Structural modification of swift heavy ion irradiated amorphous Ge layers at low temperatures

Recent room-temperature irradiation experiments exposing amorphous germanium (a-Ge) to 185-MeV heavy ions (SHI) at the ANU accelerator facility (Canberra, Australia) revealed strong volume expansions [1]. A detailed study of this effect using various irradiation parameters [2] demonstrates that this swelling is caused by the formation and growth of randomly distributed voids leading to a gradual transformation of the a-Ge layer into a porous structure. Moreover, the swelling linearly depends on the ion fluence and on the value of electronic energy deposition. This clearly demonstrates that the structural changes are determined solely by the electronic energy deposited within the amorphous layer. We thus conclude that voids are formed in a-Ge, if a specific threshold value of the energy deposited by electronic processes is exceeded [2]. In this project we investigated the effect of the irradiation temperature as well as the influence of higher electronic energy deposition on the void formation in a-Ge. Prior to the irradiation at GSI, crystalline Ge-wafers were amorphised by ion irradiation with various low energy (0.1 6.7-MeV) Ge-ions and fluences at 80 K, resulting in a 3.1 μm thick amorphous layer. The irradiations with SHI were performed at the new M3-branch at the UNILAC. For this purpose, a new target holder was constructed allowing simultaneous irradiations at room (RT) and low temperature (LT), which enables the comparison of both samples under same irradiation conditions. The sample was irradiated at T ~ 290 and 55 K with 940-MeV Au ions (projected range ~ 39 μm). The beam flux was 1 × 10 cm s and the fluence ranged between 2 × 10 and 2.3 × 10 cm (accuracy ± 30%). In order to quantify the swelling by direct comparison of irradiated and unirradiated material, one half of the sample was masked.