In situ laser recrystallization of Si layers during low-pressure chemical vapor deposition: Recrystallization dynamics a
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A. Santoni ENEA–Frascati, Via E. Fermi 45, 00040 Frascati, Italy Lab. TASC-INFM, Area Science Park, Basovizza S.S. 14, Km. 163, 5, I-34012 Trieste, Italy
L. Fornarini ENEA–Frascati, Via E. Fermi 45, 00040 Frascati, Italy
J. Lancok Academy of Science, Institute of Physics, Na Slovance 2, 182 21 Prague 8, Czech Republic
S. Loreti and I. Menicucci ENEA–Frascati, Via E. Fermi 45, 00040 Frascati, Italy
C. Minarini ENEA–Portici, Localita` Granatello, 80055 Portici, Italy (Received 14 May 2002; accepted 6 September 2002)
The growth of polycrystalline silicon on glass by low-pressure chemical vapor deposition and in situ laser induced recrystallization was investigated with the aim to study the influence of the seed layer and the mechanism of the recrystallization dynamics on the structural and morphological properties of the grown film. A seed layer was used to trigger the solidification process of many additional in situ laser-crystallized overlayers. One-dimensional calculations of the thermal flow produced by laser irradiation were used to clarify the complex interaction between the substrate and the molten silicon surface layer during nucleation and growth. The experiments show the relevant role played by the seed layer and the peculiar shaping of the film surface due to the preferential aggregation of molten silicon. Compact polysilicon films with thicknesses up to 4 m with almost monocrystalline grains of 1–2-m size were obtained.
I. INTRODUCTION
With the aim to develop fabrication technologies for low-cost high-efficiency solar cells, several methods have been proposed for producing high-quality polysilicon films as an alternative to the expensive silicon wafer solar cells (Table I). Among these methods, it is worth recalling the high-temperature processes which allow the synthesis of “crystalline silicon thin film” (c-SiTF). By this procedure, large grain poly-Si can be obtained by melting silicon powder or amorphous silicon on e.g., temperatureresistant substrates.1–3,9 The low optical absorption coefficient of crystalline silicon, in the wavelength range of interest (about 0.3 to 1 m), demands a silicon film thickness of the order of 10 m and special surface texturizations, for the efficient absorption of sunlight. A second class of materials (Table I) can be obtained by low-temperature deposition of thin silicon layers on glass or polymer substrates, by techniques like radio 2966
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J. Mater. Res., Vol. 17, No. 11, Nov 2002 Downloaded: 25 Mar 2015
frequency- and very high frequency-plasma enhanced chemical vapor deposition, the latter providing larger growth rates.4 The result is a thin-film material with an intrinsically large absorption coefficient. The material microstructure ranges from amorphous (unstable under illumination) to microcrystalline (stable under illumination), with a grain size of the order of 0.1 m at best. There is a third approach to the fabrication of silicon layers (Table I), with a low-temperature fabrication budget compatible with glass, plastic,
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