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Structural ordering of thin amorphous-nano-crystalline films studied by HRTEM, GISAXS and Raman spectroscopy

Thin nano-crystalline silicon films were deposited on glass, using the PECVD method and 13, 6 MHz (RF) gas discharge in silane gas diluted by hydrogen. By varying the dilution, the crystal to amorphous fraction was changed from a few percent up to 70%. The samples were analysed by Raman, HRTEM and GISAXS spectroscopy. The ratio of the areas under corresponding TO phonon peaks in Raman was used to estimate the ratio between the crystal and the 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 the nano-size of the crystals. However, the TO line was asymmetric and the broad line-width suggested the existence of also smaller and larger crystals. The size and size distribution of nanocrystals estimated from HRTEM and compared with Raman, show an excellent match in the average individual sizes. However, the distribution was slightly different than those obtained by Raman and suggested a larger contribution of smaller crystals. GISAXS of the deposited samples indicated particles with Gyro radii between 2 and 6 nm. Larger particles are present in samples with a higher degree of crystal fraction and larger crystals. However, the samples are porous to a certain degree and the direct correlation between particles and crystals is not easy to establish. GISAXS obtained by varying the grazing incidence angle allowed to estimate the in-depth distribution of the particles. For growth conditions that favor smaller nano-crystals, the particles were spherically symmetric and their individual sizes were uniform from the surface towards the depth of the sample. For growth conditions that resulted in larger crystals the particles were elongated parallel to the surface and smaller in the sample "bulk". These results were used in the discussion of the difference in the distributions of individual crystal sizes obtained by HRTEM and Raman.

amorphous-nano-crystalline silicon; HRTEM; GISAXS; Raman

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