Reversible changes in temperature dependence of electric conductivity of hydrogenated amorphous silicon caused by proton irradiation

We investigate temperature dependence of electric conductivity and photoconductivity of hydrogenated amorphous silicon (a-Si:H) thin films due to energetic proton irradiation and clarify the change in dominant electric conduction mechanism. We also investigate variations of the temperature dependence after proton irradiation at different temperatures and the recovery from radiation damage due to thermal annealing. The results show that the high fluence proton irradiation changes the dominant electric conduction mechanism from the band transport of thermally excited carriers through the extended states to the hopping transport based on the carrier transport via localized states near the Fermi level. The hopping conduction has the weak temperature dependence which disappears at elevated temperature and the strong temperature dependence based on the band conduction appears again. Also, the proton irradiation at high temperature never provoke the hopping conduction. However, the band conduction after irradiation is not completely restored even after thermal annealing, indicating that thermally stable radiation defects remain. We conclude that these dominant electric conduction mechanism changes originated from proton irradiation and thermal annealing, and are simply caused by the increase or decrease in localized density of states.