Charge-induced effects on the structure and properties of silane and disilane derivatives

Using ab-initio electronic structure methods, we have investigated the ground state structures and properties of neutral and charged SiHm(m=1-4) and Si2Hn(n=1-6) clusters that are produced in the plasma enhanced chemical vapor deposition process used in the preparation of hydrogenated amorphous silicon (a-Si:H). Our results show that charging a neutral cluster distorts it and the distortion mainly occurs through the orientation of a Si-H bond. We attribute structural changes in the charged clusters to electrostatic repulsion between the bonded and nonbonded electrons. We find that in addition to the usual Si-H bond, hydrogen atoms form Si-H-H and Si-H-Si bonds in some clusters. The vibrations of Si-H, Si-Si, Si-H-Si bond stretching modes show that the frequencies are shifted significantly upon charging. The frequency shifts in the charged clusters are consistent with their bond length variations. We discuss the fragmentation pathways of silane into binary products and the role of fragmented silane radicals in the cluster formation and a-Si:H film deposition process.