DIRECT REACTION PATHWAYS FOR SILICON-NITRIDE BOND FORMATION IN THE SIH4-NH3 MIXTURE IN GAS-PHASE

The reaction mechanism of silicon nitride formation in the SiH4-NH3 MiXtUre in gas phase has been studied by using ab initio quantum-chemical techniques. Only the direct Si-N bond formation has been considered, that is, the bond formation via silylenes and/or sily] radicals has not been examined. The first step of the direct Si-N bond formation in this system, SiH4+NH3-->SiH3NH2+H-2 (1), has been studied previously (J. Phys. Chem., 94, 5234 (1990)). In this work, the subsequent reactions to occur after reaction 1 have been investigated. For the sake of convenience, the subsequent reactions have been divided into two types of series of reactions. The first series of reactions is that SiH4 molecules successively attack the product of reaction (1), SiH3NH2, namely, SiH3NH2 + SiH4 --> (SiH3)2NH + H-2 (2) and (SiH3)2NH + SiH4 --> (SiH3)3N + H-2 (3), and the second one is that NH3 molecules successively attack SiH3NH2, namely, SiH3NH2 + NH3 --> SiH2(NH2)2 + H-2 (4), SiH2(NH2)2 + NH3 --> SiH(NH2)3 + H-2 (5), and SiH(NH2)3 + NH3 --> Si(NH2)4 + H-2 (6). It has been verified that the geometries obtained for the relatively small molecules included in reactions (2-6) are very close to the framework of alpha- and beta-Si3N4 crystals. Investigations of the energetics of the two series of reactions suggest that once the first step, reaction (1), occurs, the subsequent reactions readily occur and the formation of the silicon nitride crystal is favourably reached. The energetics of the Si - N bond dissociation has also been discussed by using some model reactions. The following four model reactions, SiH2(NH2)2 + SiH4 --> --> 2SiH3NH2 (7), (SiH3)2NH + NH3 --> 2SiH3NH2 (8), SiH3NH2 + SiH4 --> SiH3SiH3 + NH3 (9), and SiH3NH2 + NH2 --> SiH4 + NH2NH2 (10), have been adopted. Reactions (7) and (8) are the models for the Si-N bond dissociation with a simultaneous Si-N bond formation and reactions (9) and (10) are the ones for that with simultaneous Si-Si and N-N bond formations. The results of the calculations have predicted that the Si-N bond dissociations are unfavourable reaction pathways from the viewpoint of thermodynamic stability.