Silicon atoms in amorphous silicon mostly have the same basic structure that they have in crystal silicon: each silicon atom is connected by covalent bonds to four other silicon atoms arranged as a tetrahedron around it. However, amorphous Si has numerous atoms with only 3 bonds satisfied, the fourth bond unempty, it is a "dangeling” bond, consequently the lattice is deformed, there is no long-rang order.
In hydrogenated amorphous silicon (a-Si:H), an hydrogen atom is attached to the dangling bond. This hydrogen is invisible to X-rays, but is evident in proton magnetic resonance (Reimer and Petrich 1989) and infrared spectroscopy (Zhao et al.
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Fig. 41.3 Defect (dangling bond) density as a function of illumination time of an a-Si:H film as measured by Park et al. (1989). The intensity of illumination is indicated in the legend |
1995), secondary ion mass-spectroscopy (Santos et al. 1991), and can be collected in hydrogen evolution during annealing (Beyer et al. 1991).
Several configurations for the hydrogen in a-Si:H are possible. One is the dilute phase where each hydrogen atom is about 1 nm away from any other hydrogen atom; the other phase is the clustered phase where two or more hydrogen atoms are in close proximity. The densities of hydrogen in each of the individual phases is different from each other. In the bulk of a-Si:H the total density of hydrogen depends on the conditions under which it is made.
