Trapping of plasma produced nanocrystalline Si particles on a low temperature substrate

Nanocrystalline silicon layers deposited from silane-hydrogen plasmas were characterized by Raman and ellipsometry measurements. Films were simultaneously deposited on the conventional substrate-holder heated to 250 °C and on a second water-cooled substrate at different discharge pressures ranging from 400 mTorr to 1.2 Torr. At low pressures (400–500 mTorr), the films are amorphous on the cold substrate and microcrystalline on the hot one. As pressure increases (between 600 and 900 mTorr), gas phase reactions lead to the formation of nano-crystalline particles and thermophoresis enhances the deposition of silicon nano-crystals on the cooled substrate. The analysis of Raman spectra of the samples obtained on the cold substrate shows broadening and small spectral shifts of the TO crystalline silicon band indicating a decrease in crystal size with plasma pressure. Moreover, increasing the temperature gradient by cooling the substrate with liquid-nitrogen, we increase the trapping efficiency and achieve films containing even smaller nanometer size crystals as deduced from the position of the TO Raman peak at around 515 cm−1. At pressures above 1 Torr, we obtain amorphous silicon on both substrates and for all conditions due to the dominant formation of powders in the plasma at those relatively high pressures.