Predictive modeling of the impact of energetic particle radiation on bulk semiconductors is important to understand whether swift heavy ions exacerbate damage through cascade formation or mitigate damage through annealing. At the same time, highly energic light projectiles also significantly drive the electronic system of target materials out of equilibrium and trigger interesting dynamics. Such scenarios cannot be modeled by the Born-Oppenheimer approximation and instead require a description of the quantum dynamics of electrons. Furthermore, it is necessary to take the multi-length and time-scale character of radiation-material interactions into account. In this talk we will discuss two examples of using Ehrenfest molecular dynamics and real-time time-dependent density functional theory to describe electronic stopping during the early stages of radiation damage. First, we will illustrate how the excited-electron distribution after impact of a proton near an oxygen vacancy in MgO leads to enhanced oxygen diffusion via a novel hot-electron mediated mechanism. To this end, we propose a first-principles simulation framework that bridges time scales from ultrafast electron dynamics directly after impact, to atomic diffusion in the presence of hot electrons. We use a rate equation to estimate the conditions under which this diffusion enhancement should be directly observable in experiment. Second, we explore the ultrafast dynamics of electrons and projectile charge for self-irradiated silicon. We found that in this case, electronic stopping significantly depends on the initial charge state of the projectile ion, which was not observed for light projectiles. We explain this behavior by accounting for charge dynamics and we explicitly study the influence of the impact parameter as well as contributions of core and valence electrons. Our simulations allow us to disentangle stopping contributions due to core electrons and expose different time scales for projectile charge equilibration, depending on the trajectory.

Ultrafast electron dynamics due to electronic stopping in bulk semiconductors

2019 Spring National Meeting

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