Fermi-level dependence of the charge state of diffusing hydrogen in amorphous silicon

Abstract We observe that the charge state of diffusing hydrogen depends upon the electronic Fermi level ( E f ) in hydrogenated amorphous silicon (a-Si:H). We incorporate a thin layer of deuterium ( 2 H) at various positions between the n- and p-layers of i–n–i–p–i structures on crystalline silicon substrates. The electric field ( F ) is above 6×10 4 V/cm at each 2 H layer. After annealing, marked asymmetries in the secondary ion mass spectrometry profiles of diffused deuterium are observed. With the 2 H layer placed near the p-layer ( E f near the valence band), diffusion is into the p-layer, indicating H + moving with F . With the 2 H layer near the n-layer ( E f near the conduction band), most diffusion is into the n-layer, indicating H − moving against F . Because the Si–H bond is neutral, the charged diffusing species must be emitted mobile H. We estimate an effective correlation energy of 0.4±0.1 eV for the mobile H.