RQMP Research Seminar

Density changes in amorphous silicon provoked by swift heavy ions

Sjoerd Roorda

Université de Montréal

The tracks left by heavy energetic ions passing through a solid material are an example of the consequences of “very far from equilibrium processing”. When the material being irradiated is amorphous silicon, the story is complicated by the fact that liquid silicon is more dense than both crystalline and amorphous silicon, and that amorphous silicon has a melting point depressed by some 200 K. So, here's the experiment: Pure and gold-doped amorphous silicon membranes were irradiated with swift heavy ions (75 MeV Ag or 1.1 GeV Au ions) and studied by small angle X-ray scattering. The samples that were irradiated with 1.1 GeV Au ions produced a scattering pattern consistent with core-shell type ion tracks of 2.0+/-0.1 nm (core) and 7.0+/-0.3 nm (total) radius irrespective of gold doping and consistent with radii previously observed [Bierschenk et al:, Phys. Rev. B 88, 174111 (2013)]. However, the core must be less dense than the original amorphous silicon, not more dense as argued in the same report, because its density is nearly 4% different from that of the surrounding material. The compressive stress required to maintain the core 4% more dense would exceed the yield strength of amorphous Si. The entire track (core + shell) is slightly less dense than the surrounding material, putting it under a lateral stress consistent with the macroscopic “hammering” deformation seen when tracks overlap. No tracks were found in samples irradiated with 75 MeV Ag ions, and no signature specific to the gold impurity doping could be observed.