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The structural ordering of amorphous-nanocrystalline silicon thin films by SAXS, vibrational and optical spectroscopy

The structural properties of thin silicon films deposited with PECVD method using microwave (MW) and standard 13, 6 MHz (RF) plasma excitation were compared. The samples were deposited in variety of deposition condition in order to obtain different ratio of crystal to amorphous fraction. In the case of MW PECVD the substrate temperature, gas flows, pressure, and microwave power were varied. For RF PECVD the temperature and power were kept constant while the gas composition was varied. The structural ordering was estimated by using Raman, optical and GISAXS spectroscopy. The ratio of areas under corresponding TO phonon peaks in Raman was tentatively taken as a measure of crystal to amorphous fraction. In fitting procedure the contribution of amorphous phase to the Raman signal was estimated using the 4 Gaussian-like peaks for amorphous and 2 Voight-like for crystalline. The peak widths and frequency shifts were allowed to have variation. This approach resulted in variation of peak positions depending on degree of crystallinity. Lower crystal to amorphous fraction was connected with higher frequency TO Raman peak in both, crystal and amorphous phases, for all of deposited samples. The same was for long wavelength refraction index – the values were lower for higher crystal fraction. However, the values of refraction index for RF PECVD deposited samples were higher that those deposited by MW plasma. All of samples showed strong GISAXS signal indicating presence of "particles". While comparing the samples with similar crystal fraction, the GISAXS signal looked different for two used deposition methods The MW PECVD deposited layers showed pronounced anisotropy. The "particles", presumably voids, had smaller gyro radii, Rg, ranging from 2 to 3 nm in direction parallel to surface than in the depth of the layer, where Rg were between 4 and 6 nm. For faster growing layers, the voids were larger and the samples were les homogeneous. Slow growing samples looked the same on the surface and in the bulk while fast growing ones had pronounced roughness on the surface, with voids larger than 10 nm and the "bulk" voids smaller close to surface than in the deep of the sample. For the RF PECVD samples the GISAXS signal was spherically symmetric and showed no difference between near surface and "bulk" area, which indicates non-columnar growth. The typical voids size for these samples was up to 3 nm. The observed results, in particularly difference in RF and MW deposited samples, were discussed as a consequence of different growing kinetics: the first one assumes the strong plasma-surface interactions while in the second one the plasma is almost remote one.

amorphous silicon; nanocrystalline silicon; SAXS; Raman

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