- Email: [email protected]
Sintering effect on crystallite size, hydrogen bond structure and morphology of the silane-derived silicon powders
Powder Technology 273 (2015) 40–46
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Sintering effect on crystallite size, hydrogen bond structure and morphology of the silane-derived silicon powders Si-Si Liu, Hui Li, Wen-De Xiao ⁎ Department of Chemical Engineering, Shanghai Jiao Tong University, 800 Dong-Chuan Road, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 3 June 2014 Received in revised form 21 November 2014 Accepted 5 December 2014 Available online 15 December 2014 Keywords: Silicon powder Crystallite size Hydrogen bond structure Silane pyrolysis Sintering
a b s t r a c t The effect of sintering treatment on the amorphous silicon powders from monosilane pyrolysis was investigated to enhance polysilicon yield in the FBR-CVD process. The crystallite size, hydrogen bond structure and morphology of the powders were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Diffuse Reflection Fourier Transform Infrared Spectroscopy (DR-FT-IR) and Zeta Potential Analyzer (ZPA). The results showed that the higher sintering temperature and lower pressure were more favorable to the growth of silicon crystallites and the liberation of hydrogen from silicon hydrides. The crystallite size increased significantly with a critical low FWHM when the sintering temperature was at 750 °C, and the hydrogen releasing from polysilanes and OSiH took place remarkably with a relative flat IR spectrum when the vacuum condition was implemented. Moreover, the particle aggregation was strengthened with broad particle size distribution as long sintering duration or high temperature applied, and the fusion occurred as the temperature was high enough but still with good crystallinity. © 2014 Elsevier B.V. All rights reserved.
1. Introduction One of the most important issues for polysilicon production by chemical vapor decomposition in fluidized bed reactor (CVD-FBR) is the fine particle formation from silane vapor decomposition, which causes major negative impacts on product yield and purity and is always being a remarkable concern [1]. Many studies have been carried out to explore the detailed mechanism of silane pyrolysis. Thanks to the technology of laser intracavity absorption spectroscopy [2–5], which proves the existence of silylene (SiH2) in the gaseous products, the formation of SiH2 and H2 is believed to be the critical initial step of silane decomposition. Yuuki [6], Guinta [7], Swihart et al. and Swihart and Girshick [8,9] further propose that silane pyrolysis is mostly similar to the polymerization process with bivalent silylenes as active species, which includes the reactions of polysilane growth, hydrogen elimination, silylene isomerization and cyclization, and the heavy silicon hydrides (Si atom ≥ 10) are responsible for the powders. Beyond that, Girshick et al. and Talukdar and Swihart [10,11] also develop an aerosol dynamic model to simulate the process of silane pyrolysis considering nucleation, growth and aggregation of powders. In the experimental aspect, Onischuk et al. [12–14] make a series of characterization studies on silane vapor decomposition, which reveals three types of hydrogen bond structure containing in the silicon hydrides: poly-(SiH2)n (p-(SiH2)n), cluster-SiH (c-SiH) and isolated-SiH ⁎ Corresponding author. Tel.: +86 21 34203788. E-mail addresses: [email protected] (S.-S. Liu), [email protected] (H. Li), [email protected] (W.-D. Xiao).
http://dx.doi.org/10.1016/j.powtec.2014.12.016 0032-5910/© 2014 Elsevier B.V. All rights reserved.
(iso-SiH). Odden et al. [15], Slootman and Parent [16] and Wigger et al. [17] study the impact of operating conditions on silicon powder and find the independence of powder size on operating pressure and the inhibition of hydrogen as diluted gas on the reaction rate. While these results mentioned above provide us a profound understanding on the mechanism of silane thermal decomposition, an effective way is still unable to be figured out for fine suppression due to the complicated flow behavior in a FBR. Therefore, more detailed studies are imperatively required on the property modification of silicon powders for reutilization. Sintering is a typical treatment to modify the particle behavior at a sufficient high temperature, and frequently employed in the fields of ceramic powder synthesis, granulation process, semiconductor production and industrial catalyst manufacture [18–20]. Bellanger et al. [21] apply a two-step process (sintering and re-crystallization) to produce silicon wafers with large polysilicon grains; Wolf [22] focuses on improving the efficiency of sintered porous silicon for thin-film solar cells. Zbib et al. [23] anneal the amorphous nanopowders during the FBR-CVD process and find that the crystal transition temperature for these powders is about 650 °C by means of TEM and DSC. These studies give a great inspiration that can help to modify the crystallinity of silicon powders through sintering treatment. In the present work, we investigated the possibility of sintering treatment on modifying the properties of silicon powders prepared by monosilane pyrolysis in a quartz tube reactor, with a particular focus on the sintering conditions of temperature, duration and pressure. The as-prepared samples and the sintered powders were characterized by X-ray Diffraction (XRD), Scan Electron Microscopy (SEM), Diffuse
S.-S. Liu et al. / Powder Technology 273 (2015) 40–46
Reflection Fourier Transform Infrared Radiation (DR-FT-IR) and Zeta Potential Analyzer (ZPA) for crystal pattern, morphology, hydrogen bond structure and powder size distribution (PSD), respectively. 2. Experimental section 2.1. Materials and equipment The silicon powders were obtained by silane thermal decomposition using high purity nitrogen (99.999%) as diluted gas in a horizontal quartz tube reactor (I.D. = 0.02 m, H = 1 m) equipped within high temperature furnace (SK-G06163, Tianjin Zhonghuan Experimental Furnace Co., Ltd.). The detailed schematic of the experimental setup was displayed in Fig. 1, in which flow meters were applied for gas flux control, a thick layer of insulation cotton for temperature maintenance, a vacuum pump at the entrance of furnace for vacuum operation and a GC analyzer for the inlet and out gas compositions. Based on the temperature calibration, the temperature in the middle of the tube reactor, corresponding to the electric heating zone, was basically the same as the set one. Therefore the silicon powders in the middle of the tube reactor were collected as samples for further sintering and characterization. 2.2. General procedure The preparation procedure was generally described as follows. Before each experiment, due to the flammability of silane, a vacuum leak test was carefully carried out for the whole system followed by the blowing of the system with high purity nitrogen for oxygen replacement. Then the tube reactor was heated to a set value with the flow of nitrogen in a programmed way. When the temperature became steady, the flow rate of nitrogen was adjusted to a desired value and silane was gradually introduced. The silane reaction was carried out for 30 min with enough amount of powders for characterization and the exhausted gas passed through a lye seal vessel. When finished, the system was naturally cooled to the room temperature by nitrogen and the products were obtained and preserved in a drying oven. As for sintering, the obtained samples were put in a corundum crucible, placed in the middle of the furnace and sintered under nitrogen atmosphere or vacuum condition after getting rid of oxygen. The sintering treatment continued 30 min for all the cases unless specified otherwise and a vacuum condition of about 20 Torr, which may promote the densification of composites [24,25], was also applied. 2.3. Characterization The crystal structure of the as-prepared and sintered powders was determined by XRD measurement (D8 Advance, Bruker Co., Germany)
41
using Cu-Kα radiation (λ = 0.15404 nm) at the scan rate of 4°/min in the 2θ range of 20° to 60°. Hydrogen bond structures on the powder surface were analyzed by DR-FT-IR spectrometry (PerkinElmer Inc., Shanghai) in the absorption range of 500–4000 cm−1. The morphology of the powders treated by gold sputter coating was observed with SEM (Nova NanoSEM 450, FEI). PSD was obtained by ZPA (NANO-ZS90, Malvern Instruments Ltd., UK) after several measurements on average.
3. Results and discussions 3.1. Characterization on sample powders The silicon powders, obtained by silane pyrolysis at different temperatures as shown in Table 1 are characterized as follows. The crystal structures of the powders as XRD analysis are shown in Fig. 2. It is found that the basic diffraction peaks at 2θ = 28°, 47° and 56°, corresponding to the reflections from the lattice planes (111), (220) and (311) of typical polycrystalline silicon [26,27], can be distinguished as the operation temperature reaches 700 °C. Besides, the silicon powders are all amorphous dominated without obvious characteristic peaks. The DR-FT-IR analyses of the bond structures on the powder surface are shown in Fig. 3 in which several absorption bands due to the vibration signatures of Si–H and Si–O are observed: 600 cm− 1 (①), 900 cm− 1 (②), 1000–1200 cm− 1 (③), 1900–2000 cm− 1 (④), 2100 cm− 1 (⑤), 2200–2300 cm− 1 (⑥), 2900–3000 cm− 1 (⑦) and 1400–1500 cm−1 (⑧). Combining with the previous studies by Wigger [17] and Onischuk [13,14] as summarized in Table 2, it can be concluded that bands ③ and ⑥ are corresponding to the vibrations of SiOSi and OSiH due to inevitable air exposure during operation; the peak at position ① is attributed to the vibration of p-(SiH2)n,the intensity of which decreases with raising temperature due to hydrogen releasing with low activation energy; the peak at position ② is attributed to Si–H vibrations, which is probably formed from the hydrogen-released p-(SiH2)n and becomes obvious as the temperature over 650 °C; the peak at position ④ is attributed to the vibration of c-SiH, the intensity of which also greatly weakens with the increase in temperature; the peak at position ⑤ is attributed to the vibrations of p-(SiH2)n and iso-SiH the latter of which dominates as the temperature over 650 °C; and bands at ⑦ and ⑧ are probably attributed to the ring structure of silicon hydrides or the other impurity disturbances, which exist strongly at low temperature (600 °C). The above analyses on the Si–H bond structures at positions ①, ④ and ⑤ are in accord with those of Onischuk's [13,14], and the one at position ② which appears at relative high temperature (700 °C) agrees with those of Wigger's [17]. The amorphous silicon powders obtained at 650 °C were applied as samples for further sintering which can be regarded as fines formed in granular polysilicon production [28,29]. A further morphology
Fig. 1. Schematic diagram of experimental setup.
42
S.-S. Liu et al. / Powder Technology 273 (2015) 40–46
Table 1 Preparation conditions of silicon powder samples. Parameter
Value
Parameter
Value
Pressure (pa) Flow flux (SLM) Temperature (°C)
101,325 0.3 600, 610, 620, 650, 700
Silane mole fraction Reaction duration (min)
0.1 30
observation on the sample powders is shown in Fig. 4, in which the agglomeration phenomenon is presented with the primary powder size of 440 nm on average.
3.2. Sintering conditions on crystallite size Fig. 5a displays the XRD patterns of the sample powders under different sintering temperatures (700 °C, 750 °C, 800 °C and 900 °C). It is found that the peak intensities of lattice planes (111), (220) and (311) are gradually enhanced by raising sintering temperature, with the most intense peak of plane (111) reflection. Moreover, Fig. 5c presents the variation of full width at half maximum (FWHM) for plane (111), in which the FWHM decreases with the increasing sintering temperature, equivalent to the growth of crystallite size following Sherrer equation. The effects of sintering duration and vacuum condition on the XRD pattern of the sample powders and the FWHM for plane (111) are given in Fig. 5b and d, respectively. The results show that, besides the three strong characteristic diffraction peaks in all the cases, the narrowest FWHM is obtained under the combined condition of long sintering duration and vacuum operation, which indicates the most favor of the growth of crystal silicon grains. As to the two weak peaks near 2θ = 43°, they are probably caused by the existence of the impurity such as silicon dioxide. Based on the results above, two higher temperatures (900 °C and 1200 °C) with long sintering duration and vacuum condition were applied to investigate the variation of crystallite size. In the case of 1200 °C, close to silicon melting point, the typical XRD result in Fig. 6a presents that the diffraction peaks are obviously weakened due to the fused samples with irregular shape as shown in Fig. 12b. However, according to the normalized diffraction peaks areas in Fig. 6b, it is found
Fig. 3. DR-FT-IR analysis on the bond structure of the silicon powders obtained under different temperatures.
that the crystallinity of the sample powders obtained at 1200 °C is as high as the one obtained at 900 °C.
3.3. Sintering conditions on Si–H bond structure The existence of hydrogen is a key factor influencing the crystallinity of silicon powders from silane gaseous decomposition. Fig. 7 illustrates the effect of sintering temperature on the Si–H bond structures of the sample powders. One can notice that the vibrations of p-(SiH2)n and c-SiH at 2000 cm−1 (④) and 650 cm−1 (①) are gradually weakened with the rising sintering temperature, and the vibration peak at 2100 cm−1 (⑤) due to p-(SiH2)n and iso-SiH almost disappears in all the cases. Concurrently, the intensity of absorption peak at 950 cm−1 (②) contributed by the vibration of Si–H is gradually strengthened. These results further confirm that the hydrogen in the bond of p-
Table 2 FT-IR analysis on the bond structure of silicon powders from silane pyrolysis. Band (cm−1)
Fig. 2. XRD diagram of the silicon powders obtained under different temperatures.
① ② ③ ④ ⑤ ⑥
500–800 800–1000 1000–1250 2000 2100 2170–2300
Wigger [17] Si–H SiOSi Si–H OSiH
Onischuk [13,14]
present work
p-(SiH2)n + Si–H p-(SiH2)n SiOSi c-SiH p-(SiH2)n + iso-SiH OSiH
p-(SiH2)n Si–H SiOSi c-SiH p-(SiH2)n + iso-SiH OSiH
S.-S. Liu et al. / Powder Technology 273 (2015) 40–46
Fig. 4. SEM image of the sample powders obtained at 650 °C.
43
(SiH2)n is much easier to be released as higher sintering temperature applied, which results in the increasing amount of Si–H in the powders. Besides, higher sintering temperature also seems to benefit the hydrogen releasing from OSiH by the decreasing peak intensity at 2300 cm−1 (⑥). The typical FT-IR analyses on the Si–H bond structures of the sample powders under long duration and vacuum condition are presented in Fig. 8, respectively. The results show that much more p-(SiH2)n are formed in the case of 750 °C–2 h with the stronger absorption peak at position ①, besides no big difference is observed on the other absorption bands. However, as the vacuum condition is applied, hydrogen is obviously released from the bond structure of p-(SiH2)n (①) and OSiH (⑥) [30] with an intense vibration of Si–H at 850 cm−1 (②) as a consequence. The results suggest that the vacuum condition has a remarkable effect on the liberation of hydrogen for improving the crystallinity of the amorphous sample powders.
Fig. 5. XRD diagrams of the samples sintered under different conditions with the FWHM for lattice plane (111) (a) (c) different sintering temperatures (b) (d) long sintering duration and vacuum condition.
44
S.-S. Liu et al. / Powder Technology 273 (2015) 40–46
Fig. 7. DR-FT-IR analysis on the bond structure of the sintered samples at different temperatures.
Fig. 6. XRD diagram of the sintered samples at two higher sintering temperatures with normalized diffraction peak areas.
A further investigation on the Si–H bond structure at two higher sintering temperatures is shown in Fig. 9. It is found that the vibrations of Si–H and Si–O are all weakened as the sintering temperature is close to the melting point, especially at 1200 cm− 1 (③) contributed to the vibration of SiOSi. Besides, an extra peak at 1650 cm− 1 (⑨) is also observed, probably caused by the existence of HOH in the gaseous phase.
3.4. Sintering conditions on morphology Fig. 10 displays the surface morphologies of the sintered powders at different temperatures with the insets of PSD by a ZPA analyzer. It is observed that the powder particles are basically spherical with smooth edges in all the cases, but a distinct increase on the mean powder diameter is obtained at the sintering temperature of 750 °C (928 nm). Besides, the PSD results also show that a broad PSD with relative large powders over 4 μm is formed as the sintering temperature over 750 °C.
Fig. 8. DR-FT-IR analysis on the bond structure of the sintered samples under long duration and vacuum condition.
S.-S. Liu et al. / Powder Technology 273 (2015) 40–46
45
Fig. 11a shows the morphology and PSD of the sintered powders under a long duration. Compared with the result in the case of 750 °C–0.5 h (Fig. 10), the mean powder size after the long sintering treatment is slightly smaller (862 nm) than that after the short one (928 nm), but with the appearance of broad PSD. The result suggests that prolonging the sintering duration increases both the aggregation and breakage effects of powder particles. As the vacuum condition is applied, the chain-like aggregates are formed with uniform powder size as shown in Fig. 11b, while the powders obtained under atmospheric condition cluster into the mass. Fig. 12 represents the SEM images and PSDs of the sample powders at two higher sintering temperatures. It can be seen that the large powder particles fuse into a big mass at 1200 °C and the particle edges can be hardly distinguished. As the sintering temperature is at 900 °C, the spherical particles still can be observed but the aggregation effect is enhanced with the formation of large particles. It is also noticed that the powder particles obtained under vacuum condition at 900 °C is with much smaller (819 nm) mean powder size than that in Fig. 10d (1023 nm), which is mostly contributed to the compact of silicon powders by releasing hydrogen from p-(SiH2)n under vacuum condition.
4. Conclusion
Fig. 9. DR-FT-IR analysis on the bond structure of the sintered samples at two higher sintering temperatures.
In the present work, we studied the effect of the sintering treatment on crystallite size, hydrogen bond structure and morphology of the silicon powders from silane thermal decomposition with the aim of reutilizing the by-product fine powders during the polysilicon FBR-CVD production. The sintered silicon powders under different temperature, duration and pressure condition were characterized by XRD, SEM, DRFT-IR and ZPA methods. The main results can be concluded as the following. The higher temperature, longer duration and lower pressure are an advantage to the growth of the polycrystalline silicon grains, with a critical low FWHM at the sintering temperature of 750 °C and the releasing of hydrogen from silicon hydrides under vacuum condition. The ZPA analyses show
Fig. 10. SEM images of the sintered samples at different temperatures with the inset of PSD (a) 700 °C (b) 750 °C (c) 800 °C and (d) 900 °C.
46
S.-S. Liu et al. / Powder Technology 273 (2015) 40–46
References
Fig. 11. SEM images of the sintered samples with the inset of PSD under (a) atmospheric and (b) vacuumed.
that a broad PSD with large powder particles is formed at high sintering temperature or under long duration mostly due to the enhancement of aggregation effect. Moreover, as the sintering temperature is increased close to silicon melting point, the powders begin to fuse into a big mass but still with high crystallinity.
Fig. 12. SEM images of the sintered samples at two higher sintering temperatures (a) 900 °C with the inset of PSD and (b) 1200 °C.
[1] W.O. Filtvedt, M. Javidi, A. Holt, M.C. Malaaen, Development of fluidized bed reactors for silicon production, Sol. Energy Mater. Sol. Cells 94 (2010) 1980–1995. [2] R.T. White, R.L. Espino-Rios, D.S. Rogers, Mechanism of the silane decomposition. I. Silane loss kinetics and rate inhibition by hydrogen. II. Modeling of the silane decomposition (all stages of reactor), In. J. Chem. Kinet. 17 (1985) 1029–1065. [3] M.A. Ring, M.J. Punentes, H.E. O'Neal, Pyrolysis of silane decomposition, Proceedings of the Flat Plate Solar Array Project Workshop on the Science of Silicon, Material Preparation. DOE/JPL. 1021-81, 1983, pp. 63–75. [4] P. Ho, W.G. Breiland, Observation of Si2 in a chemical vapor deposition reactor by laser excited fluorescence, Appl. Phys. Lett. 11 (1984) 51–53. [5] J.J. O'Brien, G.H. Atkinson, Role of silylene in the pyrolysis of silane and organosilanes, J. Phys. Chem. 92 (1988) 5782–5787. [6] A. Yuuki, Y. Matsui, K. Tachibana, A numerical study on gaseous reactions in silane pyrolysis, Jpn. J. Appl. Phys. 26 (1987) 747–752. [7] C.J. Guinta, R.J. McCurdy, J.D. ChappleSokol, Gas phase kinetics in the atmospheric pressure chemical vapor deposition of silicon form silane and disilane, J. Appl. Phys. 67 (1990) 1062–1075. [8] M.T. Swihart, S. Nijhawan, M.R. Mahajan, Modeling the nucleation kinetics and aerosol dynamics of particle formation during CVD of silicon silane, J. Aerosol Sci. 29 (1998) S79–S80. [9] M.T. Swihart, S.L. Girshick, Thermochemistry and kinetics of silicon hydride cluster formation during thermal decomposition of silane, J. Phys. Chem. 103 (1999) 64–76. [10] S. Girshick, M. Swihart, S. Suh, Numerical modeling of gas-phase nucleation and particle growth during chemical vapor deposition of silicon, J. Electrochem. Soc. 147 (2000) 2303–2311. [11] S.S. Talukdar, M.T. Swihart, Aerosol dynamics modeling of silicon nanoparticles formation during silane pyrolysis: a comparison of three solution methods, J. Aerosol Sci. 35 (2004) 889–908. [12] A.A. Onischuk, V.P. Strunin, M.A. Ushakova, Analysis of hydrogen in aerosol particles of a-Si:H forming during the pyrolysis of silane, Phys. Status Solidi B 186 (1994) 43–55. [13] A.A. Onischuk, V.P. Strunin, M.A. Ushakova, On the pathways of aerosol formation by thermal decomposition of silane, J. Aerosol Sci. 28 (1997) 207–222. [14] A.A. Onischuk, V.P. Strunin, R.I. Samoilova, Chemical composition and bond structure of aerosol particles of amorphous hydrogenated silicon forming from thermal decomposition of silane, J. Aerosol Sci. 28 (1997) 1425–1441. [15] J.O. Odden, P.K. Egeberg, A. Kjekshus, From monosilane to crystalline silicon, part i: decomposition of monosilane at 690–830 K and initial pressure 0.1–6.6 Mpa in a free-space reactor, Sol. Energy Mater. Sol. Cells 86 (2005) 165–176. [16] F. Slootman, J. Parent, Homogenous gas-phase nucleation in silane pyrolysis, J. Aerosol Sci. 25 (1994) 15–21. [17] H. Wigger, R. Starke, P. Roth, Silicon particle formation by pyrolysis of silane in a hot wall gas phase reactor, Chem. Eng. Technol. 24 (2001) 261–264. [18] L. Olmos, C.L. Martin, D. Bouvard, Sintering of mixtures of powders: experiments and modeling, Powder Technol. 190 (2009) 134–140. [19] E.M. Larsson, J. Millet, S. Gustafsson, M. Skoglundh, Real time indirect nanoplasmonic in situ spectroscopy of catalyst nanoparticles sintering, Catalysis 2 (2012) 238–245. [20] M.S. Afarani, A. Samimi, E.B. Yekta, Synthesis of alumina granules by high shear mixer granulator: processing and sintering, Powder Technol. 237 (2013) 32–40. [21] P. Bellanger, M. Grau, A. Sow, A. Kaminski, D. Blangis, Multicrystalline silicon wafers prepared by sintering of silicon bed powders and re-crystallization using ZMR, 24th European Photovoltaic Solar Energy Conference. 2009, Hamburg, Germany, 2009. [22] A.K. Wolf, Sintered porous silicon — physical properties and applications for layertransfer silicon thin-film solar cells(Thesis) Gottfried Wilhelm Leibniz Universitat Hannover, Germany, 2007. [23] M.B. Zbib, M.M. Dahl, U. Sahaym, Characterization of granular silicon, powders, and agglomerates from a fluidized bed reactor, J. Mater. Sci. 47 (2012) 2583–2590. [24] E.B. Pickens, R.W. Trice, Pressureless sintering of silicon nitride/boron nitride laminate composites, J. Mater. Sci. 40 (2005) 2101–2103. [25] J. Choi, H. Lyu, W. Lee, J. Lee, Densification and microstructural development during sintering of powder injection molded Fe micro-nanopowder, Powder Technol. 253 (2014) 596–601. [26] S.H. Won, J.H. Youn, J. Jang, Study of polycrystalline silicon films deposited by inductively coupled plasma chemical vapor deposition, J. Korean Phys. Soc. 39 (2001) 123–126. [27] J.B. Dorhout, Characterization of polycrystalline silicon films grown by LPCVD of silane(Thesis) Iowa State University, USA, 2006. [28] M.P. Tejero-Ezpeleta, S. Buchholz, L. Mleczko, Optimization of reaction condition in a fluidized-bed for silane pyrolysis, Can. J. Chem. Eng. 82 (2004) 520–529. [29] G. Hsu, N. Rohatgi, J. Houseman, Silicon particle growth in a fluidized-bed reactor, AICHE J. 33 (1987) 784–791. [30] W.S. Coblenza, The physics and chemistry of the sintering of silicon, J. Mater. Sci. 25 (1990) 2754–2764.
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Sintering effect on crystallite size, hydrogen bond structure and morphology of the silane-derived silicon powders Si-Si Liu, Hui Li, Wen-De Xiao ⁎ Department of Chemical Engineering, Shanghai Jiao Tong University, 800 Dong-Chuan Road, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 3 June 2014 Received in revised form 21 November 2014 Accepted 5 December 2014 Available online 15 December 2014 Keywords: Silicon powder Crystallite size Hydrogen bond structure Silane pyrolysis Sintering
a b s t r a c t The effect of sintering treatment on the amorphous silicon powders from monosilane pyrolysis was investigated to enhance polysilicon yield in the FBR-CVD process. The crystallite size, hydrogen bond structure and morphology of the powders were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Diffuse Reflection Fourier Transform Infrared Spectroscopy (DR-FT-IR) and Zeta Potential Analyzer (ZPA). The results showed that the higher sintering temperature and lower pressure were more favorable to the growth of silicon crystallites and the liberation of hydrogen from silicon hydrides. The crystallite size increased significantly with a critical low FWHM when the sintering temperature was at 750 °C, and the hydrogen releasing from polysilanes and OSiH took place remarkably with a relative flat IR spectrum when the vacuum condition was implemented. Moreover, the particle aggregation was strengthened with broad particle size distribution as long sintering duration or high temperature applied, and the fusion occurred as the temperature was high enough but still with good crystallinity. © 2014 Elsevier B.V. All rights reserved.
1. Introduction One of the most important issues for polysilicon production by chemical vapor decomposition in fluidized bed reactor (CVD-FBR) is the fine particle formation from silane vapor decomposition, which causes major negative impacts on product yield and purity and is always being a remarkable concern [1]. Many studies have been carried out to explore the detailed mechanism of silane pyrolysis. Thanks to the technology of laser intracavity absorption spectroscopy [2–5], which proves the existence of silylene (SiH2) in the gaseous products, the formation of SiH2 and H2 is believed to be the critical initial step of silane decomposition. Yuuki [6], Guinta [7], Swihart et al. and Swihart and Girshick [8,9] further propose that silane pyrolysis is mostly similar to the polymerization process with bivalent silylenes as active species, which includes the reactions of polysilane growth, hydrogen elimination, silylene isomerization and cyclization, and the heavy silicon hydrides (Si atom ≥ 10) are responsible for the powders. Beyond that, Girshick et al. and Talukdar and Swihart [10,11] also develop an aerosol dynamic model to simulate the process of silane pyrolysis considering nucleation, growth and aggregation of powders. In the experimental aspect, Onischuk et al. [12–14] make a series of characterization studies on silane vapor decomposition, which reveals three types of hydrogen bond structure containing in the silicon hydrides: poly-(SiH2)n (p-(SiH2)n), cluster-SiH (c-SiH) and isolated-SiH ⁎ Corresponding author. Tel.: +86 21 34203788. E-mail addresses: [email protected] (S.-S. Liu), [email protected] (H. Li), [email protected] (W.-D. Xiao).
http://dx.doi.org/10.1016/j.powtec.2014.12.016 0032-5910/© 2014 Elsevier B.V. All rights reserved.
(iso-SiH). Odden et al. [15], Slootman and Parent [16] and Wigger et al. [17] study the impact of operating conditions on silicon powder and find the independence of powder size on operating pressure and the inhibition of hydrogen as diluted gas on the reaction rate. While these results mentioned above provide us a profound understanding on the mechanism of silane thermal decomposition, an effective way is still unable to be figured out for fine suppression due to the complicated flow behavior in a FBR. Therefore, more detailed studies are imperatively required on the property modification of silicon powders for reutilization. Sintering is a typical treatment to modify the particle behavior at a sufficient high temperature, and frequently employed in the fields of ceramic powder synthesis, granulation process, semiconductor production and industrial catalyst manufacture [18–20]. Bellanger et al. [21] apply a two-step process (sintering and re-crystallization) to produce silicon wafers with large polysilicon grains; Wolf [22] focuses on improving the efficiency of sintered porous silicon for thin-film solar cells. Zbib et al. [23] anneal the amorphous nanopowders during the FBR-CVD process and find that the crystal transition temperature for these powders is about 650 °C by means of TEM and DSC. These studies give a great inspiration that can help to modify the crystallinity of silicon powders through sintering treatment. In the present work, we investigated the possibility of sintering treatment on modifying the properties of silicon powders prepared by monosilane pyrolysis in a quartz tube reactor, with a particular focus on the sintering conditions of temperature, duration and pressure. The as-prepared samples and the sintered powders were characterized by X-ray Diffraction (XRD), Scan Electron Microscopy (SEM), Diffuse
S.-S. Liu et al. / Powder Technology 273 (2015) 40–46
Reflection Fourier Transform Infrared Radiation (DR-FT-IR) and Zeta Potential Analyzer (ZPA) for crystal pattern, morphology, hydrogen bond structure and powder size distribution (PSD), respectively. 2. Experimental section 2.1. Materials and equipment The silicon powders were obtained by silane thermal decomposition using high purity nitrogen (99.999%) as diluted gas in a horizontal quartz tube reactor (I.D. = 0.02 m, H = 1 m) equipped within high temperature furnace (SK-G06163, Tianjin Zhonghuan Experimental Furnace Co., Ltd.). The detailed schematic of the experimental setup was displayed in Fig. 1, in which flow meters were applied for gas flux control, a thick layer of insulation cotton for temperature maintenance, a vacuum pump at the entrance of furnace for vacuum operation and a GC analyzer for the inlet and out gas compositions. Based on the temperature calibration, the temperature in the middle of the tube reactor, corresponding to the electric heating zone, was basically the same as the set one. Therefore the silicon powders in the middle of the tube reactor were collected as samples for further sintering and characterization. 2.2. General procedure The preparation procedure was generally described as follows. Before each experiment, due to the flammability of silane, a vacuum leak test was carefully carried out for the whole system followed by the blowing of the system with high purity nitrogen for oxygen replacement. Then the tube reactor was heated to a set value with the flow of nitrogen in a programmed way. When the temperature became steady, the flow rate of nitrogen was adjusted to a desired value and silane was gradually introduced. The silane reaction was carried out for 30 min with enough amount of powders for characterization and the exhausted gas passed through a lye seal vessel. When finished, the system was naturally cooled to the room temperature by nitrogen and the products were obtained and preserved in a drying oven. As for sintering, the obtained samples were put in a corundum crucible, placed in the middle of the furnace and sintered under nitrogen atmosphere or vacuum condition after getting rid of oxygen. The sintering treatment continued 30 min for all the cases unless specified otherwise and a vacuum condition of about 20 Torr, which may promote the densification of composites [24,25], was also applied. 2.3. Characterization The crystal structure of the as-prepared and sintered powders was determined by XRD measurement (D8 Advance, Bruker Co., Germany)
41
using Cu-Kα radiation (λ = 0.15404 nm) at the scan rate of 4°/min in the 2θ range of 20° to 60°. Hydrogen bond structures on the powder surface were analyzed by DR-FT-IR spectrometry (PerkinElmer Inc., Shanghai) in the absorption range of 500–4000 cm−1. The morphology of the powders treated by gold sputter coating was observed with SEM (Nova NanoSEM 450, FEI). PSD was obtained by ZPA (NANO-ZS90, Malvern Instruments Ltd., UK) after several measurements on average.
3. Results and discussions 3.1. Characterization on sample powders The silicon powders, obtained by silane pyrolysis at different temperatures as shown in Table 1 are characterized as follows. The crystal structures of the powders as XRD analysis are shown in Fig. 2. It is found that the basic diffraction peaks at 2θ = 28°, 47° and 56°, corresponding to the reflections from the lattice planes (111), (220) and (311) of typical polycrystalline silicon [26,27], can be distinguished as the operation temperature reaches 700 °C. Besides, the silicon powders are all amorphous dominated without obvious characteristic peaks. The DR-FT-IR analyses of the bond structures on the powder surface are shown in Fig. 3 in which several absorption bands due to the vibration signatures of Si–H and Si–O are observed: 600 cm− 1 (①), 900 cm− 1 (②), 1000–1200 cm− 1 (③), 1900–2000 cm− 1 (④), 2100 cm− 1 (⑤), 2200–2300 cm− 1 (⑥), 2900–3000 cm− 1 (⑦) and 1400–1500 cm−1 (⑧). Combining with the previous studies by Wigger [17] and Onischuk [13,14] as summarized in Table 2, it can be concluded that bands ③ and ⑥ are corresponding to the vibrations of SiOSi and OSiH due to inevitable air exposure during operation; the peak at position ① is attributed to the vibration of p-(SiH2)n,the intensity of which decreases with raising temperature due to hydrogen releasing with low activation energy; the peak at position ② is attributed to Si–H vibrations, which is probably formed from the hydrogen-released p-(SiH2)n and becomes obvious as the temperature over 650 °C; the peak at position ④ is attributed to the vibration of c-SiH, the intensity of which also greatly weakens with the increase in temperature; the peak at position ⑤ is attributed to the vibrations of p-(SiH2)n and iso-SiH the latter of which dominates as the temperature over 650 °C; and bands at ⑦ and ⑧ are probably attributed to the ring structure of silicon hydrides or the other impurity disturbances, which exist strongly at low temperature (600 °C). The above analyses on the Si–H bond structures at positions ①, ④ and ⑤ are in accord with those of Onischuk's [13,14], and the one at position ② which appears at relative high temperature (700 °C) agrees with those of Wigger's [17]. The amorphous silicon powders obtained at 650 °C were applied as samples for further sintering which can be regarded as fines formed in granular polysilicon production [28,29]. A further morphology
Fig. 1. Schematic diagram of experimental setup.
42
S.-S. Liu et al. / Powder Technology 273 (2015) 40–46
Table 1 Preparation conditions of silicon powder samples. Parameter
Value
Parameter
Value
Pressure (pa) Flow flux (SLM) Temperature (°C)
101,325 0.3 600, 610, 620, 650, 700
Silane mole fraction Reaction duration (min)
0.1 30
observation on the sample powders is shown in Fig. 4, in which the agglomeration phenomenon is presented with the primary powder size of 440 nm on average.
3.2. Sintering conditions on crystallite size Fig. 5a displays the XRD patterns of the sample powders under different sintering temperatures (700 °C, 750 °C, 800 °C and 900 °C). It is found that the peak intensities of lattice planes (111), (220) and (311) are gradually enhanced by raising sintering temperature, with the most intense peak of plane (111) reflection. Moreover, Fig. 5c presents the variation of full width at half maximum (FWHM) for plane (111), in which the FWHM decreases with the increasing sintering temperature, equivalent to the growth of crystallite size following Sherrer equation. The effects of sintering duration and vacuum condition on the XRD pattern of the sample powders and the FWHM for plane (111) are given in Fig. 5b and d, respectively. The results show that, besides the three strong characteristic diffraction peaks in all the cases, the narrowest FWHM is obtained under the combined condition of long sintering duration and vacuum operation, which indicates the most favor of the growth of crystal silicon grains. As to the two weak peaks near 2θ = 43°, they are probably caused by the existence of the impurity such as silicon dioxide. Based on the results above, two higher temperatures (900 °C and 1200 °C) with long sintering duration and vacuum condition were applied to investigate the variation of crystallite size. In the case of 1200 °C, close to silicon melting point, the typical XRD result in Fig. 6a presents that the diffraction peaks are obviously weakened due to the fused samples with irregular shape as shown in Fig. 12b. However, according to the normalized diffraction peaks areas in Fig. 6b, it is found
Fig. 3. DR-FT-IR analysis on the bond structure of the silicon powders obtained under different temperatures.
that the crystallinity of the sample powders obtained at 1200 °C is as high as the one obtained at 900 °C.
3.3. Sintering conditions on Si–H bond structure The existence of hydrogen is a key factor influencing the crystallinity of silicon powders from silane gaseous decomposition. Fig. 7 illustrates the effect of sintering temperature on the Si–H bond structures of the sample powders. One can notice that the vibrations of p-(SiH2)n and c-SiH at 2000 cm−1 (④) and 650 cm−1 (①) are gradually weakened with the rising sintering temperature, and the vibration peak at 2100 cm−1 (⑤) due to p-(SiH2)n and iso-SiH almost disappears in all the cases. Concurrently, the intensity of absorption peak at 950 cm−1 (②) contributed by the vibration of Si–H is gradually strengthened. These results further confirm that the hydrogen in the bond of p-
Table 2 FT-IR analysis on the bond structure of silicon powders from silane pyrolysis. Band (cm−1)
Fig. 2. XRD diagram of the silicon powders obtained under different temperatures.
① ② ③ ④ ⑤ ⑥
500–800 800–1000 1000–1250 2000 2100 2170–2300
Wigger [17] Si–H SiOSi Si–H OSiH
Onischuk [13,14]
present work
p-(SiH2)n + Si–H p-(SiH2)n SiOSi c-SiH p-(SiH2)n + iso-SiH OSiH
p-(SiH2)n Si–H SiOSi c-SiH p-(SiH2)n + iso-SiH OSiH
S.-S. Liu et al. / Powder Technology 273 (2015) 40–46
Fig. 4. SEM image of the sample powders obtained at 650 °C.
43
(SiH2)n is much easier to be released as higher sintering temperature applied, which results in the increasing amount of Si–H in the powders. Besides, higher sintering temperature also seems to benefit the hydrogen releasing from OSiH by the decreasing peak intensity at 2300 cm−1 (⑥). The typical FT-IR analyses on the Si–H bond structures of the sample powders under long duration and vacuum condition are presented in Fig. 8, respectively. The results show that much more p-(SiH2)n are formed in the case of 750 °C–2 h with the stronger absorption peak at position ①, besides no big difference is observed on the other absorption bands. However, as the vacuum condition is applied, hydrogen is obviously released from the bond structure of p-(SiH2)n (①) and OSiH (⑥) [30] with an intense vibration of Si–H at 850 cm−1 (②) as a consequence. The results suggest that the vacuum condition has a remarkable effect on the liberation of hydrogen for improving the crystallinity of the amorphous sample powders.
Fig. 5. XRD diagrams of the samples sintered under different conditions with the FWHM for lattice plane (111) (a) (c) different sintering temperatures (b) (d) long sintering duration and vacuum condition.
44
S.-S. Liu et al. / Powder Technology 273 (2015) 40–46
Fig. 7. DR-FT-IR analysis on the bond structure of the sintered samples at different temperatures.
Fig. 6. XRD diagram of the sintered samples at two higher sintering temperatures with normalized diffraction peak areas.
A further investigation on the Si–H bond structure at two higher sintering temperatures is shown in Fig. 9. It is found that the vibrations of Si–H and Si–O are all weakened as the sintering temperature is close to the melting point, especially at 1200 cm− 1 (③) contributed to the vibration of SiOSi. Besides, an extra peak at 1650 cm− 1 (⑨) is also observed, probably caused by the existence of HOH in the gaseous phase.
3.4. Sintering conditions on morphology Fig. 10 displays the surface morphologies of the sintered powders at different temperatures with the insets of PSD by a ZPA analyzer. It is observed that the powder particles are basically spherical with smooth edges in all the cases, but a distinct increase on the mean powder diameter is obtained at the sintering temperature of 750 °C (928 nm). Besides, the PSD results also show that a broad PSD with relative large powders over 4 μm is formed as the sintering temperature over 750 °C.
Fig. 8. DR-FT-IR analysis on the bond structure of the sintered samples under long duration and vacuum condition.
S.-S. Liu et al. / Powder Technology 273 (2015) 40–46
45
Fig. 11a shows the morphology and PSD of the sintered powders under a long duration. Compared with the result in the case of 750 °C–0.5 h (Fig. 10), the mean powder size after the long sintering treatment is slightly smaller (862 nm) than that after the short one (928 nm), but with the appearance of broad PSD. The result suggests that prolonging the sintering duration increases both the aggregation and breakage effects of powder particles. As the vacuum condition is applied, the chain-like aggregates are formed with uniform powder size as shown in Fig. 11b, while the powders obtained under atmospheric condition cluster into the mass. Fig. 12 represents the SEM images and PSDs of the sample powders at two higher sintering temperatures. It can be seen that the large powder particles fuse into a big mass at 1200 °C and the particle edges can be hardly distinguished. As the sintering temperature is at 900 °C, the spherical particles still can be observed but the aggregation effect is enhanced with the formation of large particles. It is also noticed that the powder particles obtained under vacuum condition at 900 °C is with much smaller (819 nm) mean powder size than that in Fig. 10d (1023 nm), which is mostly contributed to the compact of silicon powders by releasing hydrogen from p-(SiH2)n under vacuum condition.
4. Conclusion
Fig. 9. DR-FT-IR analysis on the bond structure of the sintered samples at two higher sintering temperatures.
In the present work, we studied the effect of the sintering treatment on crystallite size, hydrogen bond structure and morphology of the silicon powders from silane thermal decomposition with the aim of reutilizing the by-product fine powders during the polysilicon FBR-CVD production. The sintered silicon powders under different temperature, duration and pressure condition were characterized by XRD, SEM, DRFT-IR and ZPA methods. The main results can be concluded as the following. The higher temperature, longer duration and lower pressure are an advantage to the growth of the polycrystalline silicon grains, with a critical low FWHM at the sintering temperature of 750 °C and the releasing of hydrogen from silicon hydrides under vacuum condition. The ZPA analyses show
Fig. 10. SEM images of the sintered samples at different temperatures with the inset of PSD (a) 700 °C (b) 750 °C (c) 800 °C and (d) 900 °C.
46
S.-S. Liu et al. / Powder Technology 273 (2015) 40–46
References
Fig. 11. SEM images of the sintered samples with the inset of PSD under (a) atmospheric and (b) vacuumed.
that a broad PSD with large powder particles is formed at high sintering temperature or under long duration mostly due to the enhancement of aggregation effect. Moreover, as the sintering temperature is increased close to silicon melting point, the powders begin to fuse into a big mass but still with high crystallinity.
Fig. 12. SEM images of the sintered samples at two higher sintering temperatures (a) 900 °C with the inset of PSD and (b) 1200 °C.
[1] W.O. Filtvedt, M. Javidi, A. Holt, M.C. Malaaen, Development of fluidized bed reactors for silicon production, Sol. Energy Mater. Sol. Cells 94 (2010) 1980–1995. [2] R.T. White, R.L. Espino-Rios, D.S. Rogers, Mechanism of the silane decomposition. I. Silane loss kinetics and rate inhibition by hydrogen. II. Modeling of the silane decomposition (all stages of reactor), In. J. Chem. Kinet. 17 (1985) 1029–1065. [3] M.A. Ring, M.J. Punentes, H.E. O'Neal, Pyrolysis of silane decomposition, Proceedings of the Flat Plate Solar Array Project Workshop on the Science of Silicon, Material Preparation. DOE/JPL. 1021-81, 1983, pp. 63–75. [4] P. Ho, W.G. Breiland, Observation of Si2 in a chemical vapor deposition reactor by laser excited fluorescence, Appl. Phys. Lett. 11 (1984) 51–53. [5] J.J. O'Brien, G.H. Atkinson, Role of silylene in the pyrolysis of silane and organosilanes, J. Phys. Chem. 92 (1988) 5782–5787. [6] A. Yuuki, Y. Matsui, K. Tachibana, A numerical study on gaseous reactions in silane pyrolysis, Jpn. J. Appl. Phys. 26 (1987) 747–752. [7] C.J. Guinta, R.J. McCurdy, J.D. ChappleSokol, Gas phase kinetics in the atmospheric pressure chemical vapor deposition of silicon form silane and disilane, J. Appl. Phys. 67 (1990) 1062–1075. [8] M.T. Swihart, S. Nijhawan, M.R. Mahajan, Modeling the nucleation kinetics and aerosol dynamics of particle formation during CVD of silicon silane, J. Aerosol Sci. 29 (1998) S79–S80. [9] M.T. Swihart, S.L. Girshick, Thermochemistry and kinetics of silicon hydride cluster formation during thermal decomposition of silane, J. Phys. Chem. 103 (1999) 64–76. [10] S. Girshick, M. Swihart, S. Suh, Numerical modeling of gas-phase nucleation and particle growth during chemical vapor deposition of silicon, J. Electrochem. Soc. 147 (2000) 2303–2311. [11] S.S. Talukdar, M.T. Swihart, Aerosol dynamics modeling of silicon nanoparticles formation during silane pyrolysis: a comparison of three solution methods, J. Aerosol Sci. 35 (2004) 889–908. [12] A.A. Onischuk, V.P. Strunin, M.A. Ushakova, Analysis of hydrogen in aerosol particles of a-Si:H forming during the pyrolysis of silane, Phys. Status Solidi B 186 (1994) 43–55. [13] A.A. Onischuk, V.P. Strunin, M.A. Ushakova, On the pathways of aerosol formation by thermal decomposition of silane, J. Aerosol Sci. 28 (1997) 207–222. [14] A.A. Onischuk, V.P. Strunin, R.I. Samoilova, Chemical composition and bond structure of aerosol particles of amorphous hydrogenated silicon forming from thermal decomposition of silane, J. Aerosol Sci. 28 (1997) 1425–1441. [15] J.O. Odden, P.K. Egeberg, A. Kjekshus, From monosilane to crystalline silicon, part i: decomposition of monosilane at 690–830 K and initial pressure 0.1–6.6 Mpa in a free-space reactor, Sol. Energy Mater. Sol. Cells 86 (2005) 165–176. [16] F. Slootman, J. Parent, Homogenous gas-phase nucleation in silane pyrolysis, J. Aerosol Sci. 25 (1994) 15–21. [17] H. Wigger, R. Starke, P. Roth, Silicon particle formation by pyrolysis of silane in a hot wall gas phase reactor, Chem. Eng. Technol. 24 (2001) 261–264. [18] L. Olmos, C.L. Martin, D. Bouvard, Sintering of mixtures of powders: experiments and modeling, Powder Technol. 190 (2009) 134–140. [19] E.M. Larsson, J. Millet, S. Gustafsson, M. Skoglundh, Real time indirect nanoplasmonic in situ spectroscopy of catalyst nanoparticles sintering, Catalysis 2 (2012) 238–245. [20] M.S. Afarani, A. Samimi, E.B. Yekta, Synthesis of alumina granules by high shear mixer granulator: processing and sintering, Powder Technol. 237 (2013) 32–40. [21] P. Bellanger, M. Grau, A. Sow, A. Kaminski, D. Blangis, Multicrystalline silicon wafers prepared by sintering of silicon bed powders and re-crystallization using ZMR, 24th European Photovoltaic Solar Energy Conference. 2009, Hamburg, Germany, 2009. [22] A.K. Wolf, Sintered porous silicon — physical properties and applications for layertransfer silicon thin-film solar cells(Thesis) Gottfried Wilhelm Leibniz Universitat Hannover, Germany, 2007. [23] M.B. Zbib, M.M. Dahl, U. Sahaym, Characterization of granular silicon, powders, and agglomerates from a fluidized bed reactor, J. Mater. Sci. 47 (2012) 2583–2590. [24] E.B. Pickens, R.W. Trice, Pressureless sintering of silicon nitride/boron nitride laminate composites, J. Mater. Sci. 40 (2005) 2101–2103. [25] J. Choi, H. Lyu, W. Lee, J. Lee, Densification and microstructural development during sintering of powder injection molded Fe micro-nanopowder, Powder Technol. 253 (2014) 596–601. [26] S.H. Won, J.H. Youn, J. Jang, Study of polycrystalline silicon films deposited by inductively coupled plasma chemical vapor deposition, J. Korean Phys. Soc. 39 (2001) 123–126. [27] J.B. Dorhout, Characterization of polycrystalline silicon films grown by LPCVD of silane(Thesis) Iowa State University, USA, 2006. [28] M.P. Tejero-Ezpeleta, S. Buchholz, L. Mleczko, Optimization of reaction condition in a fluidized-bed for silane pyrolysis, Can. J. Chem. Eng. 82 (2004) 520–529. [29] G. Hsu, N. Rohatgi, J. Houseman, Silicon particle growth in a fluidized-bed reactor, AICHE J. 33 (1987) 784–791. [30] W.S. Coblenza, The physics and chemistry of the sintering of silicon, J. Mater. Sci. 25 (1990) 2754–2764.