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Low-temperature deposition of highly crystallized silicon films on Al-coated polyethylene napthalate by inductively coupled plasma CVD
Journal of Alloys and Compounds 481 (2009) 278–282
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Low-temperature deposition of highly crystallized silicon films on Al-coated polyethylene napthalate by inductively coupled plasma CVD Jinxiao Wang a , Pingqi Gao a , Min Yin a , Yanli Qin a , Hengqing Yan a , Junshuai Li a,b , Shanglong Peng a , Deyan He a,∗ a b
Department of Physics, Lanzhou University, Lanzhou 730000, China School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
a r t i c l e
i n f o
Article history: Received 4 December 2008 Received in revised form 18 March 2009 Accepted 21 March 2009 Available online 31 March 2009 PACS: 81.10.Bk 81.15.Gh Keywords: Crystalline silicon films PEN substrates Al layer Raman spectroscopy Scanning electron microscope Inductively coupled plasma CVD
a b s t r a c t We prepared highly crystallized silicon films on Al-coated polyethylene napthalate (PEN) substrates using inductively coupled plasma chemical vapor deposition (ICP-CVD) at low temperature with a mixture of SiH4 /H2 as the source gas. The microstructure of the films was evaluated using Raman spectroscopy, scanning electron microscope (SEM) and transmission electron microscopy (TEM). The effects of deposition parameters on the crystallinity of silicon films on bare and Al-coated PEN were systematically investigated. Compared to the films deposited on bare PEN, an obvious phase transition from amorphous to crystalline occurred when decreasing the SiH4 dilution ratio [R = [SiH4 ]/([SiH4 ] + [H2 ])] to 4% for the films on Al-coated substrates. With increasing the input power from 300 W to 400 W, the crystallinity of the films on bare and Al-coated PEN are both improved at a low temperature as low as 85 ◦ C. The film on Al-coated PEN shows excellent crystallization with crystalline fraction of 82% and preferred orientation of (1 1 1). It has been found that the interaction between precursors and aluminum layers plays an important role and there should exist a different crystallization mechanism as compared to traditional annealing crystallization of amorphous Si/Al layer in the crystallization process of silicon films on Al-coated PEN at low temperature. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Low-temperature crystalline silicon films have attracted much attention due to the possibility of large-area deposition, low cost and excellent photoelectric properties for large-area electronics such as solar cells, thin-film transistor arrays of liquid crystal displays and image sensors [1–6]. Meanwhile, the portability has been in good demand for applications in personal information display such as e-book, PDA, and hand-held mounts. Hence, many researchers have focused their efforts on the preparation of highquality crystalline silicon films on plastic [7,8]. A crucial challenge, however, is the reduction of the growth temperature. The conventional fabrication techniques normally require temperatures that are much higher than the melting points of most plastic films (<200 ◦ C). Presently, excimer laser crystallization (ELC) technique appears compatible with plastic temperature limitation, since it allows the melting of the amorphous silicon film, minimizing the heat on the substrate [8]. However, in ELC process, the fabrication cost is high and the annealing procedure would prolong the
∗ Corresponding author. Tel.: +86 931 8912546 fax: +86 931 8913554. E-mail address: [email protected] (D. He). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.120
period of fabrication. In addition, to avoid deforming in the deposition progress of films on plastic, suitable plastic must be chosen carefully. In this paper, to exploit the low-cost manufacture and use of ordinary plastic substrates, we systematically investigated the effect of deposition parameters such as SiH4 dilution ratio, growth temperature and input power on the crystallinity of silicon films on bare and Al-coated PEN by ICP-CVD. As a consequence, we have successfully prepared highly crystallized silicon films on Al-coated PEN at a low temperature of 85 ◦ C. Micro Raman spectroscopy, scanning electron microscope (SEM) and transmission electron microscopy (TEM) have been used to characterize the microstructure of silicon films in detail. Based on the yielded results, the possible crystallization mechanism is proposed in terms of gas phase reactions and surface reactions for the crystallization process of silicon films on Al-coated PEN at low temperature. 2. Experimental details The plasma was generated by a built-in one-turn inductance coil (10 cm in diameter) made of a copper tube with a 13.56 MHz rf source. Water was fed in to cool the copper tube in the deposition process. In order to achieve high-density and homogeneous plasma, the inductance coil was coated with a layer of 0.1-cm-thick fiberglass as an insulator. The base vacuum was 1 × 10−3 Pa. The total flow of the
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mixture of SiH4 and H2 was fixed at 20 sccm and the working pressure was about 20 Pa. The SiH4 dilution ratio was controlled by regulating the flow ratio of SiH4 to H2 . Two kinds of substrates, bare and Al-coated PEN, were used in the experiment to investigate the effect of Al coating on the crystallization of silicon films. The Al layer (∼200 nm in thickness) was thermal evaporated on PEN. The samples were characterized by micro-Raman spectroscopy (Jobin-Yvon HR 800 with the excitation wavelength of 532 nm), scanning electron microscope (JSM-6701F) and TEM (Hitachi Model H-800).
3. Results and discussion Fig. 1 exhibits the Raman spectroscopy of silicon films grown with SiH4 dilution ratios of 4%, 8% and 12%, respectively, otherwise under identical conditions, working pressure of 20 Pa, input power of 300 W, and growth temperature of 100 ◦ C. It can be seen that, for the samples on bare PEN, a broad Raman mode centered at 480 cm−1 corresponding to amorphous phase is observed for different SiH4 dilution ratios. However, for the films on Al-coated PEN, with decreasing the SiH4 dilution ratio to 4%, a sharp Raman peak centered nearly at 520 cm−1 is present, which is attributed to the transverse optical (TO) optical phonon mode in c-Si [9,10]. Based on the results, we suggest that crystalline silicon films can be achieved on Al-coated PEN with SiH4 dilution ratio of 4% at 100 ◦ C. The crystallinity can be estimated from Raman spectra. The crystalline fraction defined as Xc =
I520 + I510 I520 + I510 + I480
Fig. 2. Raman spectra of silicon films grown on bare (a) and Al-coated (b) PEN at 300 W, SiH4 dilution ratios of 4% with growth temperature of 50 ◦ C, 70 ◦ C, 85 ◦ C and 100 ◦ C, respectively.
Fig. 1. Raman spectra of silicon films grown on bare (a) and Al-coated (b) PEN at 100 ◦ C, 300 W with SiH4 dilution ratios of 4%, 8% and 12%, respectively. The inset is a decomposed Raman spectrum as a schematic.
was deduced by deconvolution of the spectra into three Gaussian components as shown in the inset of Fig. 1(b) schematically: amorphous component (centered at 480 cm−1 ), nanocrystalline component (centered at 510 cm−1 ), and crystalline component (centered at 520 cm−1 ) [9–11]. Where I480 , I510 and I520 are integrated intensities of the peaks at 480, 510 cm−1 and 520 cm−1 , respectively, and is the ratio of the integrated Raman cross section for a-Si to c-Si. For a small grain size, we can take the correction factor as unity ( ≈ 1). Accordingly, the crystalline fraction obtained is 73% for silicon film deposited on Al-coated PEN with SiH4 dilution ratio of 4% at 100 ◦ C. As depicted in Fig. 2, at the fixed SiH4 dilution ratio of 4% and input power of 300 W, silicon films grown on bare and Al-coated PEN both show amorphous phase as the growth temperature lower than 100 ◦ C. In other words, the growth temperature of crystalline silicon films on PEN can be reduced to 100 ◦ C with assistant of Al layer at 300 W and SiH4 dilution ratio of 4%. In order to further reduce the growth temperature and improve the crystallinity of silicon films, we studied the effect of input power on the samples deposited on bare and Al-coated PEN at a lower temperature of 85 ◦ C with fixed SiH4 dilution ratio of 4%. The Raman spectra in Fig. 3 indicates a transition from amorphous phase to
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Fig. 3. Raman spectra of silicon films grown on bare (a) and Al-coated (b) PEN at 85 ◦ C, SiH4 dilution ratio of 4% with the input power of 300 W, 400 W, respectively.
crystalline phase both in the films on bare and Al-coated PEN with increasing the input power from 300 W to 400 W. At the same time, we can found that the presence of a strong Raman peak centered nearly at 520 cm−1 accompanied by a weak shoulder around 500 cm−1 (nano-crystalline phase) for the film deposited on Alcoated PEN demonstrates higher crystallization, in contrast to the sample on bare PEN. Moreover, as calculated from the corresponding Raman spectrum, the crystalline fraction for samples on bare and Al-coated PEN is about 63% and 82%, respectively. It is noteworthy that the sample using SiH4 dilution ratio of 4% at 400 W, 85 ◦ C possesses higher crystallization than the sample at 300 W, 100 ◦ C whether on bare or Al-coated PEN substrates. By optimizing deposition parameters, the growth temperature as low as 85 ◦ C is achieved for high-quality crystalline silicon films on PEN. Fig. 4 shows SEM images of silicon films deposited on bare and Al-coated PEN at 85 ◦ C, 400 W with the SiH4 dilution ratio of 4%. As demonstrated in surface images, it can be seen that the grain size is
uniform and about 80 nm and 130 nm in the lateral direction for silicon films deposited on bare and Al-coated PEN, respectively. Compared with the sample on bare PEN, the film deposited on Al-coated PEN possesses larger grain size contributed to higher crystallization. The cross-sectional SEM images perform that the thickness of silicon films on Al-coated PEN is around 580 nm, thicker than films on bare PEN (∼300 nm) which can contributed to easy diffusion of the active Si-related radicals into the Al coating once they are adsorbed on the substrate surface in the process of ICP-CVD, leading to high deposition rate. The fact that no Al layer exchange in crystallization growth on Al-coated PEN indicates that the mechanism of Al-induced crystallization growth by ICP-CVD is different from that of the traditional Al-induced annealing crystallization. Based on our experiment results, we propose crystallization growth process of silicon films on Al-coated PEN by ICP-CVD at such a low temperature. In the initial stages of ICP-CVD, the main growth precursors of SiH3 radicals [12] in the plasma of SiH4 and
Fig. 4. Surface and cross-sectional SEM images of silicon films deposited on bare (a) and Al-coated (b) PEN at 85 ◦ C, 400 W with the SiH4 dilution ratio of 4%.
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Fig. 5. Schematic diagram of Al-induced growth of crystalline silicon films by ICPCVD: (1) diffusion of SiHx (0 ≤ x ≤ 3) to the substrate; (2) diffusion of silicon atoms in the aluminum layer; (3) nucleation of silicon atoms; (4) growth of silicon grains.
H2 can easily diffuse into the Al layer when they are absorbed on the Al-coated PEN. Due to the strong nonequilibrium caused by the high electron density of ICP, the silicon concentration in Al immediately exceeds the saturation value assisted by the high-density atomic hydrogen and the substrate temperature. They will separate out from aluminum grains and nucleate in the low-energy sites, such as aluminum grain boundaries or the surface of the aluminum layer as shown in Fig. 5. The grains which nucleate in the aluminum grain boundaries will stop growing when the grains touch the upper and lower surfaces of the aluminum layer. For the silicon grains nucleated on the surface of the aluminum layer, the growth will continue along the normal of the upper surface of the aluminum layer as silicon atoms accumulate at the bottoms of the silicon grains. Such a growth process is schematically demonstrated in Fig. 5. To obtain more information about the microstructure, we also performed the TEM measurement on the films grown on Al-coated PEN using SiH4 dilution ratio of 4% at 300 W, 100 ◦ C and 400 W, 85 ◦ C, respectively. As depicted in Fig. 6(a), the selected-region electron diffraction pattern of the silicon film at 300 W, 100 ◦ C is composed of line rings. It indicates the generation of crystalline silicon accompanied with amorphous phase. For the sample at 400 W, 85 ◦ C (as shown in Fig. 6(b)), the appearing of spotted rings with weak amorphous background demonstrates its good crystalline quality, consistent with the results of Raman spectroscopy. Moreover, we calculated the interplanar distance which is about 0.313 nm consistent with that of single-crystalline silicon (1 1 1). So the preferred orientation of the highly crystallized silicon film is (1 1 1). As depicted above, the crystallization characteristics of the silicon films deposited on bare and Al-coated PEN by the ICP system are summarized as follows: (1) When the working pressure, input power and growth temperature are fixed at 20 Pa, 300 W and 100 ◦ C, respectively, the crystallinity of the resultant silicon films on Al-coated PEN is upgraded by decreasing the SiH4 dilution ratio in an appropriate range which is different from the silicon films on glass [13]. (2) Under a lower growth temperature of 85 ◦ C, the crystallinity increases with the increase of the input power from 300 W to 400 W wherever on bare or Al-coated PEN. Here, the crystallization results will be tentatively interpreted based on the unique properties of ICP used in our experiment and
Fig. 6. Selected-region electron diffraction patterns of the samples on Al-coated PEN with the SiH4 dilution ratio of 4% at 300 W, 100 ◦ C (a) and 400 W, 85 ◦ C (b), respectively.
the effect of Al layer. Meanwhile, it is noteworthy that hydrogen also plays the positive roles including enhancing surface diffusion of adsorbed precursors, preferential etching of the disordered phase, subsurface restructuring termed hydrogen chemical annealing [14–18] in the formation process of crystalline phase. Normally, the crystallization process of low-temperature silicon film deposition is strongly dependent on the plasma characteristics. In the deposition progress, the key to obtain high-quality crystallized silicon films is to increase the concentration of SiH3 and to suppress the densities of ions and SiHx (0 ≤ x ≤ 2) by adjusting the gasphase transport process [19]. As the promising plasma, ICP has the unique characteristics of high plasma density (about 1012 cm−3 at 10 mTorr), low electron temperature and the spatial confinement of the discharge [11,20], i.e. the plasma is confined around the inductance coil. In virtue of the spatial confinement of the discharge in ICP, the gas-phase transport can be easily controlled by adjusting the gas pressure and the distance between the substrate stage and
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the inductance coil plane. Due to the high activity and short lifetimes of SiHx (0 ≤ x ≤ 2) and ions, if the distance is long enough and the gas pressure is appropriate, the concentrations of SiHx (0 ≤ x ≤ 2) and ions can be effectively reduced by the reactions with SiH4 or other precursors to form high silanes in the transport process from the plasma region to substrates. Meanwhile, the low reactive SiH3 can diffuse onto the growing surface to form the ordered crystalline silicon network. When the SiH4 dilution ratio increases, the relative density of SiH3 radical increases and the densities of ions and SiHx (0 ≤ x ≤ 2) decrease. Based on this, the crystallinity of the resultant silicon films should increases with the increase of the SiH4 dilution ratio. However, the experimental result is contrary. That is because the reason mentioned above is only one aspect of improving crystallinity of silicon films. Normally, the important role of atomic H should not be neglected during deposition progress of crystalline silicon films. As higher SiH4 dilution ratio, hydrogen atoms are lacking to terminate the dangling bonds so as to the enhancing the surface diffusion of adsorbed precursors and subsurface restructuring are not carried out, leading to amorphousation. In addition, accompanied with obvious increase of SiH3 radicals, large number of precursors impact mutually and cannot effectively interact with the Al coating assisted by growth temperature, further leading to the decreasing crystallinity of the resultant films. As a result, the crystallinity of the resultant silicon films on Al-coated PEN is upgraded by decreasing the SiH4 dilution ratio in an appropriate range. With the increase of the input power, the effective atomic H termination of the dangling bonds on the growing surface and the atomic H etching to the amorphous phase are enhanced to increase the crystallinity of the resultant silicon films. Therefore, the crystallinity would upgrade with the increase of the input power as demonstrated in Fig. 4. 4. Conclusion In summary, we have successfully prepared highly crystallized silicon films on Al-coated PEN by optimizing the experimental parameters at temperature as low as 85 ◦ C by ICP-CVD. By analyzing the microstructure of the silicon films deposited at a series of condition, it is found that the crystallinity of the resultant films on Al-coated PEN increases with decreasing the SiH4 dilution ratio to 4%. However, the crystallization is not present for films on bare
PEN. Thus, Al layer plays an important role in the crystallization of silicon films. In order to further reduce the growth temperature and improve the crystallinity of films, we observe the effect of input power. When the input power increases from 300 W to 400 W, the films on bare and Al-coated PEN both exhibit crystalline phase even at low temperature of 85 ◦ C. And, the films on Al-coated PEN show more excellent crystallization with crystalline fraction of 82% and preferred orientation of (1 1 1) as compared with bare PEN. The successful deposition of highly crystallized silicon films on Alcoated PEN at low temperature of 85 ◦ C could provide more choices of substrates and effectively reduce the fabrication cost. Moreover, we propose the possible crystallization mechanism in terms of gas phase reactions and surface reactions for the crystallization process of silicon films on Al-coated PEN at low temperature. Acknowledgment This work is supported by the National Natural Science Foundation of China (No. 60776009). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
J. Jang, J. Oh, S. Kim, Y. Choi, S. Yoon, C. Kim, Nature 395 (1998) 481. K. Yamamoto, M. Yoshimi, A. Nakajima, S. Igari, Appl. Phys. A 69 (1999) 179. H. Fujiwara, M. Kondo, A. Mastuda, J. Appl. Phys. 93 (2003) 2400. C.S. McCormick, C.E. Weber, J.R. Abelson, Appl. Phys. Lett. 70 (1997) 226. A. Sazonov, A. Nathan, J. Vac. Sci. Technol. A 18 (2000) 780. N.D. Young, M.J. Trainor, S.Y. Yoon, D.J. McCulloch, R.W. Wilks, Proc. Mater. Res. Soc. Symp. H2 1 (2003) 769. E. Menard, R.G. Nuzzo, J.A. Rogers, Appl. Phys. Lett. 86 (2005) 093507. P.M. Smith, P.G. Carey, T.W. Sigmon, Appl. Phys. Lett. 70 (1997) 342. H. Campbell, P.M. Fauchet, Solid State Commun. 58 (1986) 739. H. Richter, Z.P. Wang, L. Ley, Solid State Commun. 39 (1981) 625. B. Moon, J. Youn, S. Won, J. Jang, Sol. Energy Mater. Sol. Cells 69 (2001) 139. J. Perrin, T. Broekhuizen, Appl. Phys. Lett. 50 (1987) 433. Junshuai Li, Jinxiao Wang, Min Yin, Pingqi Gao, Deyan He, J. Appl. Phys. 103 (2008) 043505. J.E. Gerbi, J.R. Abelson, J. Appl. Phys. 89 (2001) 1643. Akihisa Matsuda, Thin Solid Films 337 (1999) 1–6. S. Hamma, P. Roca i Cabarrocas, Thin Solid Films 296 (1997) 11–14. Michio Kondo, Makoto Fukawa, Lihui Gao, Akihisa Matsuda, J. Non-Cryst. Solids 266–269 (2000) 84–89. E.A.G. Hamers, A. Fontcuberta i Morral, C. Niikura, R. Brenot, P. Roca i, Cabarrocas, J. Appl. Phys. 88 (2000) 3674. S. Shimizi, M. Kondo, A. Matsuda, J. Appl. Phys. 97 (2005) 033522. R.C. Mani, I. Pavel, E.S. Aydil, J. Appl. Phys. 102 (2007) 043305.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Low-temperature deposition of highly crystallized silicon films on Al-coated polyethylene napthalate by inductively coupled plasma CVD Jinxiao Wang a , Pingqi Gao a , Min Yin a , Yanli Qin a , Hengqing Yan a , Junshuai Li a,b , Shanglong Peng a , Deyan He a,∗ a b
Department of Physics, Lanzhou University, Lanzhou 730000, China School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
a r t i c l e
i n f o
Article history: Received 4 December 2008 Received in revised form 18 March 2009 Accepted 21 March 2009 Available online 31 March 2009 PACS: 81.10.Bk 81.15.Gh Keywords: Crystalline silicon films PEN substrates Al layer Raman spectroscopy Scanning electron microscope Inductively coupled plasma CVD
a b s t r a c t We prepared highly crystallized silicon films on Al-coated polyethylene napthalate (PEN) substrates using inductively coupled plasma chemical vapor deposition (ICP-CVD) at low temperature with a mixture of SiH4 /H2 as the source gas. The microstructure of the films was evaluated using Raman spectroscopy, scanning electron microscope (SEM) and transmission electron microscopy (TEM). The effects of deposition parameters on the crystallinity of silicon films on bare and Al-coated PEN were systematically investigated. Compared to the films deposited on bare PEN, an obvious phase transition from amorphous to crystalline occurred when decreasing the SiH4 dilution ratio [R = [SiH4 ]/([SiH4 ] + [H2 ])] to 4% for the films on Al-coated substrates. With increasing the input power from 300 W to 400 W, the crystallinity of the films on bare and Al-coated PEN are both improved at a low temperature as low as 85 ◦ C. The film on Al-coated PEN shows excellent crystallization with crystalline fraction of 82% and preferred orientation of (1 1 1). It has been found that the interaction between precursors and aluminum layers plays an important role and there should exist a different crystallization mechanism as compared to traditional annealing crystallization of amorphous Si/Al layer in the crystallization process of silicon films on Al-coated PEN at low temperature. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Low-temperature crystalline silicon films have attracted much attention due to the possibility of large-area deposition, low cost and excellent photoelectric properties for large-area electronics such as solar cells, thin-film transistor arrays of liquid crystal displays and image sensors [1–6]. Meanwhile, the portability has been in good demand for applications in personal information display such as e-book, PDA, and hand-held mounts. Hence, many researchers have focused their efforts on the preparation of highquality crystalline silicon films on plastic [7,8]. A crucial challenge, however, is the reduction of the growth temperature. The conventional fabrication techniques normally require temperatures that are much higher than the melting points of most plastic films (<200 ◦ C). Presently, excimer laser crystallization (ELC) technique appears compatible with plastic temperature limitation, since it allows the melting of the amorphous silicon film, minimizing the heat on the substrate [8]. However, in ELC process, the fabrication cost is high and the annealing procedure would prolong the
∗ Corresponding author. Tel.: +86 931 8912546 fax: +86 931 8913554. E-mail address: [email protected] (D. He). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.03.120
period of fabrication. In addition, to avoid deforming in the deposition progress of films on plastic, suitable plastic must be chosen carefully. In this paper, to exploit the low-cost manufacture and use of ordinary plastic substrates, we systematically investigated the effect of deposition parameters such as SiH4 dilution ratio, growth temperature and input power on the crystallinity of silicon films on bare and Al-coated PEN by ICP-CVD. As a consequence, we have successfully prepared highly crystallized silicon films on Al-coated PEN at a low temperature of 85 ◦ C. Micro Raman spectroscopy, scanning electron microscope (SEM) and transmission electron microscopy (TEM) have been used to characterize the microstructure of silicon films in detail. Based on the yielded results, the possible crystallization mechanism is proposed in terms of gas phase reactions and surface reactions for the crystallization process of silicon films on Al-coated PEN at low temperature. 2. Experimental details The plasma was generated by a built-in one-turn inductance coil (10 cm in diameter) made of a copper tube with a 13.56 MHz rf source. Water was fed in to cool the copper tube in the deposition process. In order to achieve high-density and homogeneous plasma, the inductance coil was coated with a layer of 0.1-cm-thick fiberglass as an insulator. The base vacuum was 1 × 10−3 Pa. The total flow of the
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mixture of SiH4 and H2 was fixed at 20 sccm and the working pressure was about 20 Pa. The SiH4 dilution ratio was controlled by regulating the flow ratio of SiH4 to H2 . Two kinds of substrates, bare and Al-coated PEN, were used in the experiment to investigate the effect of Al coating on the crystallization of silicon films. The Al layer (∼200 nm in thickness) was thermal evaporated on PEN. The samples were characterized by micro-Raman spectroscopy (Jobin-Yvon HR 800 with the excitation wavelength of 532 nm), scanning electron microscope (JSM-6701F) and TEM (Hitachi Model H-800).
3. Results and discussion Fig. 1 exhibits the Raman spectroscopy of silicon films grown with SiH4 dilution ratios of 4%, 8% and 12%, respectively, otherwise under identical conditions, working pressure of 20 Pa, input power of 300 W, and growth temperature of 100 ◦ C. It can be seen that, for the samples on bare PEN, a broad Raman mode centered at 480 cm−1 corresponding to amorphous phase is observed for different SiH4 dilution ratios. However, for the films on Al-coated PEN, with decreasing the SiH4 dilution ratio to 4%, a sharp Raman peak centered nearly at 520 cm−1 is present, which is attributed to the transverse optical (TO) optical phonon mode in c-Si [9,10]. Based on the results, we suggest that crystalline silicon films can be achieved on Al-coated PEN with SiH4 dilution ratio of 4% at 100 ◦ C. The crystallinity can be estimated from Raman spectra. The crystalline fraction defined as Xc =
I520 + I510 I520 + I510 + I480
Fig. 2. Raman spectra of silicon films grown on bare (a) and Al-coated (b) PEN at 300 W, SiH4 dilution ratios of 4% with growth temperature of 50 ◦ C, 70 ◦ C, 85 ◦ C and 100 ◦ C, respectively.
Fig. 1. Raman spectra of silicon films grown on bare (a) and Al-coated (b) PEN at 100 ◦ C, 300 W with SiH4 dilution ratios of 4%, 8% and 12%, respectively. The inset is a decomposed Raman spectrum as a schematic.
was deduced by deconvolution of the spectra into three Gaussian components as shown in the inset of Fig. 1(b) schematically: amorphous component (centered at 480 cm−1 ), nanocrystalline component (centered at 510 cm−1 ), and crystalline component (centered at 520 cm−1 ) [9–11]. Where I480 , I510 and I520 are integrated intensities of the peaks at 480, 510 cm−1 and 520 cm−1 , respectively, and is the ratio of the integrated Raman cross section for a-Si to c-Si. For a small grain size, we can take the correction factor as unity ( ≈ 1). Accordingly, the crystalline fraction obtained is 73% for silicon film deposited on Al-coated PEN with SiH4 dilution ratio of 4% at 100 ◦ C. As depicted in Fig. 2, at the fixed SiH4 dilution ratio of 4% and input power of 300 W, silicon films grown on bare and Al-coated PEN both show amorphous phase as the growth temperature lower than 100 ◦ C. In other words, the growth temperature of crystalline silicon films on PEN can be reduced to 100 ◦ C with assistant of Al layer at 300 W and SiH4 dilution ratio of 4%. In order to further reduce the growth temperature and improve the crystallinity of silicon films, we studied the effect of input power on the samples deposited on bare and Al-coated PEN at a lower temperature of 85 ◦ C with fixed SiH4 dilution ratio of 4%. The Raman spectra in Fig. 3 indicates a transition from amorphous phase to
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Fig. 3. Raman spectra of silicon films grown on bare (a) and Al-coated (b) PEN at 85 ◦ C, SiH4 dilution ratio of 4% with the input power of 300 W, 400 W, respectively.
crystalline phase both in the films on bare and Al-coated PEN with increasing the input power from 300 W to 400 W. At the same time, we can found that the presence of a strong Raman peak centered nearly at 520 cm−1 accompanied by a weak shoulder around 500 cm−1 (nano-crystalline phase) for the film deposited on Alcoated PEN demonstrates higher crystallization, in contrast to the sample on bare PEN. Moreover, as calculated from the corresponding Raman spectrum, the crystalline fraction for samples on bare and Al-coated PEN is about 63% and 82%, respectively. It is noteworthy that the sample using SiH4 dilution ratio of 4% at 400 W, 85 ◦ C possesses higher crystallization than the sample at 300 W, 100 ◦ C whether on bare or Al-coated PEN substrates. By optimizing deposition parameters, the growth temperature as low as 85 ◦ C is achieved for high-quality crystalline silicon films on PEN. Fig. 4 shows SEM images of silicon films deposited on bare and Al-coated PEN at 85 ◦ C, 400 W with the SiH4 dilution ratio of 4%. As demonstrated in surface images, it can be seen that the grain size is
uniform and about 80 nm and 130 nm in the lateral direction for silicon films deposited on bare and Al-coated PEN, respectively. Compared with the sample on bare PEN, the film deposited on Al-coated PEN possesses larger grain size contributed to higher crystallization. The cross-sectional SEM images perform that the thickness of silicon films on Al-coated PEN is around 580 nm, thicker than films on bare PEN (∼300 nm) which can contributed to easy diffusion of the active Si-related radicals into the Al coating once they are adsorbed on the substrate surface in the process of ICP-CVD, leading to high deposition rate. The fact that no Al layer exchange in crystallization growth on Al-coated PEN indicates that the mechanism of Al-induced crystallization growth by ICP-CVD is different from that of the traditional Al-induced annealing crystallization. Based on our experiment results, we propose crystallization growth process of silicon films on Al-coated PEN by ICP-CVD at such a low temperature. In the initial stages of ICP-CVD, the main growth precursors of SiH3 radicals [12] in the plasma of SiH4 and
Fig. 4. Surface and cross-sectional SEM images of silicon films deposited on bare (a) and Al-coated (b) PEN at 85 ◦ C, 400 W with the SiH4 dilution ratio of 4%.
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Fig. 5. Schematic diagram of Al-induced growth of crystalline silicon films by ICPCVD: (1) diffusion of SiHx (0 ≤ x ≤ 3) to the substrate; (2) diffusion of silicon atoms in the aluminum layer; (3) nucleation of silicon atoms; (4) growth of silicon grains.
H2 can easily diffuse into the Al layer when they are absorbed on the Al-coated PEN. Due to the strong nonequilibrium caused by the high electron density of ICP, the silicon concentration in Al immediately exceeds the saturation value assisted by the high-density atomic hydrogen and the substrate temperature. They will separate out from aluminum grains and nucleate in the low-energy sites, such as aluminum grain boundaries or the surface of the aluminum layer as shown in Fig. 5. The grains which nucleate in the aluminum grain boundaries will stop growing when the grains touch the upper and lower surfaces of the aluminum layer. For the silicon grains nucleated on the surface of the aluminum layer, the growth will continue along the normal of the upper surface of the aluminum layer as silicon atoms accumulate at the bottoms of the silicon grains. Such a growth process is schematically demonstrated in Fig. 5. To obtain more information about the microstructure, we also performed the TEM measurement on the films grown on Al-coated PEN using SiH4 dilution ratio of 4% at 300 W, 100 ◦ C and 400 W, 85 ◦ C, respectively. As depicted in Fig. 6(a), the selected-region electron diffraction pattern of the silicon film at 300 W, 100 ◦ C is composed of line rings. It indicates the generation of crystalline silicon accompanied with amorphous phase. For the sample at 400 W, 85 ◦ C (as shown in Fig. 6(b)), the appearing of spotted rings with weak amorphous background demonstrates its good crystalline quality, consistent with the results of Raman spectroscopy. Moreover, we calculated the interplanar distance which is about 0.313 nm consistent with that of single-crystalline silicon (1 1 1). So the preferred orientation of the highly crystallized silicon film is (1 1 1). As depicted above, the crystallization characteristics of the silicon films deposited on bare and Al-coated PEN by the ICP system are summarized as follows: (1) When the working pressure, input power and growth temperature are fixed at 20 Pa, 300 W and 100 ◦ C, respectively, the crystallinity of the resultant silicon films on Al-coated PEN is upgraded by decreasing the SiH4 dilution ratio in an appropriate range which is different from the silicon films on glass [13]. (2) Under a lower growth temperature of 85 ◦ C, the crystallinity increases with the increase of the input power from 300 W to 400 W wherever on bare or Al-coated PEN. Here, the crystallization results will be tentatively interpreted based on the unique properties of ICP used in our experiment and
Fig. 6. Selected-region electron diffraction patterns of the samples on Al-coated PEN with the SiH4 dilution ratio of 4% at 300 W, 100 ◦ C (a) and 400 W, 85 ◦ C (b), respectively.
the effect of Al layer. Meanwhile, it is noteworthy that hydrogen also plays the positive roles including enhancing surface diffusion of adsorbed precursors, preferential etching of the disordered phase, subsurface restructuring termed hydrogen chemical annealing [14–18] in the formation process of crystalline phase. Normally, the crystallization process of low-temperature silicon film deposition is strongly dependent on the plasma characteristics. In the deposition progress, the key to obtain high-quality crystallized silicon films is to increase the concentration of SiH3 and to suppress the densities of ions and SiHx (0 ≤ x ≤ 2) by adjusting the gasphase transport process [19]. As the promising plasma, ICP has the unique characteristics of high plasma density (about 1012 cm−3 at 10 mTorr), low electron temperature and the spatial confinement of the discharge [11,20], i.e. the plasma is confined around the inductance coil. In virtue of the spatial confinement of the discharge in ICP, the gas-phase transport can be easily controlled by adjusting the gas pressure and the distance between the substrate stage and
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the inductance coil plane. Due to the high activity and short lifetimes of SiHx (0 ≤ x ≤ 2) and ions, if the distance is long enough and the gas pressure is appropriate, the concentrations of SiHx (0 ≤ x ≤ 2) and ions can be effectively reduced by the reactions with SiH4 or other precursors to form high silanes in the transport process from the plasma region to substrates. Meanwhile, the low reactive SiH3 can diffuse onto the growing surface to form the ordered crystalline silicon network. When the SiH4 dilution ratio increases, the relative density of SiH3 radical increases and the densities of ions and SiHx (0 ≤ x ≤ 2) decrease. Based on this, the crystallinity of the resultant silicon films should increases with the increase of the SiH4 dilution ratio. However, the experimental result is contrary. That is because the reason mentioned above is only one aspect of improving crystallinity of silicon films. Normally, the important role of atomic H should not be neglected during deposition progress of crystalline silicon films. As higher SiH4 dilution ratio, hydrogen atoms are lacking to terminate the dangling bonds so as to the enhancing the surface diffusion of adsorbed precursors and subsurface restructuring are not carried out, leading to amorphousation. In addition, accompanied with obvious increase of SiH3 radicals, large number of precursors impact mutually and cannot effectively interact with the Al coating assisted by growth temperature, further leading to the decreasing crystallinity of the resultant films. As a result, the crystallinity of the resultant silicon films on Al-coated PEN is upgraded by decreasing the SiH4 dilution ratio in an appropriate range. With the increase of the input power, the effective atomic H termination of the dangling bonds on the growing surface and the atomic H etching to the amorphous phase are enhanced to increase the crystallinity of the resultant silicon films. Therefore, the crystallinity would upgrade with the increase of the input power as demonstrated in Fig. 4. 4. Conclusion In summary, we have successfully prepared highly crystallized silicon films on Al-coated PEN by optimizing the experimental parameters at temperature as low as 85 ◦ C by ICP-CVD. By analyzing the microstructure of the silicon films deposited at a series of condition, it is found that the crystallinity of the resultant films on Al-coated PEN increases with decreasing the SiH4 dilution ratio to 4%. However, the crystallization is not present for films on bare
PEN. Thus, Al layer plays an important role in the crystallization of silicon films. In order to further reduce the growth temperature and improve the crystallinity of films, we observe the effect of input power. When the input power increases from 300 W to 400 W, the films on bare and Al-coated PEN both exhibit crystalline phase even at low temperature of 85 ◦ C. And, the films on Al-coated PEN show more excellent crystallization with crystalline fraction of 82% and preferred orientation of (1 1 1) as compared with bare PEN. The successful deposition of highly crystallized silicon films on Alcoated PEN at low temperature of 85 ◦ C could provide more choices of substrates and effectively reduce the fabrication cost. Moreover, we propose the possible crystallization mechanism in terms of gas phase reactions and surface reactions for the crystallization process of silicon films on Al-coated PEN at low temperature. Acknowledgment This work is supported by the National Natural Science Foundation of China (No. 60776009). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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