Doping of amorphous silicon in the hopping transport regime

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Abstract
Heavily doped thin films of monocrystalline silicon have been transformed to the amorphous state by irradiation with energetic neon ions. Electronic transport properties were measured on these hydrogen-free amorphous films, both in the as-irradiated state and after various annealing treatments. During amorphization the previously largely substitutional and electrically active impurities were mostly transferred to electrically inactive configurations. Among the different group III and V elements, sizeable doping effects were observed only for boron and phosphorus in annealed films, where hopping conductivity is enhanced by about three orders of magnitude. The fraction of electrically active impurity centres in those films is estimated to be about 1% at most. The doping effects on conductivity are discussed with respect to the hopping transport theory and the microstructure of a-Si.