Diffusion mechanism of hydrogen in amorphous silicon: ab initio molecular dynamics simulation.

The mechanism of H migration in amorphous Si has remained an unresolved problem. The main issue is the small activation energy (1.5 eV) relative to the known strength of Si-H bonds (2-3.5 eV). We report first-principles finite-temperature simulations which demonstrate vividly that H is not released spontaneously, as proposed by most models, but awaits the arrival of a floating bond (FB). The "migrating species" is an FB-H complex, with H jumping from Si to Si and the FB literally floating around it. Migration stops when the FB veers away.