Directional Mass Transport by Momentum Transfer from Ion Beam to Solid

Abstract

Irradiation of InP with MeV Se ions directed a few degrees off the surface normal leads to a lateral displacement of the implanted material. Adjacent surface regions that are not irradiated remain unaffected. The dependence of the displacement on ion energy and its directionality strongly suggest that the effect is caused by momentum transfer from the ion to the solid. The amount of displaced material as a function of ion beam energy or angle of incidence can be described with a simple model incorporating a shear stress, applied to the target as the ion slows down, and radiation enhanced viscosity.PACS numbers: 61.80.Jh, 68.55.Ln, 79.20.Nc When an energetic ion penetrates a solid it is decelerated via two different processes: elastic collisions with target nuclei and electronic excitation of target atoms. Both processes are regarded normally as energy loss processes, and for an evaluation of macroscopic consequences of ion irradiation the momentum of the incident ion is not considered. For example, a method for estimating the number of displaced atoms generated by the ion is to evaluate the rate of energy loss into elastic collisions [1]. Some results have been reported which indicate that energy loss may not be the full story, and that the ion momentum has to be considered as well. In the case of a high fluence, high Aux, and high temperature keV As implantation into Si, the material became porous near the surface, whereas deeper layers contained dislocation loops [2]. This defect structure suggested that microscopic momentum transfer from the ion to target atoms during elastic collisions resulted in a spatial separation of vacancies and interstitials, with the vacancies remaining near the surface and the interstitials kicked into deeper regions. An observation of a different nature is that during ion irradiation of amorphous metal alloys, at several hundred MeV, thin films become larger but thinner, as if the films were between a hammer and anvil [3]. It has been explained in terms of Coulomb explosions in the wake of the ions. This explanation has met with some controversy, since Coulomb explosions are more likely to occur in insulators than in conductors.In this Letter we present experimental observations that are strongly suggestive of macroscopic momentum transfer effects. Patches of InP were found to be displaced laterally following off-normal, MeV ion implantation.The displacement is orders of magnitude larger than the vacancy-interstitial separation observed as a result of microscopic momentum transfer. The energy dependence of the process shows that the momentum transfer occurs along the entire ion track, i.e. , not only in the elastic collision but also in the electronic excitation regime. 2.5 2.0-BEAM CL tt1

Coupling of morphology to surface transport in ion-beam irradiated surfaces: Oblique incidence

“…Likewise, we will neglect momentum transfer in the direction of the projection of the beam of ions to adatoms, as this is expected to be non-negligible only at higher energies (say, above 10 3 keV, see e.g. [36] and a discussion in [2]).…”

“…we seek to solve for the various orders R n , h n by substituting the above expansions into (35) and (36). Note that from Eq.…”

Anisotropic deformation of colloidal particles under MeV ion irradiation

“…From the known sample geometry during irradiation we can exclude gravity as a cause of the eect. Two possible explanations of the particle tilt come to mind: (1) momentum transfer [20] from the incident ions to the particle which then rotates around its contact point with the substrate; (2) a gradual change in the geometry of the contact area between the colloid and the substrate due to the particle expansion. Because no dierence in contact angle is observed for small (290 nm diameter) and large ($ 1 lm diameter) irradiated silica colloids, a geometric eect seems to be more likely than a ballistic eect.…”

Evolution of magnetic pillars using 200MeV Ag15+ ion irradiation

“…4), was found to be 0.29 ± 0.01, which indicate that surface is relaxed by process of surface diffusion. Therefore, it is clear from the above results that evolution of magnetic pillars on the surface films suggests a mechanism as given by Cliché et al [13] during ion irradiation. In order to understand evolution of these pillars like structure, we emphasize on the lateral mass transport and dewetting.…”