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The DC conductivity and structural ordering of thin silicon films at the amorphous to nano-crystalline phase transition

Thin silicon films were deposited by PECVD method using standard 13, 6 MHz (RF) gas discharge in silane gas, diluted by hydrogen. The deposition condition was kept constant from sample to sample for all parameters except for the degree of dilution that was varied from low values that produces amorphous layers up to the high dilution that leads to high degree of crystalline fraction. The structural properties of samples were analysed by Raman and GISAXS spectroscopy while DC conductivity was measured by standard methods. The ratio of areas under corresponding 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 estimation of crystal size, assuming only quantum effects. By increasing working gas dilution, crystalline fraction grew from 0 and 70% and the average size of crystals increased from 2 to 9 nm. However, the size distribution was broad e.g. the smaller and larger crystals are also present. The size of "particles", estimated upon GISAXS spectra by using Guinier approximation, varied from 2 to 12 nm while at the highest dilution used here, some of them reached more than 100 nm. In most of the samples particles were larger when closer to the surface. For low working gas dilution, the "particles" detected by GISAXS were spherically symmetric and showed no difference between near surface and "bulk" part. For higher dilution, the particles became asymmetric and larger when closer to the surface, which indicates columnar growth. The changes in DC conductivity do not follow the changes in crystal to amorphous fraction that grows gradually with hydrogen dilution. The DC conductivity changes abruptly by several orders of magnitude in relatively narrow dilution interval. This effect has been discussed in the framework of percolation theory.

nanocrystalline silicon; GISAXS; Raman; DC conductivity

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