Anomalous magnetoresistance in rf-sputtered hydrogenated amorphous silicon

Magnetoresistance (MR) measurements for magnetic fields up to 10 T have been made on amorphous-silicon---hydrogen (a-Si: H) thin films prepared by reactive rf sputtering. The effect of a systematic decrease of the gap state and spin densities due to the hydrogenation on MR has been investigated. Electron-spin-resonance (ESR) and dark dc conductivity measurements, which give supporting information on the electrical behavior of a-Si: H, have also been made in the temperature range 30-500 K. The Movaghar-Schweitzer-Osaka hopping model, in which MR is a result of the magnetic field dependence of the spin-flip relaxation time of the charge-carrier spin, accounts well for the results at temperatures $T>150$ K. With the fitting of this model to the MR results, values of the positive Hubbard-type repulsion energy Ueff for doubly occupied gap states near the Fermi level are obtained, Ueff=0.21 eV for the a-Si films and Ueff=0.31-0.32 eV for the a-Si: H films. It is concluded that the change of sign in MR at low fields with decreasing spin density is due merely to a narrowing in the distribution of the hopping relaxation times. Some of the parameters needed in the analysis of the MR results are determined by the fitting of Mott's variable range-hopping model to the conductivity data of the films with Nₛ×10¹⁸ spins cm^-3. Quite reasonable values of Mott's parameters are obtained if high values for jump-rate prefactors ν₀=10¹⁹ to 10²⁰ s^-1 are accepted, which values are also needed to account for the magnitudes of the MR effect and the ESR linewidth.