Temperature dependent light induced conductivity changes in hydrogenated amorphous silicon

The light induced changes for undoped and phosphorus doped hydrogenated amorphous silicon films deposited by dc glow discharge decomposition have been studied. It is observed that the conductance change after illumination is large for the undoped films deposited near the cathode, and deposited with high silane flow rate. The 0.1%-0.5% phosphorus doped samples show the increase of dark- and photoconductance after illumination. The rate of dark- and photoconductance change are observed to be accelerated as the temperature during the illumination is elevated. The activation energies for the recovery of the conductance changes are observed to be 1.5 and 1.0 eV for undoped and 0.1% phosphorus doped films, respectively. The field effect measurements for 0.1-μm-thick undoped a-Si:H film before and after the illumination show that the gap state density is not changed after illumination. The results are explained on the basis of material heterogeneity. The trapping regions, considered to be contained in tissue, should be assumed to exist in the material to explain the results. The transfer of electrons (or holes for p type samples) between islands and the trapping regions give rise to the light induced changes in conduction.