Distribution of the midgap density of states and their capture cross-sections in hydrogenated amorphous silicon deduced from space-charge-limited conduction in the dark and under illumination

Space-charge-limited current measurements in the dark and under illumination offer the means of obtaining the density of states N(E) and their capture cross-sections s(E) in a range of energy E above the equilibrium Fermi level in hydrogenated amorphous silicon (a-Si:H) n+–i–n+ structures. N(E) and s(E) have been measured in the energy region from 0.65 to 0.5 eV below the conduction-band edge E c. The decrease on s(E) observed as the energy E increases agrees with recent observations and is attributed to the coexistence of two types of defect having different capture cross-sections. This is confirmed by light soaking of a-Si:H which makes the defects with high s(E) predominant.