Optimization of plasma parameters for the production of silicon nano-crystals

We use silane-hydrogen plasmas to synthesize silicon nano-crystals in the gas phase and thermophoresis to collect them onto a cooled substrate. To distinguish between nano-crystals formed in the plasma and those grown on the substrate, as a result of surface and subsurface reactions, we have simultaneously deposited films on a conventional substrate heated at 250 deg. C and on a second substrate cooled down to 90 deg. C. A series of samples deposited at various discharge pressures, in the range of 400 mTorr to 1.2 Torr, have been characterized by Raman spectroscopy and ellipsometry. At low pressure (400-500 mTorr), the films are amorphous on the cold substrate and micro-crystalline on the hot one. As pressure increases, gas phase reactions lead to the formation of nano-crystalline particles which are attracted by the cold substrate due to thermophoresis. Consequently, we obtain nano-crystalline silicon thin films on the cold substrate and amorphous thin films on the heated one in the pressure range of 600-900 mTorr. Moreover, the analysis of the Raman spectra of the samples obtained on the cold substrate shows broadening and a small spectral shift as pressure is increased, indicating a decrease in crystal size with pressure. Finally, for yet higher discharge pressures (above 900 mTorr), the high reaction rates favour the formation of amorphous clusters resulting in the deposition of amorphous silicon films on both substrates