Coulomb explosion as a probe to understand the mechanism of electron stripping from ions interacting with crystalline solids

When an ion impinges on a solid, it rapidly undergoes a process in which its electrons are stripped away provided the velocity of the orbiting electrons is smaller than the projectile speed. Electron stripping determines any posterior behavior of the ions in the solid, and it is assumed that it takes place on the surface of the solid, but no information is available on the details of the process. Here we show, using the Coulomb explosion of ${\text{C}}_2^{+}$ ions moving in Si as a tool, that electron stripping takes place in an orderly manner and that the number of electrons stripped, before charge equilibration, depends on a characteristic length. We also propose a relation capable of quantifying this dependence. We foresee these results as a starting point to a more general understanding of ion-solid interaction, with important consequences on ion beam analysis and modification techniques, and special significance in silicon technology.

Figure 1

(Color online) Experimental and simulated backscattering yields. Left: RBS measured spectrum (open circles) with ${\text{C}}^{+}$ beams on silicon for three channeling directions. Also shown the RBS simulated spectra before (dashed line) and after convolution (fold. sim.) with the experimental resolution (solid line). From these results it is clear that the simulations describe adequately the experimental yield obtained from the atomic beams. Right: RBS measured spectrum (open circles) with ${\text{C}}_2^{+}$ beams compared with simulations, showing the effect that different values of $Q_{\alpha }{Q}_{\beta }$, the product of the individual charges of each atom that constitutes the ${\text{C}}_2$ molecule, have on the spectra. The different energy scale for $⟨100⟩$ is due to the smaller kinematic factor. For purposes of display the data has been normalized to the surface peak maximum.

Figure 2

(Color online) The signature of the Coulomb explosion. Integrated intensities of the Rutherford backscattering spectra measured in channeling configuration (RBS/C) before (top) and after (bottom) damage correction. The second irradiation step corresponds to ${\text{C}}_2^{+}$ beams. Dashed lines are a fit to the atomic irradiations, showing the linear character of the radiation-induced damage. The damage correction was carefully done by calculating the average yield increase per irradiation step of an atomic beam and subtracting this quantity times the irradiation number. The increase in yield from atomic to molecular beam is the signature of the Coulomb explosion and is the quantity to compare with the simulations.

Figure 3

(Color online) Matching of the computer simulated yield increment to the experimental values for each channel and a representation of the channels. Top: difference in integrated intensities between ${\text{C}}_2^{+}$, $I\left(m\right)$, and ${\text{C}}^{+}$, $I\left(a\right)$, in the bulk region of the simulations and their intersection with the experimental values depicted in Fig. 2. From this intersection, the ion’s charges are inferred. Bottom: image of the [111], [211], and [100] channels. For $⟨111⟩$ the fractional coordinates of the atoms that form the channel are of type: (0, 0, 0), (1/4, 1/4, 1/4), (1/2, 0, 1/2), (3/4, 1/4, 3/4), (1/2, 1/2, 1), (3/4, 3/4, 5/4), and (1, 1, 1); for $⟨100⟩$: (0, 1/2, 1/2), (1/4, 3/4, 3/4), (1/2, 1/2, 1), (3/4, 1/4, 3/4), and (1, 1/2, 1/2); and for $⟨211⟩$: (1/4, 1/4, 1/4), (0, 1/2, 1/2), ${\left(1/2,0,1/2\right)}^{\ast }$, ${\left(1/4,3/4,3/4\right)}^{\ast }$, (3/4, 1/4, 3/4), (1/2, 1/2, 1), ${\left(1,1/2,1/2\right)}^{\ast }$, ${\left(3/4,3/4,5/4\right)}^{\ast }$, (5/4, 3/4, 3/4), and (1, 1, 1). Those positions labeled with asterisks are farther from the channel center.