engleski

The structural ordering of thin silicon films at the amorphous to nano-crystalline phase transition by GISAXS and Raman spectroscopy

Thin silicon films were deposited by PECVD method using microwave (MW) and 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 the degree of dilution that was varied from low dilution that produces amorphous layers up to the high dilution that leads to high degree of crystalline fraction. The samples were analysed by Raman and GISAXS spectroscopy, while some of the samples were examined by TEM in order to check the threshold dilution for crystal formation. The ratio of areas under corresponding TO phonon peaks in Raman was taken as a ratio between crystal and amorphous fraction. In the fitting procedure the peak widths and frequency shifts were allowed to vary. This approach resulted in variation of crystalline fraction between 0 and 70% and variation of TO peak positions between 514 and 520 cm-1 that correspond to crystalline size between 3 and 9 nm. However, the line-width was broad suggesting also the existence of smaller and larger crystals. The GISAXS setup was used to detect nano-sized &laquo ; ; ; particles&raquo ; ; ; . The incident GISAX angle was varied in order to estimate depth size distribution. The 2D distribution of scattered intensities changed markedly with hydrogen dilution in particularly at angles close to critical one. For low hydrogen dilution, which results in amorphous films, the dominant contribution to the signal came from surface roughness. With increasing dilution of the working gas, the contribution of “ particles” from the bulk of material became clearly visible, while there was no evidence of crystallinity in Raman, or by TEM. At the critical dilution, where some crystals can be seen in TEM, the signal originating from “ particles” started to dominate while at highest dilution, the former surface roughness signal become relatively weak. The “ particles” in MW PECVD deposited samples showed pronounced anisotropy while for the RF PECVD samples the GISAXS signal was spherically symmetric and showed no difference between near surface and “ bulk” part, which indicates less columnar growth. The size of particles 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, particularly in the case of MW PECVD samples. The observed results, in particularly difference in RF and MW deposited samples, are discussed in relation to different growing kinetics of the used deposition methods.

silicon; PECVD; Raman; GISAXS; TEM

nije evidentirano