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Partial crystallization of silicon by high intensity laser irradiation
Applied Surface Science 255 (2009) 9783–9786
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Partial crystallization of silicon by high intensity laser irradiation Hirozumi Azuma a,*, Akito Sagisaka b, Hiroyuki Daido b, Isao Ito a, Hiroaki Kadoura a, Nobuo Kamiya a, Tadashi Ito a, Akihiko Nishimura b, Jinglong Ma b, Michiaki Mori b, Satoshi Orimo b, Koichi Ogura b a b
TOYOTA Central Research and Development Laboratories, 41-1Yokomichi, Nagakute, Aichigun, Aichi 480-1192, Japan Advanced Photon Research Center, Japan Atomic Energy Agency, 8-1 Umemidai, Kizugawa, Kyoto 619-0215, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Commercial single crystal silicon wafers and amorphous silicon films piled on single crystal silicon wafers were irradiated with a femtosecond pulsed laser and a nanosecond pulsed laser at irradiation intensities between 1017 W/cm2 and 109 W/cm2. In the single crystal silicon substrate, the irradiated area was changed to polycrystalline silicon and the piled silicon around the irradiated area has spindly column structures constructed of polycrystalline and amorphous silicon. In particular, in the case of the higher irradiation intensity of 1016 W/cm2, the irradiated area was oriented to the same crystal direction as the substrate. In the case of the lower irradiation intensity of 108 W/cm2, only amorphous silicon was observed around the irradiated area, even when the target was single crystal silicon. In contrast, only amorphous silicon particles were found to be piled on the amorphous silicon film, irrespective of the intensity and pulse duration. Three-dimensional thermal diffusion equation for the piled particles on the substrate was solved by using the finite difference methods. The results of our heat-flow simulation of the piled particles almost agree with the experimental results. ß 2009 Elsevier B.V. All rights reserved.
Available online 24 April 2009 Keywords: Silicon High intensity laser Crystalline Amorphous
1. Introduction The recent development of femtosecond (fs) pulsed lasers expanded the possible applications of laser ablation. Drilling and cutting of transparent materials and micro-processing of silicon compounds have been reported [1–3]. It is well known that different mechanisms of pulsed laser ablation of materials are realized at different laser pulse durations [4,5]. Surface modifications under fs-pulse laser irradiation of bulk silicon have also been analyzed for laser pulse durations of 5 to 400 fs for laser energy densities (fluences) of near and below the damage threshold [6]. And also, pulse laser irradiations below the ablation threshold have been used to crystallize amorphous silicon thin film for thin film transistor and silicon solar cell [7]. While, it was reported that nanosecond (ns) pulsed laser at the high fluence induces to melt and vaporize more than ten atomic layers of silicon and to form amorphous layers on the substrates [8]. We reported the single shot creation of tadpole-like silicon particles constructed with poly-crystalline heads and amorphous tails by a high brightness fspulsed laser [9]. The structures of the silicon particles depend on the irradiation intensities and pulse durations of pulsed lasers. If
* Corresponding author. E-mail address: [email protected] (H. Azuma). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.04.143
we can pattern various structures directly on substrates by highintensity laser irradiation, more kinds of applications of the substrates become possible. For example, new detectors or sensors can be fabricated. Direct patterning of some micron size crystals and an amorphous on a crystal silicon and controlling a patterning size can be used for the wavelength selected sensor or switch. And also, by using the difference of light absorption coefficients in the wavelength between a silicon crystal and an amorphous silicon, direct patterning of a silicon crystal and an amorphous silicon can realize a high efficient solar cell. Direct patterning of silicon by different pulse duration was investigated [10]. It is difficult to obtain functional materials by only using the irradiated area and the heat effective area, because that patterning of silicon is determined by melt flow and by reaction products in the case of fs and ns laser pulse, respectively. For these reasons, patterning by using not only ablated area but also ablated particles from the target which is irradiated with a pulsed laser with fs and ns pulse duration has to be studied. In experiments, comparative studies of the crystallization of ablated particles on single crystal silicon substrates and amorphous silicon substrates under the irradiation of fs pulsed lasers and ns pulse lasers are reported. In computer simulation with three-dimensional thermal diffusion equation, the transient temperature of ablated particles with various sizes and various initial temperatures on the silicon substrate were estimated.
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H. Azuma et al. / Applied Surface Science 255 (2009) 9783–9786
2. Experimental In our experiments, commercial single crystal silicon wafers (KOMATSU) and amorphous silicon films with about 100 nm thickness deposited on the single crystal silicon wafer were used as targets. In fs-pulse laser ablation experiments, the pulsed laser of 800 nm wavelength, pulsed duration of 70 fs, repetition rate of 10 Hz, and energy of 10 mJ, delivered from the Ti:sapphire laser system (JLITE-X) was used[11]. In ns-pulsed laser ablation experiments, we used the pulsed laser with the wavelength of 532 nm, the pulse duration of 7 ns, the repetition rate of 10 Hz, and the energy of 1 J delivered from the Nd-YAG laser system (Pro-290 Spectra-Physics Co.). The fs-pulsed laser was focused 20 mm in the diameter on the target at the incident angle of 458 with off-axis parabolic mirror mounted in the vacuum chamber under 103 Pa. The ns-pulsed laser was focused 0.5 mm in diameter on the target at the normal incident in the vacuum chamber under 102 Pa with spherical lens. Each irradiated intensities of the laser beam on the target were estimated to be approximately 2 1016 W/cm2 and 7 1010 W/cm2 for the fs-pulsed laser and ns-pulsed laser, respectively. Morphology and crystallization of scattered and ablated particles piled near the irradiated area on the target and the irradiated area were observed with the scanning electron microscope (FE-SEM, JSM-890 JEOL Co.) with the secondary electron image (SEI), back-scattered electron image (BEI) and electron back-scattered diffraction (EBSD) pattern mode. 3. Results and discussion The SEI, BEI and EBSD of the irradiated area on the single crystal silicon substrate with a fs-pulsed laser are shown in Fig. 1. Fig. 1(a) shows the irradiated area was hollow like a caldera and the molten silicon blew out and piled on the outskirts. The enlarged photograph of the irradiated area is shown in Fig. 1(b). The patchy
pattern was observed in BEI at the center of the irradiated area, which shows that the irradiated area was firstly melted, solidified and poly-crystallized. The enlarged photograph of the piled silicon is shown in Fig. 1(c). The BEI and EBSD around the irradiated trace show that the structure of the piled silicon was formed by blowing out the molten silicon from the irradiated area, solidifying and crystalline the blown silicon on the single crystal substrate. Fig. 1(d) shows the EBSD of the silicon substrate. Comparing the BESD in Fig. 1(d) with the BESD in Fig. 1(c), the crystal orientations of the piled part on the substrate around of the irradiated trace are almost same as the crystal orientation of the silicone substrate. Fig. 2 shows the SEI, BEI and EBSD of the irradiated area and around the irradiated trace on the silicon with a ns-pulsed laser. A comparatively small evaporated area of silicon shown in Fig. 2(a) was observed at the center of the irradiated trace. More molten silicon blew out and piled around the irradiated trace than in the case of the fs-pulse irradiation. The crystal orientations of the piled silicon parts on the substrate around the irradiated trace are almost same as the crystal orientation of the silicon substrate in the same manner as that with a fs-pulsed laser. Fig. 2(e) shows the spindly column structures piled far from the irradiated trace. From the BEI and EBSD, the spindly column structure has some polycrystalline area and some amorphous area. Especially, polycrystalline was observed in the wide column structure. These column structures with some poly-crystalline area and some amorphous area were observed in the fs-pulsed laser irradiation. In the case of the amorphous silicon film, many small undulating were observed around the irradiated area. These undulating structures were found to be amorphous by the diffraction pattern of them. And also, almost column structures had only amorphous area. For theoretical understanding of the crystallization of the spindly column structure of the piled silicon, the thermal history of the molten silicon piled on the silicon substrate was calculated.
Fig. 1. SEM images and back-scattered electron diffraction patterns for fs-pulse laser irradiation on crystal silicon substrate, (a) SEM image of irradiation area, (b) backscattered electron image of irradiated area and diffraction pattern, (c) back-scattered electron image of around the irradiated area and diffraction pattern, and (d) diffraction pattern of silicon substrate.
H. Azuma et al. / Applied Surface Science 255 (2009) 9783–9786
9785
Fig. 2. SEM images and back-scattered electron diffraction patterns for ns-pulse laser irradiation on crystal silicon substrate, (a) SEM image of irradiated area; (b) detailed SEM image of irradiated area, (c) back-scattered electron image of irradiated area and diffraction pattern; (d) back-scattered image of piled molten silicon around the irradiated area and diffraction pattern, and (e) back-scattered image and back-scattered electron diffraction patterns of the spindly column structures piled on the substrate comparatively far from the irradiated trace.
Three-dimensional thermal diffusion equation, Eq. (1), is solved by using he finite difference method: dT ¼ gradðk grad TÞ þ Q ; (1) rC dt where r, C, T, k and Q are the density, specific heat, temperature, thermal conductivity and specific latent heat, respectively. The constant values of silicon used in this calculation are shown in Table 1 [7,12]. In our calculation model of the heat transfer, we assumed that the hemispherical molten silicon piled on a single crystal silicon and the hemispherical molten silicon was divided to 10 sections in the height and to 10 sections in the radius. The initial temperatures of the molten silicon was assumed to be 2800 8C, 2350 8C and 1900 8C, whose temperature is between the boiling point and the melting point of silicon. The heat transfer of hemispherical molten silicon at the radius of between 50 mm and 10 nm just after touching on the substrate was calculated. As an example, the thermal history of the molten silicon of 50 mm radius hemispherical at the initial temperature of 2800 8C is shown in Fig. 3. Fig. 3 shows that the surface temperature of molten silicon drops to melting point during 68 ms. The surface of substrate starts to melt after 3 ms when the molten silicon droplet touches the surface and is melting for 29 ms. The duration of surface temperature of molten silicon droplets dropping to melting point was obtained from the thermal history of the molten silicon
droplets for the various sizes and various initial temperature of molten silicon droplets. The relation between the cooling time to the melting point and the radius of the hemispherical molten silicon droplets is shown in Fig. 4. Each relationship for various initial temperature of molten silicon droplets between the cooling time to the melting point and the radius of the hemispherical molten silicon droplets was linear in both log scale. From this result, the size of molten silicon droplets is more effective to crystallization of the droplets than the initial temperature, because that the crystallization is closely related with the growing rate of a silicon crystal. Fig. 4 shows that the smaller radius of molten silicon needs the higher initial temperature for crystallization. It was reported [13] that the growing rate of silicon crystal in molten silicon near the melting point is about 2 m/s, which is added in Fig. 4. This growing rate of silicon crystal indicates molten silicon droplets more than 20 mm in radius at the initial temperature of
Table 1 Material constants of silicon. Phys. Prop.
Solid
Liquid
Unit
Density Spec. heat Conductivity Melt. point
2340 712 27.3 1683
2530 968 64 1683
kg/m3 J/kg K W/m K K
Fig. 3. Calculated thermal history of the molten silicon of 60 mm radius hemispherical at the initial temperature of 2800 8C.
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H. Azuma et al. / Applied Surface Science 255 (2009) 9783–9786
4. Conclusion By high intense laser irradiation, molten silicon droplets crystallize to the almost same orientation on the single crystal silicon substrate and the piled silicon around the irradiated area has spindly column structures constructed of polycrystalline and amorphous silicon. For fs-pulsed laser irradiation, more than 1 mm size silicon was crystallized on the single crystal silicon substrate. Lower energy of fs-pulsed laser is enough to crystallize than that of ns-pulsed laser. In the case of amorphous substrate, molten silicon is hardly crystallized on it. These results almost agree with our simulation results. Acknowledgment
Fig. 4. The relation between the cooling time to the melting point and the radius of the hemispherical molten silicon.
The authors wish to thank Dr. T. Ikuno for discussion. References
2800 8C is crystallized. The growing rate of silicon crystal on a silicon crystal substrate is expected to be higher than that in molten silicon. Consequently, silicon droplets with less than 20 mm in radius are crystallized. In our experimental result, more than 1 mm size silicon for fspulse irradiation and more than 1.5 mm size silicon for ns-pulse irradiation was crystallized on the single crystal silicon substrate. In the case of fs-pulse laser irradiation, smaller size molten silicon was crystallized than that of ns-pulse laser irradiation. These results almost agree with our simulation results for different initial temperature of molten silicon, because that the temperature of molten silicon with higher irradiation intensity is higher than that with low irradiation intensity. Weingarner et al. showed the differences of the molten silicon morphology for pulse duration of irradiated laser [10]. In our experiment and heat-flow simulation, it was shown that the crystallization of silicon depends on the pulse duration and droplet size.
[1] J. Kaspar, A. Luft, S. Nolte, M. Will, E. Beyer, J. Laser Appl. 18 (2006) 85. [2] V. Zorba, I. Alexandrau, I. Zergioti, A. Manousaki, C. Ducati, A. Nevmister, C. Fotakis, G.A.J. Amaratunga, Thin Solid Film 453 (2004) 492. [3] J. Bonse, P. Rudolph, J. Kruger, S. Baudach, W. Kautek, Appl. Surf. Sci. 154 (2000) 659. [4] J. Zhu, G. Yin, M. Zhao, D. Chen, L. Zhao, Appl. Surf. Sci. 245 (2005) 102. [5] C. Chien, M. Gupta, Appl. Phys. Mat. Sci. Pros. 81 (2005) 1257. [6] K. Sokolowski-Tinten, D. von der Lide, Phys. Rev. B 61 (2000) 2643. [7] H. Azuma, A. Takeuchi, T. Ito, H. Fukushima, T. Motohiro, M. Yamaguchi, Solar Energy Mat. Solar Cells 74 (2002) 289. [8] H.O. Jeschke, M.E. Garcia, M. Lenzner, J. Bonse, J. Krunger, W. Kautek, Appl. Surf. Sci. 197 (2002) 839. [9] H. Azuma, H. Daido, N. Kamiya, T. Ito, M. Mori, K. Ogura, A. Sagisaka, S. Orimo, A. Takeuchi, N. Suzuki, Y. Hayashi, H. Hazama, K. Matukado, A. Fukumi, Jpn. J. Appl. Phys. 43 (2004) 1172. [10] M. Weingarner, R. Elschner, O. Botanjoglo, Appl. Surf. Sci. 138–139 (1999) 499. [11] M. Mori, A. Pirozhkov, M. Nishiuchi, K. Ogura, A. Sagisaka, Y. Hayashi, S. Orimo, A. Fukumi, Z. Li, M. Kado, H. Daido, Laser Phys. 16 (2006) 1092. [12] T. Tsukada, H. Fukuyama, H. Kobatake, Int. J. Heat Mass Transfer 50 (2001) 3054. [13] J. Narayan, C.W. White, O.W. Holland, M.J. Aziz, J. Appl. Phys. 56 (1984) 2121.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Partial crystallization of silicon by high intensity laser irradiation Hirozumi Azuma a,*, Akito Sagisaka b, Hiroyuki Daido b, Isao Ito a, Hiroaki Kadoura a, Nobuo Kamiya a, Tadashi Ito a, Akihiko Nishimura b, Jinglong Ma b, Michiaki Mori b, Satoshi Orimo b, Koichi Ogura b a b
TOYOTA Central Research and Development Laboratories, 41-1Yokomichi, Nagakute, Aichigun, Aichi 480-1192, Japan Advanced Photon Research Center, Japan Atomic Energy Agency, 8-1 Umemidai, Kizugawa, Kyoto 619-0215, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Commercial single crystal silicon wafers and amorphous silicon films piled on single crystal silicon wafers were irradiated with a femtosecond pulsed laser and a nanosecond pulsed laser at irradiation intensities between 1017 W/cm2 and 109 W/cm2. In the single crystal silicon substrate, the irradiated area was changed to polycrystalline silicon and the piled silicon around the irradiated area has spindly column structures constructed of polycrystalline and amorphous silicon. In particular, in the case of the higher irradiation intensity of 1016 W/cm2, the irradiated area was oriented to the same crystal direction as the substrate. In the case of the lower irradiation intensity of 108 W/cm2, only amorphous silicon was observed around the irradiated area, even when the target was single crystal silicon. In contrast, only amorphous silicon particles were found to be piled on the amorphous silicon film, irrespective of the intensity and pulse duration. Three-dimensional thermal diffusion equation for the piled particles on the substrate was solved by using the finite difference methods. The results of our heat-flow simulation of the piled particles almost agree with the experimental results. ß 2009 Elsevier B.V. All rights reserved.
Available online 24 April 2009 Keywords: Silicon High intensity laser Crystalline Amorphous
1. Introduction The recent development of femtosecond (fs) pulsed lasers expanded the possible applications of laser ablation. Drilling and cutting of transparent materials and micro-processing of silicon compounds have been reported [1–3]. It is well known that different mechanisms of pulsed laser ablation of materials are realized at different laser pulse durations [4,5]. Surface modifications under fs-pulse laser irradiation of bulk silicon have also been analyzed for laser pulse durations of 5 to 400 fs for laser energy densities (fluences) of near and below the damage threshold [6]. And also, pulse laser irradiations below the ablation threshold have been used to crystallize amorphous silicon thin film for thin film transistor and silicon solar cell [7]. While, it was reported that nanosecond (ns) pulsed laser at the high fluence induces to melt and vaporize more than ten atomic layers of silicon and to form amorphous layers on the substrates [8]. We reported the single shot creation of tadpole-like silicon particles constructed with poly-crystalline heads and amorphous tails by a high brightness fspulsed laser [9]. The structures of the silicon particles depend on the irradiation intensities and pulse durations of pulsed lasers. If
* Corresponding author. E-mail address: [email protected] (H. Azuma). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.04.143
we can pattern various structures directly on substrates by highintensity laser irradiation, more kinds of applications of the substrates become possible. For example, new detectors or sensors can be fabricated. Direct patterning of some micron size crystals and an amorphous on a crystal silicon and controlling a patterning size can be used for the wavelength selected sensor or switch. And also, by using the difference of light absorption coefficients in the wavelength between a silicon crystal and an amorphous silicon, direct patterning of a silicon crystal and an amorphous silicon can realize a high efficient solar cell. Direct patterning of silicon by different pulse duration was investigated [10]. It is difficult to obtain functional materials by only using the irradiated area and the heat effective area, because that patterning of silicon is determined by melt flow and by reaction products in the case of fs and ns laser pulse, respectively. For these reasons, patterning by using not only ablated area but also ablated particles from the target which is irradiated with a pulsed laser with fs and ns pulse duration has to be studied. In experiments, comparative studies of the crystallization of ablated particles on single crystal silicon substrates and amorphous silicon substrates under the irradiation of fs pulsed lasers and ns pulse lasers are reported. In computer simulation with three-dimensional thermal diffusion equation, the transient temperature of ablated particles with various sizes and various initial temperatures on the silicon substrate were estimated.
9784
H. Azuma et al. / Applied Surface Science 255 (2009) 9783–9786
2. Experimental In our experiments, commercial single crystal silicon wafers (KOMATSU) and amorphous silicon films with about 100 nm thickness deposited on the single crystal silicon wafer were used as targets. In fs-pulse laser ablation experiments, the pulsed laser of 800 nm wavelength, pulsed duration of 70 fs, repetition rate of 10 Hz, and energy of 10 mJ, delivered from the Ti:sapphire laser system (JLITE-X) was used[11]. In ns-pulsed laser ablation experiments, we used the pulsed laser with the wavelength of 532 nm, the pulse duration of 7 ns, the repetition rate of 10 Hz, and the energy of 1 J delivered from the Nd-YAG laser system (Pro-290 Spectra-Physics Co.). The fs-pulsed laser was focused 20 mm in the diameter on the target at the incident angle of 458 with off-axis parabolic mirror mounted in the vacuum chamber under 103 Pa. The ns-pulsed laser was focused 0.5 mm in diameter on the target at the normal incident in the vacuum chamber under 102 Pa with spherical lens. Each irradiated intensities of the laser beam on the target were estimated to be approximately 2 1016 W/cm2 and 7 1010 W/cm2 for the fs-pulsed laser and ns-pulsed laser, respectively. Morphology and crystallization of scattered and ablated particles piled near the irradiated area on the target and the irradiated area were observed with the scanning electron microscope (FE-SEM, JSM-890 JEOL Co.) with the secondary electron image (SEI), back-scattered electron image (BEI) and electron back-scattered diffraction (EBSD) pattern mode. 3. Results and discussion The SEI, BEI and EBSD of the irradiated area on the single crystal silicon substrate with a fs-pulsed laser are shown in Fig. 1. Fig. 1(a) shows the irradiated area was hollow like a caldera and the molten silicon blew out and piled on the outskirts. The enlarged photograph of the irradiated area is shown in Fig. 1(b). The patchy
pattern was observed in BEI at the center of the irradiated area, which shows that the irradiated area was firstly melted, solidified and poly-crystallized. The enlarged photograph of the piled silicon is shown in Fig. 1(c). The BEI and EBSD around the irradiated trace show that the structure of the piled silicon was formed by blowing out the molten silicon from the irradiated area, solidifying and crystalline the blown silicon on the single crystal substrate. Fig. 1(d) shows the EBSD of the silicon substrate. Comparing the BESD in Fig. 1(d) with the BESD in Fig. 1(c), the crystal orientations of the piled part on the substrate around of the irradiated trace are almost same as the crystal orientation of the silicone substrate. Fig. 2 shows the SEI, BEI and EBSD of the irradiated area and around the irradiated trace on the silicon with a ns-pulsed laser. A comparatively small evaporated area of silicon shown in Fig. 2(a) was observed at the center of the irradiated trace. More molten silicon blew out and piled around the irradiated trace than in the case of the fs-pulse irradiation. The crystal orientations of the piled silicon parts on the substrate around the irradiated trace are almost same as the crystal orientation of the silicon substrate in the same manner as that with a fs-pulsed laser. Fig. 2(e) shows the spindly column structures piled far from the irradiated trace. From the BEI and EBSD, the spindly column structure has some polycrystalline area and some amorphous area. Especially, polycrystalline was observed in the wide column structure. These column structures with some poly-crystalline area and some amorphous area were observed in the fs-pulsed laser irradiation. In the case of the amorphous silicon film, many small undulating were observed around the irradiated area. These undulating structures were found to be amorphous by the diffraction pattern of them. And also, almost column structures had only amorphous area. For theoretical understanding of the crystallization of the spindly column structure of the piled silicon, the thermal history of the molten silicon piled on the silicon substrate was calculated.
Fig. 1. SEM images and back-scattered electron diffraction patterns for fs-pulse laser irradiation on crystal silicon substrate, (a) SEM image of irradiation area, (b) backscattered electron image of irradiated area and diffraction pattern, (c) back-scattered electron image of around the irradiated area and diffraction pattern, and (d) diffraction pattern of silicon substrate.
H. Azuma et al. / Applied Surface Science 255 (2009) 9783–9786
9785
Fig. 2. SEM images and back-scattered electron diffraction patterns for ns-pulse laser irradiation on crystal silicon substrate, (a) SEM image of irradiated area; (b) detailed SEM image of irradiated area, (c) back-scattered electron image of irradiated area and diffraction pattern; (d) back-scattered image of piled molten silicon around the irradiated area and diffraction pattern, and (e) back-scattered image and back-scattered electron diffraction patterns of the spindly column structures piled on the substrate comparatively far from the irradiated trace.
Three-dimensional thermal diffusion equation, Eq. (1), is solved by using he finite difference method: dT ¼ gradðk grad TÞ þ Q ; (1) rC dt where r, C, T, k and Q are the density, specific heat, temperature, thermal conductivity and specific latent heat, respectively. The constant values of silicon used in this calculation are shown in Table 1 [7,12]. In our calculation model of the heat transfer, we assumed that the hemispherical molten silicon piled on a single crystal silicon and the hemispherical molten silicon was divided to 10 sections in the height and to 10 sections in the radius. The initial temperatures of the molten silicon was assumed to be 2800 8C, 2350 8C and 1900 8C, whose temperature is between the boiling point and the melting point of silicon. The heat transfer of hemispherical molten silicon at the radius of between 50 mm and 10 nm just after touching on the substrate was calculated. As an example, the thermal history of the molten silicon of 50 mm radius hemispherical at the initial temperature of 2800 8C is shown in Fig. 3. Fig. 3 shows that the surface temperature of molten silicon drops to melting point during 68 ms. The surface of substrate starts to melt after 3 ms when the molten silicon droplet touches the surface and is melting for 29 ms. The duration of surface temperature of molten silicon droplets dropping to melting point was obtained from the thermal history of the molten silicon
droplets for the various sizes and various initial temperature of molten silicon droplets. The relation between the cooling time to the melting point and the radius of the hemispherical molten silicon droplets is shown in Fig. 4. Each relationship for various initial temperature of molten silicon droplets between the cooling time to the melting point and the radius of the hemispherical molten silicon droplets was linear in both log scale. From this result, the size of molten silicon droplets is more effective to crystallization of the droplets than the initial temperature, because that the crystallization is closely related with the growing rate of a silicon crystal. Fig. 4 shows that the smaller radius of molten silicon needs the higher initial temperature for crystallization. It was reported [13] that the growing rate of silicon crystal in molten silicon near the melting point is about 2 m/s, which is added in Fig. 4. This growing rate of silicon crystal indicates molten silicon droplets more than 20 mm in radius at the initial temperature of
Table 1 Material constants of silicon. Phys. Prop.
Solid
Liquid
Unit
Density Spec. heat Conductivity Melt. point
2340 712 27.3 1683
2530 968 64 1683
kg/m3 J/kg K W/m K K
Fig. 3. Calculated thermal history of the molten silicon of 60 mm radius hemispherical at the initial temperature of 2800 8C.
9786
H. Azuma et al. / Applied Surface Science 255 (2009) 9783–9786
4. Conclusion By high intense laser irradiation, molten silicon droplets crystallize to the almost same orientation on the single crystal silicon substrate and the piled silicon around the irradiated area has spindly column structures constructed of polycrystalline and amorphous silicon. For fs-pulsed laser irradiation, more than 1 mm size silicon was crystallized on the single crystal silicon substrate. Lower energy of fs-pulsed laser is enough to crystallize than that of ns-pulsed laser. In the case of amorphous substrate, molten silicon is hardly crystallized on it. These results almost agree with our simulation results. Acknowledgment
Fig. 4. The relation between the cooling time to the melting point and the radius of the hemispherical molten silicon.
The authors wish to thank Dr. T. Ikuno for discussion. References
2800 8C is crystallized. The growing rate of silicon crystal on a silicon crystal substrate is expected to be higher than that in molten silicon. Consequently, silicon droplets with less than 20 mm in radius are crystallized. In our experimental result, more than 1 mm size silicon for fspulse irradiation and more than 1.5 mm size silicon for ns-pulse irradiation was crystallized on the single crystal silicon substrate. In the case of fs-pulse laser irradiation, smaller size molten silicon was crystallized than that of ns-pulse laser irradiation. These results almost agree with our simulation results for different initial temperature of molten silicon, because that the temperature of molten silicon with higher irradiation intensity is higher than that with low irradiation intensity. Weingarner et al. showed the differences of the molten silicon morphology for pulse duration of irradiated laser [10]. In our experiment and heat-flow simulation, it was shown that the crystallization of silicon depends on the pulse duration and droplet size.
[1] J. Kaspar, A. Luft, S. Nolte, M. Will, E. Beyer, J. Laser Appl. 18 (2006) 85. [2] V. Zorba, I. Alexandrau, I. Zergioti, A. Manousaki, C. Ducati, A. Nevmister, C. Fotakis, G.A.J. Amaratunga, Thin Solid Film 453 (2004) 492. [3] J. Bonse, P. Rudolph, J. Kruger, S. Baudach, W. Kautek, Appl. Surf. Sci. 154 (2000) 659. [4] J. Zhu, G. Yin, M. Zhao, D. Chen, L. Zhao, Appl. Surf. Sci. 245 (2005) 102. [5] C. Chien, M. Gupta, Appl. Phys. Mat. Sci. Pros. 81 (2005) 1257. [6] K. Sokolowski-Tinten, D. von der Lide, Phys. Rev. B 61 (2000) 2643. [7] H. Azuma, A. Takeuchi, T. Ito, H. Fukushima, T. Motohiro, M. Yamaguchi, Solar Energy Mat. Solar Cells 74 (2002) 289. [8] H.O. Jeschke, M.E. Garcia, M. Lenzner, J. Bonse, J. Krunger, W. Kautek, Appl. Surf. Sci. 197 (2002) 839. [9] H. Azuma, H. Daido, N. Kamiya, T. Ito, M. Mori, K. Ogura, A. Sagisaka, S. Orimo, A. Takeuchi, N. Suzuki, Y. Hayashi, H. Hazama, K. Matukado, A. Fukumi, Jpn. J. Appl. Phys. 43 (2004) 1172. [10] M. Weingarner, R. Elschner, O. Botanjoglo, Appl. Surf. Sci. 138–139 (1999) 499. [11] M. Mori, A. Pirozhkov, M. Nishiuchi, K. Ogura, A. Sagisaka, Y. Hayashi, S. Orimo, A. Fukumi, Z. Li, M. Kado, H. Daido, Laser Phys. 16 (2006) 1092. [12] T. Tsukada, H. Fukuyama, H. Kobatake, Int. J. Heat Mass Transfer 50 (2001) 3054. [13] J. Narayan, C.W. White, O.W. Holland, M.J. Aziz, J. Appl. Phys. 56 (1984) 2121.