Roles of implantation temperature and ion dose rate in ion-beam synthesis of buried Si3N3 layers

High doses of 135–150 keV N+ ions were implanted in silicon using low (j = 3−5 μA/cm2) and high (j = 100 μA/cm2) dose rates. Target temperature interval was from Ti = 600 to 900°C. The implanted samples were examined by IR transmission spectroscopy, TEM and light reflection spectroscopy. At Ti < 700°C, implantations produced amorphous buried layers. Above 700°C, but still much lower than the normal nitride crystallization temperature, formation of numerous Si3N4 crystallites was observed. Low j caused the formation of α-Si3N4 precipitates and provided for the growth of single-crystal matrix-oriented buried α-Si3N4 layers during post-implantation annealing. High j implantations led in the early stages to the formation of more than 1011 cm−2 tiny β-Si3N4 precipitates which enlarged and combined with increasing dose. It is assumed that the role of Ti is in providing sufficient mobility to the nitrogen atoms and suppression of defect accumulation in silicon. An increase in j favours the accumulation of defects, which serve as nucleation sites for nitride, and enhances the supply rate of nitrogen atoms to the growing precipitates. Thus, low j implantations result in the formation of α-Si3N4 better adapted to the Si matrix and allow single-crystal buried nitride layers to be obtained after annealing. Under high j the silicon lattice fails to orient the growing nitride, therefore the energetically more favourable β-phase forms.