Elctronical conduction in hydrogenated amorphous silicon

Abstract

Electrical conduction in hydrogenated amorphous silicon films prepared by glow discharge decomposition of silane has been studied. The conductivity activation energies and downward conductivity kinds of undoped films were observed to depend on the preparation conditions such as substrate temperature, annealing temperature and discharge geometry, and on the measurement methods such as plane or sandwich geometry. The variation of conductivity activation energy values measured by plane geometry is interpreted by the transfer of electrons between the tissue and the island. The conductivity vs. inverse temperature curves of good quality samples observed by the sandwich geometry are explained by the fact that the top and bottom electrodes can be percolated by the tissue while they can not be percolated by the islands only. The carrier transport studies near the interface in the insulated gate field effect structure reveral that the fixed or movable charge density in the substrate of the order of $10^{10}cm^{-2}$ gives rise to the significant effect on the plane geometry transport. It was observed from the temperature dependence of field effect conductance that the exponential prefactor of conductance vs. the conductance activation energy curve satisfies the Meyer-Neldel rule, which is basically the same as that obtained either by changing the conductivity activation energy by different amounts of doping or by the shifts of Fermi level by illumination. The method to determine the size of mobility gap of thin a-Si:H film is suggested. The size of the mobility gap of a-Si:H film, containing 10 at. \% hydrogen, was observed to be 1.87 eV at zero Kelvin from the temperature dependence of field effect conductance. As the thickness of a-Si:H layer in insulated gate field effect geometry changes from 0.8 ㎛ to 0.1 ㎛, the ratio of the maximum to the minimum point in the plot of conductance vs. applied field voltage change from $~10^4$ to $~10^6$, resulting in the conduction ...