AVS 46th International Symposium
    Thin Films Division Monday Sessions
       Session TF-MoM

Paper TF-MoM7
Ab-initio Study of H Abstraction from Amorphous Silicon Surface By Hydrogen and silyl(SiH@sub 3@) Radicals: Implications for Stability of 3-center Bond Formation

Monday, October 25, 1999, 10:20 am, Room 615

Session: Fundamentals of PECVD
Presenter: A. Gupta, North Carolina State University
Authors: A. Gupta, North Carolina State University
H. Yang, North Carolina State University
G.N. Parsons, North Carolina State University
Correspondent: Click to Email
Plasma deposition of amorphous and micro-crystalline silicon from silane/hydrogen gas mixtures is widely utilized in the manufacture of solar cells and in thin film transistors (TFT) required in the active matrix liquid crystal displays (AMLCD). A fundamental understanding of the surface reactions that may result in the removal of bonded H will be useful to further optimize the deposition process. The surface H may be abstracted by H or SiH@sub 3@ radicals to form H@sub 2@ or SiH@sub 4@ respectively leaving behind a dangling bond which provides an active site for chemisorption of SiH@sub n@ radicals. It has been proposed that the first step in a-Si deposition is the formation of a stable 3-center (Si-H-Si) bond by the SiH@sub 3@ radicals and the surface hydrogen groups but the energetics of this elementary step are unknown. We have used configuration interaction (CI) and density functional (DFT-BLYP) calculations to compare the abstraction of surface hydrogen by H as well as SiH@sub 3@ radicals. Our results indicate that the H radicals are more likely to abstract the surface H compared to the SiH@sub 3@ radicals. The activation energy for the former reaction was determined to be ~5.5 kcal/mol and the heat of reaction was ~-11 kcal/mol while the latter reaction had an activation energy ~9.4 kcal/mol and an energy change ~-6 kcal/mol. Thus we can conclude that the H radicals are more likely to abstract the hydrogen from the Si(111) surface as compared to SiH@sub 3@ radicals. The potential energy surface for the reaction of SiH@sub 3@ radicals with the surface H generated using the DFT method does not indicate the presence of a stable 3-center bond within the accuracy of the method. The activation energy using this method was ~6 kcal/mol and the reaction energy was ~-7 kcal/mol. These results indicate that long-standing models for surface and bulk bond structure development need to be re-examined and support the possibility of other reaction schemes viz. SiH@sub 3@ insertion into Si-Si bonds on the surface.