Ion irradiation effects on electric properties of hydrogenated amorphous silicon thin films

Change in the dominant electronic conduction mechanism of hydrogenated amorphous silicon (a-Si:H) thin films from the band transport to the hopping transport due to ion irradiation is investigated. The change is clarified by the experimental study of electric conductivity of a-Si:H irradiated with energetic protons. Dark electric conductivity (DC) and photoconductivity (PC) variations as a function of 100 keV proton fluence, and variations of temperature dependence of DC due to 100 keV proton irradiation are investigated in detail. As a result, the decrease in DC and PC due to reduction of the band transport is observed at the fluence of less than 1014 cm-2, and the drastic increase in DC and the loss of photoconduction due to enhancement of the hopping transport are observed in the high fluence regime. However, the hopping transport induced by proton irradiation easily disappears at above 300 K and after that, the band transport dominates the electric conduction again. The conductivity based on the band transport after irradiation is not completely restored even after thermal annealing, indicating that thermally stable dangling bonds remain. It is concluded that these electronic transport changes originated from ion irradiation and thermal annealing are caused by the increase or decrease in dangling bond density (localized density of states).