growth process on bare PEN, the amorphous silicon layer firstly forms prior to the incubation of crystalline silicon. It indicates that the amorphous incubation layer formed under the assistance of the aluminum layer is very important to obtain highly crystallized silicon films in our experiment. Thus, the selective deposition of highly crystallized silicon films on plastic substrates can be
ARTICLE IN PRESS J. Li et al. / Journal of Crystal Growth 308 (2007) 330–333
performed by modifying the distribution of aluminum layers at room temperature in ICP-CVD process. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 10175030) and the Natural Science Foundation of Gansu Province under grant no. 4WS035-A72-134. References [1] J. Jang, J. Oh, S. Kim, Y. Choi, S. Yoon, C. Kim, Nature 395 (1998) 481. [2] A. Shah, P. Torres, R. Tscharner, N. Wyrsch, H. Keppner, Science 285 (1999) 692. [3] R.B. Bergmann, Appl. Phys. A 69 (1999) 187. [4] K. Yamamoto, M. Yoshimi, Y. Tawada, Y. Okamoto, A. Nakajima, S. Igari, Appl. Phys. A 69 (1999) 179. [5] L. Guo, Y. Toyoshima, M. Kondo, A. Matsuda, Appl. Phys. Lett. 75 (1999) 3515. [6] H. Liu, S. Jung, Y. Fujimura, C. Fukai, H. Shirai, Y. Toyoshima, Jpn. J. Appl. Phys. 40 (2001) 44. [7] H. Fujiwara, M. Kondo, A. Mastuda, J. Appl. Phys. 93 (2003) 2400. [8] A. Matsuda, Thin Solid Films 337 (1999) 1. [9] A. Matsuda, J. Non-Cryst. Solids 59/60 (1983) 767. [10] C.C. Tsai, G.B. Anderson, R. Thompson, B. Wacker, J. Non-Cryst. Solids 114 (1989) 151.
333
[11] K. Nakamura, K. Yoshida, S. Takeoka, I. Shimizu, Jpn. J. Appl. Phys. 34 (1995) 442. [12] A.K. Kalkan, S.H. Bae, H.D. Li, D.J. Hayes, S.J. Fonash, J. Appl. Phys. 88 (2000) 555. [13] S. Wagner, S.J. Fonash, T.N. Jakson, J.C. Sturm, Proc. SPIE 4362 (2001) 226. [14] D.Y. Kim, M.S. Shim, C.H. Kim, J. Yi, Thin Solid Films 453/454 (2004) 100. [15] E. Menard, R.G. Nuzzo, J.A. Rogers, Appl. Phys. Lett. 86 (2005) 093507. [16] J.Y. Kwon, D.Y. Kim, H.S. Cho, K.B. Park, J.S. Jung, J.M. Kim, Y.S. Park, T. Noguchi, IEICE Trans. Electron. E88-C (4) (2005) 667. [17] P. Alpuim, V. Chu, J.P. Conde, J. Non-Cryst. Solids 266–269 (2000) 110. [18] M. Losurdo, M. Giangregorio, A. Grimaldi, P. Capezzuto, G. Bruno, Eur. Phys. J. Appl. Phys. 26 (2004) 187. [19] J. Li, J. Wang, M. Yin, P. Gao, D. He, Q. Chen, H. Shirai, J. Crystal Growth 306 (2007) 1. [20] H. Campbell, P.M. Fauchet, Solid State Commun. 58 (1986) 739. [21] H. Richter, Z.P. Wang, L. Ley, Solid State Commun. 39 (1981) 625. [22] J. Li, M. Yin, J. Wang, D. He, Chin. Phys. Lett. 22 (2005) 3130. [23] V. Paillard, P. Puech, R. Sirvin, S. Hamma, P. Roca, I. Cabarrocas, J. Appl. Phys. 90 (2001) 3276. [24] E. Amanatides, E. Katsia, D. Mataras, A. Soto, G.A. Voyiatzis, Thin Solid Films 511/512 (2006) 603. [25] J. Rath, Sol. Energy Mater. Sol. Cells 76 (2003) 431. [26] A. Matsuda, Jpn. J. Appl. Phys. 43 (2004) 7909. [27] H. Shirai, C. Fukai, Y. Sakuma, Y. Moriya, J. Non-Cryst. Solids 266–269 (2000) 131.