Low-energy primary knock on atom damage distributions near MeV proton beams focused to nanometre dimensions

Abstract

In this preliminary study, the spatial extent of the defects introduced in Si by a normally incident 1 MeV H + nanobeam was investigated using a hybrid approach. First, the standard SRIM code was employed to calculate the angular and energy distributions of Primary Knock-on Atoms (PKA) using the Binary Collision Approximation (BCA). The long mean free path and the kinematics of scattering in a screened Coulomb potential resulted in an anisotropic PKA distribution that was mainly directed perpendicular to the primary ion trajectories. The PKA energy E dependence was close to 1/ E n with 1.5 < n < 2 such that the vast majority of PKAs had low energies (⩽40 eV) close to the threshold displacement energy. Subsequently, the low energy PKA data was used as input to a Molecular Dynamics (MD) code that was used to follow the recoil cascades. The force field resulting from Si–Si atom interactions was represented in the MD calculations by the Environmental-Dependant Interaction Potential (EDIP). Finally, the spatial distribution of Self Interstitial Atoms (SIA) and vacancies was characterised using the Pixel Mapping (PM) method. The results revealed that the low energy PKA’s resulted in localised damaged regions with Si Self-Interstitial Atoms (SIA) and associated vacancies concentrated within a few nm of the ion track. Although some clustering occurred, the majority of SIA were present as monomers. This is in agreement with previous work on low energy B + implanted Si. The contribution from the spatial distribution of displacements from high energy (>40 eV) PKAs was investigated using SRIM. It was found that within the maximum region of the SIA concentration (1–2 nm radius of the ion track) the contribution from high energy PKA’s is relatively unimportant (∼5%).