Surface Reaction Mechanism During Deposition of a-Si: 11 by PECVD
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Hydrogen contents were investigated in a-Si:H films prepared by PECVD under various conditions. The H atom contents were divided into SiH and SiH Dehydrogenation of densities by spectral deconvolution of the IR absorption. The SiHl simultaneously occurs with Si-Si bonding in the surface reaction. mechanisms of surface reaction were proposed based on an analysis of the There are two kinds variation in SiH2 density with the deposition condition. The fast process incorof processes, the fast process and the slow process. The activation energy of the slow process is porates SiH2 into the network. 0.3 eV, which is due to the surface migration of adsorbed radicals. INTRODUCTION Hydrogenated amorphous Si (a-Si:H) films are used as the semiconductor material for producing large area devices such as TFT array for liquid crystal display. This material is prepared by the plasma-enhanced chemical vapor deposition (PECVD) of SiH gas. Understanding of this deposition mechanism is essential for both control of the deposition process and improvement of the film quality. The deposition process is very complicated. There has been no definite clue to correlate the deposition conditions with the characteristics of the film. It is very recently that the SiH3 radical has been determined as the main During the deposition, the growing surprecursor in PECVD of SiH4 gas [1]. Incident SiH3 radical is face of a-Si:H film is terminated by H atoms [2]. adsorbed first on this surface and then causes a surface reaction to form a In the previous studies of Si-Si bond during its migration on the surface. the surface reaction [3,41, attention has been paid only to the Si-Si bond formation in the growing surface. Since a-Si:H films of good quality contain H atoms of about 10 at % of Si, the dehydrogenation reaction of SiH 3 should be made simultaneously with the Si-Si bond formation. It has been considered that H atoms are released by forming a Si-Si bond However, between a pair of Si-H bonds already incorporated into the network. such a bond formation is considered to be unlikely because when the coordinaion number becomes large these network-forming Si atoms cannot move freely to approach a position satisfying the short-range order. It was found experimenof multiply Htally by Gallagher et al [5] that the H-rich surface layer bonded Si atoms is one monolayer during deposition at 240 0 C substrate temperature. Therefore, H atom incorporation into the network is considered to be closely correlated with the initial bond formation process. In this work we studied H atom concentrations in a-Si:H films deposited under various conditions; at various substrate temperature T. and under varThe H atom concentration ious Sil 4 partial pressure Ps in the reactant gas. detected by IR absorption was divided into SiH and SiH2 configuration by spectral decomposition. On the basis of an analysis of the H atom concentrations, the mechanisms of the dehydrogenation reaction as well as the Si-Si bonding reaction are proposed. Detailed description of this work was publishe
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