DC conductivity of amorphous–nanocrystalline silicon thin films

Abstract

The direct current (DC) conductivity of amorphous–nanocrystalline Si films deposited by the plasma enhanced chemical vapour deposition method was studied as a function of the structural properties obtained by Raman spectroscopy and grazing incidence small angle X-ray scattering (GISAXS). The crystalline fraction estimated from the Raman spectra altered between 0 and 60% while the average size of the crystals varied from 2 to 7 nm, however, the size distribution was wide i.e. smaller and larger crystals were also present. GISAXS showed a signal that corresponds to “particles” with values for the gyration radius close to the average crystal sizes, between 2 and 6 nm. Samples with higher crystalline fraction had elongated “particles” that are larger when situated closer to the sample surface, which indicates a columnar structure. The DC conductivity had a nearly constant, low value up to some 30% of crystal fraction. A further increase of the crystal fraction resulted in an abrupt increase of the conductivity in a narrow interval of crystal fraction. Above this interval, conductivity was much higher and remained constant in that range. This result is in perfect agreement with the percolation threshold obtained by model calculation for a six-fold coordinated cubic lattice that appears at 32% of crystal fraction. A certain scattering of the experimental data around the predicted values was discussed as a possible consequence of the variation of the individual crystal size and shape by change of the crystal fraction and/or non-uniformity of depth distribution.