engleski

The influence of structural ordering on DC conductivity of the amorphous-nano-crystalline silicon thin films

Thin silicon films were deposited by plasma enhanced chemical vapour deposition method using radio frequency (RF) gas discharge in silane gas, diluted by hydrogen. The gas dilution was varied in order to obtain variety of structural ordering, starting from pure amorphous to the high degree of nano-crystalline phase. The structural properties of samples were analysed by Raman spectroscopy and high resolution transmission microscopy (HRTEM). The direct current (DC) conductivity was measured by standard methods. The ratio of areas under corresponding transversal optical (TO) phonon peaks in Raman was taken as a ratio between crystal and amorphous volume fraction. The shift of TO peak position was used for the estimation of crystal size, assuming only quantum effects. By increasing working the gas dilution, the crystalline fraction grew from 0 and 70% and the average size of crystals increased from 2 to 9 nm. However, the size distribution was wide i.e. the smaller and larger crystals were also present. The width of corresponding TO phonon peak and the ratio between transversal optical and transversal acoustical phonon peaks were used as a measure of amorphous silicon network ordering. HRTEM confirmed the presence of nano-crystals with dimensions between 1 and some 20 nm. It showed that the crystal fraction increased, from the substrate towards the films surface. Furthermore, for the lower crystal fractions, the crystals were fully surrounded by amorphous matrix while at highest crystal fraction ; coalescence of individual crystals became visible. The DC conductivity is a complex function of films ordering, that includes the amorphous to crystalline ratio, ordering of amorphous matrix, the presence of voids. However, we demonstrated the possibility of using different percolation theories in explanation of abrupt change in DC conductivity by several orders of magnitude in the relatively narrow interval of amorphous to nano-crystalline transition. The modeling estimations were based on the measured structural properties.

silicon thin films; structuraal ordering; DC conductivity

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