Nanoscale modification of silicon surfaces via Coulomb explosion

Abstract

Coulomb explosions on silicon surfaces are studied using large-scale molecular-dynamics simulations. Processes under investigation begin by embedding a region consisting of 265-365 singly charged Si ϩ ions on a Si ͓111͔ surface. The repulsive electrostatic energy, initially stored in the charged region, leads to a local state with ultrahigh pressure and stress. During the relaxation process, part of the potential energy propagates into the surrounding region while the remainder is converted to kinetic energy, resulting in a Coulomb explosion. Within less than 1.0 ps, a nanometer-sized hole on the surface is formed. A full analysis of the density, temperature, pressure, and energy distribution as functions of time reveals the time evolution of physical properties of the systems related to the violent explosive event. A shock wave that propagates in the substrate is formed during the first stage of the explosion, 0ϽtϽ100 fs. The speed of the shock wave is twice the average speed of sound. After the initial shock the extreme nonequilibrium conditions leads to ultrarapid evaporation of Si atoms from the surface. Qualitatively similar features are observed on a smaller scale when the number of initial surface charges is reduced to 100. Our simulations demonstrate the details of a process that can lead to permanent structure on a semiconductor surface at the nanoscale level. The work reported here provides physical insights for experimental investigations of the effects of slow, highly charged ions (Q Ͼ40, e.g.͒ on semiconductor materials.

Kinetically Assisted Potential Sputtering of Insulators by Highly Charged Ions

“…Depending on the surface material and/or the charge state and impact energy of the projectiles, several complementary models have been suggested to explain PS. The "Coulomb explosion" model [9,10] has long been favored, but with the exception of proton sputtering from hydrocarbon covered surfaces [8,11] has failed to provide even a semiquantitative interpretation of experimental data [12]. For GaAs a model to explain the observed high sputtering yields [5] was recently suggested, which involves structural instabilities arising from the destabilization of atomic bonds due to a high density of electronic excitation [13] produced during the neutralization and penetration of very highly charged ions with typically 500 keV where the kinetic energy exceeds the available potential energy.…”

First Direct Observation of Coulomb Explosion during the Formation of Exotic Atoms

“…In molecules, the emission of binding electrons is ultimately accompanied by an energetic fragmentation of the atoms, a phenomenon referred to as Coulomb explosion. The fragmentation is usually achieved by intense laser fields [1,2], heavy-ion bombardment [3,4], stripping in thin foils [5], or soft x rays [6].…”

Threshold for Potential Sputtering of LiF

“…For impact of ions in very high charge states q (up to q 70 for Th) on uranium oxide, it has been speculated [3] that a "Coulomb explosion" mechanism [9,10] is responsible for the observed strong increase of ablation and secondary ion yields with q. For comparably highly charged ions on GaAs, a model involving structural instabilities arising from the destabilization of atomic bonds due to a high density of electronic excitations [11] was invoked to explain the observed high sputtering yields [4].…”