The electron elevator: excitations across the band gap via a dynamical gap state?

TL;DR

This study uses time-dependent density functional theory to analyze electronic stopping in silicon, revealing a defect-induced gap state that facilitates electron excitation across the band gap at low ion velocities.

Contribution

It identifies a defect state acting as an electron elevator, providing a new mechanism for electronic excitations during ion irradiation in silicon.

Findings

Electronic stopping power is significant below the traditional threshold velocity.

A structured crossover replaces the hard threshold in low-velocity regimes.

A defect-induced gap state enables electron transfer across the band gap.

Abstract

We have used time-dependent density functional theory to study self-irradiated Si. We calculate the electronic stopping power of Si in Si by evaluating the energy transferred to the electrons per unit path length by an ion of kinetic energy from $1$ eV to $100 $ keV moving through the host. Electronic stopping is found to be significant below the threshold velocity normally identified with direct transitions across the band gap. A structured crossover at low velocity exists in place of a hard threshold. An analysis of the time dependence of the transition rates using coupled linear rate equations enables one of the excitation mechanisms to be clearly identified: a defect state induced in the gap by the moving ion acts like an elevator and carries electrons across the band gap.