It is known that amorphous Ge (a-Ge) layers become porous when irradiated with ions both in the nuclear and electronic stopping power regimes (provided the minimum irradiation fluence threshold is achieved). In the nuclear stopping regime, the formation of pores is usually interpreted as the agglomeration of vacancies due to collision cascades. In the electronic stopping regime, however, this interpretation cannot be used, and the mechanisms for pore formation remain unidentified. Here we report on the early stages of pore formation in a-Ge layers irradiated with 185 MeV Au ions (for which the electronic stopping is dominant), by means of small-angle X-ray scattering (SAXS). We demonstrate the irradiation with swift heavy ions leads to the formation of two distinct populations of features: one that is highly anisotropic, with the longest dimension parallel to the direction of the incident beam, which can be assigned to long cylinders of approximately 4 nm radius; and another that is isotropic and can be depicted as nearly spherical regions, possibly voids, of 3 nm mean radius. The scattering amplitude of the latter increases proportionally to the irradiation fluence up to 3x10¹² cm^-2, suggesting these regions could be interpreted as the embryonic precursors for pore formation in the electronic stopping power regime. Comparisons are made with a-Si irradiated under similar conditions, for which the presence of tracks is clearly detected by SAXS analysis, but with no evidence of voids or porous structure formation.

Raquel Giulian*, Australian National University, Australia
Leandro Langie Araujo, Australian National University, Australia
Patrick Kluth, Australian National University, Australia
Boshra Afra, Australian National University, Australia
Matias Daniel Rodriguez, Australian National University, Australia
Aidan P. Byrne, Australian National University, Australia
Stephen Mudie, Australian Synchrotron, Australia
Nigel Kirby, Australian Synchrotron, Australia
Mark Cameron Ridgway, Australian National University, Australia

*presenting author