Nucleation and Growth Rates in Isothermal Crystallization of Amorphous Si 50 Ge 5o Films
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J. H. Song and James S. Im Columbia University, Department of Chemical Engineering, Materials Science, and Mining Engineering, New York, NY ABSTRACT
Isothermal crystallization behavior of as-deposited thin amorphous Si 50 Ge 50 films (-1000A-thick) at 5801C has been investigated using transmission electron microscopy (TEM). The crystal counting method was employed in order to obtain directly the two-dimensional steady-state crystal nucleation rate of 3.9x10 3 #/cm 2 sec (equivalent volumetric nucleation rate of 3.4x10 8 #/cm 3 sec). The modified twodimensional Johnson-Mehl-Avrami analysis, in which the growth rate of the crystals was the only adjustable parameter, and in which the time-dependent nucleation rate and the size effect associated with the onset of the observation are considered, was developed in order to extract the crystal growth rate of 16.5 A/sec. When compared to the crystallization of a-Si films, these nucleation and growth rates confirm the observation that it is possible to achieve significantly faster crystallization at lower temperatures while producing substantially better microstructures (i.e., > 5 gm grainsized poly-Si 5 o0 e 5 o obtained within two hours at 5800C vs. 1-2gm grain-sized poly-Si obtained in about > 10 hours at 6000 C). INTRODUCTION
Poly-SiGe films are promising materials for the fabrication of thin-film transistor (TFT) devices, and are beginning to demonstrate potential as a legitimate contender against poly-Si films. However, because poly-SiGe TFT devices tested to date have been fabricated on grain-boundary-rich (i.e., small-grained) poly-SiGe films [11, the full potential of the material has not yet been tapped, and remains to be further explored. It is likely that improvement in the microstructure of poly-SiGe films would lead to a corresponding improvement in the performance of poly-SiGe TFT devices. To its credit, SiGe is a processing-friendly material, which is compatible with existing Si processing technologies 121. But most significantly, poly-crystal SiGe films may be an attractive material for TFT devices as they could hold property and process advantages over their Si counterparts. The materials process currently utilized to produce the best low temperature TFT devices - i.e., solid-phase crystallization of a-Si films - is not entirely satisfactory from a processing perspective as the method requires a long heat treatment period (several tens of hours) at a high processing temperature of 6000C 13]. Although there exists a small number of articles on the solid-phase crystallization (SPC) of a-SiGe films, they provide no information pertaining to the microstructure of the crystallized material 14, 51. In particular, these investigations did not utilize TEM-based "crystal counting" analysis 131, and as a result, failed to provide any independent information about the incubation period, nucleation rate,
307 Mat. Res. Soc. Symp. Proc. Vol. 321. @1994 Materials Research Society
and growth rate that characterize crystallization; such information is precisely what is
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