Induced defects in a-Si:H and a-Ge:H calculated by molecular orbital theory

Gap states induced by weak bond and hydrogen movement in a-Si:H have been investigated on the basis of DV-X/spl alpha/ molecular orbital theory. Two energy levels appear near both sides of band edge, and shift to the gap center, while the weak bond is stretched and the bonded H atom is moved to the neighboring Si or Ge atoms. These additional states deep in the energy gap are due to the modification of bonding states, decreasing the electrical conductivity of a-Si:H. These facts give a possible explanation of light-induced degradation, when the weak bond and the movement of bonded H atoms are induced by prolonged exposure to intense light.