Trajectory dependence of electronic energy-loss straggling at keV ion energies

TL;DR

This study measures electronic energy-loss straggling of various ions in silicon, revealing velocity-dependent behavior and the influence of trajectory and charge exchange processes at keV energies.

Contribution

It provides new experimental data on ion energy-loss straggling in silicon, highlighting trajectory dependence and the limitations of existing models for heavier ions.

Findings

Straggling increases with ion velocity in channeling trajectories.

Electronic straggling plateaus or increases at low velocities for heavier ions.

Models based solely on electron-hole pair excitations do not fully explain the results.

Abstract

We have measured the electronic energy-loss straggling of protons, helium, boron and silicon ions in silicon using a transmission time-of-flight approach. Ions with velocities between 0.25 and 1.6 times the Bohr velocity were transmitted through single-crystalline Si(100) nanomembranes in either channelling or random geometry to study the impact parameter dependence of energy-loss straggling. Nuclear and path length contributions to the straggling were determined with the help of Monte Carlo simulations. Our results exhibit an increase in straggling with increasing ion velocity for channelled trajectories for all projectiles as well as for protons and helium in random geometry. In contrast for heavier ions, electronic straggling at low velocities does not decrease further but plateaus and even seems to increase again. We compare our experimental results with transport cross section calculations. The satisfying agreement for helium shows that electronic stopping for light ions is dominated by electron-hole pair excitations, and that the previously observed trajectory dependence can indeed be attributed to a higher mean charge state for random trajectories. No agreement is found for boron and silicon indicating the breakdown of models based solely on electron-hole pair excitations, and that local electron-promotion and charge-exchange events significantly contribute to energy loss at low velocities.