Doping and the Fermi Energy in Amorphous Silicon

The doping of hydrogenated amorphous silicon is discussed within the context of Mott's $8\ensuremath{-}N$ rule, and found to have properties distinctly different from dopants in crystalline silicon. A unifying description of dopant and defect states based on the position of the Fermi energy is given. Some possible consequences of the $8\ensuremath{-}N$ rule for the deposition and structure of $a$-Si: H are also discussed.