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Laser-assisted chemical vapor deposition of thick poly-Si layers for solar cells
Thin Solid Films 403 – 404 (2002) 302–306
Laser-assisted chemical vapor deposition of thick poly-Si layers for solar cells D. Della Salaa, S. Loretib, L. Fornarinib,*, I. Menicuccib, A. Santonib, P. Delli Veneric, C. Minarinic, C. Privatoc, J. Lancockd a
ENEA–Casaccia, Via Anguillarese 301, 00060 Santa Maria di Galeria, Italy b ENEA–Frascati, Via E. Fermi 45, 00040 Frascati, Italy c ENEA–Portici, Via Vecchio Macello snc, 80055 Portici, Italy d Academy of Science, Institute of Physics, Na Slovance 2, 182 21 Prague 8, Czech Republic
Abstract The growth of polycrystalline silicon on glass by laser-assisted chemical vapor deposition has been studied with the aim of identifying a light absorber layer for solar cells, with superior material quality compared to other technologies available for lowtemperature substrates. One-dimensional calculations of the thermal wave produced by laser irradiation have been used to elucidate the complex interaction of the molten silicon surface layer with the substrate during the growth. The experiments show the relevant role played by the seed layer used as the growth initiator. The morphology of the laser-crystallized films has been analysed by scanning electron microscopy and X-ray diffraction. Polysilicon films, 2 mm thick, with a compact structure consisting of 1–2-mm grains that are almost monocrystalline, have been obtained. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser crystallization; Poly-silicon; Thin film; Solar cells
1. Introduction Among the technologies devoted to the fabrication of low-cost high-efficiency solar cells, many methods have been proposed for producing high-quality silicon layers as an alternative to the usual silicon wafer solar cells (Table 1). The first concept is the ‘thin film crystalline silicon’ (TFCS), i.e. large-grained poly-Si obtained by hightemperature processing with crystallization of Si powder or amorphous silicon on temperature-resistant glass w1x, insulating and conductive ceramics w2–4x, alumina or mullite w5x, or encapsulated graphite w6x. The low optical absorption coefficient of crystalline silicon requires a silicon film thickness of the order of 10 mm and special surface texturizations for the efficient absorption of sunlight. * Corresponding author. Tel.: q39-06-94005196; fax: q39-0694005400. E-mail address: [email protected] (L. Fornarini).
The second class of materials is based on the lowtemperature deposition of thin silicon layers on glass or polymer substrates, with techniques such as radio frequency — and very high frequency — plasma-enhanced chemical vapor deposition, the latter providing greater growth rates w7x. The result is a thin film material with an intrinsically large absorption coefficient. The internal structure ranges from amorphous (unstable under illumination) to microcrystalline (stable under illumination), with a grain size that is of the order of 0.1 mm, at best. There might be a third approach to the fabrication of silicon layers with a safe, low-temperature fabrication budget compatible with glass, plastic and thin foils. It is based on the irradiation of amorphous silicon by laser sources. If the irradiation is very short, the laser light can melt the surface of a pre-deposited amorphous film, preserving the delicate substrate underneath. Such a method, based mainly on excimer laser sources (pulse duration f20–100 ns, melt duration f1 ms), has been successfully applied to thin (-0.1 mm) amorphous
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 5 6 8 - 1
D. Della Sala et al. / Thin Solid Films 403 – 404 (2002) 302–306
303
Table 1 Characteristics of the fabrication methods of thin silicon films Process
Process temperature (8C) Grain size (mm) Stability Growth rate (mmymin) Film thickness (mm)
High temperature w2–6x
Low temperature w7 x
Laser-enhanced low temperature w11,12x, current study
900–1400 )100 OK 1–4 10–100
150–300 0–0.1 Only for mc-Si 0.001–0.06 -3
150–550 0.1–10 OK 0.02–0.03 1–10
silicon films deposited on glass w8x and plastics w9,10x, and in the fabrication of thin film transistors for flat panel displays, and has begun to be extended to the typical film thickness required for solar cells (a few mm), based on CW Arq and pulsed KrF lasers w11,12x. This requires that the pulsed laser irradiation be carried out sequentially with the film deposition on a crystalline seed layer. This method is the most recent and is less proved for solar cell fabrication, so conversion efficiency data are still lacking. In the following, we describe a study on the morphology of silicon films obtained with a method belonging to the third group: laser-assisted chemical vapor deposition (LACVD) of silicon from disilane gas.
beam energy of 100–200 mJ. The beam is homogenized and focused to a spot of 5=5 mm2. The surface morphology of the films was investigated by conventional scanning electron microscopy (SEM). The crystalline structure of the films was investigated by an X’Pert-MPD (Philips) diffractometer using CuKa radiation. The patterns were acquired by a u–2u
2. Experimental Silicon films were deposited on Corning glass by CVD at 800 K using Si2H6 as the gas source. The growth rate measured by real-time in situ reflectance was approximately 20 nmymin. The precursor films are amorphous and the hydrogen content is below the detection limit of the FTIR analysis performed on some initial samples. The CVD growth was assisted by irradiation with a KrFNe-filled (248 nm) Lambda Physik LPX-100 laser system (pulse duration 20 ns), with a
Fig. 1. The fluence necessary to melt a 2.5-nm-thick a-Si film deposited between two laser pulses, with and without Ag contact.
Fig. 2. (a) Fracture SEM micrograph of a Secco-etched silicon film grown directly on a Corning glass substrate. Some large grains in the microcrystalline matrix are clearly visible. (b) Surface SEM micrograph of the same film. The surface appears quite smooth, with some microcrystalline agglomerates.
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D. Della Sala et al. / Thin Solid Films 403 – 404 (2002) 302–306
Fig. 3. Micrograph of the as-grown seed layer surface obtained using the back scattered electron detector. The channeling contrast makes visible the sample microcrystal structure. The mean grain dimension is approximately 3 mm
goniometer mounted on the lineshape radiation and equipped with a monochromator, a programmable receiving slit and a Xe-filled proportional detector. The XRD spectra were collected in the range 20–608 for 2u in the Bragg–Brentano configuration, with a measurement step of 0.058 and a step time of 20 s. The mean value of the crystallite size was determined by the Debye–Scherrer formula for the main diffraction peaks. 3. Results and discussion The best grain size in the laser crystallization of a-Si films for TFTs is obtained in the so-called ‘super lateral growth’ regime, where the laser intensity is just capable of melting the whole precursor layer, leaving a few crystalline seeds at the interface with the substrate that trigger the crystallization of the liquid silicon during solidification. The same regime should be the best for the LACVD of silicon, when applied to all the individual amorphous layers deposited between successive laser pulses. The ‘full melting intensity’ IFM was therefore calculated with one-dimensional codes using specific models w13x for the appropriate ‘thickness per pulse’ t obtained in the LACVD process, which is the material thickness deposited between two successive laser pulses. t is a function of the deposition rate Rd and the pulse repetition rate PRR: tsRd yPRR. By changing the gas pressure in the deposition chamber, the deposition rate was Rds 15–30 nmymin, and with the typical value PRRs0.1 Hz, ts2.5–5 nm was achieved. Fig. 1 shows the IFM calculated for an amorphous film of fixed thickness t on the growing crystalline layer. IFM first decreases as a function of thickness for very thin buffer crystalline layers, due to the fact that
Fig. 4. (a) Surface SEM micrograph of a Secco-etched LACVD silicon film grown on the microcrystalline seed layer. The surface shows a regular structure with a dome diameter of approximately 1 mm. (b) The higher magnification demonstrates the grain boundaries inside the domes.
the underlying silicon is not capable to absorb all the incident radiation. For larger values of the buffer thickness, IFM increases again because of an appreciable heatdrain effect of the crystalline silicon buffer. When the layer reaches a thickness of approximately 1 mm, the crystalline silicon buffer can be considered infinitely thick from the point of view of the temperature, and IFM is constant. Similar simulations were performed considering an additional 200-nm-thick Ag film under the crystalline buffer to simulate a realistic electrical back contact for the solar cell. The increased heat-sink effect of the electrical contact for the solar cell is evident. Due to the fact that in a realistic LACVD process the fluence is constant, it cannot be appropriate to the superal lateral growth regime during the entire deposition process. The use of a crystalline seed layer a few 100 nm thick has two beneficial effects: the obvious propagation of its crystalline structure in the deposited
D. Della Sala et al. / Thin Solid Films 403 – 404 (2002) 302–306
305
Fig. 5. Schematic image of the formation of domes around microcrystalline grains, due to the superior adhesion force of the top liquidsilicon layer to crystal silicon, rather than to liquid silicon itself.
material, and the narrowing of the range of IFM required for the deposited layers, as shown in Fig. 1. As a term of comparison, a 2-mm LACVD film was directly grown on Corning glass with a pulse period of 30 s. The film exhibits a few large crystals embedded in a fine-grained poly-Si matrix (Fig. 2a,b), probably triggered by explosive crystallization w14x. This finegrained material can be obtained with many low-temperature methods, and LACVD is useless in this case. To increase the grain size, highly crystalline Si seed layers were prepared by thermal crystallization of 200nm a-Si deposited by CVD on glass, with XRD peaks showing that the volume fraction of crystal Si is 100%. To observe the grain pattern of the seed layer (average size 3 mm), SEM analysis was carried out with the backscattered detector in special conditions to improve the channeling contrast w15x (Fig. 3), as an alternative to the destructive Secco etching. We deposited 500 nm of LACVD poly-Si on the seed layer, with the purpose of investigating the early stages of the growth. In this case, the crystallized silicon assumes a dome-like shape with average size of 1 mm (Fig. 4a). The detailed SEM image (Fig. 4b) shows that each dome is made up by many individual grains. This type of morphology has already been reported under different experimental conditions by Shih et al. w16x. To explain this phenomenon, it has been suggested that the adhesion force between the molten silicon and the
Fig. 6. (a) Micrograph of a thick silicon film grown on the seed layer. The surface shows the presence of close-standing smooth domes. (b) A selected area of (a) at higher magnification. The domes are very close to each other and the seed layer is completely covered.
underlying microcrystalline Si is greater than the agglomeration force of the molten silicon itself, and the liquid silicon layer generated by the laser pulse sticks on hillocks and pits rather than smoothing out the surface. The nucleation starts inside the crystalline seed sites rather than in defective areas, such as the grain boundary, and the mechanism is self-sustaining (Fig. 5). In order to reduce the number of the grains inside the domes and point to monocrystalline grains, we increased the pulse repetition rate PRR. This allows melting of all the a-Si deposited during the delay between two laser shots. A series of samples was prepared using a crystalline seed layer, two different pulse repetition rates and a LACVD film thickness greater than 1 mm.Fig. 6a,b show the sample growth with PRRs0.1 Hz. The domes have a smooth surface and they are close to each other. The SEM micrograph does not show grain boundaries inside the grains. The XRD analysis shown in Fig. 7 confirms the good quality of the silicon crystallites. The experimental
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D. Della Sala et al. / Thin Solid Films 403 – 404 (2002) 302–306
obtained with other low-temperature methods, where the grain size is lower, but grain boundaries are effectively passivated by hydrogen atoms. Acknowledgements This work has been supported by Project FOTO, cofinanced by the Multi-Regional Operative Programme ‘Research, Technological Development and Higher Education’ of the European Regional Development Fund (FESR No 94.05.09.013yARINCO No 94.IT.16.028). References
Fig. 7. XRD pattern of the sample grown on the seed layer. The (111) and (222) silicon diffraction peaks confirm the crystalline structure of the sample. The inset shows the profile analysis of the line shape. The experimental peak (squares) is the convolution of a broad peak (dashed line) related to small crystallites and a sharp peak (continuous line) due to large crystallites.
diffraction peaks were fitted by a pseudo-Voigt function and the presence of two phases with different crystallite sizes is deduced by analysing the peak lineshape. The first is a crystalline phase with a mean crystallite size in excess of 200 nm (which is the limiting size that can be detected with this method), and the other has an average size of 7 nm, and probably corresponds to veryfine-grained polysilicon hidden in the grain boundaries or in the seed layer. 4. Conclusions We have investigated the morphology of poly-silicon films obtained by LACVD under different experimental conditions. The presence of a seed layer underneath the growing LACVD film is essential for stabilizing the laser intensity required for complete melting of the individual layers deposited between successive laser pulses. The seed layer is also essential in increasing the crystal fraction of the poly-Si layer. With the optimum experimental parameters, large-grained polysilicon (grain size 1 mm) is obtained at a thickness useful for photovoltaic applications (2 mm). If this material will also show good grain-boundary quality (e.g. good lattice reconstruction, low stress, good lattice defect passivation), it might be superior to small-grained polysilicon
w1x R.B. Bergmann, J.G. Darrant, A.R. Hyde, J.H. Werner, J. NonCryst. Solids 218 (1997) 388. w2x C. Hebling, S. Reber, K. Schmidt, R. Luedemann, F. Lutz, Proceedings of the 26th IEEE Photovoltaics Specialist Conference, Anaheim, CA, 1997. w3x J.A. Rand, Y. Bai, J.C. Checchi, J.S. Culik, D.H. Ford, C.L. Kendall, P.E. Sims, R.B. Hall, A.M. Barnett, Proceedings of the 26th IEEE Photovoltaics Specialist Conference, Anaheim, CA, 1997. w4x G. Beaucarne, J. Poortmans, M. Caymax, J. Nijs, R. Mertens, R. Monna, D. Angermeier, A. Slaoui, Proceedings of the 14th European Photovoltaic Solar Energy Conference, Barcelona, Spain, 30 June–4 July, 1997. w5x D. Angermeier, R. Monna, A. Slaoui, J.C. Muller, C.J.J. Tool, J.A.M. Roosmalen, S. Acosta, A. Ayral, Proceedings of the 14th European Photovoltaic Solar Energy Conference, Barcelona, Spain, 30 June–4 July, 1997. w6x R. Luedemann, S. Schaefer, C. Schele, C. Hebling, Proceedings of the 26th IEEE Photovoltaics Specialist Conference, Anaheim, CA, 1997. w7x J. Meier, S. Dubail, J. Cuperus, U. Kroll, R. Platz, P. Torres, J.A. Anna Selvan, P. Perret, N. Beck, N. Pellaton Vaucher, C. Hof, D. Fischer, H. Keppner, A. Shah, J. Non-Cryst. Solids 227–230 (1998) 1250. w8x J.S. Im, H.J. Kim, M.O. Thompson, Appl. Phys. Lett. 63 (1993) 1969. w9x P.G. Carey, P.M. Smith, S.D. Theiss, P. Wickboldt, J. Vac. Sci. Technol. A 17 (1999) 1946. w10x N.D. Young, D.J. McCullch, R.M. Bunn, Proceedings of AMLCD ’87, Tokyo, 1987, p. 47. w11x G. Andrae, J. Bergmann, F. Falk, E. Ose, Thin Solid Films 318 (1998) 42. w12x G. Andrae, J. Bergmann, F. Falk, E. Ose, N.D. Sinh, S. Christiansen, M. Nerding, H.P. Strunk, Proceedings of the 28th IEEE Photovoltaics Specialist Conference, Anchorage, 15–20 Sept. 2000 p. 217. w13x A. Mittiga, L. Fornarini, R. Carluccio, Appl. Surf. Sci. 154 (2000) 112. w14x V.V. Gupta, H.J. Song, J.S. Im, Appl. Phys. Lett. 71 (1997) 99. w15x S. Loreti, D. Della Sala, M. Garozzo, Micron 31 (2000) 299. w16x A. Shih, C.Y. Meng, S.C. Lee, M.Y. Chern, J. Appl. Phys. 88 (2000) 3725.
Laser-assisted chemical vapor deposition of thick poly-Si layers for solar cells D. Della Salaa, S. Loretib, L. Fornarinib,*, I. Menicuccib, A. Santonib, P. Delli Veneric, C. Minarinic, C. Privatoc, J. Lancockd a
ENEA–Casaccia, Via Anguillarese 301, 00060 Santa Maria di Galeria, Italy b ENEA–Frascati, Via E. Fermi 45, 00040 Frascati, Italy c ENEA–Portici, Via Vecchio Macello snc, 80055 Portici, Italy d Academy of Science, Institute of Physics, Na Slovance 2, 182 21 Prague 8, Czech Republic
Abstract The growth of polycrystalline silicon on glass by laser-assisted chemical vapor deposition has been studied with the aim of identifying a light absorber layer for solar cells, with superior material quality compared to other technologies available for lowtemperature substrates. One-dimensional calculations of the thermal wave produced by laser irradiation have been used to elucidate the complex interaction of the molten silicon surface layer with the substrate during the growth. The experiments show the relevant role played by the seed layer used as the growth initiator. The morphology of the laser-crystallized films has been analysed by scanning electron microscopy and X-ray diffraction. Polysilicon films, 2 mm thick, with a compact structure consisting of 1–2-mm grains that are almost monocrystalline, have been obtained. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser crystallization; Poly-silicon; Thin film; Solar cells
1. Introduction Among the technologies devoted to the fabrication of low-cost high-efficiency solar cells, many methods have been proposed for producing high-quality silicon layers as an alternative to the usual silicon wafer solar cells (Table 1). The first concept is the ‘thin film crystalline silicon’ (TFCS), i.e. large-grained poly-Si obtained by hightemperature processing with crystallization of Si powder or amorphous silicon on temperature-resistant glass w1x, insulating and conductive ceramics w2–4x, alumina or mullite w5x, or encapsulated graphite w6x. The low optical absorption coefficient of crystalline silicon requires a silicon film thickness of the order of 10 mm and special surface texturizations for the efficient absorption of sunlight. * Corresponding author. Tel.: q39-06-94005196; fax: q39-0694005400. E-mail address: [email protected] (L. Fornarini).
The second class of materials is based on the lowtemperature deposition of thin silicon layers on glass or polymer substrates, with techniques such as radio frequency — and very high frequency — plasma-enhanced chemical vapor deposition, the latter providing greater growth rates w7x. The result is a thin film material with an intrinsically large absorption coefficient. The internal structure ranges from amorphous (unstable under illumination) to microcrystalline (stable under illumination), with a grain size that is of the order of 0.1 mm, at best. There might be a third approach to the fabrication of silicon layers with a safe, low-temperature fabrication budget compatible with glass, plastic and thin foils. It is based on the irradiation of amorphous silicon by laser sources. If the irradiation is very short, the laser light can melt the surface of a pre-deposited amorphous film, preserving the delicate substrate underneath. Such a method, based mainly on excimer laser sources (pulse duration f20–100 ns, melt duration f1 ms), has been successfully applied to thin (-0.1 mm) amorphous
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 5 6 8 - 1
D. Della Sala et al. / Thin Solid Films 403 – 404 (2002) 302–306
303
Table 1 Characteristics of the fabrication methods of thin silicon films Process
Process temperature (8C) Grain size (mm) Stability Growth rate (mmymin) Film thickness (mm)
High temperature w2–6x
Low temperature w7 x
Laser-enhanced low temperature w11,12x, current study
900–1400 )100 OK 1–4 10–100
150–300 0–0.1 Only for mc-Si 0.001–0.06 -3
150–550 0.1–10 OK 0.02–0.03 1–10
silicon films deposited on glass w8x and plastics w9,10x, and in the fabrication of thin film transistors for flat panel displays, and has begun to be extended to the typical film thickness required for solar cells (a few mm), based on CW Arq and pulsed KrF lasers w11,12x. This requires that the pulsed laser irradiation be carried out sequentially with the film deposition on a crystalline seed layer. This method is the most recent and is less proved for solar cell fabrication, so conversion efficiency data are still lacking. In the following, we describe a study on the morphology of silicon films obtained with a method belonging to the third group: laser-assisted chemical vapor deposition (LACVD) of silicon from disilane gas.
beam energy of 100–200 mJ. The beam is homogenized and focused to a spot of 5=5 mm2. The surface morphology of the films was investigated by conventional scanning electron microscopy (SEM). The crystalline structure of the films was investigated by an X’Pert-MPD (Philips) diffractometer using CuKa radiation. The patterns were acquired by a u–2u
2. Experimental Silicon films were deposited on Corning glass by CVD at 800 K using Si2H6 as the gas source. The growth rate measured by real-time in situ reflectance was approximately 20 nmymin. The precursor films are amorphous and the hydrogen content is below the detection limit of the FTIR analysis performed on some initial samples. The CVD growth was assisted by irradiation with a KrFNe-filled (248 nm) Lambda Physik LPX-100 laser system (pulse duration 20 ns), with a
Fig. 1. The fluence necessary to melt a 2.5-nm-thick a-Si film deposited between two laser pulses, with and without Ag contact.
Fig. 2. (a) Fracture SEM micrograph of a Secco-etched silicon film grown directly on a Corning glass substrate. Some large grains in the microcrystalline matrix are clearly visible. (b) Surface SEM micrograph of the same film. The surface appears quite smooth, with some microcrystalline agglomerates.
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D. Della Sala et al. / Thin Solid Films 403 – 404 (2002) 302–306
Fig. 3. Micrograph of the as-grown seed layer surface obtained using the back scattered electron detector. The channeling contrast makes visible the sample microcrystal structure. The mean grain dimension is approximately 3 mm
goniometer mounted on the lineshape radiation and equipped with a monochromator, a programmable receiving slit and a Xe-filled proportional detector. The XRD spectra were collected in the range 20–608 for 2u in the Bragg–Brentano configuration, with a measurement step of 0.058 and a step time of 20 s. The mean value of the crystallite size was determined by the Debye–Scherrer formula for the main diffraction peaks. 3. Results and discussion The best grain size in the laser crystallization of a-Si films for TFTs is obtained in the so-called ‘super lateral growth’ regime, where the laser intensity is just capable of melting the whole precursor layer, leaving a few crystalline seeds at the interface with the substrate that trigger the crystallization of the liquid silicon during solidification. The same regime should be the best for the LACVD of silicon, when applied to all the individual amorphous layers deposited between successive laser pulses. The ‘full melting intensity’ IFM was therefore calculated with one-dimensional codes using specific models w13x for the appropriate ‘thickness per pulse’ t obtained in the LACVD process, which is the material thickness deposited between two successive laser pulses. t is a function of the deposition rate Rd and the pulse repetition rate PRR: tsRd yPRR. By changing the gas pressure in the deposition chamber, the deposition rate was Rds 15–30 nmymin, and with the typical value PRRs0.1 Hz, ts2.5–5 nm was achieved. Fig. 1 shows the IFM calculated for an amorphous film of fixed thickness t on the growing crystalline layer. IFM first decreases as a function of thickness for very thin buffer crystalline layers, due to the fact that
Fig. 4. (a) Surface SEM micrograph of a Secco-etched LACVD silicon film grown on the microcrystalline seed layer. The surface shows a regular structure with a dome diameter of approximately 1 mm. (b) The higher magnification demonstrates the grain boundaries inside the domes.
the underlying silicon is not capable to absorb all the incident radiation. For larger values of the buffer thickness, IFM increases again because of an appreciable heatdrain effect of the crystalline silicon buffer. When the layer reaches a thickness of approximately 1 mm, the crystalline silicon buffer can be considered infinitely thick from the point of view of the temperature, and IFM is constant. Similar simulations were performed considering an additional 200-nm-thick Ag film under the crystalline buffer to simulate a realistic electrical back contact for the solar cell. The increased heat-sink effect of the electrical contact for the solar cell is evident. Due to the fact that in a realistic LACVD process the fluence is constant, it cannot be appropriate to the superal lateral growth regime during the entire deposition process. The use of a crystalline seed layer a few 100 nm thick has two beneficial effects: the obvious propagation of its crystalline structure in the deposited
D. Della Sala et al. / Thin Solid Films 403 – 404 (2002) 302–306
305
Fig. 5. Schematic image of the formation of domes around microcrystalline grains, due to the superior adhesion force of the top liquidsilicon layer to crystal silicon, rather than to liquid silicon itself.
material, and the narrowing of the range of IFM required for the deposited layers, as shown in Fig. 1. As a term of comparison, a 2-mm LACVD film was directly grown on Corning glass with a pulse period of 30 s. The film exhibits a few large crystals embedded in a fine-grained poly-Si matrix (Fig. 2a,b), probably triggered by explosive crystallization w14x. This finegrained material can be obtained with many low-temperature methods, and LACVD is useless in this case. To increase the grain size, highly crystalline Si seed layers were prepared by thermal crystallization of 200nm a-Si deposited by CVD on glass, with XRD peaks showing that the volume fraction of crystal Si is 100%. To observe the grain pattern of the seed layer (average size 3 mm), SEM analysis was carried out with the backscattered detector in special conditions to improve the channeling contrast w15x (Fig. 3), as an alternative to the destructive Secco etching. We deposited 500 nm of LACVD poly-Si on the seed layer, with the purpose of investigating the early stages of the growth. In this case, the crystallized silicon assumes a dome-like shape with average size of 1 mm (Fig. 4a). The detailed SEM image (Fig. 4b) shows that each dome is made up by many individual grains. This type of morphology has already been reported under different experimental conditions by Shih et al. w16x. To explain this phenomenon, it has been suggested that the adhesion force between the molten silicon and the
Fig. 6. (a) Micrograph of a thick silicon film grown on the seed layer. The surface shows the presence of close-standing smooth domes. (b) A selected area of (a) at higher magnification. The domes are very close to each other and the seed layer is completely covered.
underlying microcrystalline Si is greater than the agglomeration force of the molten silicon itself, and the liquid silicon layer generated by the laser pulse sticks on hillocks and pits rather than smoothing out the surface. The nucleation starts inside the crystalline seed sites rather than in defective areas, such as the grain boundary, and the mechanism is self-sustaining (Fig. 5). In order to reduce the number of the grains inside the domes and point to monocrystalline grains, we increased the pulse repetition rate PRR. This allows melting of all the a-Si deposited during the delay between two laser shots. A series of samples was prepared using a crystalline seed layer, two different pulse repetition rates and a LACVD film thickness greater than 1 mm.Fig. 6a,b show the sample growth with PRRs0.1 Hz. The domes have a smooth surface and they are close to each other. The SEM micrograph does not show grain boundaries inside the grains. The XRD analysis shown in Fig. 7 confirms the good quality of the silicon crystallites. The experimental
306
D. Della Sala et al. / Thin Solid Films 403 – 404 (2002) 302–306
obtained with other low-temperature methods, where the grain size is lower, but grain boundaries are effectively passivated by hydrogen atoms. Acknowledgements This work has been supported by Project FOTO, cofinanced by the Multi-Regional Operative Programme ‘Research, Technological Development and Higher Education’ of the European Regional Development Fund (FESR No 94.05.09.013yARINCO No 94.IT.16.028). References
Fig. 7. XRD pattern of the sample grown on the seed layer. The (111) and (222) silicon diffraction peaks confirm the crystalline structure of the sample. The inset shows the profile analysis of the line shape. The experimental peak (squares) is the convolution of a broad peak (dashed line) related to small crystallites and a sharp peak (continuous line) due to large crystallites.
diffraction peaks were fitted by a pseudo-Voigt function and the presence of two phases with different crystallite sizes is deduced by analysing the peak lineshape. The first is a crystalline phase with a mean crystallite size in excess of 200 nm (which is the limiting size that can be detected with this method), and the other has an average size of 7 nm, and probably corresponds to veryfine-grained polysilicon hidden in the grain boundaries or in the seed layer. 4. Conclusions We have investigated the morphology of poly-silicon films obtained by LACVD under different experimental conditions. The presence of a seed layer underneath the growing LACVD film is essential for stabilizing the laser intensity required for complete melting of the individual layers deposited between successive laser pulses. The seed layer is also essential in increasing the crystal fraction of the poly-Si layer. With the optimum experimental parameters, large-grained polysilicon (grain size 1 mm) is obtained at a thickness useful for photovoltaic applications (2 mm). If this material will also show good grain-boundary quality (e.g. good lattice reconstruction, low stress, good lattice defect passivation), it might be superior to small-grained polysilicon
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