What is the origin of tail states in amorphous semiconductors?

The tight-binding molecular dynamics simulations and reverse Monte Carlo structural modeling method were applied in order to investigate the existence of small bond angles (like those in triangles and squares) in amorphous silicon networks. The influence of small bond angles on the electronic density of states was analyzed. The presence of a number of smaller bond angles is necessary for a proper reproduction of the neutron diffraction data of amorphous silicon. Semi-empirical Hartree-Fock calculations show that these arrangements provide higher energy states in electronic density of states which are localized on these local structures. We consider that smaller bond angles are a new type of defects. These defect states are highly energetic and they are the origin of tail in amorphous semiconductors.