Damage generation mechanisms

During irradiation, energetic particles penetrate into the target and lose their energy through interactions with its atoms. A target atom is displaced from its lattice position when it receives from these interactions an energy higher than the displacement energy threshold E_d. For energy transfers above E_d, the target atom can become a recoil leaving behind a vacancy and generating an interstitial defect where it stops, in addition to further displacements that can be produced as it travels through the material. When energy transfers to target atoms are well above E_d, damage generation can be regarded as a ballistic process. As energy transfers decreases in magnitude and approach E_d, multiple interactions with target atoms become important. These multiple interactions can result in a local melting (thermal spike regime) and the subsequent generation of amorphous regions at energy transfers per atom much lower than E_d.

We have used classical molecular dynamics simulations to study damage generation in Si and Ge. We have developed a general framework for damage generation that captures the transition from the ballistic to the thermal regime. It has been successfully applied to monatomic and molecular implantations, where non-linear effects on damage generation occur. Although the model was based on semiconductors, it can be applied with appropriate calibration to other materials.

Displaced atoms from a cascade of 1 keV of B (light ion) and Ge (heavy ion) into Si: light ions produce scattered damage (ballistic regime), while heavy ones generate compact amorphous pockets (thermal spike regime). Arrows indicate the ion impact position, and the line the ion trajectory. Si atoms at lattice positions are not shown for clarity.