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Abstract The role of hydrogen atoms in the formation process of hydrogenated microcrystalline silicon (gc-Si:H) by plasma enhanced chemical vapor deposition method has been investigated. Under the present conditions, the etching and the permeation of hydrogen atoms in the subsurface region do not cause the crystallization. The kinetics study of surface morphology and structure in the initial growth of gc-Si:H on an atomically flat substrate indicates that the onset thickness of island coalescence reduced under gic-Si:H formation condition. The results support the 'surface diffusion model' in which the surface diffusion of film precursors is enhanced by the sufficient hydrogen coverage of surface and by hydrogen atom recombination energy on the growing surface of the film. 1. Introduction Extensive efforts to develop hydrogenated microcrystalline silicon (jic-Si:H) materials has been made with a plasma enhanced chemical vapor deposition (PECVD) method for applications in thin film silicon solar cells [1]. To enhance their performance, it is important to improve the crystallinity of jic-Si:H film. Several deposition parameters have been reported to affect the crystallinity. The ion bombardment deteriorates both the crystallinity of bulk and an initially grown layer. To avoid this problem, one can use a mesh electrode between the cathode and anode electrodes with negative bias voltage, resulting in better film crystallinity [2]. Another crucial factor for the crystal formation is atomic hydrogen flux density onto the growing surface. Below a critical flux density an amorphous phase appears. The role of hydrogen is, therefore, a key issue in understanding the formation mechanism of crystalline phase of silicon at much lower temperatures than that for thermal equilibrium growth, but it is still controversial. There have been three predominant models proposed; 'etching' [31, 'growth zone' [4,5] and 'surface diffusion' [2] models. In the 'etching' model, the formation of crystallites is determined by balance between deposition and 'etching' on the growing surface due to hydrogen. This model originates from a chemical transport method which was used for the first time to fabricate jic-Si:H at around 300 'C [6]. Hydrogen atoms generated by remote plasma etch a c-Si source and transport Sispecies to the substrate which is kept at -300 'C. The etching rate of the Si at room temperature is much higher than that at 300 'C. As a consequence, the deposition of crystalline Si occurs on the substrate because this process is nearly equilibrium process. Hence, under the conditions used in the chemical transport, both the 843 Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society

etching and deposition take place and are equilibrated. Under the usual deposition conditions for jic-Si:H, however, atomic hydrogen density is expected to be much lower than that for chemical transport. Therefore, to test the presence of the etching process, we have studied the relation between the deposition rate and the generation rate of S