The conductivity of undoped polycrystalline material was measured  in situ  during exposure to monatomic H at high temperatures. At first, the conductivity increased due to electrons which were contributed by in-diffusing H donors. After long exposure times, the conductivity decayed exponentially. Hall-effect data revealed that the Fermi energy shifted towards the valence band, and that the majority carriers changed from electrons to holes; thus indicating the creation of acceptor states. The observed type-conversion was attributed to the diffusion of excess H from the plasma, since it did not occur during exposure to species such as O. Acceptor creation was thermally activated, with an energy of 1.62eV, and the acceptors annealed out with an activation energy of 2.75eV.

N.H.Nickel, N.M.Johnson, J.Walker: Physical Review Letters, 1995, 75[20], 3720-3