Role of Hydrogen in the Grain Growth in Microcrystalline Silicon Films
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0910-A08-02
Role of Hydrogen in the Grain Growth in Microcrystalline Silicon Films Gyu-Hyun Lee, and Jong-Hwan Yoon Department of Physics, Kangwon National University, 192-1 Hyoja-dong, Chuncheon, Kangwon-do, 200-701, Korea, Republic of
ABSTRACT Thick microcrystalline silicon (µc-Si:H) films were exposed to atomic hydrogen plasma at the substrate temperature of 220 oC after deposition. The microstructure of µc-Si:H films after exposure was characterized using Raman back scattering spectroscopy and transmission electron microscopy (TEM). Raman spectra reveal that the intensity near 520 cm-1 significantly increases after hydrogen exposure, indicating an increase of crystallinity in the films. TEM micrographs of µc-Si:H films exposed to atomic hydrogen also show an increase in the size of grains and a growth of crystalline grains ranging from surface to bulk. These results suggest that crystalline grain formation in µc-Si:H films is likely to be caused by chemical annealing. INTRODUCTION Microcrystalline silicon (µc-Si:H), as a good substitute for amorphous silicon (a-Si:H), is very promising for large-area electronic applications such as solar cells and thin film transistors because of its stable electronic properties against light exposure, high doping efficiency and high mobility. Moreover, µc-Si:H films can be easily grown at substrate temperatures of 200-300 oC by plasma-enhanced chemical vapor deposition (PECVD) using either high hydrogen- and argon-diluted silane[1-3] or layer-by-layer (LBL) deposition technique[4,5]. As a result, many studies have been devoted to understanding the µc-Si:H formation. The detailed formation mechanism of µc-Si:H, however, still remains unclear. Understanding of the formation of µc-Si:H can be divided into two categories largely: one is the growth precursor for grains and the other is the growth mechanism for grains. The growth precursor is generally believed to be the SiH3 radical[6,7], but there are various suggestions for the growth mechanism for grains: typically the surface diffusion model which involves the formation of crystalline grains due to the surface diffusion of the growth precursors enhanced by the hydrogen coverage[3], the etching model which involves the formation of crystalline grain due to the selective etching of the a-Si:H phase by hydrogen atoms[8], and the chemical annealing model which suggests that atomic hydrogen penetrates into the a-Si:H subsurface region and breaks some fraction of the Si-Si bonds, allowing the structural relaxation of amorphous phase and a transition to the crystalline phase[5]. The former two models are based on a surface reaction of precursors during deposition. In these cases crystalline grains continue to grow as long as the precursors are supplied, causing an epitaxial-like growth of grain. On the other hand, the latter, in particular, useful for explaining the µc-Si:H formation by the LBL technique, is based on a restructuring of subsurface after deposition. These suggestions basically demonstrate the formation of crystallites
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