Assessing trajectory-dependent electronic energy loss of keV ions by a binary collision approximation code

Abstract

The inelastic energy deposition of energetic ions is a decisive quantity for numerous industrial-scale applications, such as sputtering and ion implantation, yet the underlying physics being governed by dynamic many-particle processes is commonly only qualitatively understood. Recently, transmission experiments on single-crystalline targets (Phys. Rev. Lett. 124, 096601 & Phys. Rev. A 102, 062803) revealed a complex energy scaling of the inelastic energy loss of low-energy ions heavier than protons along different trajectories. We use a Monte Carlo like binary collision approximation code equipped with an impact-parameter-dependent modeling of the inelastic energy losses to assess the role of local contributions to electronic excitations in these cases. We compare angular intensity distributions of calculated trajectories with experimental results for 50-keV 4He and 100-keV 29Si ions transmitted in a time-of-flight setup through single-crystalline silicon (001) foils with nominal thicknesses of 200 and 50 nm, respectively. In these calculations, we employ different models of electronic energy loss, i.e., local and nonlocal forms for light and heavy projectiles. We find that the vast number of projectiles are eventually channeled along their trajectories, regardless of the alignment of the crystal with respect to the incident beam. It is, however, only when local electronic energy loss is considered that the simulated two-dimensional maps and energy distributions show excellent agreement with the experimental results, where channeling leads to significantly reduced stopping, especially for heavier projectiles. We demonstrate the relevance of these effects for ion implantations by assessing the nonlinear and nonmonotonic scaling of the ion range with the thickness of a random surface layer. Published by the American Physical Society 2024