Electron Elevator: Excitations across the Band Gap via a Dynamical Gap State

Area of Science:

• Materials Science
• Condensed Matter Physics
• Computational Physics

Background:

• Understanding ion-solid interactions is crucial for materials modification and device performance.
• Electronic stopping power, the energy transferred to electrons, is a key parameter in ion-matter interactions.
• The traditional view assumes electronic stopping is negligible below a specific threshold velocity related to band gap transitions.

Purpose of the Study:

• To investigate the electronic stopping power of silicon (Si) in Si using time-dependent density functional theory.
• To determine the energy transferred to electrons by ions moving through silicon across a range of kinetic energies (1 eV to 100 keV).
• To analyze the velocity dependence of electronic stopping and identify underlying excitation mechanisms.

Main Methods:

• Time-dependent density functional theory (TDDFT) calculations.
• Evaluation of energy transfer to electrons per unit path length for ions in silicon.
• Analysis of transition rates using coupled linear rate equations to study time dependence.

Main Results:

• Electronic stopping power in silicon is significant even at velocities below the conventional band gap transition threshold.
• A structured crossover, rather than a sharp threshold, characterizes electronic stopping at low velocities.
• A defect state induced within the band gap by the moving ion was identified as a primary mechanism for electron excitation.

Conclusions:

• The traditional threshold velocity model for electronic stopping in silicon is insufficient.
• Ion-induced defect states play a critical role in facilitating electronic excitations across the band gap.
• This finding has implications for understanding radiation damage and ion implantation processes in semiconductors.