Swift heavy ion irradiation of amorphous ge, amorphous si and amorphous si1-xgex alloys

Abstract

The interaction of swift heavy ions (SHIs) with solids is characterised by inelastic collisions between the ions and the target electrons. SHIs typically deposit tens of keV/nm of energy in the target material causing extreme excitation of the electronic subsystem that result in different damage formation mechanisms than for ion implantation such as the formation of ion tracks, plastic deformation or porous layer formation. The crystalline phases of the elemental semiconductors Ge and Si are relatively resistant to SHI irradiation induced damage. In contrast, their amorphous counterparts (a-Ge and a-Si, respectively) are subject to SHI irradiation induced plastic deformation and porous layer formation. The former is caused by ion hammering and entirely predicated on the creation of ion tracks and thus provides indirect evidence for ion-track formation in a-Ge and a-Si. SHI irradiation was performed on a-Ge, a-Si and amorphous Si(1-x)Ge(x) (a-Si(1-x)Ge(x)) with different stoichiometries to study the study ion-track and porous layer formation in these materials. Synchrotron-based small-angle x-ray scattering was utilised to characterise the structure of ion tracks in the amorphous materials and the formation of nanoporosity was investigated by scanning electron microscopy. The experimental observations were complemented by a novel theoretical approach comprising a Monte Carlo calculation of the electron dynamics, a Two-Temperature Model description of the heat dissipation, and Molecular Dynamics simulations of the atom dynamics. Ion-track formation has been identified for a-Ge, a-Si and a-Si(1-x)Ge(x) alloys. While SHI irradiation of all such materials results in the formation of overall densified ion tracks with an underlying core-shell morphology, different mechanisms for the formation of ion tracks are revealed. Ion tracks in a-Ge are comprised of an under-dense shell surrounded by an over-dense core. The formation of ion-tracks is accompanied by the formation of non-spherical voids which are identified as the precursors for the porous layer formation under continuing SHI irradiation. On the contrary, ion tracks in a-Si and a-Si(1-x)Ge(x) alloys feature a dominant over-dense core surrounded by an under-dense shell. The formation of non-spherical voids was observed for a-Si(0.2)Ge(0.8), however, voids are absent in all other SHI irradiated alloys and a-Si. Continuous SHI irradiation leads to the formation of porosity in a-Ge, a-Si and a-Si(1-x)Ge(x) alloys. In a-Ge and Ge-rich a-Si(1-x)Ge(x) alloys, self-organisation of pores into well separated porous layers occurs. The layering effect depends on the irradiation energy, angle of incidence and thickness of the amorphous layer. SHI irradiation induced void formation in a-Si occurs at much higher ion fluences relative to a-Ge. In contrast to a-Ge, no self-organisation of pores is apparent in a-Si and the voids exhibit a cavity-like morphology, which implies a different void formation mechanism in the two elemental semiconductors. The results on ion-track and porous layer formation in a-Ge, a-Si and a-Si(1-x)Ge(x) alloys shed new light on the damage evolution in amorphous semiconductors due to high electronic excitation and may provide a pathway for the development of novel materials. Show more