Structure dependence of the electrical conductivity of hydrogenated nanocrystalline silicon films

Hydrogenated nanocrystalline silicon films were deposited by reactive sputtering in a pure hydrogen plasma at various substrate temperatures T-s. The layers were investigated by infrared spectroscopy, Raman scattering, transmission electron microscopy and electrical conductivity measurements. Upon increasing T-s from 50 degrees C to 250 degrees C, the crystalline fraction was found to grow from a value less than 50 % to about 75 %. Additionally, the room-temperature conductivity exhibit an exponential increase from some 10(-10) Ohm(-1).cm(-1) to 10(-3)-10(-2) Ohm(-1).cm(-1) while the activation energy decreases from 1.25 eV to a value as low as 0.13 eV. These results were correlated with crystalline fraction that seems to govern the nature and properties of the material. The high conductivity (10(-3)-10(-2) Ohm(-1).cm(-1)) and the corresponding low activation energy (0.13 eV) for T-s = 250 degrees C, suggest a thermally assisted tunneling conductivity process.