Thermal stability of Al/nanocrystalline-Si bilayers investigated by in situ heating energy-filtered transmission electron microscopy

Abstract

In situ heating energy-filtered transmission electron microscopy was employed to investigate the interfacial intermixing/reactions during thermal annealing of Al/nanocrystalline-Si (nc-Si) bilayers in the temperature range of 150–500 °C. In comparison with the Al/amorphous-Si (a-Si) bilayer, the Al/nc-Si bilayers were found to be much more stable against thermal annealing. Wetting and c-Si growth processes along Al grain boundaries, which take place during annealing of Al/a-Si bilayers, do not occur in Al/nc-Si bilayers, because of the lack of thermodynamic driving forces in the latter case. As a consequence, also in contrast with Al/a-Si bilayers, no layer exchange occurs in Al/nc-Si bilayers, not even after annealing at 500 °C. Instead, intermixing of Al/nc-Si is realized at the Al/nc-Si interface by the formation of Al spikes growing into the nc-Si sublayers at temperatures higher than 300 °C. The relatively low Al-spike formation temperature in Al/nc-Si systems, as compared with that for Al/single-crystalline Si systems, is ascribed to the higher Gibbs energy of nanocrystalline Si as compared to single-crystalline Si.