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 (E_f) 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 (E_f near the valence band), diffusion is into the p-layer, indicating H⁺ moving with F. With the ²H 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.