Annealing and light induced changes in the field effect conductance of amorphous silicon

Abstract

We have measured the field effect conductance of α-Si:H and its dependence on annealing to 180 °C and illumination with white light. We find that illumination produces changes in the field effect conductance which are completely reversed by annealing. Illumination produces a decrease in the off conductance between a factor of 5 and 30 times, together with a shift of the threshold voltage. We interpret the effect of illumination as a modification of the density or occupancy of deep states in such a way that the bulk Fermi level is moved with respect to the conduction band mobility edge. Space-charge regions can exist at either or both interfaces of the α-Si:H film and these are very sensitive to interface conditions. The magnitude of the surface band bending is modified by electron trapping in the gate insulator (SiN), which is brought about by bias- temperature stressing. Annealing to 180 °C causes the detrapping of excess electrons in the SiN layer, reversing the effect of positive bias-temperature stressing.