Advances in understanding the defects contributing to the tail states in pure amorphous silicon

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. Semiempirical Hartree–Fock calculations show that these arrangements provide higher energy levels in electronic density of states which are localized on these local structures. Accepting this result we must reconsider the electronic density of states of amorphous semiconductors. The localized mobility gap has structure i.e. two characteristic peaks can be found inside the tail. First larger peak belongs to the squares while the peak at larger energy is formed by triangles.