Mechanism and kinetics of the ion-assisted nucleation in amorphous silicon

The processes of nucleation and growth of crystalline silicon grains in amorphous Si layers irradiated at temperatures between 450 and 530 degrees C with a 3-MeV Pt or a 1-MeV Si ion beam have been investigated in details using transmission electron microscopy. The crystal-growth kinetics exhibits a behavior similar to that observed during ion-assisted epitaxial crystallization, i.e., it increases by increasing the energy deposited into nuclear elastic collisions by the impinging ions, and weakly decreases by increasing the irradiation dose rate. Vice versa, the nucleation rate decreases by increasing the energy deposition of the ion and strongly decreases by increasing the dose rate. These results can be quantitatively explained by following a description already tested for the ion-assisted solid-phase epitaxy. Under ion irradiation, the evolution of a crystal-amorphous interface is controlled by the competition between a prompt amorphization and a subsequent crystallization due to long-living defects generated by the ion beam. At high temperatures the former term is much smaller than the latter and therefore the variations of the relative weights produce weak effects on the crystal-growth velocity. However, the same variations cause appreciable shifts of the critical crystalline cluster size for the nucleation process. This effect produces a strong dependence of the steady-state concentration of the critical crystalline embryos and of the nucleation rate on the irradiation conditions.