  The nanostructure of hydrogenated amorphous silicon (a Si:H) is studied by a
combination of small-angle X-ray (SAXS) and neutron scattering (SANS) with a
spatial resolution of 0.8 nm. The a-Si:H materials were deposited using a range
of widely varied conditions and are representative for this class of materials.
We identify two different phases which are embedded in the a-Si:H matrix and
quantified both according to their scattering cross-sections. First, 1.2 nm
sized voids (multivacancies with more than 10 missing atoms) which form a
superlattice with 1.6 nm void-to-void distance are detected. The voids are
found in concentrations as high as 6*10^19 ccm in a-Si:H material that is
deposited at a high rate. Second, dense ordered domains (DOD) that are depleted
of hydrogen with 1 nm average diameter are found. The DOD tend to form 10-15 nm
sized aggregates and are largely found in all a-Si:H materials considered here.
These quantitative findings make it possible to understand the complex
correlation between structure and electronic properties of a-Si:H and directly
link them to the light-induced formation of defects. Finally, a structural
model is derived, which verifies theoretical predictions about the
nanostructure of a-Si:H.
