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

We observe that the charge state of diffusing hydrogen depends upon the electronic Fermi level (Ef) in hydrogenated amorphous silicon (a-Si:H). We incorporate a thin layer of deuterium (2H) 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 6×104 V/cm at each 2H layer. After annealing, marked asymmetries in the secondary ion mass spectrometry profiles of diffused deuterium are observed. With the 2H layer placed near the p-layer (Ef near the valence band), diffusion is into the p-layer, indicating H+ moving with F. With the 2H layer near the n-layer (Ef 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.