Ion channelling and electronic excitations in silicon

Abstract

When a high-energy (MeV) atom or neutron collides with an atom in a solid, a region of radiation damage is formed. The intruding atom may cause a large number of atoms to leave their lattice sites with low energies (keV) in a collision cascade. Alternatively, it may cause a single lattice atom to be removed from its crystal lattice site and travel large distances without undergoing any further atomic collisions/scatterings in a process known as channelling. Any moving atom may lose energy via collisions with other atoms or by the excitation of electrons. The microstructural evolution of the irradiated material depends on the rate at which the damaged region cools, which in turn depends on the rate at which electrons are excited and carry energy away. Since silicon is a semiconductor with a band gap of 1.1 eV, it is normally assumed that the rate of excitation of electrons by atoms with low kinetic energies (< 100 eV) is negligible. However, the atomic kinetic energy threshold required for electronic excitation is not understood. This thesis uses large-scale quantum mechanical simulations to investigate how a moving atom loses energy to the electrons in a crystal of silicon. It is possible to calculate the energy transfer from a channelling atom to the host material’s electrons using time-dependent density functional theory (TDDFT) and we have done so. However, these highly accurate calculations are computationally expensive and cannot be used to study very large systems. Hence, we also use a less accurate method called time-dependent tight binding (TDTB). We show that TDTB and TDDFT simulations of channelling in small silicon systems are in good qualitative agreement and use the cheaper TDTB method to investigate finite-size effects. We also investigate how an atom oscillating around its lattice site transfers energy to the crystal’s electrons. To understand the complex behaviour of the electronic energy transfer, we utilise non-adiabatic perturbation theory. Our simulations and the perturbative analysis both show that the presence of a gap state with a time-dependent energy eigenvalue allows electronic excitations for very low energy (eV) channelling atoms.