engleski

DC conductivity of amorphous-nano-crystalline silicon thin films

We studied structural properties and direct current (DC) conductivity of nano-crystalline-amorphous Si films deposited by plasma enhanced chemical vapor deposition method using radio frequency (RF) gas discharge in silane - hydrogen gas mixture. The gas composition 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 analyzed by Raman spectroscopy and grazing incidence x-ray scattering (GISAXS) while direct current (DC) conductivity was measured by standard methods. The crystal to amorphous volume fraction and average crystal sizes were estimated by analysing ratio of areas under corresponding transversal optical (TO) phonon peaks and TO peak position in Raman spectra. In that way estimated crystalline fraction was between 0 and 70% while average size of crystals varied from 2 to over 15 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 additional measure of amorphous silicon network ordering. GISAXS showed strong signal that corresponds to "particles" with Gyro radii up to 20 nm. For lower crystal-to amorphous fraction, GISAXS corresponded to spherically symmetric "particles", or arbitrary shaped but statistically oriented particles which is consistent with model of isolated nano-crystals embedded in amorphous matrix. Samples with higher crystalline fraction had elongated "particles" that are larger when places closer to surface, which indicates columnar structure.

DC conductivity; structural properties; silicon thin films

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