Structure and Electronic Properties of Amorphous Silicon

This thesis is a study of the relationship between the structural disorder and the electronic properties in amorphous silicon. We present the solution of the electronic density of states (DOS) for clusters embedded in a Bethe lattice with first and second neighbor interactions. DOS of clusters with disordered bond angles show that breaking the T_{d} symmetry predominately affects the top of the valence band while changes in the second neighbor distances will influence the states at the bottom of the conduction band. Dihedral angle variations narrow the p-like peaks, but do not contribute states at the band edges. We also show that bond angle disorder will widen the band of dangling bond defect states in the gap. Band tail widths and optical band gaps are obtained as a function of the standard deviations in the bond angles. Averaged DOS of clusters and Cayley trees with distorted bond angles have wider band tails in the conduction band than in the valence band. The addition of hydrogen will decrease the band tail width in the conduction band resulting in a band tail ~1.5 times smaller at the conduction band edge than at the valence band edge (for standard deviation of 8^circ in bond angle). DOS of hydrogen bonds show a peak at -5 eV below the Fermi level with an additional peak at -10 eV for poly-hydride bonds. Fractions of mono- to poly-hydride bonding can vary for different thin-film deposition techniques. Absorption coefficients in the infra-red region are used to determine the ratio of mono- to poly-hydride bonding and a statistical model is presented to demonstrate that this ratio is dependent on the total hydrogen concentration. Force constant calculations are presented that determine shifts in the hydrogen vibration frequencies due to neighboring impurities. Also, an empirical model, applied to experimental data of changes in the reflectivity from ultra-fast free carrier excitations, is used to describe dispersion in the complex dielectric function in the energy region near the optical band gap.