Structure and Carrier Transport in Amorphous Semiconductors

First-principles methods are used to study the network dynamics of hydrogenated amorphous silicon, including the motion of hydrogen. In addition to studies of atomic dynamics in the electronic ground state, a simple procedure is adopted to track the H dynamics in light-excited states. Consistent with recent experiments and computer simulations, formation of dihydride structures are observed for dynamics in the light-excited states, and explicit examples of pathways to these states are presented. My results appear to be consistent with aspects of the Staebler-Wronski effect, such as the light-induced creation of well-separated dangling bonds.

The network topology and defects in the amorphous Si_1-xGe_x:H alloys have been analyzed. Structural changes, particularly an increase in the number of defects and strained bond angles, have been found as the Ge content increases from x=0.1 to 0.5. The electronic density of states exhibits a decreasing band gap and additional midgap and band-tail defect states as the Ge concentration increases. The network structures, which are responsible for midgap and band-tail states, are presented. The band tails show an exponential “Urbach” behavior. The mobility gap as a function of Ge concentration is also estimated.

Simulation of dynamics of localized states in a presence of thermal disorder is studied by integrating the time dependent Kohn-Sham equation and density functional Hamiltonian. A rapid diffusion of the localized state to the extended state in a very short time step is observed. This diffusion is explained to be due to quantum mechanical mixing when another states gets close in energy to the state that is being tracked. Finally, a study of transport in amorphous semiconductors is presented. Kubo- Greenwood formula is used for computing the DC conductivity of elemental and hydrogenated amorphous silicon for different temperatures. The results from this method are in good agreement with the experiment. The effect of doping on the DC conductivity is also presented. As the E_f level gets close to either the conduction edge or valence edge the DC conductivity increases. Once E_f exceeds the “mobility edge” there exist a weak temperature dependence on the DC conductivity.