Hydrogen-mediated models for metastability in a-Si:H: role of dihydride bonding

Two hydrogen-mediated models for defects in hydrogenated amorphous silicon (a-Si:H) based on hydrogen pairing have previously been proposed. One model based on the clustered hydrogen phase detected by nuclear magnetic resonance accounts well for several near-equilibrium properties, and in particular for the defect creation during hydrogen effusion. The second model based on hydrogen-collisions forming metastable pairs at arbitrary sites accounts for aspects of light-induced generation of metastable defects. We describe a unified model subsuming both the clustered-phase and hydrogen-collision models. The model is based on sites which bond two pairs of hydrogens. We discuss evidence that the site may be involved with dihydride bonding observed by infrared absorption spectroscopy.