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Size distribution of Si-nano-particles in amorphous matrix by Raman, HRTEM and XRD

Thin nano-crystalline silicon particles embedded in amorphous silicon matrix were deposited on glass, using plasma enhanced chemical vapour deposition method and 13, 6 MHz (RF) gas discharge in silane gas diluted by hydrogen. By variation of dilution, the crystal to amorphous volume fraction was changed from few percent up to some 70%. The samples were analyzed by Raman, high resolution transmission electron microscopy (HRTEM) and x-ray diffraction (XRD) spectroscopy. The ratio of areas under corresponding TO phonon peaks in Raman was taken as a volume ratio between crystal and amorphous fraction. The crystalline TO peak positions varied between 500 and 521 cm-1 which corresponds to average crystal sizes between 2 and 20 nm, considering only phonon confinement due to nano-size of crystals. However, the TO line was asymmetric and line-width was broad suggesting also the existence of smaller and larger crystals. The size distribution of nano-crystals were estimated from HRTEM and compared with Raman. The average individual sizes showed excellent match when HRTEM results were fitted as two size distribution. However, the distribution was slightly different than those obtained by Raman and suggested larger contribution of smaller crystals. Furthermore, HRTEM micrographs suggested presence of certain strain in the nano-crystals. XRD spectra show Voigt- like peaks assigned to crystalline phase and broad Gaussian- like peaks assigned to amorphous phase. Areas under these peaks were used as a measure of crystalline to amorphous fraction and corresponded well to the results obtained by analogous approach in analysis of Raman results. Voigt-like peaks were analyzed assuming strain and size broadening of diffraction lines. In that way obtained size distribution corresponded well to the distributions obtained using other two methods. The differences in results obtained by applied methods were discussed as a consequence of oversimplified models that neglects influence of strain and shape of nano-crystals (Raman) and possible in-homogeneity of samples that makes "bulk methods" (XRD and Raman) not easy comparable to microscopic methods (HRTEM).

Si nano-particles; Raman; HRTEM; XRD

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