Plasma-chemical and photo-chemical vapor decomposition mechanism of disilane in the lowest triplet state

Abstract

The decomposition mechanism of disilane, Si 2H 6, in the lowest triplet state has been investigated by ab initio molecular orbital calculations with 3–21G+ and 3–21G *+ basis sets. The vertical transition energy is calculated to be 6.85 eV from the ground state 1 1A 1g of disilane, which belongs to the D 3d point group, to the lowest triplet state 3A 2u. Starting from the vertically excited disilane, two silyl radicals having a large amount of excess energy are generated following the steepest path of the lowest potential energy in the lowest triplet state. These silyl radicals finally produce Si 2H 5 radicals by reacting with parent disilane molecules. This reaction process for disilane is very different from that for monosilane SiH 4, which decomposes to a silyl radical having only a small amount of excess energy (M. Tsuda, S. Oikawa and K. Nagayama, Chem. Phys. Lett., 118 (1986) 498). The saddle point was decided on the lowest triplet potential energy hypersurface 3A 2u, where the energy level is 2.5 eV higher than the ground state. Two degradation paths are from the saddle point; one is Si−Si bond scission producing two silyl radicals and the other is Si−H bond degradation forming an Si 2H 5 radical and a hydrogen atom. The reaction mechanism elucidated in this research is in good agreement with various experimental observations in disilane plasma.