Gap states in undoped amorphous silicon studied by below-gap modulated photocurrent spectroscopy

The energetic distribution and nature of dangling-bond states in undoped a-Si: H have been investigated by a newly developed gap-state spectroscopy which is based on the frequency-resolved spectrum of modulated photocurrent with below-gap photoexcitation. The energy scale is directly specified by the energy of the below-gap light populating the particular gap states. The result has led to the conclusion that the peak of the gap-state distribution associated with doubly occupied dangling bonds (D-) is located about 0•5 eV below the conduction band edge. The analysis for the pre-exponential factor of the thermal emission rate of electron from the D- centre suggests that the emission occurs in two stages through excited states.