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

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 channeling 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 channeled 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.

Automated summary

We measured the energy-loss straggling of different ions in silicon using a transmission time-of-flight method. The straggling showed an increase with increasing ion velocities for channeled trajectories and for protons and helium in random geometry. However, for heavier ions, the electronic straggling did not decrease further but plateaued or even increased again at low velocities. Comparison with calculations showed that electronic stopping for light ions is dominated by electron-hole pair excitations, but for boron and silicon, local electron-promotion and charge-exchange events significantly contribute to energy loss at low velocities.