Mediation of light-induced metastable defect formation in hydrogenated amorphous silicon by interstitial hydrogen and voids

A large set of samples was deposited by radio frequency powered glow discharge under various conditions of temperature, power, gas pressure and dilution of silane. Density of states of each sample was studied in the as-deposited, light-soaked and annealed states by means of the modulated photocurrent and constant photocurrent methods. Upon light-soaking we have systematically found an increase of both the deep defect density and of the conduction band tail states. Moreover, for samples deposited on the edge of crystallinity and for polymorphous materials, irreversible modifications of the density of states were observed after light-soaking followed by annealing. None of the existing models of the metastability accounting for these behaviors we propose a new model in which light-induced creation of dangling bonds is mediated by interstitial hydrogen, During light-soaking the hydrogen atoms generated by the breaking of Si-H bonds gather to form interstitial hydrogen molecules that are trapped into voids or platelets. During annealing, these molecules are broken and H atoms are released in the lattice passivating the metastable dangling bonds. This qualitative model fully explains our experimental results and also many other experimental observations reported in the literature.