Effect of hydrogenation on the conductivity of UHV-deposited amorphous silicon

The d.c. plasma post-hydrogenation of pure amorphous silicon (a-Si) strongly modifies its conductivity. The low-temperature hopping conductivity is reduced by several orders of magnitude, whereas the high-temperature activated regime remains nearly unchanged. After hydrogenation an extra conductivity regime is observed which is dependent on the incorporated hydrogen concentration and is attributed to band-tail conduction. The conductivity is discussed in terms of the structural defects of the a-Si matrix, the associated gap states and their modification upon incorporation of hydrogen.