Depth profiling of defects in a-Si:H films by means of modulated photocurrent

This work reports about experimental findings obtained by means of Modulated Photocurrent (MPC) spectroscopy as applied to a-Si:H films in sandwich configuration. The gap density of states from 0.4 to 0.7 eV below the conduction band edge has been reconstructed for three excitation wavelengths. This allows for a spatial depth profiling of unoccupied defects. It is shown that the defects peak apparently shifts towards midgap when probing the region near a-Si:H-metal interface. This result is interpreted by means of two different models. The likelihood of these models is discussed by comparing the results with other spectroscopic data.