Molecular dynamics collision cascade

SIMS is a very surface-sensitive technique because the emitted particles originate from the uppermost one or two monolayers. The dimensions of the collision cascade are rather small and the particles are emitted within an area of a few nanometers diameter. Hence, SIMS can be used for microanalysis with very high lateral resolution (50 nm to 1 pm), provided such finely focused primary ion beams can be formed. Furthermore, SIMS is destructive in nature because particles are removed from the surface. This can be used to erode the solid in a controlled manner to obtain information on the in-depth distribution of elements.109 This dynamic SIMS mode is widely applied to analyze thin films, layer structures, and dopant profiles. To receive chemical information on the original undamaged surface, the primary ion dose density must be kept low enough (<1013 cm-2) to prevent a surface area from being hit more than once. This so-called static SIMS mode is used widely for the characterization of molecular surfaces (see Figure 3.10). 

The assumption of one collision partner being at rest initially, assumption (d), has been made in all previous work except molecular-dynamics computations. It is not fulfilled in very dense collision cascades, especially when the process of energy dissipation has proceeded to the point where most of the atoms in the cascade are in motion. 

The energy provided by the impacting primary particle causes a collision cascade in the upper atomic or molecular layers of the sample. Within 30-60 ps, a cylindrical expansion is effected in the sample along the path of penetration [22]. Not all of this energy is dissipated and absorbed in deeper sample layers. A portion is directed toward the surface, where it effects ejection of material into the vacuum (Fig. 10.4) [24]. Due to the primary particle flux employed, this mode of operation corresponds to the dynamic SIMS mode as described in Chap. 15.5.1 in more detail. 

Molecular-dynamics calculations provide valuable insight into the evolution with time of defect structures created in the collision caiscade. Consider, for example, the molecular-dynamics simulations of low-energy displacement cascades in the Bll-ordered compound CuTi (Figure 7) by Zhu et al, (1992). Figure 8 shows the number of Frenkel pairs produced by a Cu primary knock-on atom (PKA) as a function of recoil energy at the end of the collisional phase (0.2 p ) and at the end of the cooling phase (2.5 ps). The number of Frenkd... 

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